Polymorphism and evolution of vulval precursor cell lineages within two nematode genera, Caenorhabditis and Oscheius

Background: The cell lineage of nematodes is mostly invariant for a given s pecies, but varies between species. One can thus wonder how a cell lineage varies during evolution. We have started a microevolutionary approach withi n two genera by observing lineage variations of vulval precursor cells in d ifferent natural nematode populations of the same and closely related speci es. Results: In Caenorhabditis elegans, the P3.p cell lineage is variable withi n a genetically homogeneous population and polymorphic between wild strains . Irrespective of its division pattern, P3.p is competent to form vulval ti ssue in different C. elegans strains, whereas it is not competent in C. bri ggsae. In Oscheius sp. 1, P4.p and P8.p lineages are strongly polymorphic. Within each genus, these intraspecies polymorphisms in cell lineages are am plified between closely related species. In Oscheius sp. 1, the large polym orphisms in P4.p and P8.p lineages allowed us to undertake a genetic analys is of the variation between two pairs of strains. Multiple loci are involve d in cell lineage differences, and variation at one locus appears to have a relatively strong effect. In addition to these large lineage variations in cells that do not normally contribute to the vulva, we find minor variatio ns (errors) in vulval lineages, which represent the precision level of the vulval-patterning process and point to a selection pressure for maintenance of a large vulval equivalence group. Conclusions: Polymorphisms in vulval cell lineage are found within a given nematode species, and could be instrumental in explaining evolutionary vari ations between closely related species.