The origin and evolution of the pseudoautosomal regions of human sex chromosomes

The human X and Y chromosomes share two homologous pseudoautosomal regions (PARs) which pair and recombine at meiosis. PAR1 lies at the tips of the sh ort arms, and the smaller PARS at the tips of the long arms. PAR1 contains several active genes, and has been thought to be critical for pairing and f ertility. The inconsistent gene content of the PARs between different speci es of eutherian ('placental') mammals suggests that gene content is immater ial to function, and the failure to detect a PAR at all in some rodents and all marsupials implies that homologous pairing is not universally essentia l for fertility. The autosomal localization of marsupial homologues of huma n PAR1 genes and their co-localization with human Xp22 genes implies that t he human PAR1 represents a relic of part of an autosomal region added to bo th X and Y chromosomes between 80 and 130 MYrBP. The same argument may be m ade for part of PARS. Independent additions to the sex chromosomes of macro podid marsupials and monotremes can also be inferred from comparative mappi ng. We conclude that the PARs are relies of differential additions, loss, r earrangement and degradation of the Y chromosome in different mammalian lin eages.