Smith Jeramiah J, Voss S Randal
Department of Biology and Spinal Cord and Brian Injury Research Center, University of Kentucky, Lexington, Kentucky 40506, USA.
Genetics. 2007 Sep;177(1):607-13. doi: 10.1534/genetics.107.072033. Epub 2007 Jul 29.
We tested hypotheses concerning the origin of bird and mammal sex chromosomes by mapping the location of amniote sex-chromosome loci in a salamander amphibian (Ambystoma). We found that ambystomatid orthologs of human X and chicken Z sex chromosomes map to neighboring regions of a common Ambystoma linkage group 2 (ALG2). We show statistically that the proportion of human X and chicken Z orthologs observed on ALG2 is significantly different from the proportion that would be expected by chance. We further show that conserved syntenies between ALG2 and amniote chromosomes are identified as overlapping conserved syntenies when all available chicken (N = 3120) and human (N = 14,922) RefSeq orthologs are reciprocally compared. In particular, the data suggest that chromosomal regions from chicken chromosomes (GGA) Z and 4 and from human chromosomes (HSA) 9, 4, X, 5, and 8 were linked ancestrally. A more distant outgroup comparison with the pufferfish Tetraodon nigroviridis reveals ALG2/GGAZ/HSAX syntenies among three pairs of ancestral chromosome duplicates. Overall, our results suggest that sex chromosomal regions of birds and mammals were recruited from a common ancestral chromosome, and thus our findings conflict with the currently accepted hypothesis of separate autosomal origins. We note that our results were obtained using the most immediate outgroup to the amniote clade (mammals, birds, and other reptiles) while the currently accepted hypothesis is primarily based upon conserved syntenies between in-group taxa (birds and mammals). Our study illustrates the importance of an amphibian outgroup perspective in identifying ancestral amniote gene orders and in reconstructing patterns of vertebrate sex-chromosome evolution.
我们通过绘制蝾螈两栖动物（美西螈）中羊膜动物性染色体基因座的位置，来检验有关鸟类和哺乳动物性染色体起源的假说。我们发现，人类X染色体和鸡Z染色体的美西螈直系同源基因定位于美西螈共同连锁群2（ALG2）的相邻区域。我们通过统计学方法表明，在ALG2上观察到的人类X染色体和鸡Z染色体直系同源基因的比例，与随机预期的比例存在显著差异。我们进一步表明，当对所有可用的鸡（N = 3120）和人类（N = 14,922）RefSeq直系同源基因进行相互比较时，ALG2与羊膜动物染色体之间的保守同线性被识别为重叠的保守同线性。特别是，数据表明鸡染色体（GGA）Z和4以及人类染色体（HSA）9、4、X、5和8的染色体区域在祖先中是相连的。与河豚黑青斑河鲀进行更遥远的外类群比较，揭示了三对祖先染色体重复序列之间的ALG2/GGAZ/HSAX同线性。总体而言，我们的结果表明，鸟类和哺乳动物的性染色体区域是从共同的祖先染色体中招募而来的，因此我们的发现与目前被广泛接受的独立常染色体起源假说相冲突。我们注意到我们的结果是使用羊膜动物类群（哺乳动物、鸟类和其他爬行动物）最直接的外类群获得的，而目前被广泛接受的假说主要基于类群内部（鸟类和哺乳动物）之间的保守同线性。我们的研究说明了从两栖动物外类群角度在识别祖先羊膜动物基因顺序以及重建脊椎动物性染色体进化模式方面的重要性。
