Roundworm: Caenorhabditis elegans
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.