The evolution of novel traits relies on heritable changes in gene content 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. By comparing the genetic regulatory network allowing for androdioecy in two species, C. elegans and C. briggsae, to their dioecious close relatives, we hope to understand how the novelty arose. Sex determination is well understood in C. elegans, and we have been using similar molecular and genetic tools to understand hermaphroditism in C. briggsae.
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. Mutant alleles with this phenotype have been identified both within know sex determination genes, such as fem-3, and in novel regulatory loci. Through examination of these alleles, we will gain key insights into differences in C. briggsae sex determination in both the soma and the germ line. We will present our current understanding of somatic and germ line sex determination in C. briggsae and compare that to what is known in C. elegans. This illustrates how we can use comparative genomics within the Caenorhabditis clade to better understand the evolution of genetic regulatory network.