Department of Molecular Biology and Genetics, Cornell University, Ithaca, New York 14853, USA.
Genome Res. 2012 Jul;22(7):1255-65. doi: 10.1101/gr.132100.111. Epub 2012 Apr 12.
X chromosomes are preferentially transmitted through females, which may favor the accumulation of X-linked alleles/genes with female-beneficial effects. Numerous studies have shown that genes with sex-biased expression are under- or over-represented on the X chromosomes of a wide variety of organisms. The patterns, however, vary between different animal species, and the causes of these differences are unresolved. Additionally, genes with sex-biased expression tend to be narrowly expressed in a limited number of tissues, and narrowly expressed genes are also non-randomly X-linked in a taxon-specific manner. It is therefore unclear whether the unique gene content of the X chromosome is the result of selection on genes with sex-biased expression, narrowly expressed genes, or some combination of the two. To address this problem, we measured sex-biased expression in multiple Drosophila species and at different developmental time points. These data were combined with available expression measurements from Drosophila melanogaster and mouse to reconcile the inconsistencies in X-chromosome content among taxa. Our results suggest that most of the differences between Drosophila and mammals are confounded by disparate data collection/analysis approaches as well as the correlation between sex bias and expression breadth. Both the Drosophila and mouse X chromosomes harbor an excess of genes with female-biased expression after controlling for the confounding factors, suggesting that the asymmetrical transmission of the X chromosome favors the accumulation of female-beneficial mutations in X-linked genes. However, some taxon-specific patterns remain, and we provide evidence that these are in part a consequence of constraints imposed by the dosage compensation mechanism in Drosophila.
X 染色体优先通过女性传递，这可能有利于具有女性有益效应的 X 连锁等位基因/基因的积累。许多研究表明，具有性别偏向表达的基因在各种生物体的 X 染色体上的表达水平较低或过高。然而，这些模式在不同的动物物种之间存在差异，这些差异的原因尚未解决。此外，具有性别偏向表达的基因往往在有限数量的组织中表达狭窄，并且以分类群特异性的方式窄表达的基因也不是随机的 X 连锁。因此，尚不清楚 X 染色体的独特基因含量是由于对具有性别偏向表达的基因、窄表达基因还是两者的组合进行选择的结果。为了解决这个问题，我们在多个果蝇物种和不同的发育时间点测量了性别偏向表达。这些数据与来自果蝇和小鼠的可用表达测量值相结合，以协调分类群之间 X 染色体含量的不一致性。我们的结果表明，果蝇和哺乳动物之间的大多数差异都受到不同的数据收集/分析方法以及性别偏见和表达广度之间的相关性的混淆。在控制混杂因素后，果蝇和小鼠的 X 染色体都含有过多的具有雌性偏倚表达的基因，这表明 X 染色体的不对称传递有利于 X 连锁基因中雌性有益突变的积累。然而，一些特定于分类群的模式仍然存在，我们提供的证据表明，这些部分是由于果蝇中剂量补偿机制施加的限制所致。
