EVOLUTION OF SEX DETERMINATION IN CAENORHABDITIS - UNUSUALLY HIGH DIVERGENCE OF TRA-1 AND ITS FUNCTIONAL CONSEQUENCES

The tra-1 gene is a terminal regulator of somatic sex in Caenorhabditi s elegans: high tra-1 activity elicits female development, low tra-l a ctivity elicits male development. To investigate the function and evol ution of tra-l, we examined the tra-l gene from the closely related ne matode C. briggsae. Ce-tra-1 and Cb-tra-1 are unusually divergent. Eac h gene generates two transcripts, but only one of these is present in both species. This common transcript encodes TRA-1A, which shows only 44% amino acid identity between the species, a figure much lower than that for previously compared genes. A Cb-tra-1 transgene rescues many tissues of tra-1(null) mutants of C. elegans but not the somatic gonad or germ line. This transgene also causes nongonadal feminization of X O animals, indicating incorrect sexual regulation. Alignment of Ce-TRA -1A and Cb-TRA-1A defines several conserved regions likely to be impor tant for tra-1 function. The phenotypic differences between Ce-tra-l(n ull) mutants rescued by Cb-tra-1 transgenes and wild-type C. elegans i ndicate significant divergence of regulatory regions. These molecular and functional studies suggest that evolution of sex determination in nematodes is rapid and genetically complex.