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Evidence for a selectively favourable reduction in the mutation rate of the X chromosome

机译：X染色体突变率选择性降低的证据

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The equilibrium per-genome mutation rate in sexual species is thought to result from a trade-off between the benefits of reducing the deleterious mutation rate and the costs of increasing fidelity. We propose that selection will often favour a lower mutation rate on the X chromosome than on autosomes, owing to the exposure of deleterious recessive mutations on hemizygous chromosomes. We tested this hypothesis by examining 33 X-linked genes that have been sequenced in both mouse and rat, and compared their rate of evolution against 238 autosomal genes. The X-linked genes were found to have a significantly lower rate of synonymous substitution than the autosomal genes. Neither the supposed higher mutation rate in males nor stronger purifying selection against slightly deleterious mutations on the X chromosome can account for the low value. The most parsimonious explanation is that rodents have a lower mutation rate on the X chromosome than on autosomes. It is therefore likely that previous indirect estimates of the excess male mutation rate are inaccurate. Indeed, after correction we find no evidence for a male-biased mutation rate in rodents. Furthermore, the rate of synonymous substitution in Y-linked genes is not significantly different from that in autosomal ones. The extent to which enhanced male mutation rates are problematic for the mutational deterministic model of the evolution of sex must, in turn, be questioned.
机译：人们认为，有性物种中平衡的每个基因组的突变率是在降低有害突变率的好处与提高保真度的成本之间进行权衡的结果。我们建议，由于半合子染色体上有害的隐性突变的暴露，选择通常会倾向于X染色体上的突变率低于常染色体。我们通过检查已在小鼠和大鼠中测序的33个X连锁基因并比较了它们与238个常染色体基因的进化速率，验证了这一假设。发现X连锁基因的同义替换率比常染色体基因低得多。男性预期的较高突变率或针对X染色体上的轻微有害突变的更强纯化选择均不能解释该低值。最简约的解释是啮齿动物在X染色体上的突变率低于常染色体。因此，先前对过量男性突变率的间接估计可能不准确。确实，校正后，我们没有发现任何证据表明啮齿动物有男性偏见的突变率。此外，Y连锁基因中的同义替换率与常染色体基因中的同义替换率没有显着差异。反过来，必须提高男性突变率在多大程度上对性别进化的突变确定性模型造成问题。

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