The Hox gene lin-39 plays a crucial role in the establishment of the nematode vulva equivalence group. Mutations in lin-39 in Caenorhabditis elegans and Pristionchus pacificus result in a vulvaless phenotype because presumptive vulva precursor cells adopt non-vulval fates. Interestingly, the non-vulval fate of anterior and posterior epidermal cells differs between Caenorhabditis and Pristionchus ; in C. elegans , non-vulval cells fuse with the hypodermis, whereas, in P. pacificus , they die as a result of programmed cell death. C. elegans lin-39 ( Cel-lin-39 )indirectly controls the cell fusion gene eff-1 by regulating the GATA transcription factors egl-18 and elt-6 . In P. pacificus , the genetic context of its lin-39 ( Ppa-lin-39 ) function was unknown. Here, we describe the isolation and characterization of gev-2 , a second generation-vulvaless mutant in P. pacificus. We show that gev-2 is the Ppa-pax-3 gene and that it has distinct functions in the cell fate specification of epidermal cells. Whereas Ppa-pax-3 regulates cell survival of the presumptive vulval precursor cells, it controls cell death of posterior epidermal cells. Molecular studies indicate that Ppa-pax-3 is a direct target of Ppa -LIN-39. Thus, we describe the first specific developmental defect of a nematode pax-3 gene and our data reveal different regulatory networks for the specification of the vulva equivalence group.

Anteroposterior cell migration and patterning in C. elegans are governed by multiple, interacting signaling pathways and transcription factors. In this study, we have investigated the role of ceh-20 , the C. elegans ortholog of the HOX co-factor Extradenticle (Exd/Pbx), and unc-62 , the C. elegans ortholog of Homothorax(Hth/Meis/Prep), in two processes that are regulated by Hox gene lin-39 : cell migration and vulva formation. As in lin-39 mutants, the anterior migrations of neuroblasts in the Q lineage are truncated in Hox co-factor mutants. Surprisingly, though, our findings suggested that the roles of ceh-20 and unc-62 are different from that of lin-39 ; specifically, ceh-20 and unc-62 but not lin-39 are required for the transmembrane protein MIG-13 to promote anterior migration. To our knowledge, ceh-20 and unc-62 are the only genes that have been implicated in the mig-13 pathway. We find that ceh-20 and unc-62 are also required for several steps in vulva development. Surprisingly, ceh-20 and unc-62 mutants have phenotypes that are starkly different from those of lin-39 mutants. Thus, in this process, too, ceh-20 and unc-62 are likely to have functions that are independent of lin-39 .