Rens Willem, O'Brien Patricia C M, Grützner Frank, Clarke Oliver, Graphodatskaya Daria, Tsend-Ayush Enkhjargal, Trifonov Vladimir A, Skelton Helen, Wallis Mary C, Johnston Steve, Veyrunes Frederic, Graves Jennifer A M, Ferguson-Smith Malcolm A
Department of Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 OES, UK.
Genome Biol. 2007;8(11):R243. doi: 10.1186/gb-2007-8-11-r243.
Sex-determining systems have evolved independently in vertebrates. Placental mammals and marsupials have an XY system, birds have a ZW system. Reptiles and amphibians have different systems, including temperature-dependent sex determination, and XY and ZW systems that differ in origin from birds and placental mammals. Monotremes diverged early in mammalian evolution, just after the mammalian clade diverged from the sauropsid clade. Our previous studies showed that male platypus has five X and five Y chromosomes, no SRY, and DMRT1 on an X chromosome. In order to investigate monotreme sex chromosome evolution, we performed a comparative study of platypus and echidna by chromosome painting and comparative gene mapping.
Chromosome painting reveals a meiotic chain of nine sex chromosomes in the male echidna and establishes their order in the chain. Two of those differ from those in the platypus, three of the platypus sex chromosomes differ from those of the echidna and the order of several chromosomes is rearranged. Comparative gene mapping shows that, in addition to bird autosome regions, regions of bird Z chromosomes are homologous to regions in four platypus X chromosomes, that is, X1, X2, X3, X5, and in chromosome Y1.
Monotreme sex chromosomes are easiest to explain on the hypothesis that autosomes were added sequentially to the translocation chain, with the final additions after platypus and echidna divergence. Genome sequencing and contig anchoring show no homology yet between platypus and therian Xs; thus, monotremes have a unique XY sex chromosome system that shares some homology with the avian Z.
脊椎动物的性别决定系统是独立进化的。胎盘哺乳动物和有袋动物具有XY系统，鸟类具有ZW系统。爬行动物和两栖动物有不同的系统，包括温度依赖型性别决定，以及起源不同于鸟类和胎盘哺乳动物的XY和ZW系统。单孔目动物在哺乳动物进化早期就已分化，就在哺乳动物类群从蜥形类群分化之后。我们之前的研究表明，雄性鸭嘴兽有五条X染色体和五条Y染色体，没有SRY基因，且X染色体上有DMRT1基因。为了研究单孔目动物性染色体的进化，我们通过染色体涂染和比较基因定位对鸭嘴兽和针鼹进行了比较研究。
染色体涂染揭示了雄性针鼹中九条性染色体的减数分裂链，并确定了它们在链中的顺序。其中两条与鸭嘴兽的不同，鸭嘴兽的三条性染色体与针鼹的不同，并且几条染色体的顺序发生了重排。比较基因定位表明，除了鸟类常染色体区域外，鸟类Z染色体区域与鸭嘴兽的四条X染色体（即X1、X2、X3、X5）以及Y1染色体上的区域同源。
基于常染色体依次添加到易位链上这一假设，最容易解释单孔目动物的性染色体，且最终的添加发生在鸭嘴兽和针鼹分化之后。基因组测序和重叠群锚定显示，鸭嘴兽和有胎盘类动物的X染色体之间尚未发现同源性；因此，单孔目动物具有独特的XY性染色体系统，与鸟类的Z染色体有一些同源性。
