THE EVOLUTION OF CELL LINEAGE IN NEMATODES

The invariant development of free-living nematodes combined with the e xtensive knowledge of Caenorhabditis elegans developmental biology pro vides an experimental system for an analysis of the evolution of devel opmental mechanisms, We have collected a number of new nematode specie s from soil samples. Most are easily cultured and their development ca n be analyzed at the level of individual cells using techniques standa rd to Caenorhabditis. So far, we have focused on differences in the de velopment of the vulva among species of the families Rhabditidae and P anagrolaimidae. Preceding vulval development, twelve Pn cells migrate into the ventral cord and divide to produce posterior daughters [Pn.p cells] whose fates vary in a position specific manner [from P1.p anter ior to P12.p posterior]. In C. elegans hermaphrodites, P(3-8).p are tr ipotent and form an equivalence group. These cells can express either of two vulval fates (1 degrees or 2 degrees) in response to a signal f rom the anchor cell of the somatic gonad, or a non-vulval fate (3 degr ees), resulting in a 3 degrees-3 degrees-2 degrees-1 degrees-2 degrees -3 degrees pattern of cell fates, Evolutionary differences in vulval d evelopment include the number of cells in the vulval equivalence group .