Supplementary Material for: Characterization of the Bovine Pseudoautosomal Region and Comparison with Sheep, Goat, and Other Mammalian Pseudoautosomal Regions

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.

假常染色体区域（pseudoautosomal region, PAR）是哺乳动物X与Y染色体之间一段短小的序列同源区域，是雄性减数分裂中性染色体分离所必需的遗传区段。尽管目前仅在少数物种中对该区域开展过研究，但已发现其在长度与基因组成上存在显著差异。本研究通过分离并定位18个细菌人工染色体（Bacterial Artificial Chromosome, BAC）克隆，构建了牛假常染色体区域及其邻近X染色体特异性区段的中等密度基因图谱，这些克隆共包含20个PAR相关基因与5个X染色体特异性基因。其中1个携带TBL1XY与GPR143基因的BAC克隆，跨越了近期划定的牛假常染色体边界（PAB）。将牛PAR的基因图谱与人类、马及犬的PAR高分辨率图谱进行比对后，估算出牛PAR的长度约为5~9 Mb。BAC末端序列分析结果显示，从PAR末端向PAB方向存在GC含量逐渐降低的梯度分布，这一发现与人类、小鼠和马中的研究结果一致。本研究还在山羊与绵羊中定位了20个牛PAR相关基因及5个X染色体特异性基因，结果表明这三个物种的PAR在长度、基因组成与基因排列顺序上均较为相似。此外，本研究首次在山羊性染色体中确定了假常染色体边界。将牛、山羊与绵羊的PAR与人类及马X染色体上的同源区域进行比对后发现，所有物种间均存在高度保守的连锁关系。不过，人类、马和犬PAR中最末端的基因PLCXD1，在反刍动物中却属于X染色体特异性基因。由于包含PLCXD1的人类/马连锁群属于祖先起源的连锁群，因此PLCXD1的定位可被视为反刍动物性染色体演化过程中的一次全新事件。牛PAR的基因图谱丰富了我们对真兽类哺乳动物PAR比较基因组组织与演化的认知，同时可为牛X染色体长臂（BTAXq）末端区域的测序、序列组装与注释工作提供辅助支持。