THE EVOLUTION OF CELL LINEAGE IN NEMATODES

The invariant development of free-living nematodes combined with the extensive knowledge of Caenorhabditis elegans developmental biology provides an experimental system for an analysis of the evolution of developmental mechanisms, We have collected a number of new nematode species from soil samples. Most are easily cultured and their development can be analyzed at the level of individual cells using techniques standard to Caenorhabditis. So far, we have focused on differences in the development of the vulva among species of the families Rhabditidae and Panagrolaimidae. 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 anterior to P12.p posterior]. In C. elegans hermaphrodites, P(3-8).p are tripotent and form an equivalence group. These cells can express either of two vulval fates (1 degrees or 2 degrees) in response to a signal from the anchor cell of the somatic gonad, or a non-vulval fate (3 degrees), resulting in a 3 degrees-3 degrees-2 degrees-1 degrees-2 degrees-3 degrees pattern of cell fates, Evolutionary differences in vulval development include the number of cells in the vulval equivalence group.