Veyrunes Frédéric, Waters Paul D, Miethke Pat, Rens Willem, McMillan Daniel, Alsop Amber E, Grützner Frank, Deakin Janine E, Whittington Camilla M, Schatzkamer Kyriena, Kremitzki Colin L, Graves Tina, Ferguson-Smith Malcolm A, Warren Wes, Marshall Graves Jennifer A
Research School of Biological Sciences, Australian National University, Canberra 2601, Australia.
Genome Res. 2008 Jun;18(6):965-73. doi: 10.1101/gr.7101908. Epub 2008 May 7.
In therian mammals (placentals and marsupials), sex is determined by an XX female: XY male system, in which a gene (SRY) on the Y affects male determination. There is no equivalent in other amniotes, although some taxa (notably birds and snakes) have differentiated sex chromosomes. Birds have a ZW female: ZZ male system with no homology with mammal sex chromosomes, in which dosage of a Z-borne gene (possibly DMRT1) affects male determination. As the most basal mammal group, the egg-laying monotremes are ideal for determining how the therian XY system evolved. The platypus has an extraordinary sex chromosome complex, in which five X and five Y chromosomes pair in a translocation chain of alternating X and Y chromosomes. We used physical mapping to identify genes on the pairing regions between adjacent X and Y chromosomes. Most significantly, comparative mapping shows that, contrary to earlier reports, there is no homology between the platypus and therian X chromosomes. Orthologs of genes in the conserved region of the human X (including SOX3, the gene from which SRY evolved) all map to platypus chromosome 6, which therefore represents the ancestral autosome from which the therian X and Y pair derived. Rather, the platypus X chromosomes have substantial homology with the bird Z chromosome (including DMRT1) and to segments syntenic with this region in the human genome. Thus, platypus sex chromosomes have strong homology with bird, but not to therian sex chromosomes, implying that the therian X and Y chromosomes (and the SRY gene) evolved from an autosomal pair after the divergence of monotremes only 166 million years ago. Therefore, the therian X and Y are more than 145 million years younger than previously thought.
在有胎盘类哺乳动物（胎盘哺乳动物和有袋类动物）中，性别由XX雌性：XY雄性系统决定，其中Y染色体上的一个基因（SRY）影响雄性的决定。在其他羊膜动物中不存在类似情况，尽管一些分类群（特别是鸟类和蛇类）有分化的性染色体。鸟类具有ZW雌性：ZZ雄性系统，与哺乳动物的性染色体没有同源性，其中一个Z连锁基因（可能是DMRT1）的剂量影响雄性的决定。作为最原始的哺乳动物类群，产卵的单孔目动物对于确定有胎盘类XY系统的进化方式非常理想。鸭嘴兽有一个特殊的性染色体复合体，其中五条X染色体和五条Y染色体在由交替的X和Y染色体组成的易位链中配对。我们使用物理图谱来鉴定相邻X和Y染色体配对区域上的基因。最显著的是，比较图谱显示，与早期报道相反，鸭嘴兽和有胎盘类动物的X染色体之间没有同源性。人类X染色体保守区域中的基因直系同源物（包括SRY基因所起源的SOX3基因）都定位在鸭嘴兽的6号染色体上，因此该染色体代表了有胎盘类X和Y染色体对所起源的祖先常染色体。相反，鸭嘴兽的X染色体与鸟类的Z染色体（包括DMRT1）有大量同源性，并且与人类基因组中该区域的同线区段也有同源性。因此，鸭嘴兽的性染色体与鸟类有很强的同源性，但与有胎盘类动物的性染色体没有同源性，这意味着有胎盘类动物的X和Y染色体（以及SRY基因）是在仅1.66亿年前单孔目动物分化后从一对常染色体进化而来的。因此，有胎盘类动物的X和Y染色体比之前认为的要年轻超过1.45亿年。
