Formation of the egg-laying system in Pristionchus pacificus requires complex interactions between gonadal,...

The invariant cell lineage of nematodes allows the formation of organ systems, like the egg-laying system, to be studied at a single cell level. The Caenorhabditis elegans egg-laying system is made up of the vulva, the mesodermal gonad and muscles and several neurons. The gonad plays a central role in patterning the underlying ectoderm to form the vulva and guiding the migration of the sex myoblasts to their final position. In Pristionchus pacificus, the egg-laying system is homologous to C. elegans, but comparative studies revealed several differences at the cellular and molecular levels during vulval formation. For example, the mesoblast M participates in lateral inhibition, a process that influences the fate of two vulval precursor cells. Here, we describe the M lineage in Pristionchus and show that both the dorsal and ventral M sublineages are involved in lateral inhibition. Mutations in the homeotic gene Ppa-mab-5 cause severe misspecification of the M lineage, resembling more the C. elegans Twist than the mab-5 phenotype. Ectopic differentiation of P8.p in Ppa-mab-5 results from at least two separate interactions between M and P8.p. Thus, interactions among the Pristionchus egg-laying system are complex, involving multiple cells of different tissues occurring over a distance.