Complex Events In The Evolution Of The Human Pseudoautosomal Region 2 (PAR2)

The 320 kb human pseudoautosomal region 2 (PAR2) at the tips of the long arms of the X and Y chromosomes is thought to have been duplicated onto the Y recently in primate evolution. The four genes within PAR2 have been proposed to constitute two zones with different base ratios and transcription, one of which was added recently to the X. To test this hypothesis, we cloned and mapped PAR2 genes in other species, the lemur, the cat and a marsupial, the tammar wallaby. None of the human PAR2 genes co-localized with human PAR1 genes in the marsupial genome, confirming that the human PAR1 and PAR2 evolved independently. Of the four PAR2 genes, only SYBL1 was located on the X chromosome in all species, including marsupials, so was part of the ancient X. HSPRY3 localized to the X in all the eutherians, but not marsupial, so must have been added to the X 80-130 million years ago. CXYorf1 was present in primates and also mouse, but autosomal in wallaby, suggesting a later additon 70-130MYA, and IL9R was on the X only in primates, suggesting addition 60-70MYA. The results therefore implies at least three independent additions necessary for PAR2 evolution. The present gene order on the human X also requires two inversions. The complicated evolutionary pathway supports the hypothesis that terminal interchromosomal rearrangements are common in regions unpaired at meiosis.