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 degree or 2 degrees) in response to a signal from the anchor cell of the somatic gonad, or a nonvulval fate (3 degrees), resulting in a 3 degrees-3 degrees-2 degrees-1 degree-2 degrees-3 degrees pattern of cell fates. Evolutionary differences in vulval development include the number of cells in the vulval equivalence group, the number of 1 degree cells, the number of progeny generated by each vulval precursor cell, and the position of VPCs before morphogenesis. Examples of three Rhabditidae genera have a posterior vulva in the position of P9-P11 ectoblasts. In Cruznema tripartitum, P(5-7).p form the vulva as in Caenorhabditis, but they migrate posteriorly before dividing. Induction occurs after the gonad grows posteriorly to the position of P(5-7).p cells. In two other species, Mesorhabditis sp. PS 1179 and Teratorhabditis palmarum, we have found changes in induction and competence with respect to their presumably more C. elegans-like ancestor. In Mesorhabditis, P(5-7).p form the vulva after migrating to a posterior position. However, the gonad is not required to specify the pattern of cell fates 3 degrees-2 degrees-1 degree-2 degrees-3 degrees. Moreover, the Pn.p cells are not equivalent in their potentials to form the vulva. A regulatory constraint in this family thus forces the same set of precursors to generate the vulva, rather than more appropriately positioned Pn.p cells.