Schematic diagram of sex chromosome evolution in Theria.

After the divergence of Theria from monotremes, recombination was suppressed in at least 10 genes on the proto-XY chromosome in the therian ancestor; this is indicated by gray color on the chromosome. In the stem lineage of marsupials and eutherians, a gene conversion occurred in RPS4X/Y or both SMCX/Y and UBE1X/Y, respectively; this is indicated by back color on the chromosome. An asterisk means that it is not clear whether PHF6X/Y diverged at the same time as the differentiation of the other gametologs (see text).