Formation of the egg-laying system in Pristionchus pacificus requires complex interactions between gonadal, mesodermal and epidermal tissues and doesnot rely on single cell inductions

The invariant cell lineage of nematodes allows the formation of organ syste ms, like the egg-laying system, to be studied at a single cell level. The C aenorhabditis elegans egg-laying system is made up of the vulva, the mesode rmal gonad and muscles and several neurons. The gonad plays a central role in patterning the underlying ectoderm to form the vulva and guiding the mig ration of the sex myoblasts to their final position. In Pristionchus pacifi cus, the egg-laying system is homologous to C elegans, but comparative stud ies revealed several differences at the cellular and molecular levels durin g vulval formation. For example, the mesoblast M participates in lateral in hibition, a process that influences the fate of two vulval precursor cells. Here, we describe the M lineage in Pristionchus and show that both the dor sal and ventral M sublineages are involved in lateral inhibition. Mutations in the homeotic gene Ppa-mab-5 cause severe misspecification of the M line age, 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 int eractions between M and P8.p. Thus, interactions among the Pristionchus egg -laying system are complex, involving multiple cells of different tissues o ccurring over a distance.