High sensitivity of C. elegans vulval precursor cells to the dose of posterior Wnts

Cell competence is a key developmental property. The Caenorhabditis elegans vulval competence group consists of P(3–8).p, six cells aligned along the antero-posterior axis in a wide central body region. The six cells are not equal in their competence: 1) P3.p quits the competence group in half of the individuals; 2) the posterior cells P7.p and P8.p are less competent than central vulval precursor cells. Competence to adopt a vulval fate is controlled by expression of the HOM-C gene lin-39, and maintained through Wnt signals that are secreted from the tail in a long-range gradient. Here we quantify the LIN-39 protein profile in vulval precursor cells of early L2 stage larvae, prior to P3.p fusion and inductive signaling. We show that LIN-39 levels are very low in P3.p and P4.p, peak in P5.p and progressively decrease until P8.p. This unexpectedly centered profile arises independently from the gonad. Posterior Wnt signaling reduces LIN-39 level in the posterior cells by activating the next-posterior HOM-C gene, mab-5. On the anterior side, P3.p and P4.p competence and division are sensitive to the already low LIN-39 and Wnt doses; most dramatically, each of the cwn-1/Wnt and egl-20/Wnt genes show haplo-insufficience for P3.p fate. In contrast to previous results, we find that these Wnts maintain P3.p and P4.p competence without affecting their LIN-39 level. The centered vulval competence profile is thus under the control of the posterior Wnts and of cross-regulation of three HOM-C genes and prepatterns the later induction of vulval fates.