Primate DAX1, SRY and SOX9: Evolutionary stratification of sex-determination pathway

The molecular evolution of DAX1, SRY, and SOX9, genes involved in mammalian sex determination, was examined in six primate species. DAX1 and SRY have been added to the X and Y chromosomes, respectively, during mammalian evolu tion, whereas SOX9 remains autosomal. We determined the genomic sequences o f DAX1, SRY, and SOX9 in all six species, and calculated K-s, the number of nonsynonymous substitutions per nonsynonymous site, and compared this with the K-s, the number of synonymous substitutions per synonymous site. Phylo genetic trees were constructed by means of the DAX1, SRE: and SOX9 coding s equences, and phylogenetic analysis was performed using maximum likelihood. Overall measures of gene and protein similarity were closer for DAX1 and S OX9, but DAX1 exhibited nonsynonymous amino acid substitutions at an accele rated frequency relative to synonymous changes, similar to SRY and signific antly higher than SOX9. We conclude that, at the protein level, DAX1 and SR Y are under less selective pressure to remain conserved than SOX9, and, the refore, diverge more across species than does SOX9. These results are consi stent with evolutionary stratification of the mammalian sex determination p athway, analogous to that for sex chromosomes.