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Curr Top Dev Biol,
1991]
The striking invariance of nematode development has inspired and intrigues developmental biologists for 100 years (e.g., Boveri, 1892; zur Strassen, 1892; Pai, 1928). We now know that this invariant development is the result of both intrinsic and extrinsic controls over cell fate. Highly reproducible cell interactions occur since most cells do not migrate, and are thus always subject to signals from the same neighbors. Some of the very features that make the nematode Caenorhabditis elegans an attractive organism with which to study development - its small cell number and essentially invariant development- make one wonder how relevant studies of this nematode will be to animals whose development is not invariant: To what extent will conclusions based on studies with C. elegans apply to organisms with a large number of cells and variable development? One can answer that nematode development is not wholly invariant. One can also answer this question of relevance by arguing that molecular mechanisms are conserved even if developmental phenomena are not, and what one learns about a particular gene product in one organism can be extended to other organisms. This "model system" argument is certainly meritorious, and the success of yeast molecular genetics in unraveling cell biological problems might well be repeated by C. elegans molecular genetics for problems specific to metazoans. Moreover, since nematodes diverged from the their coelomate relatives prior to the arthropod - chordate split, nematodes afford useful molecular biological comparisons for genes conserved between, for example, insects and mammals. Caenorhabditis elegans will provide such a comparison, given the intensive molecular biological studies driven by the genome-mapping project (Coulson et al., 1986, 1988; reviewed by Robertson, 1990).