pvl-4, which encodes the Paired-box protein PAX-3, functions in hypodermal cell fate patterning in C. elegans

The development of the C. elegans hypodermis serves as an excellent system to study cell fate specification and morphogenesis. During embryogenesis, three major hypodermal cell types are born (dorsal hyp, lateral hyp, and ventral hyp) that undergo invariant morphogenetic processes that result in the embryo being completely surrounded by hypodermal cells. Improper specification of the major hypodermal cell types affects morphogenesis, and can lead to embryonic lethality. Several GATA-like transcription factors have been identified that function in specifying the major hypodermal cell fates in the embryo. elt-1, egl-18, and elt-6 specify the lateral (seam) fate and elt-1 and elt-3 along with unidentified factor(s) specify the ventral and dorsal fates. Absent from our knowledge of hypodermal cell fate specification is how the ventral and dorsal hyp distinguish themselves from one another and how they are prevented from adopting a seam cell fate. Here, I present findings that begin to address how the ventral hypodermis does not adopt a seam cell fate. In a mutagenesis screen for animals with a protruding vulva phenotype, a single allele (ga96) in a gene named pvl-4 was identified. Mapping experiments and whole genome sequencing revealed that the pvl-4(ga96) allele is a missense mutation in an exon of the Paired-box gene, pax-3. To support that pvl-4(ga96) is a mutation in pax-3, a pax-3 transgene was introduced into pvl-4(ga96) animals and rescued multiple phenotypes. Also, I found that pax-3(RNAi) phenocopies all pvl-4(ga96) phenotypes. Based on this we concluded that pvl-4 is pax-3. pax-3(ga96) animals have body morphology defects which suggests that pax-3 may be playing a role in hypodermal development. Using a combination of hypodermal reporter genes we found that the P cells in pax-3 reduction-of-function animals undergo a cell fate transformation and adopt a seam cell-like fate. Further, both ectopic and seam cell specific expression of pax-3 caused the seam cells to lose expression of a seam cell-specific reporter gene. Based on these findings we propose a model by which pax-3 acts in the P cells to repress the seam cell fate.