Developmental Biology of Caenorhabditis elegans: Symposium Introduction.

T h e comple te pos tembryon ic ceil l ineages of the free-l iving nenta todes Caenorhabditis elegans and Panagrellus redivivus are known. Pos tembryonic cell divisions lead to subs tan t ia l increases in the n u m b e r of cells and, in mos t cases, in the n u m b e r of types of cells in the neurona l , muscu la r , hypodermal , and digestive systems. T h e pa t te rns of pos tembyron ic cell divisions are essentially invar ian t and genera te a fixed n u m b e r of progeny cells of strictly specified fates. Cell fates depend u p o n bo th l ineage his tory and cell-cell in teract ions: l ineage l imits the developm e n t a l po tent ia l of each cell and, for cer tain cells, cell-cell in terac t ions specify which of a smal l n u m b e r of a l te rnat ive potent ia l fates is acqui red . Relat ively s imple differences in cell l ineage account for some of the s t r ik ing differences in gross morpho logy bo th between sexes and be tween species. Genet ic s tmties indicate tha t these cell l ineage differences reflect one o r a few relatively s imple m u t a t i o n a l events, ht terspecif ic differences in cell l ineage are likely to be good indicators of evolu t ionary dis tance and may be he lp fu l in def in ing t axonomic re la t ionships . Both the techn iques uti l ized in, and the in fo rma t ion acqui red from, s tudies of cell l ineages in C. elegam attd P. redivivus may prove useful to o the r hematologis ts .