The sexual fate of the nematode Caenorhabditis elegans is controlled by a complex signal transduction pathway. Previous studies suggested that some of the proteins in this pathway have evolved more quickly than non-sex-determining proteins. To investigate the causes and functional significance of rapid sex-determining protein evolution, three C. elegans genes encoding members of the protein phosphatase 2C (PP2C) family were compared with their orthologs from another Caenorhabditis species (strain CB5161). One of the genes encodes FEM-2, a sex-determining protein, while the others have no known sex-determining role. FEM-2's PP2C domain was found to be more diverged than the other PP2C domains, supporting the notion that sex-determining proteins are subjected to selective pressures that allow for or cause rapid divergence. Comparison of the positions of amino acid substitutions in FEM-2 with a solved three-dimensional structure suggests that the catalytic face of the protein is highly conserved among C. elegans, CB5161, and another closely related species C. briggsae. However, the non-conserved regions of FEM-2 cannot be said to lack functional importance, since fem-2 transgenes from the other species were unable to rescue the germ-line defect caused by a C. elegans fem-2 mutation. RNA-mediated interference (RNAi) was used to examine whether fem-2 functions as a sex-determining gene in the other Caenorhabditis species. fem-2(RNAi) caused germ-line feminization in C. elegans hermaphrodites and males, while CB5161 worms were unaffected. In C. briggsae, fem-2(RNAi) feminized the male but not the hermaphrodite germ line. These results suggest that the role of fem-2 in sex determination has also evolved rapidly, and may provide an example of orthologous genes regulating different processes between closely related species.
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