Metazoan sex determination genes evolve rapidly. One such rapidly evolving gene,
fog-2, is required for C. elegans hermaphrodite spermatogenesis. The
fog-2 gene is part of a large and highly diverged family sharing a similar structure; an N-terminal F-box domain and C-terminal Duf38/FTH domain. In the C. elegans germline sex determination pathway, FOG-2 and GLD-1 form a ternary complex with the
tra-2 mRNA 3UTR to promote the male germ cell fate in the hermaphrodite allowing self-fertility. Here we present data that
fog-2 represents one of the most rapidly evolving C. elegans sex determination genes identified to date. The
fog-2 family (F-box + Duf38/FTH) is present in C. briggsae and also appears to be expanded, however, no clear orthologous relationships between
fog-2 family members could be unambiguously identified between C. briggsae and C. elegans using reciprocal BLAST, unlike the other sex determination genes. Analysis of syntenic relationships surrounding
fog-2 revealed a breakdown in local synteny and an absence
fog-2 and duplicated family members in the corresponding C. briggsae sequence. To resolve the evolutionary relationships between C. elegans and C. briggsase
fog-2 family members we performed phylogenetic analysis using neighbor-joining, maximum parsimony, and maximum-likelihood methods. All trees were in good agreement showing a separation between C. elegans and C. briggsae lineages. This suggests two possibilities about the etiology of the
fog-2 family. One, expansion of the
fog-2 family occurred before the species split and was followed by rapid divergence and presumably the acquisition of new roles such as
fog-2 in C. elegans sex determination. Alternatively, while there was clearly a common ancestral sequence, the majority of the expansion in the
fog-2 family occurred after the species split suggesting that the germline sex determination function of
fog-2 is recently evolved. Providing support for the latter is that sequence analysis from C. elegans suggests that duplications involving
fog-2 occurred very recently. Additionally, using the yeast two-hybrid system we have identified unique sequences in the final exon of
fog-2, absent from the highly similar neighboring gene
ftr-2, that are required for GLD-1 interaction and thus incorporation into the
tra-2 3UTR mRNA translational repression complex. Our data suggest that establishment of hermaphrodite spermatogenesis may fundamentally different between C. elegans and C. briggsae. The
fog-2 gene family may represent molecular evidence that, similar to parasitism, the evolution of hermaphroditic species from ancestral gonochoristic species has arisen independently even in closely related lineages.