Sex chromosomes contribute substantially to key evolutionary processes such as speciation and adaptation. Several theories suggest that evolution could occur more rapidly on sex chromosomes, but currently our understanding of whether and how this occurs is limited. Here, we present an analysis of the great tit (Parus major) genome, aiming to detect signals of faster-Z evolution. We find mixed evidence of faster divergence on the Z chromosome than autosomes, with significantly higher divergence being found in ancestral repeats, but not at 4-fold or 0-fold degenerate sites. Interestingly, some 4-fold sites appear to be selectively constrained, which may mislead analyses that use these sites as the neutral reference (e.g., dN/dS). Consistent with other studies in birds, the mutation rate is significantly higher in males than females, and the long-term Z-to-autosome effective population size ratio is only 0.5, significantly lower than the expected value of 0.75. These are indicative of male-driven evolution and high variance in male reproductive success, respectively. We find no evidence for an increased efficacy of positive selection on the Z chromosome. In contrast, the Z chromosome in great tits appears to be affected by increased genetic drift, which has led to detectable signals of weakened intensity of purifying selection. These results provide further evidence that the Z chromosome often has a low effective population size, and that this has important consequences for its evolution. They also highlight the importance of considering multiple factors that can affect the rate of evolution and effective population sizes of sex chromosomes.

中文翻译：

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在大山雀（Parus major）中快速Z演变的研究。

性染色体在诸如物种形成和适应等关键进化过程中起着重要作用。几种理论表明，进化可能会在性染色体上更快地发生，但是目前我们对这种情况是否发生以及如何发生的理解是有限的。在这里，我们提出了对大山雀（大山雀）基因组的分析，旨在检测Z轴快速进化的信号。我们发现混杂的证据表明Z染色体的发散速度快于常染色体，在祖先重复序列中发现的发散明显更高，但在4倍或0倍的简并位点却没有。有趣的是，某些4折位点似乎被选择性地限制，这可能会误导使用这些位点作为中性参考的分析（例如dN / dS）。与其他鸟类研究相一致，男性的突变率明显高于女性，Z与常染色体的长期有效种群大小之比仅为0.5，大大低于期望值0.75。这些分别表示男性驱动的进化和男性生殖成功的高度差异。我们没有证据显示Z染色体上阳性选择的功效增加。相反，大山雀中的Z染色体似乎受遗传漂移增加的影响，这导致可检测到的信号减弱了纯化选择的强度。这些结果提供了进一步的证据，表明Z染色体的有效种群数量通常很低，这对其进化具有重要的影响。他们还强调了考虑多种因素的重要性，这些因素可能影响性染色体的进化速率和有效种群大小。