Evolution of cell lineage and pattern formation in the vulval equivalence group of rhabditid nematodes.

During the formation of the vulva in many nematode hermaphrodites or females, pattern formation, induction, and cell specification can readily be studied at a single-cell level. Nematodes thus allow an evolutionary analysis of developmental processes. We have analyzed cell lineages and pattern formation in the vulva equivalence group of six rhabditid nematodes of the genera Oscheius, Rhabditella, Rhabditoides, Pelodera, and Protorhabditis. The comparison of these species with four previously analyzed species of this family reveals evolutionary modification at several levels. The number of vulva precursor cells (VPCs) differ among species. Of the three particular cell lineages (1 degree, 2 degrees, and 3 degrees) generated by the vulva precursor cells in Caenorhabditis, two (2 degrees and 3 degrees) are altered, whereas the third lineage (1 degree) is conserved among the analyzed species. While most vulval lineages are invariant, we observe variability of the 3 degrees 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 additional or altered set of VPCs forming vulval tissue.