Handley Lori-Jayne Lawson, Ceplitis Helene, Ellegren Hans
Department of Evolutionary Biology, Evolutionary Biology Centre, Uppsala University, SE-752 36 Uppsala, Sweden.
Genetics. 2004 May;167(1):367-76. doi: 10.1534/genetics.167.1.367.
The human X chromosome exhibits four "evolutionary strata," interpreted to represent distinct steps in the process whereby recombination became arrested between the proto X and proto Y. To test if this is a general feature of sex chromosome evolution, we studied the Z-W sex chromosomes of birds, which have female rather than male heterogamety and evolved from a different autosome pair than the mammalian X and Y. Here we analyze all five known gametologous Z-W gene pairs to investigate the "strata" hypothesis in birds. Comparisons of the rates of synonymous substitution and intronic divergence between Z and W gametologs reveal the presence of at least two evolutionary strata spread over the p and q arms of the chicken Z chromosome. A phylogenetic analysis of intronic sequence data from different avian lineages indicates that Z-W recombination ceased in the oldest stratum (on Zq; CHD1Z, HINTZ, and SPINZ) 102-170 million years ago (MYA), before the split of the Neoaves and Eoaves. However, recombination continued in the second stratum (on Zp; UBAP2Z and ATP5A1Z) until after the divergence of extant avian orders, with Z and W diverging 58-85 MYA. Our data suggest that progressive and stepwise cessation of recombination is a general feature behind sex chromosome evolution.
人类X染色体呈现出四个“进化层”，这被解释为代表了原X染色体和原Y染色体之间重组停止过程中的不同阶段。为了检验这是否是性染色体进化的普遍特征，我们研究了鸟类的Z-W性染色体，鸟类具有雌性异配性别而非雄性异配性别，并且是从与哺乳动物X和Y不同的常染色体对进化而来。在这里，我们分析了所有五个已知的同源Z-W基因对，以研究鸟类中的“进化层”假说。对Z和W同源基因之间同义替换率和内含子分歧的比较揭示了在鸡Z染色体的p臂和q臂上至少存在两个进化层。对来自不同鸟类谱系的内含子序列数据进行的系统发育分析表明，Z-W重组在最古老的进化层（位于Zq；CHD1Z、HINTZ和SPINZ）中于1.02亿至1.7亿年前（百万年前）停止，这发生在新鸟亚纲和古鸟亚纲分化之前。然而，重组在第二个进化层（位于Zp；UBAP2Z和ATP5A1Z）中一直持续到现存鸟类目分化之后，Z和W在5800万至8500万年前开始分化。我们的数据表明，重组的逐步停止是性染色体进化背后的一个普遍特征。
