FELIX, M.-A.; Institut Jacques Monod, CNRS: Evolution of nematode vulva development

The nematode vulva is formed by precursors in the ventral epithelium, called Pn.p cells. Each Pn.p cell has a specific fate, determined in Caenorhabditis elegans by an inductive signal from the gonadal anchor cell and a lateral signal between the Pn.p cells. The Pn.p cells then undergo a fate-specific pattern of divisions. The cell lineage of nematodes is mostly invariant for a given species, but varies between species. One can thus wonder how evolutionary changes in cell lineages appear. Within the Caenorhabditis and Oscheius genera, we determined vulval lineages in different natural nematode populations of the same species and in closely related species. Within both genera, the same vulva lineage characters that are polymorphic within a species diverge between closely related species. Genetic analyses of the variation between several pairs of strains show that this vulval lineage difference is due to variations at several loci. Although the pattern of fates is conserved, the mechanisms of specification of this pattern vary between different nematodes. For example, in Oscheius tipulae CEW1, as in many other species, the same fate pattern is specified by two successive nested inductions by the anchor cell. A phylogenetic analysis seems to indicate that the patterning mechanism found in C. elegans is a derived character (an “innovation”) within the family. We performed a screen for vulva mutants in Oscheius tipulae. The spectrum of vulval phenotypes is very different from that found by others in C. elegans. Such a comparative mutagenesis approach allows to assess the available phenotypic neighborhood of the two wild genotypes, hence their evolvability. We can then compare the phenotypes of induced mutants (or of mutation accumulation lines) to phenotypes of wild isolates of the same species.