Necessity and contingency in developmental genetic screens: LIN-3, Wnt and semaphorin pathways in vulval induction of the nematode Oscheius tipulae
AbstractGenetic screens in the nematode Caenorhabditis elegans 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 1{degrees} vulval fate; P6.p in turn induces its neighbors to a 2{degrees} fate through Delta-Notch signaling and represses Ras signaling. In the nematode Oscheius tipulae, the anchor cell successively induces 2{degrees} then 1{degrees} vulval fates. Here we report on the molecular identification of mutations affecting vulval induction in O. tipulae. A single Induction Vulvaless mutation was found, which we identify as a cis-regulatory deletion in a tissue-specific enhancer of the O. tipulae lin-3 homolog, confirmed by CRISPR/Cas9 mutation. In contrast to this predictable Vulvaless mutation, mutations resulting in an excess of 2{degrees} fates unexpectedly correspond to the plexin/semaphorin pathway, which was not implicated in vulval fate induction in C. elegans. 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 O. tipulae is the Wnt pathway: decrease in Wnt pathway activity results in loss of vulval precursor competence and induction, and 1{degrees} fate miscentering on P5.p. Our results suggest that the EGF and Wnt pathways have qualitatively similar activities in vulval induction in C. elegans and O. tipulae, albeit with quantitative differences in the effects of mutation. This study highlights both necessity and contingency in forward genetic screens.\n\n100-word summaryGenetic screens in the nematode Caenorhabditis elegans identified EGF and Notch pathways as key for vulval precursor cell fate patterning. Here we report on the molecular identification of mutations affecting vulval induction in another nematode, Oscheius tipulae. The single mutation with reduced induction is identified as a cis-regulatory deletion in the O. tipulae lin-3 homolog, confirmed by CRISPR/Cas9 mutation. In contrast to this predictable Vulvaless mutation, mutations resulting in an excess of 2{degrees} vulval fates unexpectedly correspond to the plexin/semaphorin pathway, not implicated in vulval induction in C. elegans. This study highlights both necessity and contingency in forward genetic screens.