The pseudoautosomal region (PAR) is a small region of sequence homology between mammalian X and Y chromosomes and is needed for sex chromosome segregation in male meiosis. The region, though studied as yet in only a few species, shows considerable variation in size and gene content. We have constructed a medium-density gene map for the cattle PAR and the adjacent X-specific region by isolating and mapping 18 BAC clones which contain 20 PAR- and 5 X-specific genes. One BAC clone containing TBL1XY and GPR143 spanned the recently demarcated bovine pseudoautosomal boundary (PAB). Comparing the gene map of cattle PAR with the high-resolution maps of human, horse, and dog PAR allowed to estimate that the size of cattle PAR is approximately 5–9 Mb. BAC end sequence analysis showed that there is a gradient of decreasing GC content from PARter towards the PAB which is consistent with findings in human, mouse, and horse. The 20 PAR- and 5 X-specific cattle genes were mapped also in goat and sheep, showing that PAR in the 3 species is similar in size, gene content, and gene order. For the first time the PAB was determined in goat sex chromosomes. Comparison of cattle, goat, and sheep PAR with homologous regions on human and horse X chromosomes showed a high degree of linkage conservation between all species. However, the most terminal human, horse, and dog PAR gene, PLCXD1, is X-specific in ruminants. Since the human/horse linkage group containing PLCXD1 is of ancestral origin, the location of PLCXD1 can be considered as a de novo event in ruminant sex chromosome evolution. The gene map of the cattle PAR adds to our knowledge about the comparative organization and evolution of the eutherian PAR and aids the sequencing, sequence assembly, and annotation of the terminal region of BTAXq.

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