Figure 2.

Evolution of SRY and the mammal XY sex chromosome pair. A: Evolution of SRY from an allele of autosomal SOX3. SOX3 is highly conserved throughout vertebrates, with functions in brain and germcells, but is sex reversing when mis‐expressed in the bipotential gonad. SOX3 shares with SRY a moderately conserved HMG box (grey), involved with DNA binding and bending through a specific angle. We speculate that SOX3 was truncated by breakage at the 5′ end, and rejoining with a promotor that drove its expression into the bipotential gonad. B: Differentiation of the mammal X and Y chromosomes. Originally the X and Y were an ordinary pair of autosomes bearing SOX3. Mutation of one allele to a sex‐determining SRY ensured that the chromosome that bore it became a male‐specific Y. It degenerated rapidly as nearby genes acquired male‐specific functions, and recombination (represented by dotted lines) in the region was suppressed to preserve a male‐specific gene package. Within the region of low recombination, mutation, deletion amplification and insertion of repetitive sequence was rife, hastening progressive degeneration. Genes that acquired a male‐specific function (grey) were retained. The XY of eutherian mammals retains a short pseudoautosomal segment that still recombines. However, the marsupial X and Y have lost all homology, and some rodents have completely lost the Y.