EVOLUTION OF CELL LINEAGE AND PATTERN-FORMATION IN THE VULVAR EQUIVALENCE GROUP OF RHABDITID NEMATODES

During the formation of the vulva in many nematode hermaphrodites or f emales, pattern formation, induction, and cell specification can readi ly be studied at a single-cell level. Nematodes thus allow an evolutio nary analysis of developmental processes. We have analyzed cell lineag es and pattern formation in the vulva equivalence group of six rhabdit id nematodes of the genera Oscheius, Rhabditella, Rhabditoides, Pelode ra, and Protorhabditis. The comparison of these species with four prev iously analyzed species of this family reveals evolutionary modificati on at several levels. The number of vulva precursor cells (VPCs) diffe r among species. Of the three particular cell lineages (1 degrees, 2 d egrees, and 3 degrees) generated by the vulva precursor cells in Caeno rhabditis, two (2 degrees and 3 degrees) are altered, whereas the thir d lineage (1 degrees) is conserved among the analyzed species. While m ost vulval lineages are invariant, we observe variability of the 3 deg rees lineage in Pelodera with respect to the number of precursor cells adopting this fate and the number of progeny formed. In two species, the 3 degrees lineage generates an asymmetrical set of cells, oriented by the gonad. In Protorhabditis we frequently find animals with an ad ditional or altered set of VPCs forming vulval tissue. (C) 1995 Academ ic Press, Inc.