To ensure correct partition of X and Y into sperm, genetic exchange (crossover) must occur between these very different chromosomes in a short, shared region. However, women and men differ in their sex chromosomes: while women have two X chromosomes, men have an X and a smaller, sex-determining Y chromosome. We conclude ePAR very likely contributes to the critical crossover function of PAR1.ĩ5% of our genome is contained in 22 pairs of chromosomes shared by all humans. Extensive resequencing and inference of ePAR haplotypes, placed in the framework of the Y phylogeny as ascertained by both Y microsatellites and single nucleotide polymorphisms, allowed us to estimate a minimum rate of crossover over the entire ePAR region of 6-fold greater than genome average, comparable with pedigree estimates of PAR1 activity generally.
We observed good correspondence between the male DSB clusters and historical recombination activity of this region in the X chromosomes of females, as ascertained from linkage disequilibrium analysis this suggests that this region is similarly primed for crossover in both male and female germlines, although sex-specific differences may also exist.
Other hallmarks of classic recombination hotspots included evidence of transmission distortion and GC-biased gene conversion. Crossover activity comparable to that seen at autosomal hotspots was observed between the X and the ePAR borne on the Y chromosome both at a distal and a proximal site within the 110-kb extension. Here, we sought direct proof by examining thousands of gametes from each of two ePAR-carrying men, for two subregions chosen on the basis of previously published male X-chromosomal meiotic double-strand break (DSB) maps. So far, only limited evidence has been presented to suggest this extension is recombinationally active. This event has occurred at least twice in human evolution. Although the boundary between pseudoautosomal and sex-specific DNA has traditionally been regarded as conserved among primates, it was recently discovered that the boundary position varies among human males, due to a translocation of ~110 kb from the X to the Y chromosome that creates an extended PAR1 (ePAR). The human X and Y chromosomes are heteromorphic but share a region of homology at the tips of their short arms, pseudoautosomal region 1 (PAR1), that supports obligate crossover in male meiosis.
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