Caenorhabditis elegans is a free-living, nonparasitic nematode. It is a self-fertilizing hermaphrodite. Males arise spontaneously by nondisjunction of X-chromosomes. Of all eukaryotic organisms C. elegans has probably been most extensively studied at the cellular level. Within 12 hours the fertilized egg develops into a young larva with 558 nuclei (560 in the male). During postembryonic development the animal proceeds through four larval stages increasing its number of nuclei to 959 (1,031 in the male) plus some 2,000 germ cells (about 1,000 in the male). The cell lineages from fertilization to adulthood have been completely analyzed in living embryos and animals. This and its well-established genetics (more than 300 genes have been mapped on the six linkage groups) make it a suitable model organism to study problems of gene action and development. Various techniques have been used to interfere with normal development (including laser-induced cell ablations) and to analyze development on the subcellular level (including recombinant DNA technology). The characteristic features of rigidly determined development, the low cell number, and the knowledge of cellular events should make it possible to identify molecular action in situ and relate it to the structure and
The dauer, or "enduring," larval stage of Caenorhabditis elegans is a form of facultative diapause. Environmental factors act as signals to a receptive developmental stage (the J1 juvenile) resulting in altered physiology and developmental potential, so that a third-stage juvenile can be formed that is specialized for dispersal and long-term survival. Dauer larvae are capable of active movement, but they do not feed. They have a unique morphology and resistance to stress, they are altered in energy metabolism, and they are arrested in development. Dauer larvae survive four to eight times the 3-week life span of animals that have bypassed the dauer stage. The dauer stage itself has been considered to be nonaging, because the duration of the dauer stage does not affect postdauer life span. As far as is known, the consumption of stored energy may be the major factor limiting dauer larva life expectancy. Diapause may be ended in response to conditions improved for growth and reproduction. Developmental commitment to recovery (exit) from the dauer stage occurs within 1 hour after the animal is placed in a fresh environment with food. After 2-3 hours it begins to feed, resumes development, and molts to the J4 stage after
Nematodes have been cultured continuously in the laboratory since 1944 when Margaret Briggs Gochnauer isolated and cultured the free-living hermaphroditic species Caenorhabditis briggsae. Work with C. briggsae and other rhabditid nematodes, C. elegans, Rhabditis anomala, and R. pellio, demonstrated the relative ease with which they could be cultured. The culturing techniques described here were developed for C. elegans, but are generally suitable (to varying degrees) for other free-living nematodes. Whereas much of the early work involved axenic culturing, most of these techniques are no longer in common use and are not included here. In the 1970s C. elegans became the predominant research model due to work by Brenner and co-workers on the genetics and development of this species. An adult C. elegans is about 1.5 mm long, and under optimal laboratory conditions has a life cycle of approximately 3 days. There are two sexes, males and self-fertile hermaphrodites, that are readily distinguishable as adults. The animals are transparent throughout the life cycle, permitting observation of cell divisions in living animals using differential interference microscopy. The complete cell lineage and neural circuitry have been determined and a large collection of behavioral and anatomical mutants have been isolated. C. elegans has six developmental stages: egg, four larval stages (L1-L4), and adult. Under starvation conditions or specific manipulations of the culture conditions a developmentally arrested dispersal stage, the dauer larva, can be formed as an alternative third larval stage. Many of the protocols included here and other experimental protocols have been summarized in "The Nematode Caenorhabditis elegans". We also include a previously unpublished method for long-term chemostat cultures of C. elegans. General laboratory culture conditions for nematode parasites of animals have been described, but none of these nematodes can be cultured in the laboratory through more than one life cycle. Marine nematodes and some plant parasites have been cultured xenically or with fungi. Laboratory cultivation of several plant parasites on Arabidopsis thaliana seedlings in agar petri plates has also been reported.