RAPID-SEQUENCE EVOLUTION OF THE MAMMALIAN SEX-DETERMINING GENE SRY

IN mammals, induction of male sex determination requires the Y-chromosome gene SRY1. SRY encodes a protein with a central 'high mobility group' domain (HMG box) of about 78 amino acids1-3 . HMG boxes are found in a wide variety of proteins that bind to DNA with high affinity but differing degrees of sequence specificity4. The human SRY protein binds to linear DNA with sequence specificity5 and to cruciform DNA structures without sequence specificity6. The DNA-binding activity of the SRY protein resides in the HMG box and mutations in this region are associated with sex reversal in XY females6-8. No function has been ascribed to the portions of the SRY protein outside the HMG box. SRY belongs to a family of genes that are related by sequence homology within the DNA-binding domain: the genes most similar to SRY (>60%) have been named SOX genes (SRY box genes). None of the known SOX genes is homologous to SRY outside the HMG-box region. Although SRY is an important developmental regulator, its sequence is poorly conserved between species apart from the HMG-box domain. Here we investigate the coding sequence of SR Y in primates and find that evolution has been rapid in the regions flanking the conserved domain. The high degree of sequence divergence and the frequency of non-synonymous mutations suggest either that the majority of the coding sequence has no functional significance or that directional selection has occurred.