Roundworm: Caenorhabditis elegans

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Roundworm: Caenorhabditis elegans

Caenorhabditis elegans is a nonparasitic nematode that normally lives in the soil. Although studied since the 1800s, the modern use of C. elegans as a model system dates to the mid-1960s. By the start of that decade, scientists thought that the "classical" problems in molecular biology were about to be solved. They began to search for a model system that would support the new challenges presented by research on multicellular organisms, particularly in developmental biology and neurology. In 1965 Sydney Brenner proposed what he thought was the ideal model organism, one that provided the best compromise between biological complexity and ease of manipulation: C. elegans.

Useful Characteristics

C. elegans is hermaphroditic, meaning that almost all of the 300 progeny produced in a single clutch of eggs are females, capable of self-68fertilization. In essence, C. elegans clones itself. One or two progeny are morphologically distinct males that can be collected and used for standard genetic studies. This permits rapid chromosomal mapping of the many mutations that occur in any one of the organism's six chromosomes. The C. elegans genome was sequenced in 1998, predicting 19,099 genes. Currently, 3,000 of these are considered "essential" genes, 900 of which had already been identified through chromosomal mapping.

In the laboratory, C. elegans is easily cultivated on simple growth media on petri dishes. Only 1 millimeter long, its transparent nature permits an investigator to trace the origin and determine the exact anatomical position of any one of its 959 cells at any stage in development from fertilized egg to death. This has become known as "cell lineage." Cell abnormalities, as well as abnormal migration of normal cells, are readily discovered, which has led to the identification and cataloging of a large number of mutants. The ability to directly see the individual sarcomeres (protein subunits) within a muscle cell quickly established C. elegans as a model system for the study of muscle structure and function. The gene for the muscle protein myosin, which could not be identified or cloned in other organisms, was readily cloned from C. elegans. This accomplishment, along with the study of the genes that give rise to "paralyzed" mutants of C. elegans, has had a major impact on the study of muscle development and diseases in humans. The transparency of C. elegans continues to expand the universality of this model system through the use of techniques that were not available in 1965, such as microinjection or laser microsurgery.

Insights from Laboratory Observation

Although anatomically and genetically simple, C. elegans mimics the life cycle of humans. Starting from a fertilized egg, it undergoes a complex development that gives rise to excretory, reproductive, digestive, and neuromuscular organ systems. The cell lineages of each of the 959 adult somatic cells have been directly observed, as has the fact that an additional unique set of 131 cells die during development. These deaths would have gone unnoticed, except for their demise was observable under the microscope. These observations led to the concept that apoptosis is a vital feature of development in multicellular organisms, and that it enables the shaping and carving of organs and tissues.

Today C. elegans remains one of the foremost model systems used in genetic research. Advanced descriptions have been made of its anatomy, cell lineages, developmental genetics, neural development, and reproductive cycle. The recent discovery of a mutation that doubles its life expectancy suggests that this "worm" will continue to expand its usefulness as a model system for the study of aging. Its growing significance as a member of the large, evolutionarily successful Nematoda phylum highlights the importance of C. elegans to the study of the genes involved in adaptive evolution.

see also Apoptosis; Cloning Genes; Genome; Mapping; Model Organisms.

Diane C. Rein