Existing DNA sequence data on the Sry gene, the mammalian sex-determin
ing locus in the Y chromosome, were analyzed for primates, rodents, an
d bovids. In all three taxonomic groups, the terminal sequences evolve
d faster than the HMG (high mobility group) boxes, and this applies bo
th to synonymous (K-s) and nonsynonymous (K-a) nucleotide substitution
s. Similar intragenic correlation between synonymous and nonsynonymous
substitution rates was not found either in other mammalian genes that
contain a conservative box (Sox, Msx) or in the MADS-box genes of pla
nts. The rate of nonsynonymous substitutions exceeds significantly tha
t of synonymous substitutions in the terminal Sry sequences of apes. W
e did not find good support for the hypothesis that the high evolution
ary rate of Sry would be associated with a promiscuous mating system.