Adaptive Evolution of Genes Duplicated from the Drosophila pseudoobscura neo-X Chromosome
Adaptive Evolution of Genes Duplicated from the Drosophila pseudoobscura neo-X Chromosome
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DOI：
10.1093/molbev/msq085
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发表时间：
2010-08-01
影响因子：
10.7
通讯作者：
Schaeffer, Stephen W.
中科院分区：
文献类型：
--
作者：
Meisel, Richard P.;Hilldorfer, Benedict B.;Schaeffer, Stephen W.
果蝇X染色体是不成比例的重复基因的来源，这些重复通常是X连锁基因反转录转座到常染色体的结果。过量复制被认为是由自然选择驱动的，原因有二：X染色体在精子发生过程中失活，以及逆转录复制的衍生拷贝倾向于在睾丸中表达。因此，常染色体衍生的逆转录基因拷贝提供了一种机制，为他们的X-连锁旁系同源物“逃脱”X失活。一旦这些重复固定下来，它们就可以被选择用于男性特有的功能。在果蝇属的整个进化过程中，常染色体与X染色体沿着多个谱系融合，产生新X染色体。另外，两条独立的neo-X染色体也存在着过量的重复--一条出现在willistoni种群的共同祖先之前，另一条出现在沿着果蝇的谱系中。为了确定自然选择在D. pseudobscura neo-X染色体上，我们分析了旁系同源物之间的DNA序列差异，每个拷贝内的多态性，以及这些重复基因的表达谱。我们发现，所有重复基因的衍生副本有升高的非同义多态性，这表明他们是在宽松的选择性约束。衍生的拷贝也倾向于具有睾丸或男性偏好的表达谱，而不管它们的染色体来源。从新X染色体复制的基因似乎比从其他染色体臂复制的基因受到的限制要少。我们还发现了更多的证据，从neo-X染色体复制的基因的历史适应性进化，这表明他们是在一个独特的选择制度，其中升高的非同义多态性提供了一个大水库的功能变体，其中一些是固定的自然选择。
Drosophila X chromosomes are disproportionate sources of duplicated genes, and these duplications are usually the result of retrotransposition of X-linked genes to the autosomes. The excess duplication is thought to be driven by natural selection for two reasons: X chromosomes are inactivated during spermatogenesis, and the derived copies of retroposed duplications tend to be testis expressed. Therefore, autosomal derived copies of retroposed genes provide a mechanism for their X-linked paralogs to "escape" X inactivation. Once these duplications have fixed, they may then be selected for male-specific functions. Throughout the evolution of the Drosophila genus, autosomes have fused with X chromosomes along multiple lineages giving rise to neo-X chromosomes. There has also been excess duplication from the two independent neo-X chromosomes that have been examined-one that occurred prior to the common ancestor of the willistoni species group and another that occurred along the lineage leading to Drosophila pseudoobscura. To determine what role natural selection plays in the evolution of genes duplicated from the D. pseudoobscura neo-X chromosome, we analyzed DNA sequence divergence between paralogs, polymorphism within each copy, and the expression profiles of these duplicated genes. We found that the derived copies of all duplicated genes have elevated nonsynonymous polymorphism, suggesting that they are under relaxed selective constraints. The derived copies also tend to have testis- or male-biased expression profiles regardless of their chromosome of origin. Genes duplicated from the neo-X chromosome appear to be under less constraints than those duplicated from other chromosome arms. We also find more evidence for historical adaptive evolution in genes duplicated from the neo-X chromosome, suggesting that they are under a unique selection regime in which elevated nonsynonymous polymorphism provides a large reservoir of functional variants, some of which are fixed by natural selection.