Genetic flexibility in the convergent evolution of hermaphroditism in Caenorhabditis nematodes.

The self-fertile hermaphrodites of C. elegans and C. briggsae evolved from female ancestors by acquiring limited spermatogenesis. Initiation of C. elegans hermaphrodite spermatogenesis requires germline translational repression of the female-promoting gene tra-2, which allows derepression of the three male-promoting fem genes. Cessation of hermaphrodite spermatogenesis requires fem-3 translational repression. We show that C. briggsae requires neither fem-2 nor fem-3 for hermaphrodite development, and that XO Cb-fem-2/3 animals are transformed into hermaphrodites, not females as in C. elegans. Exhaustive screens for Cb-tra-2 suppressors identified another 75 fem-like mutants, but all are self-fertile hermaphrodites rather than females. Control of hermaphrodite spermatogenesis therefore acts downstream of the fem genes in C. briggsae. The outwardly similar hermaphrodites of C. elegans and C. briggsae thus achieve self-fertility via intervention at different points in the core sex determination pathway. These findings are consistent with convergent evolution of hermaphroditism, which is marked by considerable developmental genetic flexibility.

A sensitized genetic background reveals evolution near the terminus of the Caenorhabditis germline sex determination pathway.

Caenorhabditis elegans and Caenorhabditis briggsae are both self-fertile hermaphroditic nematodes that evolved independently from male/female ancestors. In C. elegans, FEM-1, FEM-2, and FEM-3 specify male fates by promoting proteolysis of the male-repressing transcription factor, TRA-1. Phenotypes of tra-1 and fem mutants are consistent with this simple linear model in the soma, but not in the germline. While both XX and XO tra-1(lf) mutants have functional male somas, they produce both sperm and oocytes. Further, all three tra-1; fem double mutants retain the expected male soma, but make only oocytes (the germline fem phenotype). Thus, a poorly characterized tra-1 activity is important for sustained male spermatogenesis, and the fem genes affect germline sexual fate independently of their role in regulating TRA-1. C. briggsae tra-1 mutants are phenotypically identical to their C. elegans counterparts, while the fem mutants differ in the germline: XX and XO C. elegans fem mutants are true females, but in C. briggsae they are self-fertile hermaphrodites. To further explore how C. briggsae hermaphrodites regulate germline sex, we analyzed Cb-tra-1/Cb-fem interactions. Cb-tra-1 is fully epistatic to Cb-fem-2 in the germline, unlike the orthologous C. elegans combination. In contrast, Cb-fem-3 shifts the Cb-tra-1(lf) germline phenotype to that of a nearly normal hermaphrodite in the context of a male somatic gonad. This suggests that Cb-fem-3 is epistatic to Cb-tra-1(lf) (as in C. elegans), and that the normal control of C. briggsae XX spermatogenesis targets Cb-tra-1-independent factors downstream of Cb-fem-3. The effect of Cb-fem-3(lf) on Cb-tra-1(lf) is not mediated by change in the expression of Cb-fog-3, a likely direct germline target of Cb-tra-1. As Cb-fem-2 and Cb-fem-3 have identical single mutant phenotypes, Cb-tra-1 provides a sensitized background that reveals differences in how they promote male germline development. These results represent another way in which C. briggsae germline sex determination is incongruent with that of the outwardly similar C. elegans.