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.