Data from: Sex-linked genomic variation and its relationship to avian plumage dichromatism and sexual selection

Background: Sexual dichromatism is the tendency for sexes to differ in color pattern and represents a striking form of within-species morphological variation. Conspicuous intersexual differences in avian plumage are generally thought to result from Darwinian sexual selection, to the extent that dichromatism is often treated as a surrogate for the intensity of sexual selection in phylogenetic comparative studies. Intense sexual selection is predicted to leave a footprint on genetic evolution by reducing the relative genetic diversity on sex chromosome to that on the autosomes. Results: In this study, we test the association between plumage dichromatism and sex-linked genetic diversity using eight species pairs with contrasting levels of dichromatism. We estimated Z-linked and autosomal genetic diversity for these non-model avian species using restriction-site associated (RAD) loci that covered ~3 % of the genome. We find that monochromatic birds consistently have reduced sex-linked genomic variation relative to phylogenetically-paired dichromatic species and this pattern is robust to mutational biases. Conclusions: Our results are consistent with several interpretations. If present-day sexual selection is stronger in dichromatic birds, our results suggest that its impact on sex-linked genomic variation is offset by other processes that lead to proportionately lower Z-linked variation in monochromatic species. We discuss possible factors that may contribute to this discrepancy between phenotypes and genomic variation. Conversely, it is possible that present-day sexual selection -- as measured by the variance in male reproductive success -- is stronger in the set of monochromatic taxa we have examined, potentially reflecting the importance of song, behavior and other non-plumage associated traits as targets of sexual selection. This counterintuitive finding suggests that the relationship between genomic variation and sexual selection is complex and highlights the need for a more comprehensive survey of genomic variation in avian taxa that vary markedly in social and genetic mating systems.研究背景：性二色性（Sexual dichromatism）指两性间色彩模式存在差异的现象，是物种内形态变异的一种显著形式。学界普遍认为，鸟类羽毛呈现出的显著两性差异源自达尔文性选择，以至于在系统发育比较研究中，性二色性常被用作性选择强度的替代指标。强烈的性选择被认为会通过降低性染色体（sex chromosome）相较于常染色体（autosomes）的相对遗传多样性，从而在遗传演化中留下印记。 研究结果：本研究选取8对性二色性水平存在显著差异的鸟类物种对，以此检验羽毛性二色性与性连锁遗传多样性之间的关联。我们通过覆盖基因组约3%的限制性酶切位点关联（RAD）位点，对这些非模式鸟类物种的Z染色体连锁遗传多样性与常染色体遗传多样性进行了估算。研究发现，相较于系统发育匹配的二色性鸟类物种，单色性鸟类的性连锁基因组变异水平普遍更低，且该结论不受突变偏好的影响。 研究结论：本研究结果与多种解释框架相符。若当前二色性鸟类的性选择更强，那么本研究结果表明，性选择对性连锁基因组变异的影响，会被其他导致单色性物种Z染色体连锁变异比例更低的过程所抵消。我们对可能造成表型与基因组变异间这种不一致的潜在因素进行了讨论。反之，我们所研究的单色性类群中，以雄性繁殖成功率方差衡量的当前性选择强度可能更强，这或许反映了鸣唱、行为及其他非羽毛相关性状作为性选择靶标的重要性。这一反直觉的发现表明，基因组变异与性选择之间的关系十分复杂，同时也凸显了对社会交配系统与遗传交配系统差异显著的鸟类类群开展更全面的基因组变异调查的必要性。