Necessity and Contingency in Developmental Genetic Screens: EGF, Wnt, and Semaphorin Pathways in Vulval Induction of the Nematode <i>Oscheius tipulae</i>

Abstract

Genetic screens in the nematode <i>Caenorhabditis elegan</i>s identified the EGF/Ras and Notch pathways as central for vulval precursor cell fate patterning. Schematically, the anchor cell secretes EGF, inducing the P6.p cell to a primary (1°) vulval fate; P6.p in turn induces its neighbors to a secondary (2°) fate through Delta-Notch signaling and represses Ras signaling. In the nematode <i>Oscheius tipulae</i>, the anchor cell successively induces 2° then 1° vulval fates. Here, we report on the molecular identification of mutations affecting vulval induction in <i>O. tipulae</i> A single Induction Vulvaless mutation was found, which we identify as a <i>cis</i>-regulatory deletion in a tissue-specific enhancer of the <i>O</i><i>. tipulae lin-3</i> homolog, confirmed by clustered regularly interspaced short palindromic repeats/Cas9 mutation. In contrast to this predictable Vulvaless mutation, mutations resulting in an excess of 2° fates unexpectedly correspond to the plexin/semaphorin pathway. Hyperinduction of P4.p and P8.p in these mutants likely results from mispositioning of these cells due to a lack of contact inhibition. The third signaling pathway found by forward genetics in <i>O. tipulae</i> is the Wnt pathway; a decrease in Wnt pathway activity results in loss of vulval precursor competence and induction, and 1° fate miscentering on P5.p. Our results suggest that the EGF and Wnt pathways have qualitatively similar activities in vulval induction in <i>C. elegans</i> and <i>O. tipulae</i>, albeit with quantitative differences in the effects of mutation. Thus, the derived induction process in <i>C. elegans</i> with an early induction of the 1° fate appeared during evolution, after the recruitment of the EGF pathway for vulval induction.