sem-4/spalt and egl-17/FGF have a conserved role in sex myoblast specification and migration in P. pacificus and C. elegans

Evolutionary comparisons between Caenorhabditis elegans and the satellite organism Pristionchus pacificus revealed major differences in the regulation of nematode vulva development. For example, Wnt signaling is part of a negative signaling system that prevents vulva formation in P. pacificus, whereas it plays a positive role in C. elegans. We wondered if the genetic control of the second major part of the nematode egg-laying system, the sex muscles, has diverged similarly between P. pacificus and C. elegans. The sex muscles derive from the mesoblast M, which has an identical lineage in both species. Here, we describe a large-scale mutagenesis screen for mutations that disrupt the M lineage and the sex myoblast (SM) sublineage. We isolated and characterized mutations that result in a failure of proper SM fate specification and SM migration and showed that the corresponding genes encode Ppa-sem-4 and Ppa-egl-17, respectively. Ppa-sem-4 mutants have additional defects in the specification of the vulva precursor cells P(5, 7).p and experimental studies in the Ppa-egl-17 mutant background indicate a complex set of gonad-dependent and gonad-independent mechanisms required for SM migration. Mutations in Cel-sem-4 and Cel-egl-17 cause similar defects. Thus, the molecular mechanisms of SM cell specification and migration are conserved between P. pacificus and C. elegans.