Introduction to C. elegans
Caenorhabditis elegans is a small (1 mm) free-living nematode worm that has served as a model organism for understanding many biological processes. The “worm” is easy to grow in the lab, crawling on the surface of agar plates and eating E. coli. Since the generation time is only about four days at room temperature and each worm will have as many as 300 offspring, C. elegans is an ideal organism for genetics. Many mutants are known, and mutant strains are available from the Caenorhabditis Genetics Center.
C. elegans has two sexes, males and hermaphrodites. A hermaphrodite has the anatomical features of a female and lays eggs. However, at the beginning of meiosis, it makes sperm. It then switches off spermatogenesis (permanently) and begins oogenesis, which continues until all sperm are exhausted. If no male are available as sperm donors, the hermaphrodite will self-fertilize its own oocytes with its own sperm. When males mate with an hermaphrodite, the male sperm out-compete the hermaphrodite sperm and cross-fertilization occurs.
In addition to its genetic advantages, C. elegans is cellularly simple. For example, it accomplishes complex behaviors and movements with only 302 neurons and 152 muscle cells. The complete developmental lineage of the cells is known, as is the complete wiring diagram of the nervous system. With a few (programmed) alternatives, the cell lineages are invariant. This allows us to a compile a cellular parts list of the worm, in principle knowing the ancestry and descendents of every cell. Much more information about the development and neurobiology of C. elegans is available on the web from links at the Caenorhabditis Genetics Center.
C. elegans was also the first multicellular organism to have its genome completely sequenced. It is estimated to have about 17,500 genes in a genome of slightly less than 100 million base pairs. Detailed information about the worm genome can be found at Wormbase. There are six chromosomes, five autosomes and the X chromosome. Hermaphrodites have five pairs of autosomes and a pair of X chromosomes, whereas males have five pairs of autosomes and a single X chromosome; there is no Y chromosome, so the males are X0. Thus, when a hermaphrodite self-fertilizes and uses its own sperm to fertilize its own ova, the offspring will be XX and hermaphrodite. However, about one in 550 gametes is nullo-X , resulting in an X0 (male) offspring. The spontaneous loss of the X chromosome appears to occur principally during spermatogenesis in hermaphrodites. Half the offspring of a male to hermaphrodite mating are males and half are hermaphrodites, indicating that the X chromosome in the male goes through meiosis normally despite the absence of a pairing partner.
This mode of reproduction has made it easy to identify mutations that affect meiosis. Such mutations will have a high incidence of male offspring (a Him phenotype). Many such mutations are known, although only a few have been thoroughly studied. Most of the genes affect all of the chromosomes and result in aneuploidy embryos that do not survive; among the survivors, there is high frequency of males. Mutations in a few genes do not have a lot of aneuploid offspring, although they do have a high frequency of males. These genes may affect the X chromosome more strongly than the autosomes, and have formed the basis for most of our recent research.