Crop Improvement

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CROP IMPROVEMENT

CROP IMPROVEMENT. Crop improvement refers to the genetic alteration of plants to satisfy human needs. In prehistory, human forebears in various parts of the world brought into cultivation a few hundred species from the hundreds of thousands available. In the process they transformed elements of these species into crops though genetic alterations that involved conscious and unconscious selection, the differential reproduction of variants. Through a long history of trial and error, a relatively few plant species have become the mainstay of agriculture and thus the world's food supply. This process of domestication involved the identification of certain useful wild species combined with a process of selection that brought about changes in appearance, quality, and productivity. The exact details of the process that altered the major crops is not fully understood, but it is clear that the genetic changes were enormous in many cases. In fact some crop plants have been so changed that for many of them, maize, for example, their origins are obscure, with no extant close wild relatives.

The selection process was unconscious in many cases. For example, in wild wheats, the grains scatter by disarticulation, separation of the seed from the seed head. When these grains were harvested by cutting the heads with a sickle, an unconscious selection occurred for "nonshattering" types that would then be continually replanted. For some crops a clear conscious selection occurred, especially when the variant was obvious and would be maintained by vegetative propagation. Something so clearly useful as a seedless banana must have been immediately seized upon and maintained ("fixed") by planting offshoots of the plant. The changes wrought in domestication included alteration in organ size and shape; loss of many survival characters, such as bitter or toxic substances; disarticulation of seeds in grains; protective structures, such as spines and thorns; seed dormancy; and change in life span—increased in crops grown for roots or tubers and decreased in crops grown for seed or fruit. Selection by bulking desirable types (mass selection) is a powerful technique for making rapid changes easily while maintaining genetic diversity in the population.

The selection of naturally occurring variants is the basis of crop improvement. Over thousands of years this technique resulted in the development of modern basic crops. The discovery of techniques for asexual (vegetative) propagation, such as by using natural offshoots, rooting stem cuttings, or various grafting techniques, made it possible to "fix" genetic variants. This was the technique used for many tree fruits, enabling identical plants to be cultivated in orchards. Naturally produced seedlings derived from intercrosses of these selected plants were then available for selection again. Many present-day fruit crops are similar to those cultivated in antiquity, and some ancient selections are still cultivated—dates, for example. As humans carried improved crops to new locations, opportunities opened to increase genetic variation from natural intercrosses with new wild populations.

The changes that occur can be dramatic over time, as seen in the proliferation of breeds of animals and especially the wide range of changes brought about by fanciers of dogs, chickens, and pigeons. The observation of these changes influenced the thinking of Charles Darwin to suggest that natural selection, the survival of the fittest, could lead to enormous genetic changes if carried out over a long enough time, and could lead to the origin of new species.

In the eighteenth and nineteenth centuries a conscious attempt was made to predict the performance of plants that could be expected from one seed generation to the next. The concept that ancestry was important in crop improvement led to refinement in the selection process, brought about by keeping records and the assessment of lineage. Furthermore it became obvious that variation could be managed by controlling the mating process, an extension of what had long been known in animal breeding. This new type of selection, termed pedigree selection, was found to increase the efficiency of the process. Progeny testing (evaluating the genetic worth by progeny performance) increased efficiency of this process. The origins of commercial plant breeding began in the second half of the nineteenth century among seed producers. It involved controlled crosses (hybridization) between selections to control genetic recombination, followed by selection of improved types. This is still the basis of traditional plant breeding. Interestingly much of this early type of plant breeding was carried out without a clear understanding of the genetic mechanism involved in inheritance.

Until the famous experiments with the garden pea by Gregor Mendel (1822–1884), a Catholic priest in Brünn, a Moravian town then in the Austro-Hungarian Empire, the basic theory of inheritance involved the concept of blending. Mendel unraveled the basic concept of inheritance and clearly showed that characters in the pea were due to elements, later called genes, that remained unaltered as they were inherited. Many characters in peas, such as tallness and dwarfness, were shown to be controlled by a pair of genes, of which one member was not always expressed (the concept of dominance and recessiveness). Mendel demonstrated that the gametes of the pea contained one member of the gene pairs that controlled characters and that recombined randomly at fertilization. Mendel's paper was published in 1866, but it had no impact until the paper was "rediscovered" in 1900, when it created a sensation. It was soon obvious that the differences in appearance among plants (phenotypes) could be explained by the interaction of various genes (genotypes) as well as interaction with the environment.

Twentieth-Century Developments

In the twentieth century plant breeding developed a scientific basis, and crop improvement was understood to be brought about by achieving favorable accumulations and combinations of genes. Taking advantage of known genetic diversity could facilitate this, and appropriate combinations were achieved through recombinations brought about by the sexual process (hybridization). Furthermore it was possible to move useful genes by special breeding strategies. Thus a gene discovered in a wild plant could be transferred to a suitable adapted type by a technique known as the backcross method. A sexual hybrid was made, followed by a series of backcrosses to the desirable (recurrent) parent, while selecting for the new gene in each generation. After about five or six back-crosses, the offspring resembled the recurrent parent but contained the selected gene.

In the early twentieth century it was demonstrated that the extra vigor long associated with wide crosses (called hybrid vigor or heterosis), particularly in naturally cross-pollinated crops, could be exploited in plant breeding. For maize, a new system of hybrid breeding was developed, using a combination of inbreeding and outbreeding. Inbreeding was accomplished by crossing the plant with itself. This led to a decline in vigor as the step was repeated over several generations. Outbreeding was achieved by intercrossing the inbred lines to restore vigor. The hybrid between inbreds derived from divergent inbreds (called a single cross or F₁ hybrid) was uniform (homogeneous), and some were superior to the original populations before inbreeding. During the process of inbreeding, the inbreds became weak, but vigor was restored in the F₁. To increase seed set from weak inbreds, two hybrids were crossed; this was known as the double cross method.

Hybrid breeding technique in a sense is similar to arranging a Rubic's cube, where contradictory steps need to be taken to achieve the appropriate reformulation. In hybrid breeding, the first step produces a series of weak inbreds, followed by a series of specific combination, to produce a series of new hybrids. Hybrid maize breeding led to enormous increases in productivity, which were soon exploited in a wide variety of seed-propagated crops, including naturally self-pollinated ones, such as tomato and rice.

A number of genetic techniques were developed and refined in twentieth-century breeding, such as improved techniques to search for and store increased genetic variability, different techniques to develop variable populations for selection, and improved methods of testing to separate genetic from environmental effects. The exact details of the process for crops necessarily differed among naturally cross-pollinated plants (such as maize) and naturally self-pollinated plants (such as soybean or tomato) as well as those plants in which vegetative propagation (usually cross-pollinated) permitted the fixing of improved types directly.

Conventional plant breeding can be defined as systems for selection of superior genotypes from genetically variable populations derived from sexual recombination. The system is powerful because it is evolutionary; progress can be cumulative, with improved individuals continually serving as parents for subsequent cycles of breeding. Genetic improvement by conventional breeding has made substantial changes when the efforts have been long-term. Characters improved include productivity, quality, and resistance to diseases, insects, and stress. There are, however, limits to the progress of conventional breeding. These are due to limitations of the sexual system, because it is usually not possible to incorporate genes from nonrelated species or to incorporate small changes without disturbing the particular combination of genes that make a particular type unique. Thus a useful gene in cabbage cannot be transferred to wheat. Limitations of conventional breeding are particularly apparent when a needed character (such as disease or insect resistance) is unavailable in populations that can be incorporated by sexual crosses. Mutations may be induced, but they are often deleterious or connected with undesirable effects.

With conventional breeding, it is also not possible to improve a unique genotype, such as "Bartlett" pear, by adding a single character, since the recombination that results from hybridization makes it impossible to recon-figure this cultivar exactly. Finally, conventional breeding has technical or economic limitations to detect infrequent or rare recombinants, the lack of sufficient time to generate cycles of recombination, space to grow necessary populations to recover superior recombinants, or resources to be able to select, identify, evaluate, and fix desired recombinants.

Developments Using DNA

It has been suggested that many of the limitations of conventional breeding can be overcome with advances in molecular biology that rely on DNA, the genetic material.

Recombinant DNA technology, called transgene technology or genetic engineering, is the most powerful and revolutionary of the new genetics developed in the last half of the twentieth century. It is possible to isolate stretches of DNA from one organism, store it in a bacterial host, select unique combinations, and then incorporate them into the DNA of another species, where it can be expressed. This technique, which relies on cell and tissue culture, is truly a marvelous process. Refinements in the technique make it possible to concentrate mutations in desired genes, further increasing variability. Other uses of molecular biology known as genomics involve the detailed mapping of the DNA and the identification of useful stretches of the molecule. This makes it possible to improve the efficiency of selection, because it is based directly on the genes rather than the organism, where the effects may be confounded by environment and genetic interactions.

The limitations of the new breeding methods include technical problems, such as the difficulty of transformation, problems of gene expression, or the lack of knowledge concerning suitable genes to transfer. There are also nontechnical issues, such as legal problems, since the techniques and the genes are usually patented. However, in the short run the greatest impediment has been problems of consumer acceptability and fear of the unknown. The term "Frankenfood" has been coined to refer to food altered by the process of using exotic genes incorporated by transgene technology. No convincing evidence shows that genetic engineering has produced harmful products, and an abundance of evidence shows that many foods derived from traditional systems have inherent problems (consider the allergic reactions of many people to peanuts). Nevertheless, molecular techniques have incited fear of this new technology in many people. Moreover, the surplus of food in the West has reduced the imperative to make the case for the need for new technology to consumers.

The biotechnology industry has sold the technology to farmers (who have accepted it) and ignored consumers. They have not been sophisticated in exploiting the environmental virtues inherent in the new technology, such as reducing pesticides, or in making the case that increased yield could free up the agricultural use of fragile environments. Because of the benefits that could accrue from this new technology, especially in the problem areas of the world, it seems certain that future progress in plant breeding will involve both conventional and unconventional techniques, but the immediate course of events is fraught with uncertainty.

See also Agronomy; Genetic Engineering; Green Revolution; High-Technology Farming; Horticulture.