Feminizing mutations in Caenorhabditis briggsae indicate novel regulation of ovotestis development

Abstract

The evolution of novel traits relies on heritable changes in gene structure or gene expression, but the processes by which these occur is not always clear. Sex determination is a particularly interesting trait with which to model these processes because its regulation seems to be subject to rapid evolution. Androdioecy, or a species’ ability to make an ovotestis in an otherwise female animal, has independently evolved three times in the Caenorhabditis nematode clade from a dioecious ancestor. We want to understand this regulation at the genomic level in C. briggsae and compare it to C. elegans to provide insight into the way a common set of genes can be modified to produce a novel trait. Forward genetic screens allowed genetic identification of the C. briggsae orthologs of C. elegans genes (tra, fem) required for somatic and gonadal sex determination, but unlike in C. elegans, none of the feminizing mutants blocked spermatogenesis in the female ovotestis. A second screen, looking for suppressors of masculinizing mutants identified several phenotypes not seen in C. elegans. Many of these suppressors permit the development of XX hermaphrodites and XO males in a masculinized tra-2 background, in contrast to C. elegans feminizing mutants where XO animals are feminized. A loss of all cbr-fem genes results in the development of XX and XO hermaphrodites, indicating that the regulatory locus controlling ovotestis development is downstream of the fems. tra-2;fem-X/+ animals are partially feminized males, providing more evidence that the regulation is downstream of the fems. Finally, cbr-ubxn-3 was indicated as a novel member of the C. briggsae sex determination regulatory network. Together these findings indicate that the regulatory locus in C. briggsae that permits ovotestis development is downstream of the fem genes, different than in C. elegans.