Waters Paul D, Delbridge Margaret L, Deakin Janine E, El-Mogharbel Nisrine, Kirby Patrick J, Carvalho-Silva Denise R, Graves Jennifer A Marshall
Comparative Genomics Group, Research Group of Biological Science, The Australian National University, Canberra, ACT, 2601, Australia.
Chromosome Res. 2005;13(4):401-10. doi: 10.1007/s10577-005-0978-5.
Mammalian sex chromosomes evolved from an ancient autosomal pair. Mapping of human X- and Y-borne genes in distantly related mammals and non-mammalian vertebrates has proved valuable to help deduce the evolution of this unique part of the genome. The platypus, a monotreme mammal distantly related to eutherians and marsupials, has an extraordinary sex chromosome system comprising five X and five Y chromosomes that form a translocation chain at male meiosis. The largest X chromosome (X1), which lies at one end of the chain, has considerable homology to the human X. Using comparative mapping and the emerging chicken database, we demonstrate that part of the therian X chromosome, previously thought to be conserved across all mammals, was lost from the platypus X1 to an autosome. This region included genes flanking the XIST locus, and also genes with Y-linked homologues that are important to male reproduction in therians. Since these genes lie on the X in marsupials and eutherians, and also on the homologous region of chicken chromosome 4, this represents a loss from the monotreme X rather than an additional evolutionary stratum of the human X.
哺乳动物的性染色体由一对古老的常染色体进化而来。在远缘哺乳动物和非哺乳动物脊椎动物中对人类X染色体和Y染色体携带的基因进行定位，已被证明有助于推断基因组这一独特部分的进化。鸭嘴兽是一种与有胎盘类动物和有袋类动物亲缘关系较远的单孔目哺乳动物，它拥有一个特殊的性染色体系统，由五条X染色体和五条Y染色体组成，在雄性减数分裂时形成一个易位链。最大的X染色体（X1）位于链的一端，与人类X染色体有相当的同源性。利用比较定位和新兴的鸡数据库，我们证明了有胎盘类动物X染色体上先前被认为在所有哺乳动物中都保守的一部分，已从鸭嘴兽的X1染色体丢失到一条常染色体上。这个区域包括XIST基因座侧翼的基因，以及在有胎盘类动物中对雄性生殖很重要的具有Y连锁同源物的基因。由于这些基因在有袋类动物和有胎盘类动物中位于X染色体上，在鸡的4号染色体同源区域也有，这代表着从单孔目动物的X染色体上丢失，而不是人类X染色体额外的进化层次。
