Conservation and diversification of Wnt signaling function during the evolution of nematode vulva development

Cell-fate specification and cell-cell signaling have been well studied during vulva development in Caenorhabditis elegans and provide a paradigm in evolutionary developmental biology(1,2). Pristionchus pacificus has been developed as a 'satellite' organism with an integrated physical and genetic map that allows detailed comparisons to C. elegans(3-5).A common aspect of vulva formation in both species is the polarization of the P7. p lineage, which is responsible for vulval symmetry. In C. elegans, Wnt signaling is crucial for P7. p cell-fate patterning(6); nothing is known about vulval symmetry in P. pacificus. We isolated mutations that disrupt polarization of the P7. p lineage in P. pacificus and found that the corresponding gene encodes a Frizzled-like molecule. In addition, mutations in Ppa-lin-17 (encoding Frizzled) and morpholino knock-down of Ppa-lin-44 (encoding Wnt), Ppa-egl-20 (encoding Wnt), Ppa-mig-5 (encoding Dsh), Ppa-apr-1 (encoding APC) and Ppa-bar-1 (encoding beta-catenin) results in gonad-independent vulva differentiation, indicating that these genes have a role in a negative signaling process. In contrast, in C. elegans, Wnt signaling has a positive role in vulva induction, and mutations in bar-1 result in a hypoinduced phenotype(7). Therefore, whereas the molecular mechanisms that generate vulval symmetry are conserved, the genetic control of vulva induction diversified during evolution.