Caenorhabditis tropicalis followed a unique evolutionary path to self-fertility

Caenorhabditis nematodes provide 3 natural experiments in which a male/female species evolved to produce 1 with males and self-fertile hermaphrodites. Such changes in mating systems are among the most complex and consequential transformations to occur in evolution and provide excellent models for evolutionary change. Studies of Caenorhabditis elegans and Caenorhabditis briggsae have shown that each species followed a different path to self-fertility, in part by altering 1 of 2 alternate branches of the sex-determination pathway, and in part by recruiting unique modifier genes. Here we examine sex-determination in a third hermaphroditic species, Caenorhabditis tropicalis. Several illuminating results stand out. First, although the core genes of the sex-determination pathway act in the same order in the soma of these 3 species, C. tropicalis differs from all Caenorhabditis species previously studied in some aspects of somatic development-tra-2 XX mutants resemble tra-1 XX mutants instead of differing in tail development, and neither gene regulates key aspects of male behavior that are controlled by the sex-determination pathway in other species. Second, the control of germ cell fates in C. tropicalis follows a simple linear process not seen in either C. elegans or C. briggsae. Third, genetic epistasis suggests that the decision to produce sperm and then oocytes in XX hermaphrodites is made at the level of tra-2 in all 3 species, implying that this gene might occupy a place uniquely sensitive to regulatory change. These results, when considered alongside those from other Caenorhabditis species, help elucidate how genetic pathways change during natural selection.