FUNGI

FUNGI. Fungi—sing. fungus; from the Greek sphongis (sponge)—are nonphotosynthetic and thus must absorb nutrients from organic matter formed by other organisms. The great majority of fungi obtain their food from dead organic matter and hence are known as saprophytes; a relatively small percentage derive their food from other living organisms and are known as parasites. Fungi may be unicellular (yeasts) or multicellular (mushrooms) and their cell walls usually contain chitin or cellulose and bglucan. They may produce sexually or asexually by means of spores that are roughly comparable with the seeds of higher plants.

The fungal kingdom offers enormous biodiversity with over seventy thousand known species and an estimated 1.5 million species. According to molecular evidence (16S-like ribosome RNA sequences), the fungi may have originated from protozoan ancestors before the kingdoms Animalia and Plantae split; there is strong evidence that Fungi are closer to Animalia than Plantae (Hawksworth et al.). Fungi are associated with some of the earliest remains of land plants. Some scientists believe that lichens (a stable self-supporting association of a fungus and an alga) might be transmigrants, the earliest colonizers of land.

Fungi have contributed to the shaping of humankind's welfare since the beginning of civilization. Fungi are recognized as both beneficial and harmful in their relationship to humans although this role is predominantly beneficial. They are responsible for a major portion of food deterioration in developing countries; however, the preservative effects of fermentation of foods and beverages with fungi are well-known benefits, including organic acids, alcohol, antibiotics, pigments, vitamins, growth regulators, immunomodulating agents, and enzymes. Finally, various types of edible mushrooms are consumed as an important part of human diets in many countries.

Fungi and Food Processing

Fungi used in food processing have been an integral part of the human diet since the beginning of civilization. In such foods, fungi are the agents responsible for imparting special flavors, textures, odors, or consistencies to food products. Fungi such as Aspergillus spp., Rhizopus spp., Penicillium spp., Neurospora spp., Cladosporium spp., and Mucor spp., as well as yeasts and many others have long been used to process a number of food products from soybeans to peanuts, rice, gram, maize, cassava, taro, and cacao beans.

Fungal enzymes. Food formulation using enzymes derived from fungi has undergone a rebirth in recent years. Enzyme suppliers have improved their ability to supply single-activity enzymes that do not have undesirable side activities (see Table 1 for a list of commercial fungal enzymes and their uses). Enzyme products have found increasing application for improving product clarity and yield and in replacing costly physical processes such as heating.

Cheese manufacture. Two general types of cheese are made with fungi as the ripening agents. Roquefort cheese is an example of cheese that is ripened primarily by growth of fungi (Penicillium roquefortii ) throughout the cheese mass. Brie cheese is an example of one type of soft cheese that is ripened by the growth of fungi (Penicillium camemberti ) on the outside of the cheese mass. In both types of cheeses, the fungi grow and release protein and fat-degrading enzymes that soften and ripen the cheese. Roquefort cheese requires about two months to ripen while Brie cheese requires only about one month to ripen.

Baker's yeast. Leavening, a process whereby batter or dough is caused to rise via the production of gas, especially carbon dioxide, was first discovered in Egypt. Today, most of the bread, cakes, cookies, and the like consumed by the public are prepared from leavened batter

or dough. Most cakes and cookies are leavened chemically (by using baking powder) while most bread is leavened by yeasts (such as Saccharomyces cerevisiae ). Yeasts develop and reproduce by producing buds on mother cells that subsequently enlarge and produce more buds. During growth, carbohydrates in the dough are metabolized to carbon dioxide that is trapped in the dough in the form of bubbles. During the leavening process, alcohol may accumulate in the dough to as high as 0.5 percent. The alcohol is driven off during baking and helps give the bread a pleasant aroma.

Edible Mushrooms

Mushrooms have a long history of human consumption. Traces of puffball fungi have been found in Stone Age settlements. Over 4,500 years ago in ancient Egypt only pharaohs were permitted to eat mushrooms, which they believed were "sons of the gods" sent down to earth on lightning bolts announced by claps of thunder. The legend that mushrooms may have originated from thunder and lightning also existed among people of other ethnic groups. In Roman folklore, some fungi were believed to spring from the ground in places struck by a thunderbolt. In the Hindu tradition, there was a god named Soma that manifested himself to the priests in the form of hallucinogenic fluids. Some scientists believe that Soma was the fly mushroom, Amanita muscaria. A similar legend may have existed among the inhabitants of the highlands of Guatemala and Mexico, where even today the people refer to A. muscaria by a common name meaning thunderbolt (Lowy).

Cultivated species. The cultivation of edible mushrooms worldwide reached 6.16 million metric tons in 1997, up from 1.26 million tons in 1981 (Table 2; Chang). This represents a 12 percent annual increase. Six mushroom genera accounted for 87 percent of the total mushroom supply (Table 2). These were Agaricus (31.8%), Lentinula (25.4%), Pleurotus (14.2%), Auricularia (7.9%), Flammulina (4.6%), and Volvariella (3%). China produced 3.92 million tons of mushrooms in 1997, or 63.6 percent of the total world output. The major mushroom of commerce in China is L. edodes, which accounts for 35 percent of the total output for that country. China currently produces 88 percent of the total world production of L. edodes.

Agaricus bisporus (button mushroom). The cultivation of the button mushroom originated in the Paris region in France. Melon growers in this region discovered how mushrooms could be grown and started cultivating them in 1650. By the mid 1700s it was discovered that A. bisporus could grow without light, and that very favorable conditions for growing mushrooms prevailed in subterranean tunnels and caves. As a result of this discovery,

successful culture was undertaken inside the numerous caves that were excavated for building stones and for gypsum. The caves presented, from a climatic point of view, several advantages over the previous growing conditions in open air. Factors such as temperature and relative humidity were much more constant in caves compared with aboveground conditions.

From France, mushroom cultivation spread to other parts of the world. The business grew and soon spread to England and other countries. By 1825, the first mushroom crops were being produced in caves in Holland. In 1865, mushroom culture entered the United States via England and the first mushrooms were grown on a small scale on Long Island, New York; by 1870 the industry had begun to develop.

The button mushroom is produced commercially on a selective substrate prepared by composting mixtures of wheat straw, hay, corncobs, horse manure, or combinations thereof. The finished compost should have a nitrogen (N) content of 2–2.5 percent, and to reach such a level, nitrogen-rich supplements must be added. Inorganic nitrogen supplements can be added but only to provide part of the necessary amount. Organic sources of nitrogren include oilseed meal, brewers' grain, malt sprouts, and poultry manure.

Once the compost has been prepared, it is seeded with mushroom spawn that is prepared from a mother culture maintained by a spawn laboratory. Spawn is prepared by inoculating a pure culture of the mushroom onto steam-sterilized grain, usually rye or millet. Approximately one liter (500 g) of spawn is used to seed 0.5 m² of production surface that is contained in trays or beds inside environment-controlled production houses. Spawn run (vegetative growth of the mycelium) lasts ten to fourteen days, then a layer of neutralized peat moss (casing) is placed on top of the colonized compost to stimulate production of mushrooms. Approximately ten to fourteen days after casing, mushrooms are ready for harvest.

Lentinula edodes (shiitake). Production of shiitake worldwide increased more than sevenfold in the fourteen-year period from 1983 (207,000 t) to 1997 (1,573,000 t; Chang). Most of this increase occurred in China, where more than ten million part-and full-time farmers cultivate shiitake. Shiitake is widely consumed in China, yet one-third of production is exported. In 1997, China produced approximately 88 percent of the total world output (Chang). In the United States, production of shiitake is a relatively new enterprise, having begun only in the late 1970s. In 1990, the United States produced 1,123 tons of shiitake and by 1999 production reached 3,941 tons, a 3.5-fold increase (USDA). This increase in production was due, in part, to increased production efficiency and to increasing consumer demand. Farmers have learned to provide the specialized management this crop requires, thereby reducing production costs. The amount of controlled-environment production surface devoted to growing shiitake on synthetic logs has increased 2.9 fold from 1990 to 1999 (74,200 m² to 212,400 m², respectively).

Sawdust is the most popular basal ingredient used in synthetic formulations of substrate for producing shiitake in the United States, but other basal ingredients may include straw, corncobs, or both. Starch-based supplements (20–60 percent dry weight) such as wheat bran, rice bran, millet, rye, and maize may be added to the mix. These supplements serve as nutrients to provide a more optimal growth medium (Royse).

Pleurotus spp. (oyster mushroom). Oyster mushroom production increased at a rapid rate worldwide during the 1980s and then decreased slightly during the 1990s (Table 2). From 1986 to 1997, oyster mushroom production increased from 169,000 tons to 917,000 tons (a 5.4-fold increase). China was responsible for most of the production increase. In the United States, production of oyster mushrooms was 1,647 tons in 2001, up 2 percent from the previous year (USDA).

In the United States, the primary ingredients used for Pleurotus spp. production are chopped wheat straw or cottonseed hulls or mixtures thereof. After completion of pasteurization (140°F [60°C] for one to two hours) the substrate is cooled and spawned with the desired strain. There are several species of oyster mushrooms cultivated, with various colors of fruiting body. In Japan, bottle production of oyster mushrooms is most common. Substrate is filled into bottles, sterilized, and inoculated with Pleurotus spawn. Upon completion of the spawn run, bottle lids are removed and mushrooms emerge from the surface of the substrate. After the mushrooms are harvested they are weighed and packaged for shipment to market.

Auricularia spp. (wood ear mushroom). Total production of Auricularia spp. in 1997 exceeded 485,000 metric tons (fresh weight; Table 2). This value is an increase of 366,000 tons or fourfold over 1986 levels (Chang). Auricularia spp. production now represents about 8 percent of the total cultivated mushroom supply worldwide.

Auricularia auricula and A. polytricha commonly are produced on a synthetic medium consisting of sawdust, cottonseed hulls, bran, and other cereal grains or on natural logs of broadleaf trees. For synthetic medium production of Auricularias, the substrate may be composted for up to five days or used directly after mixing. The medium is filled into heat-resistant polypropylene bags and sterilized (substrate temperature 240°F [121°C]) for sixty minutes. After the substrate has cooled, it is inoculated with either grain or sawdust spawn. Light intensity of more than 500 lux during the spawn run may result in premature formation of primordia. Temperature, light intensity, and relative humidity all interact to influence the nature and quality of the mushrooms.

Flammulina velutipes (enokitake) . Worldwide production of F. velutipes has increased from about 100,000 metric tons in 1986 to about 285,000 tons in 1997. Japan is the main producer of enokitake. In the United States, enokitake production has increased at an estimated rate of 25 percent or more per year for the last four years. However, only about 60 tons of enokitake were produced in the United States in 2001.

Production of most enokitake in Japan is based on synthetic substrate contained in polypropylene bottles. Substrates (primarily sawdust and rice bran; 4:1 ratio) are mechanically mixed and filled into heat-resistant bottles with a capacity of 800 to 1,000 ml. Sawdust primarily from Cryptomeria japonica, Chamaecyparis obtusa, or aged (nine to twelve months) Pinus spp. appears to offer the best yields. In the United States, a sterilized, bran-supplemented medium, consisting primarily of corncobs, serves as the primary medium. When the substrate is fully colonized, the original inoculum is removed mechanically from the surface of the substrate and the bottles may be placed upside down for a few days.

To further improve quality during fruiting, temperatures are lowered to 37° to 46°F (3 to 8°C) until harvest. As the mushrooms begin to elongate above the lip of the bottle, a plastic collar is placed around the neck and secured with a Velcro® strip. This collar serves to hold the mushrooms in place so that they are long and straight. When the mushrooms are thirteen to fourteen cm long, the collars are removed and the mushrooms are pulled as a bunch from the substrate. The mushrooms then are vacuum packed and placed into boxes for shipment to market.

Grifola frondosa (maitake). Japan is the major producer and consumer of maitake. Commercial production of maitake in Japan began in 1981 (325 t) and by 1997 reached 32,000 tons (a 98-fold increase). Maitake is produced primarily in the Japanese provinces of Niigata, Nagano, Gunnma, and Shizuoka. Other countries, such as the United States, began maitake production in the early 1990s. Maitake production in the United States in 2001 was estimated at about 84 tons.

Most maitake is marketed as food. However, maitake has been shown to have both antitumor and antiviral properties. Powdered fruit bodies are used in the production of many health foods such as maitake tea, whole powder, granules, drinks, and tablets. Maitake also is believed to lower blood pressure, reduce cholesterol, and reduce the symptoms of chronic fatigue syndrome.

Commercial production of most G. frondosa is on synthetic substrate contained in polypropylene bottles or bags. A common substrate used for production is hardwood sawdust supplemented with rice bran or wheat bran in a 5:1 ratio, respectively. Other formulas include hardwood sawdust (70 percent based on oven dry weight basis) supplemented with white millet (20 percent) and wheat bran (10 percent). Some growers may add soil to the mix to stimulate fruit body formation. For production in bags, the moistened substrate is filled into micro-filtered polypropylene bags and sterilized to kill unwanted competitive microorganisms. After cooling (sixteen to twenty hours), the substrate is inoculated and the bags are heat-sealed and shaken to uniformly distribute the spawn throughout the substrate. Spawn run lasts about thirty to fifty days depending on strain and substrate formulation.

Volvariella volvacea (straw mushroom). Cultivation of V. volvaceae is believed to have begun in China as early as 1822. In the 1930s, straw mushroom cultivation began in the Philippines, Malaysia, and other Southeast Asian countries. Production of the straw mushroom increased from 54,000 tons in 1981 to about 181,000 tons in 1997 (about 3 percent of the total mushroom supply).

Many agricultural by-products and waste materials have been used to produce the straw mushroom. These include paddy straw, water hyacinth, oil palm bunch, oil palm pericarp waste, banana leaves and sawdust, cotton waste, and sugarcane waste. Volvariella is well suited for cultivation in the tropics because of its requirement for higher production temperatures. In addition, the mushroom can be grown on nonpasteurized substrate, which is more desirable for low-input agricultural practices.

In recent years, cotton wastes (discarded after sorting in textile mills) have become popular as substrates for straw mushroom production. Cotton waste gives higher and more stable biological efficiencies (30 to 45 percent), earlier fructification (four days after spawning) and harvesting (first nine days after spawning) than that obtained using straw as a substratum. Semi-industrialization of paddy straw cultivation on cotton wastes has occurred in Hong Kong, Taiwan, and Indonesia as a result of the introduction of this method.

Wild mushrooms. In many developing countries, the collection and sale of wild edible mushrooms has become an important source of income for many people in remote forested regions. Despite a relatively short growing season, wild mushrooms provide many families with 50 to 100 percent of their income. World trade in wild, edible mushrooms is estimated at more than $7 billion annually (Arora). The global trade in matsutake (Tricholoma matsutake ), the most expensive wild mushrooms after truffles, is estimated at $3 to $5 billion. Matsutake may sell for as much as $200 apiece in Tokyo markets. The King Bolete (Boletus edulis ; also known as porcini, cepe, borovik, etc.) is the most popular wild mushroom of Europe. These may be served fresh in some upscale restaurants. Dried boletes are famous for their concentrated flavor and choice aroma and are available year round from almost anywhere in the world. Other wild mushrooms available on world markets include chanterelles (Cantharellus cibarius ), morels (Morchella spp.), hedgehog mushrooms (Hydnum repandum ), lobster mushrooms (Hypomyces lactifluorum ), candy caps (Lactarius fragilis ), and cauliflower mushrooms (Sparassis crispa ).

WARNING: Collecting and ingesting wild mushrooms without the presence of an expert to correctly identify specimens can be very dangerous and should be discouraged since there are several deadly mushrooms that look like edible wild ones.

Mycotoxins

Mycotoxins are chemical compounds produced by fungi growing on organic substances such as corn, cottonseed, or peanuts that, when ingested, have some undesirable effect on humans or on an animal consuming them. Adverse effects can range from vomiting to weight loss, various types of tumors, and in some cases, death. Over one hundred toxic compounds produced by fungi have been identified, and about forty-five of these occur in grain crops. Some mycotoxins are rare in occurrence while others such as aflatoxin are common in some years. The seriousness of the mycotoxin problem varies with the year, the crop being grown, and the intended use of the crop product. Most mycotoxins affect the blood, kidneys, skin, or central nervous system, and some may cause cancer.

The genera of fungi of greatest importance to humans with respect to natural poisoning outbreaks are Aspergillus, Penicillium, and Fusarium. The Aspergillus flavus group produces aflatoxins (at least eighteen types known) that are considered the most important from the viewpoint of a direct hazard to human health. Aspergillus flavus is a common fungus that is found in soil, air, and decaying plant residues. Infection by A. flavus and subsequent aflatoxin production can occur in the field, in transit, or in storage. Most reports indicate that infection occurs in the field, while aflatoxin production can occur whenever the product is exposed to favorable conditions, either in the field or in storage.

Control of aflatoxin includes prevention of fungal growth, removal of toxins, and inactivation of toxin. Most control efforts have been directed toward control of aflatoxins in peanuts and corn. Hand picking, electronic sorting, and air classification accomplish control of aflatoxin in processed peanut products. Removal of shriveled, rancid, or discolored kernels has proven the most practical way of limiting aflatoxin contamination in peanuts.

BIBLIOGRAPHY

Arora, D. "The Global Mushroom Trade." California Wild 52, no. 4 (fall 1999):16–17.

Beuchat, Larry R. Food and Beverage Mycology. 2d ed. New York: Van Nostrand Reinhold, 1987.

Chang, S. T. "World Production of Cultivated Edible and Medicinal Mushrooms in 1997 with Emphasis on Lentinus edodes (Berk.) Sing. in China." International Journal of Medicinal Mushrooms 1 (1999):273–282.

Findlay, W. P. K. Fungi: Folklore, Fiction, and Fact. Eureka, Calif.: Mad River Press, 1982.

Fine, Gary Alan. Morel Tales: The Culture of Mushrooming. Cambridge: Harvard University Press, 1998.

Friedman, Sara Ann. Celebrating the Wild Mushroom. New York: Dodd, Mead, 1986.

Hawksworth, D. L., P. M. Kirk, B. C. Sutton, and D. N. Pegler. Ainsworth and Bisby's Dictionary of the Fungi. Wallingford, U.K.: CAB International, 1995.

Lowy, B. "Amanita muscaria and the Thunderbolt Legend in Guatemala and Mexico." Mycologia 66 (1974): 188–190.

Moore-Landecker, Elizabeth M. Fundamentals of the Fungi. 4th ed. Upper Saddle River, N.J.: Simon & Schuster, 1996.

Royse, Daniel J. "Specialty Mushrooms and Their Cultivation." Horticultural Reviews 19 (1997): 59–97.

United States Department of Agriculture. Mushrooms. Washington, D.C.: National Agricultural Statistics Service, Agricultural Statistics Board, 2001.

Wasson, R. Gordon. Soma: Divine Mushroom of Immortality. New York: Harcourt Brace Jovanovich, 1968.

Daniel J. Royse

Mushroom Collectors

The consumption of wild mushrooms has a lengthy history, dating back well over two millennia and extending throughout the world. For over two hundred years, mushrooms have been a cultivated crop as well. Despite somewhat negative images of mushrooms in the popular imagination and despite the possibility of real danger in their consumption, they have long been valued for their culinary and psychedelic properties. In 300 b.c. Theophrastus recorded that mushrooms were valued as food and for trade. Pliny, Juvenal, Martial, and Cicero all considered mushrooms to be great delicacies, and the Roman emperor Claudius was allegedly poisoned by a plate of mushrooms. Mushrooms are also mentioned in the Hindu Rig Veda and were eaten on the Indian subcontinent. Mushrooms were probably consumed for food and for their psychedelic properties in Mesoamerica, Siberia, and Scandinavia. Some suggest that the biblical "manna from heaven" was a fungus. By the eighteenth-century reign of Louis XIV, mushrooms were cultivated in caves near Paris. During the nineteenth century mushrooming became a popular leisure pursuit in Europe and America, and by the end of the century mushroom societies were formed.

One estimate placed the number of mushroomers in the United States at thirty million in the early 1980s. A survey conducted at the same time found that 22 percent of Americans collect wild mushrooms, and 15 percent consume mushrooms they find. In the nations of eastern, central, and southern Europe with stronger mushroom cultures, these figures would likely be higher. Mushroom societies are found in every region of the United States, as well as Canada and Europe. In the United States, mushroom societies were founded in Boston and Minneapolis in the late nineteenth century. The North American Mycological Association, covering the United States and Canada, has approximately 2,000 members. These clubs organize talks, dinners, sharing of advice, and forays to mushroom collecting sites.

Novices worry about the toxic qualities of wild mushrooms. Despite this, the number of mushroom fatalities, at least in the United States, is very low. In some years, there are no fatalities although illnesses or hospitalizations might occur as a result of the misidentification of mushrooms, the contamination of otherwise edible specimens, or allergic reactions. Among the edible wild mushrooms that are most widely collected in the United States and Europe are morels, chanterelles, puffballs, boletes, and coral mushrooms. While the collection of wild mushrooms has increased in the past decades, the hobby is limited, and the greatest growth in "wild mushrooms" is likely to occur when these foods become cultivated and therefore perceived as safe to consume.

Gary Alan Fine

Fungi

General characteristics

Nutrition and ecology

Evolution

Classification

Zygomycota, conjugating fungi

Ascomycota, sac fungi

Basidiomycota, club fungi

Deuteromycota, imperfect fungi

Mycophycophyta, lichens

Resources

Fungi are one of the five kingdoms of organisms. Like higher plants (of the kingdom Plantae), most fungi are attached to the substrate they grow on. Unlike plants, fungi do not have chlorophyll and are not photosynthetic. Another key difference from plants is that fungi have cell walls composed of chitin, a nitrogen containing carbohydrate. All fungi have nuclei and the nuclei of most species are haploid at most times. Many species have two or more haploid nuclei per cell during most of the life cycle. All fungi reproduce asexually by spore production. Most species reproduce sexually as well.

Fungi are very important in human activities. For example, some species of yeast, a type of fungus, is responsible for the brewing of beer and bread making. In another example, some types of mushrooms, another kind of fungus, are edible and have been eaten raw or used as ingredients in other food dishes for millenia.

Reflecting their importance, a branch of science called mycology evolved. Mycology is the study of fungi.

General characteristics

The different taxonomic groups of fungi have different levels of cellular organization. Some groups, such as the yeasts, consist of single-celled organisms, which have a single nucleus per cell. Some groups, such as the conjugating fungi, consist of single-celled organisms in which each cell has hundreds or thousands of nuclei. Groups such as the mushrooms, consist of multicellular, filamentous organisms that have

one or two nuclei per cell. These multicellular fungi are composed of branched filaments of cells called hyphae. The hyphae, in turn, often mass together to form a tissue called mycelium.

Mycology, the study of fungi, has traditionally included groups such as the cellular slime molds, plasmodial slime molds, water molds, chytrids, and several other groups of funguslike organisms. Most modern biologists consider these groups as diverse assemblages of organisms unrelated to the true fungi considered here. However, it should be emphasized that biologists are very uncertain about the evolutionary relationships of these other groups and the true fungi.

Nutrition and ecology

Most species of fungi grow on land and obtain their nutrients from dead organic matter. Some fungi are symbionts or parasites on other organisms. The majority of species feed by secreting enzymes, which partially digest the food extracellularly, and then absorbing the partially digested food to complete digestion internally. As with animals, the major storage carbohydrate of fungi is glycogen. Fungi lack the complex vascular system found in higher plants, so their transport of food and water is less efficient.

Along with bacteria, fungi have an important ecological role in the decomposition of dead plants, animals, and other organic matter. Thus, fungi are ecologically important because they release large amounts of carbon dioxide into the atmosphere and recycle nitrogen and other important nutrients within ecosystems for use by plants and other organisms. Some fungi are parasitic, in that they obtain their nutrients from a living host organism, a relationship which usually harms the host. Such parasitic fungi usually have specialized tissues called haustoria, which penetrate the host’s body. Parasitic fungi cause most of the diseases that afflict agricultural plants. Some examples are corn smut, black stem rust of wheat and barley, and cotton root rot. Some species of fungi can also parasitize animals. Even humans can be parasitized by fungi which cause diseases such as athlete’s foot, ringworm, and yeast infections.

Evolution

The Fungi group constitute a large and diverse group of organisms. Until the 1960s, fungi were considered members of the plant kingdom. With the advent of the five-kingdom system of biological classification, fungi were assembled into a single kingdom because of their similar ecological roles as primary decomposers of organic matter and their similar anatomical and biochemical features. Recent studies that compare the sequence of amino acids in proteins from fungi, plants, and animals now indicate that fungi share a closer evolutionary relationship to animals than to plants.

The evolutionary ancestry and relationships of the different fungi are not well understood. There are few fossils of fungi, presumably because their relatively soft tissues are not well preserved. There is some fossil evidence that they existed in the Precambrian era (over four billion years ago), although identification of these very early fossils is uncertain. There is definite fossil evidence for fungi in the lower Devonian (about 400 million years ago) period. Fossils of all the major groups of fungi are found in the Carboniferous period (about 300 million years ago). By the late Tertiary period (about 20 million years ago), the fossil record shows a rapid and divergent evolution of fungi. Many of the Tertiary fossils of fungi are similar to existing species.

In addition to the incomplete fossil record, there are at least two other reasons why the evolutionary relationships of fungi are not well-known: they tend to have simple morphologies and they lack embryos which follow a definite sequence of developmental stages. Biologists often use these two features to determine evolutionary relationships of animals and plants.

Another complication is that many fungi presumably evolved similar morphologies by convergent evolution. That is, unrelated species may share a common morphology because they have been subjected to similar selective pressures.

The relatively new technique of molecular system-atics is particularly useful in the study of the evolution of fungi. This technique compares the sequence of DNA segments of different species to determine evolutionary relationships. One important finding from this new technique is that the plasmodial slime molds, cellular slime molds, and water molds are only distantly related to the true fungi (the taxonomic groups considered here). Evolutionary relationships among organisms also are being studied by comparing the sequences of ribosomal RNAs and transfer RNAs from different organisms. Although these RNAs are similar to DNA, they have structural roles in cells, rather than coding for proteins, as do DNA and messenger RNAs. Therefore, the sequences of ribosomal and transfer RNAs tend to be more conserved through evolution. Comparison of these RNAs among fungi, plants, and animals also suggests that fungi are more closely related to animals than to plants. Future work in molecular systematics is expected to tell us more about the evolution and relationships of the fungi.

Classification

Biologists have estimated that over 200,000 species of fungi exist in nature, although only about 100,000 have been identified so far. Since classification schemes of organisms are usually based on evolutionary relationships, and the evolutionary relationships of fungi are not well known, biologists have proposed numerous classification schemes for fungi over the years. Below, we consider the five major phyla that nearly all mycologists would agree belong in the kingdom of Fungi.

Zygomycota, conjugating fungi

Species in this phylum reproduce sexually by forming a zygospore, a thick-walled, diploid cell which contains thousands of nuclei. There are about 600 species in this phylum. Most species are terrestrial and feed on organic matter, although there are a few parasitic species. The conjugating fungi are coeno-cytic, in that they have a continuous mycelium, containing hundreds or thousands of haploid nuclei, with no divisions between them. However, the Zygomycota do have septa (cross walls) between their reproductive structures and the rest of their mycelium.

The conjugating fungi have a life cycle that includes a sexual phase and an asexual phase. In the asexual phase, thousands of spores develop inside a sporangium, a small spherical structure. The sporangium grows on the tip of a sporangiophore, a specialized aerial hypha, typically about as thin as a hair.

In the sexual phase of their life cycle, these fungi form specialized hyphae, called gametangia, which are of two different strains (sexes), plus and minus. The plus and minus strains are very similar morphologically, but differ physiologically and biochemically. Plus and minus gametangia conjugate with one another and form a structure with hundreds or thousands of nuclei from each strain.

Then, a thick-walled structure, called the zygo-spore, develops from the conjugated gametangia. Inside the zygospore, the many thousands of nuclei from the plus and minus strains pair off and fuse together to form thousands of diploid nuclei. The zygospore is typically spherical in shape and has a thick, dark outer wall. It usually remains dormant for several months or more before development continues.

As the zygospore germinates, it produces germ-sporangia which are born on germsporangiophores, structures morphologically similar to the asexual sporangium and sporangiophore (see above). The germsporangium contains thousands of haploid germ-spores which arose from the diploid nuclei of the zygospore by meiosis. Each germspore is liberated, germinates, and gives rise to a new haploid mycelium.

One of the best known of the conjugating fungi is Phycomyces blakesleeanus, a species which grows on animal feces in nature. The sporangiophores of Phycomyces respond to a variety of sensory stimuli. For example, they bend in response to light (photo-tropism), gravity (gravitropism), wind (anemotrop-ism), and nearby objects (avoidance response). Physiologists and biophysicists have intensively studied the response to light. One important finding is that the light sensitivity of the sporangiophore is about the same as the eyes of humans. Furthermore, like humans, the sporangiophore can adapt to a one-billion-fold change in ambient light intensity. One of the pigments involved in the extraordinary light responses of Phycomyces is a flavin (vitamin B2) bound to a special protein. This pigment is commonly called the blue light photoreceptor, since it is most sensitive to blue light.

Ascomycota, sac fungi

Species in this phylum reproduce sexually by forming a spore-filled structure called an ascus, which means literally “a sac.” The hyphae of the sac fungi are divided by septa with pores, that is, they have perforated walls between adjacent cells. They reproduce asexually by producing spores, called conidia, which are born on specialized erect hyphae, called conidiophores. The sac fungi are typically prolific producers of conidia.

The sac fungi also have a sexual reproduction phase of their life cycles. In the first step of this process, compatible hyphae fuse together by one of several different methods. Second, the nuclei from the different hyphae move together into one cell to form a dikaryon, a cell with two haploid nuclei. Third, several cell divisions occur, resulting in several cells with two different haploid nuclei per cell. Fourth, nuclear fusion of the two haploid nuclei occurs in one of these cells, the ascus mother cell. Fifth, the ascus mother cell develops into an ascus. Then, meiosis occurs in the diploid cells and, depending on the species, four or eight haploid ascospores form inside the ascus. In some species, such as the fleshy and edible morels, a large number of asci are massed together to form an ascocarp.

This large phylum of fungi includes many species which are beneficial to humans. For example, the yeasts are a major group of ascomycetes. Different yeasts in the genus Saccharomyces are employed by bakers, brewers, and vintners to make their bread, beer, or wine. Truffles are subterranean ascomycetes which grow in association with tree roots. Traditionally, pigs have been used to sniff out these underground fungi, so that French chefs could use truffles to complement their finest cuisine.

Some other ascomycetes are significant plant pathogens. For example, Endothia parasitica is an ascomycete which causes chestnut blight, a disease which virtually extirpated the American chestnut as a mature forest tree. Ceratocystis ulmi is a pathogenic ascomycete which causes Dutch elm disease, a scourge of American elm trees. Claviceps purpurea, the ergot fungus, infects agricultural grains, and when ingested can cause intense hallucinations or death due to the presence of LSD (D-Lysergic acid diethylamide).

Another well known ascomycete is Neurospora crassa, the red bread mold. The ordered manner in which the eight spores of this fungus align during sexual reproduction allows geneticists to construct a map of the genes on its chromosomes. Earlier in this century, biologists used Neurospora as a model organism to investigate some of the basic principles of genetics and heredity. More recently, biologists have shown that the mycelium of this species can produce spores at approximately 24 hour intervals, a circadian rhythm, in a constant environment. Many biologists are currently using Neurospora crassa as a model organism for investigation of circadian rhythms, which occur in a wide diversity of organisms, including humans.

Basidiomycota, club fungi

Species in this phylum reproduce sexually by forming spores on top of club-shaped structures called basidia. The club fungi are believed to be closely related to the sac fungi. Both groups have cells which are separated by septa (walls), and both have a dikary-otic phase in their life cycle; a phase with two haploid nuclei per cell. The septum of the club fungi is somewhat different from those of sac fungi and is referred to as a dolipore septum. The dolipore septum has a bagel-shaped pore in its center.

The club fungi reproduce asexually by producing asexual spores or by fragmentation of mycelium.

The sexual reproduction phase of the club fungi involves three developmental stages of the mycelium. In the primary stage, a haploid spore germinates and grows a germ tube, which develops into mycelium. The mycelium initially contains a single haploid nucleus. Then, its haploid nucleus divides and septa form between the nuclei.

A secondary mycelium forms upon conjugation of two sexually compatible hyphae. The secondary mycelium is dikaryotic, in that it has two haploid nuclei, one from each parent. As the dikaryotic mycelium grows, the cells divide and more septa are formed between the new cells.

Each of the new cells in the secondary mycelium has one haploid nucleus from each parent. This is assured by clamp connections, specialized structures unique to the club fungi. These are loop-like hyphae, which connect the cytoplasm of adjacent cells and through which nuclei move during cell division. In particular, during cell division, one nucleus divides directly into the newly formed cell; the other nucleus divides inside the clamp connection and the two daughter nuclei migrate through the clamp connection in opposite directions to the two daughter cells.

The tertiary mycelium is simply an organized mass of secondary mycelium. It is a morphologically complex tissue and forms structures such as the typically mushroom-shaped basidiocarps commonly seen in nature.

Sexual reproduction of the club fungi begins upon fusion of two primary hyphae to form a club-shaped structure, known as a basidium. Second, the two haploid nuclei inside the basidium fuse together to form a diploid zygote. Third, the zygote undergoes meiosis to form two haploid nuclei. Fourth, these two haploid nuclei undergo mitosis to form a total of four haploid nuclei. These four nuclei then migrate into projections, which form on the tip of the basidium. These projections then develop into four separate haploid spores, each with a single nucleus.

In the species of club fungi which are large and fleshy, such as the mushrooms, a mass of basidia form a structure called a basidiocarp. The spores on the basidia are released from the underside of the fleshy gills of the mushroom. The color and shape of the basidiocarp, as well as the color of the spores are often diagnostic for species identification.

This large phylum includes species which are known as mushrooms, toadstools, earthstars, stink-horns, puffballs, jelly fungi, coral fungi, and many other interesting common names. Some species, such as the rusts and smuts, are pathogens that attack agricultural grains. Other species, such as the fly agaric (Agaricus muscaria ) and some species in the genus Psilocybe, produce chemical hallucinogens and have been used by numerous cultures in their religious ceremonies. Another species, Agaricus bisporus, is the common edible mushroom found in supermarkets.

An important aspect of the club fungi is the great diversity of alkaloids and other toxic and psychogenic chemicals produced by some species. For example, Amanita virosa, a mushroom colloquially known as “death angel,” is so deadly poisonous that a small bite can kill a person. A related mushroom is Amanita muscaria, known as “fly agaric,” which is hallucinogenic. Over the millennia, numerous cultures have eaten the fly agaric as part of their religious ceremonies. For example, R. Gordon Wasson has shown that Amanita muscaria is the hallucinogenic plant referred to as “Soma” throughout Rg Veda, the ancient religious text. According to Rg Veda, the ancient Aryans who invaded India about four millennia ago ingested “Soma” as a euphoriant.

While mushrooms are the best-known club fungi, many other club fungi grow underground as mycor-rhizae. Mycorrhizae result from a symbiosis between a plant root and a fungus. In mycorrhizae, the fungus typically supplies nitrogen-containing compounds to

KEY TERMS

Biomass— Total weight, volume, or energy equivalent of all living organisms within a given area.

Clamp connection— Loop-like hypha which connects the cytoplasm of adjacent cells. Characteristic feature of Basidiomycota.

Coenocytic— Lacking walls for separation of the nuclei of cytoplasm.

Cyanobacteria (singular, cyanobacterium)— Photo synthetic bacteria, commonly known as blue-green alga.

Diploid— Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.

Haploid— Nucleus or cell containing one copy of each chromosome.

Hypha (plural, hyphae)— Cellular unit of a fungus, typically a branched and tubular filament. Many strands (hyphae) together are called mycelium.

Mycorrhiza— Subterranean symbiotic relationship between a fungus, typically a species of Basidiomycota, and a plant root.

Phylum— Broadest taxonomic category within a kingdom.

Septum— Wall that separates the cells of a fungal hypha into segments.

Symbiogenesis— Evolutionary origin of a completely new life form from the symbiosis of two or more independent species.

Symbiosis— A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

the plant, and the plant supplies carbohydrates and other organic compounds to the fungus. Mycorrhizae are very important for the growth of orchids. One reason many orchids are difficult to grow is because they require particular fungal species to form mycorrhizae on their roots.

A recent report investigated a subterranean club fungus, Armillaria bulbosa, which is a pathogen on tree roots. The investigators used molecular biology techniques to demonstrate that a single subterranean “individual” of this species in Northern Michigan was spread out over 37 acres (15 ha) and weighed an estimated 22,000 lb (10,000 kg). Based on the estimated growth rate of this species, of about 0.7 ft (0.2 m) per year, this individual was about 1,500 years old.

Deuteromycota, imperfect fungi

The Deuteromycota is a heterogeneous group of unrelated species in which sexual reproduction has never been observed. Since mycologists refer to the “perfect phase” of a life cycle as the phase in which sexual reproduction occurs, these fungi are often referred to as imperfect fungi. These fungi may have lost their sexual phase through the course of evolution. Alternatively, biologists simply may not have found the appropriate environmental conditions to observe development of the sexual phase of their life cycle.

The Deuteromycota are classified as fungi for two main reasons. First, their multicellular tissue is similar to the hyphae of sac fungi and club fungi. Second, they have erect hyphae with asexual spores, called conidio-phores, which are similar to those of the sac fungi and club fungi.

Most imperfect fungi are believed to be related to the sac fungi because their conidiophores closely resemble those produced by the sac fungi during their sexual phase. The imperfect fungi are not placed in the Ascomycota phylum because classification of that group is based on the morphology of sexual structures which the Deuteromycota do not have.

The best known fungus in this phylum is Penicillium. Some species in this genus appear as pathogenic, blue-green molds on fruits, vegetables, and cheeses. Several other species are important for the making of cheeses, such as blue cheese, Roquefort, and Camembert. Certainly the best-known product from this genus is penicillin, the first widely-used antibiotic. Penicillin was first discovered in Penicillium notatum over 50 years ago, but is now known to be produced by many other species in this genus.

Mycophycophyta, lichens

A lichen is a symbiotic relationship between a fungus and an alga, or between a fungus and a photo-synthetic cyanobacterium. They constitute a very diverse and polyphyletic group of organisms and are classified together simply because they all result from a fungus-alga symbiosis. In most lichens, the fungal species is in the Ascomycota phylum and the photo-synthetic species is a green alga from the Chlorophyta phylum. Typically, the photosynthetic species supplies carbohydrates to the fungus and the fungus supplies nitrogen and other nutrients to the alga. The morphology of a lichen differs from its component species.

Lichens can reproduce by several methods. The fungal component of the lichen can produce spores, which are dispersed, germinate, and then recombine with the algal component. Alternatively, the lichen can produce soredia, specialized reproductive and dispersal structures in which the algal component is engulfed by fungal mycelium. Typically, the soredia break off from the thallus, the main body of the lichen.

Ecologists have shown that many species of lichens are very sensitive to air pollutants, such as sulfur dioxide. Thus, they are often used as indicator species for air pollution; the presence of certain lichen species correlates with the cleanliness of the air.

Many lichens can inhabit harsh environments and withstand prolonged periods of desiccation. In the temperate region of North America, lichens often grow on tree trunks and bare rocks and soil. In the arctic and antarctic regions, lichens constitute a large proportion of the ecosystembiomass. Many lichens are even found growing upon and within rocks in Antarctica. In the arctic region, the lichen species known colloquially as reindeer mosses (Cladonia rangifera and several other species) are an important food for caribou and reindeer.

Studies of the symbiotic nature of lichens in the late 1800s laid an important foundation for development of the theory of symbiogenesis. This theory says that new life forms can evolve from the symbiotic relationship of two or more independent species. Nearly all modern biologists now agree that symbiogenesis of different bacteria led to the origin of eukary-otic cells, which contain many different organelles, intracellular “small organs” which are specialized for different functions.

Resources

BOOKS

Money, Nicholas P. The Triumph of the Fungi: A Rotten History. New York: Oxford University Press, USA, 2006.

Phillips, Roger. Mushrooms and Other Fungi of North America. Tonawanda: Firefly Books, 2005.

Spooner, Brian, and Peter Roberts. Fungi (Collins New Naturalist Series ). New York: Harper Collins Publishers, 2005.

Peter A. Ensminger

Fungi

Fungi are one of the five kingdoms of organisms. Like higher plants (of the kingdom Plantae), most fungi are attached to the substrate they grow on. Unlike plants, fungi do not have chlorophyll and are not photosynthetic. Another key difference from plants is that fungi have cell walls composed of chitin, a nitrogen containing carbohydrate . All fungi have nuclei and the nuclei of most species are haploid at most times. Many species have two or more haploid nuclei per cell during most of the life cycle. All fungi reproduce asexually by spore production. Most species reproduce sexually as well.

General characteristics

The different taxonomic groups of fungi have different levels of cellular organization. Some groups, such as the yeasts, consist of single-celled organisms, which have a single nucleus per cell. Some groups, such as the conjugating fungi, consist of single-celled organisms in which each cell has hundreds or thousands of nuclei. Groups such as the mushrooms , consist of multicellular, filamentous organisms which have one or two nuclei per cell. These multicellular fungi are composed of branched filaments of cells called hyphae. The hyphae, in turn, often mass together to form a tissue called mycelium.

Mycology, the study of fungi, has traditionally included groups such as the cellular slime molds , plasmodial slime molds, water molds, chytrids, and several other groups of fungus-like organisms. Most modern biologists consider these groups as diverse assemblages of organisms unrelated to the true fungi considered here. However, it should be emphasized that biologists are very uncertain about the evolutionary relationships of these other groups and the true fungi.

Nutrition and ecology

Most species of fungi grow on land and obtain their nutrients from dead organic matter . Some fungi are symbionts or parasites on other organisms. The majority of species feed by secreting enzymes, which partially digest the food extracellularly, and then absorbing the partially digested food to complete digestion internally. As with animals, the major storage carbohydrate of fungi is glycogen. Fungi lack the complex vascular system found in higher plants, so their transport of food and water is less efficient.

Along with bacteria , fungi have an important ecological role in the decomposition of dead plants, animals, and other organic matter. Thus, fungi are ecologically important because they release large amounts of carbon dioxide into the atmosphere and recycle nitrogen and other important nutrients within ecosystems for use by plants and other organisms. Some fungi are parasitic, in that they obtain their nutrients from a living host organism , a relationship which usually harms the host. Such parasitic fungi usually have specialized tissues called haustoria, which penetrate the host's body. Most of the diseases which afflict agricultural plants are caused by parasitic fungi. Some examples are corn smut, black stem rust of wheat and barley , and cotton root rot. Some species of fungi can also parasitize animals. Even humans can be parasitized by fungi which cause diseases such as athlete's foot, ringworm, and yeast infections.

Evolution

The Fungi constitute a large and diverse group of organisms. Until the 1960s, fungi were considered members of the plant kingdom. With the advent of the five-kingdom system of biological classification, fungi were assembled into a single kingdom because of their similar ecological roles as primary decomposers of organic matter and their similar anatomical and biochemical features. Recent studies that compare the sequence of amino acids in proteins from fungi, plants, and animals now indicate that fungi share a closer evolutionary relationship to animals than to plants.

The evolutionary ancestry and relationships of the different fungi are not well understood. There are few fossils of fungi, presumably because their relatively soft tissues are not well preserved. There is some fossil evidence that they existed in the Precambrian era (over four billion years ago), although identification of these very early fossils is uncertain. There is definite fossil evidence for fungi in the lower Devonian (about 400 million years ago) period. Fossils of all the major groups of fungi are found in the Carboniferous period (about 300 million years ago). By the late Tertiary period (about 20 million years ago), the fossil record shows a rapid and divergent evolution of fungi. Many of the Tertiary fossils of fungi are similar to existing species.

In addition to the incomplete fossil record, there are at least two other reasons why the evolutionary relationships of fungi are not well-known: they tend to have simple morphologies and they lack embryos which follow a definite sequence of developmental stages. Biologists often use these two features to determine evolutionary relationships of animals and plants.

Another complication is that many fungi presumably evolved similar morphologies by convergent evolution. That is, unrelated species may share a common morphology because they have been subjected to similar selective pressures.

The relatively new technique of molecular systematics is particularly useful in the study of the evolution of fungi. This technique compares the sequence of DNA segments of different species to determine evolutionary relationships. One important finding from this new technique is that the plasmodial slime molds, cellular slime molds, and water molds are only distantly related to the true fungi (the taxonomic groups considered here). Evolutionary relationships among organisms also are being studied by comparing the sequences of ribosomal RNAs and transfer RNAs from different organisms. Although these RNAs are similar to DNA, they have structural roles in cells, rather than coding for proteins as do DNA and messenger RNAs. Therefore, the sequences of ribosomal and transfer RNAs tend to be more conserved through evolution. Comparison of these RNAs among fungi, plants, and animals also suggests that fungi are more closely related to animals than to plants. Future work in molecular systematics is expected to tell us more about the evolution and relationships of the fungi.

Classification

Biologists have estimated that over 200,000 species of fungi exist in nature, although only about 100,000 have been identified so far. Since classification schemes of organisms are usually based on evolutionary relationships, and the evolutionary relationships of fungi are not well known, biologists have proposed numerous classification schemes for fungi over the years. Below, we consider the five major phyla that nearly all mycologists would agree belong in the kingdom of Fungi.

Zygomycota, conjugating fungi

Species in this phylum reproduce sexually by forming a zygospore, a thick-walled, diploid cell which contains thousands of nuclei. There are about 600 species in this phylum. Most species are terrestrial and feed on organic matter, although there are a few parasitic species. The conjugating fungi are coenocytic, in that they have a continuous mycelium, containing hundreds or thousands of haploid nuclei, with no divisions between them. However, the Zygomycota do have septa (cross walls) between their reproductive structures and the rest of their mycelium.

The conjugating fungi have a life cycle that includes a sexual phase and an asexual phase. In the asexual phase, thousands of spores develop inside a sporangium, a small spherical structure. The sporangium grows on the tip of a sporangiophore, a specialized aerial hypha, typically about as thin as a hair.

In the sexual phase of their life cycle, these fungi form specialized hyphae, called gametangia, which are of two different strains (sexes), plus and minus. The plus and minus strains are very similar morphologically, but differ physiologically and biochemically. Plus and minus gametangia conjugate with one another and form a structure with hundreds or thousands of nuclei from each strain.

Then, a thick-walled structure, called the zygospore, develops from the conjugated gametangia. Inside the zygospore, the many thousands of nuclei from the plus and minus strains pair off and fuse together to form thousands of diploid nuclei. The zygospore is typically spherical in shape and has a thick, dark outer wall. It usually remains dormant for several months or more before development continues.

As the zygospore germinates, it produces germsporangia which are born on germsporangiophores, structures morphologically similar to the asexual sporangium and sporangiophore (see above). The germsporangium contains thousands of haploid germspores which arose from the diploid nuclei of the zygospore by meiosis . Each germspore is liberated, germinates, and gives rise to a new haploid mycelium.

One of the best known of the conjugating fungi is Phycomyces blakesleeanus, a species which grows on animal feces in nature. The sporangiophores of Phycomyces respond to a variety of sensory stimuli. For example, they bend in response to light (phototropism ), gravity (gravitropism), wind (anemotropism), and nearby objects (avoidance response). Physiologists and biophysicists have intensively studied the response to light. One important finding is that the light sensitivity of the sporangiophore is about the same as the eyes of humans. Furthermore, like humans, the sporangiophore can adapt to a one-billion-fold change in ambient light intensity. One of the pigments involved in the extraordinary light responses of Phycomyces is a flavin (vitamin B2) bound to a special protein. This pigment is commonly called the blue light photoreceptor, since it is most sensitive to blue light.

Ascomycota, sac fungi

Species in this phylum reproduce sexually by forming a spore-filled structure called an ascus, which means literally "a sac." The hyphae of the sac fungi are divided by septa with pores, that is, they have perforated walls between adjacent cells. They reproduce asexually by producing spores, called conidia, which are born on specialized erect hyphae, called conidiophores. The sac fungi are typically prolific producers of conidia.

The sac fungi also have a sexual reproduction phase of their life cycles. In the first step of this process, compatible hyphae fuse together by one of several different methods. Second, the nuclei from the different hyphae move together into one cell to form a dikaryon, a cell with two haploid nuclei. Third, several cell divisions occur, resulting in several cells with two different haploid nuclei per cell. Fourth, nuclear fusion of the two haploid nuclei occurs in one of these cells, the ascus mother cell. Fifth, the ascus mother cell develops into an ascus. Then, meiosis occurs in the diploid cells and, depending on the species, four or eight haploid ascospores form inside the ascus. In some species, such as the fleshy and edible morels, a large number of asci are massed together to form an ascocarp.

This large phylum of fungi includes many species which are beneficial to humans. For example, the yeasts are a major group of ascomycetes. Different yeasts in the genus Saccharomyces are employed by bakers, brewers, and vintners to make their bread, beer, or wine. Truffles are subterranean ascomycetes which grow in association with tree roots. Traditionally, pigs have been used to sniff out these underground fungi, so that French chefs could use truffles to complement their finest cuisine.

Some other ascomycetes are significant plant pathogens . For example, Endothia parasitica is an ascomycete which causes chestnut blight, a disease which virtually extirpated the American chestnut as a mature forest tree. Ceratocystis ulmi is a pathogenic ascomycete which causes Dutch elm disease, a scourge of American elm trees. Claviceps purpurea, the ergot fungus, infects agricultural grains, and when ingested can cause intense hallucinations or death due to the presence of LSD (D-Lysergic acid diethylamide).

Another well known ascomycete is Neurospora crassa, the red bread mold . The ordered manner in which the eight spores of this fungus align during sexual reproduction allows geneticists to construct a map of the genes on its chromosomes. Earlier in this century, biologists used Neurospora as a model organism to investigate some of the basic principles of genetics and heredity. More recently, biologists have shown that the mycelium of this species can produce spores at approximately 24 hour intervals, a circadian rhythm, in a constant environment. Many biologists are currently using Neurospora crassa as a model organism for investigation of circadian rhythms, which occur in a wide diversity of organisms, including humans.

Basidiomycota, club fungi

Species in this phylum reproduce sexually by forming spores on top of club-shaped structures called basidia. The club fungi are believed to be closely related to the sac fungi. Both groups have cells which are separated by septa (walls), and both have a dikaryotic phase in their life cycle; a phase with two haploid nuclei per cell. The septum of the club fungi is somewhat different from those of sac fungi and is referred to as a dolipore septum. The dolipore septum has a bagel-shaped pore in its center.

The club fungi reproduce asexually by producing asexual spores or by fragmentation of mycelium.

The sexual reproduction phase of the club fungi involves three developmental stages of the mycelium. In the primary stage, a haploid spore germinates and grows a germ tube, which develops into mycelium. The mycelium initially contains a single haploid nucleus. Then, its haploid nucleus divides and septa form between the nuclei.

A secondary mycelium forms upon conjugation of two sexually compatible hyphae. The secondary mycelium is dikaryotic, in that it has two haploid nuclei, one from each parent. As the dikaryotic mycelium grows, the cells divide and more septa are formed between the new cells.

Each of the new cells in the secondary mycelium has one haploid nucleus from each parent. This is assured by clamp connections, specialized structures unique to the club fungi. These are loop-like hyphae which connect the cytoplasm of adjacent cells and through which nuclei move during cell division . In particular, during cell division, one nucleus divides directly into the newly formed cell; the other nucleus divides inside the clamp connection and the two daughter nuclei migrate through the clamp connection in opposite directions to the two daughter cells.

The tertiary mycelium is simply an organized mass of secondary mycelium. It is a morphologically complex tissue and forms structures such as the typically mushroom-shaped basidiocarps commonly seen in nature.

Sexual reproduction of the club fungi begins upon fusion of two primary hyphae to form a club-shaped structure, known as a basidium. Second, the two haploid nuclei inside the basidium fuse together to form a diploid zygote. Third, the zygote undergoes meiosis to form two haploid nuclei. Fourth, these two haploid nuclei undergo mitosis to form a total of four haploid nuclei. These four nuclei then migrate into projections, which form on the tip of the basidium. These projections then develop into four separate haploid spores, each with a single nucleus.

In the species of club fungi which are large and fleshy, such as the mushrooms, a mass of basidia form a structure called a basidiocarp. The spores on the basidia are released from the underside of the fleshy gills of the mushroom. The color and shape of the basidiocarp, as well as the color of the spores are often diagnostic for species identification.

This large phylum includes species which are known as mushrooms, toadstools, earthstars, stinkhorns, puffballs, jelly fungi, coral fungi, and many other interesting common names. Some species, such as the rusts and smuts , are pathogens which attack agricultural grains. Other species, such as the fly agaric (Agaricus muscaria) and some species in the genus Psilocybe, produce chemical hallucinogens and have been used by numerous cultures in their religious ceremonies. Another species, Agaricus bisporus, is the common edible mushroom found in supermarkets.

An important aspect of the club fungi is the great diversity of alkaloids and other toxic and psychogenic chemicals produced by some species. For example, Amanita virosa, a mushroom colloquially known as "death angel," is so deadly poisonous that a small bite can kill a person. A related mushroom is Amanita muscaria, known as "fly agaric," which is hallucinogenic. Over the millennia, numerous cultures have eaten the fly agaric as part of their religious ceremonies. For example, R. Gordon Wasson has shown that Amanita muscaria is the hallucinogenic plant referred to as "Soma" throughout Rg Veda, the ancient religious text. According to Rg Veda, the ancient Aryans who invaded India about four millennia ago ingested "Soma" as a euphoriant.

While mushrooms are the best-known club fungi, many other club fungi grow underground as mycorrhizae. Mycorrhizae result from a symbiosis between a plant root and a fungus. In mycorrhizae, the fungus typically supplies nitrogen-containing compounds to the plant, and the plant supplies carbohydrates and other organic compounds to the fungus. Mycorrhizae are very important for the growth of orchids. One reason many orchids are difficult to grow is because they require particular fungal species to form mycorrhizae on their roots.

A recent report investigated a subterranean club fungus, Armillaria bulbosa, which is a pathogen on tree roots. The investigators used molecular biology techniques to demonstrate that a single subterranean "indi vidual" of this species in Northern Michigan was spread out over 37 acres (15 ha) and weighed an estimated 22,000 lb (10,000 kg). Based on the estimated growth rate of this species, of about 0.7 ft (0.2 m) per year, this individual was about 1,500 years old.

Deuteromycota, imperfect fungi

The Deuteromycota is a heterogeneous group of unrelated species in which sexual reproduction has never been observed. Since mycologists refer to the "perfect phase" of a life cycle as the phase in which sexual reproduction occurs, these fungi are often referred to as imperfect fungi. These fungi may have lost their sexual phase through the course of evolution. Alternatively, biologists simply may not have found the appropriate environmental conditions to observe development of the sexual phase of their life cycle.

The Deuteromycota are classified as fungi for two main reasons. First, their multicellular tissue is similar to the hyphae of sac fungi and club fungi. Second, they have erect hyphae with asexual spores, called conidiophores, which are similar to those of the sac fungi and club fungi.

Most imperfect fungi are believed to be related to the sac fungi because their conidiophores closely resemble those produced by the sac fungi during their sexual phase. The imperfect fungi are not placed in the Ascomycota phylum because classification of that group is based on the morphology of sexual structures which the Deuteromycota do not have.

The best known fungus in this phylum is Penicillium. Some species in this genus appear as pathogenic, blue-green molds on fruits , vegetables , and cheeses. Several other species are important for the making of cheeses, such as blue cheese, Roquefort, and Camembert. Certainly the best known product from this genus is penicillin, the first widely-used antibiotic. Penicillin was first discovered in Penicillium notatum over 50 years ago, but is now known to be produced by many other species in this genus.

Mycophycophyta, lichens

A lichen is a symbiotic relationship between a fungus and an alga, or between a fungus and a photosynthetic cyanobacterium. They constitute a very diverse and polyphyletic group of organisms and are classified together simply because they all result from a fungus-alga symbiosis. In most lichens , the fungal species is in the Ascomycota phylum and the photosynthetic species is a green alga from the Chlorophyta phylum. Typically, the photosynthetic species supplies carbohydrates to the fungus and the fungus supplies nitrogen and other nutrients to the alga. The morphology of a lichen differs from its component species.

Lichens can reproduce by several methods. The fungal component of the lichen can produce spores which are dispersed, germinate, and then recombine with the algal component. Alternatively, the lichen can produce soredia, specialized reproductive and dispersal structures in which the algal component is engulfed by fungal mycelium. Typically, the soredia break off from the thallus, the main body of the lichen.

Ecologists have shown that many species of lichens are very sensitive to air pollutants, such as sulfur dioxide . Thus, they are often used as indicator species for air pollution ; the presence of certain lichen species correlates with the cleanliness of the air.

Many lichens can inhabit harsh environments and withstand prolonged periods of desiccation. In the temperate region of North America , lichens often grow on tree trunks and bare rocks and soil . In the arctic and antarctic regions, lichens constitute a large proportion of the ecosystem biomass . Many lichens are even found growing upon and within rocks in Antarctica . In the arctic region, the lichen species known colloquially as reindeer mosses (Cladonia rangifera and several other species) are an important food for caribou and reindeer.

Studies of the symbiotic nature of lichens in the late 1800s laid an important foundation for development of the theory of symbiogenesis. This theory says that new life forms can evolve from the symbiotic relationship of two or more independent species. Nearly all modern biologists now agree that symbiogenesis of different bacteria led to the origin of eukaryotic cells, which contain many different organelles, intracellular" small organs" which are specialized for different functions.

Resources

books

Griffin, D.H. Fungal Physiology. New York: Wiley-Liss, 1993.

Margulis, L., and K.V. Schwartz. Five Kingdoms. New York: W. H. Freeman and Company, 1988.

Soothill, E., and A. Fairhurst. The New Field Guide to Fungi. Transatlantic Arts, 1993.

Peter A. Ensminger

KEY TERMS

Biomass

—Total weight, volume, or energy equivalent of all living organisms within a given area.

Clamp connection

—Loop-like hypha which connects the cytoplasm of adjacent cells. Characteristic feature of Basidiomycota.

Coenocytic

—Lacking walls for separation of the nuclei of cytoplasm.

Cyanobacteria (singular, cyanobacterium)

—Photosynthetic bacteria, commonly known as blue-green alga.

Diploid

—Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.

Haploid

—Nucleus or cell containing one copy of each chromosome.

Hypha (plural, hyphae)

—Cellular unit of a fungus, typically a branched and tubular filament. Many strands (hyphae) together are called mycelium.

Mycorrhiza

—Subterranean symbiotic relationship between a fungus, typically a species of Basidiomycota, and a plant root.

Phylum

—Broadest taxonomic category within a kingdom.

Septum

—Wall that separates the cells of a fungal hypha into segments.

Symbiogenesis

—Evolutionary origin of a completely new life form from the symbiosis of two or more independent species.

Symbiosis

—A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

Fungi

Fungi are eukaryotic organisms distinct from plants and animals and members of several other smaller kingdoms. Common fungi include mushrooms, conks, corals, jellies, puffballs, stinkhorns, morels, cups, truffles, lichens, yeasts, rusts, smuts, bread molds, mildews, and molds on bathroom tiles.

In 1959, R. H. Whittaker introduced a five-kingdom taxonomy that granted fungi equal status with plants and animals. The five-kingdom system has been supplanted by a multiple-kingdom classification, and species traditionally treated as fungi are now distributed across several kingdoms. Those believed to form a monophyletic lineage are assigned to kingdom Eumycota (often called kingdom Fungi). Mycology, the science devoted to fungi, still covers all traditional fungi.

Characteristics of Fungi

The Eumycota consist of eukaryotic, nonchlorophyllous heterotrophs that absorb nutrients from dead or living organic matter, have cell walls composed of chitin , and store excess energy as glycogen . The kingdom contains four phyla: Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota. All true fungi have a definite cell wall throughout all developmental stages. Fungal cell walls are composed of chitin, the compound also found in arthropod exoskeletons (for example, lobster shells). Most fungi produce a vegetative mycelium (filamentous thallus) composed of hyphae that branch and extend via tip elongation, although some groups (like yeasts) consist only of individual cells. Hyphae (singular, hypha) are tube-like filaments with either single multinucleate cells (coenocytes) that lack septa (cross-walls) separating nuclei, or many septate cells containing one, two, or more nuclei.

Fungal Nutrition: Saprobes, Parasites, and Mutualists

A fungal thallus may be as small as a single microscopic cell (baker's yeast, Saccharomyces cerevesiae ) or exceedingly large (Armillaria gallica, the several-acre-sized "humungous fungus" reported in 1992 as the world's largest organism). Sporocarp (fruit body, or "mushroom" size) also ranges from microscopic to meters in diameter (Bridgeoporus nobilissimus, an endangered bracket fungus found on noble fir trees).

Not unexpectedly, such a diverse kingdom manifests several different life cycles. Virtually all fungi produce spores. Both asexual and sexual spores may germinate to form vegetative thalli from primary and secondary mycelia. Thalli may be haploid dominant, diploid dominant, or exhibit haplo-diploid alternation of generations. Here it is important not to confuse the chromosomal state of individual nuclei (haploid versus diploid) with the number of nuclei per cell (monokartyotic versus dikaryotic). Fungi are unusual in that they often exhibit dikaryotamy, wherein hyphal cells contain two (usually haploid) nuclei that migrate, multiply, and divide together.

Although superficially similar to plants, fungi are probably more closely related to animals. Like animals, fungi lack chlorophyll and do not photo-synthesize, must obtain nutrients from organic sources, and store energy as glycogen instead of starch. Unlike animals, however, fungi do not engulf, but rather absorb, their nutrients after breaking them down via enzymatic action, earning them the nickname "absorbotrophs."

Fungi absorb their nutrients in three different ways: (1) saprobes decompose dead organic matter; (2) parasites feed on living hosts; and (3) mutualists live in symbiotic unions with other living organisms. Saprophytic fungi, such as edible meadow mushrooms (Agaricus campestris ), shiitake (Lentinula edodes ), and oyster mushrooms (Pleurotus ostreatus ), decompose dead plant and animal tissue by releasing enzymes from hyphal tips, thereby recycling organic materials back into the surrounding environment. Parasitic fungi also use enzymes to break down living tissue, usually sapping the energy of the host and frequently causing its demise.

Lichens and mycorrhizae are two important mutualistic associations. Lichens represent partnerships between a fungus (mycobiont) and one or more algae (phycobiont). Although there are a few basidiolichens, almost all lichen mycobionts are Ascomycota (approximately 20,000 ascolichens described thus far). Mycorrhizae are symbiotic or non to slightly pathogenic fungus-plant unions formed with approximately 85 percent of the vascular plants. Mycorrhizae are identified as ectomycorrhizal, arbuscular mycorrhizal (AM), ericoid, orchid, arbutoid, and monotropoid based on anatomical form and association. Ectomycorrhizal fungi (predominantly basidiomycetes such as boletes, amanitas, and coral fungi) form thick mycelial mantles around rootlets of many trees (oaks, firs, pines, poplars) to which they transport water and minerals from the soil, receiving sugars and other organic nutrients in return. AM fungi (in the Zygomycota order Glomales) form an endo-infection by penetrating rootlets to form coils and vesicles or finely branched arbuscules. The last four mycorrhizal types are specific to individual plant groups.

Ecological and Economic Importance

Fungi have a profound biological and economic impact. As decomposers, plant pathogens, and symbiotic partners, their ability to grow anywhere, on anything, makes them both beneficial and harmful recyclers of carbon and nitrogen. Beneficially, they are used as food (mushrooms, truffles) and in baking and brewing (yeasts). They are being developed to detoxify pollutants (soil fungi), control insects (pathogenic Zygomycota), and regulate plant growth (pathogenic Ascomycota). Detrimentally, rusts, smuts, and molds cost billions of dollars through crop disease and spoilage while forest pathogens such as the honey mushroom (Armillaria ostoyae ) and root-butt rot (Heterobasidion annosum ) similarly threaten the timber industry. Some are toxic when eaten, such as the infamous destroying angel (Amanita phalloides ). Natural LSD, a hallucinogen produced by ergot (Claviceps purpurea ), is associated with medieval hysterical frenzies produced by consumption of infected grain, and the aflatoxin produced by Aspergillus flavus in improperly stored grain is one of the most potent carcinogens yet discovered. As human and animal pathogens, fungi cause infections that range from the vexing (athlete's foot, yeast infections) to life threatening (histoplasmosis). Fortunately, other fungi (such as Penicillium ) have been used to develop modern antibiotics and beneficial immunosuppressants .

Classification

With the introduction of deoxyribonucleic acid (DNA) sequence analyses, previous fungal evolutionary theory is undergoing rapid transformation, not surprising when one considers the extremely fragmentary fungal fossil record. Mycologists generally agree that true fungi (with animals and plants) diverged from the protozoan fungi (for example, slime molds and Oomycota) before fungi and animals diverged from plants. The chytrids separated from the remaining phyla approximately 550 million years ago, followed by splintering of the Zygomycota from the other two phyla, which share septate hyphae and dikaryotic stages in their life cycles. The Ascomycota split from the Basidiomycota approximately 400 million years ago, followed by an increase in fungal diversity throughout the Paleozoic. Most yeasts and "asco-molds" are thought to have evolved in approximately the last 200 million years.

While most plants and vertebrates appear to have been described, most fungi have yet to be discovered or named. The 1995 edition of the Dictionary of Fungi counts 566,360 described species (about 100,000 in the Eumycota) but notes that there may be approximately 1.5 million species of fungi in the world. Elias Fries (1794–1878), often regarded as the "father of mycology," classified fungi based primarily on spore print color and sporocarp appearance. Friesian-based nomenclature, reflecting similarity of form (phenetic) rather than genetic relationships, is still used in many field guides. Now, however, taxonomists are integrating molecular and morphological characters to develop natural classifications that more adequately reflect evolutionary relationships. Realizing that fungal taxonomy and nomenclature will remain somewhat fluid until new species and data are analyzed and integrated, most mycologists generally accept the classification below.

Chytridiomycota. The fact that chytrids alone among the Eumycota produce motile zoospores explains why their phylum is sometimes assigned with the flagellate oomycetes to kingdom Chromista. Chytrids possess posteriorly uniflagellate spores, mitochondria with flattened cristae, and cell walls composed of glucan and chitin. Among the simplest and smallest fungi, they live as saprobes in water and damp organic-rich habitats, or as parasites on invertebrates, plants, and other fungi. The so-called "frog chytrids" (such as Batrachochytrium dendrobatidis ) are implicated in the current worldwide decline of amphibian species. Other chytrids are host-specific "rumen fungi" (such as Piromyces communis ) that thrive in anaerobic conditions in the guts of herbivores, such as cattle and sheep. There are five orders and about 800 species of chytrids recognized thus far.

Unicellular members of the order Chytridiales lack a mycelial stage and consist of a central body with a few rudimentary appendages (haustoria) that attach to and invade the host tissue. Other chytrids, such as the Blastocladiales, develop true mycelia with sporangia and male/female gametangia that produce uniflagellate zoospores. Chytrid thalli may be either haploid or diploid, and some, like the aquatic chytrid Allomyces, exhibit an isomorphic alternation of generations with similar appearing sexual and asexual zoospores.

Zygomycota. The chytrids and this phylum are assigned the two "bottom" branches of the fungal evolutionary tree. There are more than 1,000 species in two classes (Trichomycetes, Zygomycetes) and ten orders, representing a diverse assemblage of saprobes, soil fungi, obligate insect and fungal parasites, and mycorrhizal formers. Common representatives of the saprobic order Mucorales include the dung-inhabiting "cap-thrower" (Pilobolus ), the black bread mold (Rhizopus stolonifera ), and Phycomyces blakesleeanus, sometimes referred to as the "body in the basement" because of the rapid growth of long, hairlike sporangiophores over a substrate under the right conditions.

Zygomycota are characterized by large, thick-walled, coenocytic zygospores and hyphae with relatively thin walls composed of chitin and chitosan. Both asexual sporangiospores and sexual zygospores germinate into haploid mycelia, with the hyphae functioning as gametangia during the sexual stage. In Rhizopus, for instance, close proximity of two hyphal strands of different mating types chemically triggers each to grow branches toward the other to form septate suspensor cells and gametangia. Eventual fusion produces a diploid zygosporangium that undergoes meiosis to become a thick-walled zygospore with large numbers of haploid nuclei.

Ascomycota. In addition to most lichens and so-called "imperfect fungi," about 33,000 species of unicellular yeasts, green and black molds, powdery mildews, morels, cup fungi, and ascotruffles ("true" truffles) belong to this phylum. The phylum is characterized by ascospores produced within a saclike sporangium called an ascus. Mycelia (more complex than Zygomycota mycelia) are composed of septate hyphae with chitin-glucan hyphal walls. Most species produce specialized fruiting bodies called ascocarps whose details of structure help define different species, classes, or orders. Nonascocarpic representatives (such as unicellular yeasts and mildews that reproduce primarily by budding) do not form mycelia. Both sexual and asexual reproduction are found within this phylum.

Basidiomycota. This phylum, which also features septate hyphae and chitin-glucan cell walls, is characterized by basidiospores borne upon a club-like structure called a basidium. Approximately 22,500 species are assigned to three classes: Basidiomycetes, Teliomycetes, and Ustomycetes. Basidiomycetes include mushrooms, polypores, crusts, corals, clubs, basidiolichens, and jellies, which propel their spores, and "gastromycetes" (or "stomach fungi") that passively release their spores (puffballs, basidiotruffles ["false truffles"], stinkhorns, and birds' nests). Teliomycetes (rusts) and Ustomycetes (smuts) are obligate parasites of insects or plants. Rusts and smuts have exceedingly complex cycles involving up to five separate spore stages and multiple hosts. This ability to produce spores on different hosts in multiple ways presents a significant economic challenge to agriculture.

Oomycota. Oomycetes (kingdom Chromista) are distinguished from true fungi by having glucan-cellulose cell walls that only occasionally incorporate small amounts of chitin. These algae-like fungi occur in aquatic or moist terrestrial habitats as single cells or mycelial mats composed of multinucleate, nonseptate hyphae. Their life cycle generally mirrors that of plants, with a transitory haploid stage. Both resting oospores and motile zoospores are diploid, the latter propelled by two unequal flagella (tinsel type plus whiplash). This phylum contains about 700 species in nine orders, including the generally saprophytic water molds (Saprolegniales) and the pathogenic Peronosporales and Pythiales. Most water molds are saprophytic, but there are a number of parasites that invade plants (white rusts, downy mildews, tobacco blue mold) or fish. Among the economically significant wilts, blights, and pathogens are Phytophthora infestans (responsible for the Irish potato famine in the 1850s), Plasmopara viticola (the causative agent of downy mildew of grapes), and the fish parasite Saprolegnia parasitica, a twenty-first-century threat to salmon migrating through dams in western North America.

see also Alternation of Generations; Kingdom; Mycorrhizae; Plant Pathogens and Pests; Symbiosis

Lorelei L. Norvell

Bibliography

Alexopoulos, C. J., C. W. Mims, and M. Blackwell. Introductory Mycology, 4th ed. New York: John Wiley & Sons, 1996.

Arora, D. Mushrooms Demystified: A Comprehensive Guide to the Fleshy Fungi. Berkeley, CA: Ten Speed Press, 1986.

Hanlin, R.T., and M. Ulloa. Atlas of Introductory Mycology, 2nd ed. Winston-Salem, MA: Hunter Textbooks Inc., 1978.

Fungi

Definition

Fungi are eukaryotic organisms (each containing a membrane-bound nucleus) that develop from reproductive bodies called spores. Fungi may be the cause of any number of diseases in humans, animals, and plants; fungal infections are called mycoses (singular, mycosis).

Description

Mycology is the branch of science that studies organisms of the kingdom Fungi. Scientists estimate that over 200,000 species of fungus exist in nature. These species include yeasts, moulds, mildews, mushrooms, lichens, and smuts.

There are a number of characteristics that fungi share: they are eukaryotic (containing a nucleus that is bound by a nuclear membrane); they develop from reproductive bodies called spores; their cell walls are composed mostly of chitin, a nitrogen-containing carbohydrate; and they are heterotrophic (they cannot synthesize their own food and therefore absorb food from an external source through their cell walls).

Most fungi obtain their nutrients from dead organic matter and are called saphrophytes. Saphrophytes play an important ecological role in the decomposition of dead plants, animals, and other organic matter: they release large amounts of carbon dioxide into the atmosphere and recycle nitrogen and other important nutrients for use by plants and other organisms. Other fungi are parasites (obtaining their nutrients from a living host organism in a relationship that usually harms the host) or mutualists (involved in a mutually beneficial relationship with another organism).

Another important characteristic of fungi is that they do not contain chlorophyll. Chlorophyll is a green pigment that enables plants and such other photosynthetic organisms as algae and cyanobacteria to absorb energy from sunlight and use it to synthesize carbohydrates (photosynthesis). Because fungi are not reliant on sunlight as an energy source, they can grow in dark or low-light environments and in directions not normally observed in plants.

Morphology

Most fungi may be classified according to two major growth forms: yeasts or molds. Yeasts are round, unicellular (single-celled) organisms that form a vegetative body called a thallus. The thallus may consist of cells in groups or in branched chains called pseudo-hyphae. Examples of yeasts include Saccharomyces cerevisiae, used in making bread and alcoholic beverages; and Candida albicans, the causative agent of yeast infections.

Molds, on the other hand, are composed of long filaments called hyphae (singular, hypha). Hyphae may be further classified as septate (containing cross walls) or aseptate. A mass of hyphae is called a mycelium.

Whereas yeast cells each contain a single nucleus, cells in septate hyphae may be uninucleate (containing one nucleus), binucleate (containing two nuclei), or multinucleate (containing many nuclei). An example of a mold is Penicillium roqueforti, used to make blue cheese.

Some fungi are dimorphic: they may exist in either yeast or mold form. What form a fungus assumes depends on such environmental factors as the temperature or nutrients present. Some examples of dimorphic fungi include Histoplasma capsulatum and Coccidioides immitis.

Reproduction

All fungi can reproduce asexually by the production of single-celled structures called spores. The number of chromosomes (structures in the nucleus containing genetic material) remains unchanged when cells duplicate their genetic material and then divide. This is not the ideal state for a fungus and is thus called the imperfect state. (It is often observed in the laboratory when fungi that are normally pathogenic to humans are allowed to reproduce.)

Sexual reproduction can also occur in most fungi and is called the perfect state. In this process, one cell divides to become two haploid cells (each containing a single set of unpaired chromosomes). Two cells can then fuse together to become a diploid cell (containing a full set of chromosomes); that cell can then divide.

Role in human health

Some fungi have been found to be directly or indirectly beneficial to humans, while others are pathogenic (disease-causing). Still others are pathogenic to plants and animals important in the food chain.

Food manufacturing

Different yeasts in the genus Saccharomyces are employed by bakers, brewers, and vintners to make their bread, beer, or wine. For instance, S. cerevisiae is commonly used as baker's yeast and in the production of ales. Candida milleri is a yeast used in conjunction with an acid-producing bacteria to yield sourdough bread.

Various species of mushrooms are cultivated specifically for human consumption. These include Agaricus bisporus (accounting for 38% of the world's cultivated mushroom supply), Lentinus edodes (shiitake mushrooms), Volvariella volvacae (the paddy straw mushroom), and the Pleurotus family (oyster mushrooms). Other edible fungi include truffles (fungi of the family Tuber that grow in a special subterranean (mycorrhizal) association with certain trees), morels (of the Morchella family), and the blue-green mold of the Penicillium family that is essential in the production of certain cheeses.

Medicinal and recreational drugs

Discovered in 1929, a metabolite of the fungus Penicillium notatum (later to be called penicillin) became the first antibiotic (a substance produced by a microorganism that can selectively treat an infectious disease). Other fungi that are the source of clinically important antibiotics include those in the family Streptomyces: S. nodosus (amphotericin B), S. erythreus (erythromycin), S. fradiae (neomycin), S. griseus (streptomycin), S. orientalis (vancomycin), and S. rimosus (tetracycline).

Some species of fungi are known as hallucinogens (substances inducing false sensations in the absence of true stimuli) and have been used by many cultures during religious ceremonies (for example, Amanita muscaria). Claviceps purpurea (the ergot fungus) and fungi of the Psilocybe family are also known for their hallucinogenic effects.

Biomedical research

Phycomyces blakesleeanus is a fungus that grows on animal feces in nature. The sporangiophores (the stalks on which spores are produced) of Phycomyces have been shown to respond to a variety of stimuli, including light, gravity, wind, and nearby objects. One important finding was that the light sensitivity of the sporangiophore is about the same as the eyes of humans. Furthermore, like humans, the sporangiophore can adapt to a one-billion-fold change in ambient light intensity.

Biologists have recently shown that Neurospora crassa, also known as red bread mold, can produce spores at approximately 24-hour intervals (known as a circadian rhythm) when in a constant environment. The fungus is therefore being used as a model organism for investigating circadian rhythms, which occur in many different organisms including humans.

Common disease and disorders

Human mycoses can be classified as superficial, cutaneous, subcutaneous, systemic, or opportunistic.

Superficial and cutaneous mycoses

These fungal infections do not invade underlying muscle or bone and are mostly restricted to the outer layers of the skin, nails, and/or hair. Superficial mycoses involve only the outermost layers of skin and result in a change in hair or skin pigment. For example, tinea nigra, caused by Exophiala werneckii, results in black lesions on the skin. Piedraia hortai, the causative agent of black piedra, creates hard dark-colored nodules on scalp hair, eyebrows, and/or eyelashes.

Cutaneous mycoses are generally caused by infection with a dermatophyte (a skin-infecting fungus). Common families of dermatophytes are Epidermophyton, Microsporum, and Trichophyton. Some of the more commonly seen cutaneous infections include:

• tinea corporis (body, "ringworm")
• tinea capitis (scalp, eyebrows, eyelashes)
• tinea barbae (beard, "barber's itch")
• tinea cruris (groin, "jock itch")
• tinea inguium (nails)
• tinea pedis (feet, "athlete's foot")

Subcutaneous mycoses

In the case of subcutaneous fungal infection, muscle, bone, connective tissue, and/or overlying skin may be involved. Subcutaneous mycoses may begin at the site of a laceration or even a seemingly innocuous scratch or puncture wound; fungi are introduced from soil or plant material. These mycoses, however, typically remain localized rather than spread from the site of infection. Sporotrichosis (caused by Sporothrix schenckii) and mycetoma (caused by Madurella grisea, among others) are two noted exceptions; sporotrichosis may spread along the lymphatic system , and mycetoma to deeper muscle and bone.

KEY TERMS

Cyanobacteria —Photosynthetic bacteria, commonly known as blue-green algae.

Dermatophyte —A fungus that can cause a skin infection.

Hypha —Cellular unit of the fungi; typically a branched and tubular filament.

Lichen —A fungus that grows a symbiotic relationship with algae.

Meningitis —Inflammation of the meninges, the membranes surrounding the brain and spinal cord.

Metabolite —A substance produced by way of a metabolic process.

Morphology —The study of the shape and structure of an organism.

Mutualism —Close relationship of two or more organisms, which typically involves exchange of food or other resources.

Mycorrhiza —Subterranean symbiotic relationship between a fungus and a plant root.

Septum —Wall that separates the cells of a fungal hypha into segments.

Systemic mycoses

Systemic fungal infections usually involve more than one type of body tissue. The lungs are often a site of primary infection when airborne spores are inhaled. Often the primary infection is asymptomatic (shows no signs of infection) or resolves quickly. If the fungus spreads to the bloodstream, however, it may disseminate to other organs or systems. The following are the most commonly seen systemic mycoses:

• Blastomycosis, caused by Blastomyces dermatitidis; begins as a pulmonary infection but may disseminate to bone and/or skin.
• Coccidioidomycosis, caused by Coccidioides immitis; begins as a pulmonary infection (although 60% of infections are asymptomatic) but may disseminate to the central nervous system , bone, and/or skin.
• Cryptococcosis, caused by Cryptococcus neoformans; begins as a pulmonary infection but may disseminate to the central nervous system to cause meningitis.
• Histoplasmosis, caused by Histoplasma capsulatum; begins as a pulmonary infection but may disseminate to the lymph nodes, spleen, and/or liver .
• Paracoccidioidomycosis, caused by Paracoccidioides immitis; begins as a pulmonary infection but may disseminate to the mucous membranes, lymph nodes, and/or skin.

Opportunistic mycoses

Opportunistic fungi do not normally cause disease in healthy humans, but can cause infection in individuals who are immunocompromised, such as those with acquired immune deficiency syndrome [AIDS] or those who have undergone organ transplantation. Some important opportunistic mycoses include:

• Aspergillosis, a mycosis caused by members of the Aspergillus family. Common mechanisms of infection include hypersensitivity (an allergic reaction); local pulmonary infection; opportunistic infection (leading to pneumonia and the development of a characteristic "fungal ball"); and systemic infection (leading to abscesses in the brain , liver, kidneys , skin, or bone.
• Candidiasis. Candida albicans is a yeast that causes oropharyngeal candidiasis, also known as thrush. Thrush is an often-seen opportunistic infection in patients with acquired immune deficiency syndrome (AIDS ). Candida albicans is also the cause of the majority of cases of vulvovaginitis (yeast infection).

Resources

BOOKS

Fisher, Frances W. Fundamentals of Diagnostic Mycology. Philadelphia: W. B. Saunders Company, 1998.

Spicer, W. John. Clinical Bacteriology, Mycology, and Parasitology. London, UK: Churchill Livingstone, 2000.

OTHER

"Mycology." Microbiology and Immunology On-line Textbook. University of South Carolina. 3 April 2001. 2 August 2001. <http://www.med.sc.edu:85/book/mycolsta.htm>.

"Mycoses."Doctor Fungus Website. 12 April 2001. 2 August 2001. <http://www.doctorfungus.org/Mycoses/index.htm>.

"Penicillin and Other Antibiotics." The Microbial World

Website. University of Edinburgh. 2 August 2001. <http://helios.bto.ed.ac.uk/bto/microbes/penicill.htm>. "

What are yeasts?" Yeast WWW Virtual Library. Stanford University. 2 August 2001. <http://genome-www.stanford.edu/Saccharomyces/VL-what_are_yeast.html>.

Stéphanie Islane Dionne