ARTICLE Reconstructing hominid Y evolution: X-homologous block, created by X–Y transposition, was disrupted

The human X and Y chromosomes share many blocks of similar DNA sequence. We conducted mapping and nucleotide sequencing studies of extensive, multi-megabase homologies between Yp and Xq21, which do not recombine during male meiosis. We confirmed and built upon previous evidence that a Yp inversion had occurred during evolution: a single contiguous segment of Xq21 is homologous to two non-contiguous segments of Yp. We precisely defined and sequenced the inversion breakpoints, obtaining evidence that the inversion was mediated by recombination between LINE-1 elements in otherwise non-homologous regions. This inversion appears to have followed a single transposition of an ∼4 Mb segment from the X to the Y chromosome. These events jointly account for the present arrangement of Yp–Xq21 homologous sequences. Based on Southern blotting studies of primates and of humans drawn from diverse populations, we conclude that both the X–Y transposition and the subsequent, LINE-mediated Yp inversion occurred after the divergence of hominid and chimp lineages but before the radiation of extant human populations. This evolutionary scenario is consistent with our finding of 99.3 ± 0.2 % nucleotide identity between the X and Y chromosomes within the transposed region, which suggests that the transposition occurred ∼3–4 million years ago, near the time of emergence of Homo. Comparative sequencing of the entire human X and Y chromosomes may reveal a succession o