Comparative Genomics Group, Research School of Biology, The Australian National University, GPO Box 475, ACT 2601, Canberra, Australia.
Chromosome Res. 2009;17(7):917-26. doi: 10.1007/s10577-009-9079-1. Epub 2009 Sep 30.
All therian mammals (eutherians and marsupials) have an XX female/XY male sex chromosome system or some variant of it. The X and Y evolved from a homologous pair of autosomes over the 166 million years since therian mammals diverged from monotremes. Comparing the sex chromosomes of eutherians and marsupials defined an ancient X conserved region that is shared between species of these mammalian clades. However, the eutherian X (and the Y) was augmented by a recent addition (XAR) that is autosomal in marsupials. XAR is part of the X in primates, rodents, and artiodactyls (which belong to the eutherian clade Boreoeutheria), but it is uncertain whether XAR is part of the X chromosome in more distantly related eutherian mammals. Here we report on the gene content and order on the X of the elephant (Loxodonta africana)-a representative of Afrotheria, a basal endemic clade of African mammals-and compare these findings to those of other documented eutherian species. A total of 17 genes were mapped to the elephant X chromosome. Our results support the hypothesis that the eutherian X and Y chromosomes were augmented by the addition of autosomal material prior to eutherian radiation. Not only does the elephant X bear the same suite of genes as other eutherian X chromosomes, but gene order appears to have been maintained across 105 million years of evolution, perhaps reflecting strong constraints posed by the eutherian X inactivation system.
所有真兽类哺乳动物（有胎盘类和有袋类）都具有 XX 雌性/XY 雄性性染色体系统或其某种变体。X 和 Y 染色体是在 1.66 亿年以来从单孔类动物分化出来的真兽类哺乳动物中，从同源的一对常染色体进化而来的。比较有胎盘类和有袋类的性染色体确定了一个古老的 X 保守区，该区域在这些哺乳动物分支中物种之间共享。然而，有胎盘类的 X（和 Y）被最近添加的（XAR）所增加，而 XAR 在有袋类动物中是常染色体。XAR 是灵长类动物、啮齿动物和偶蹄目动物（属于有胎盘类 Boreoeutheria 分支）X 染色体的一部分，但尚不确定 XAR 是否是与更遥远相关的有胎盘类哺乳动物的 X 染色体的一部分。在这里，我们报告了非洲象（Loxodonta africana）的 X 染色体上的基因含量和顺序-非洲哺乳动物的一个基础特有分支 Afrotheria 的代表，并将这些发现与其他有记录的有胎盘类物种进行了比较。总共将 17 个基因映射到了大象的 X 染色体上。我们的结果支持这样的假说，即在有胎盘类辐射之前，性染色体 X 和 Y 是通过添加常染色体物质而增加的。大象的 X 染色体不仅具有与其他有胎盘类 X 染色体相同的基因套件，而且基因顺序似乎在 1.05 亿年的进化过程中得到了维持，这可能反映了由有胎盘类 X 失活系统所带来的强烈约束。
