Island organisms often evolve phenotypes divergent from their mainland counterparts, providing a useful system for studying adaptation under differential selection. In the white-winged fairywren (Malurus leucopterus), subspecies on two islands have a black nuptial plumage whereas the subspecies on the Australian mainland has a blue nuptial plumage. The black subspecies have a feather nanostructure that could in principle produce a blue structural color, suggesting a blue ancestor. An earlier study proposed independent evolution of melanism on the islands based on the history of subspecies divergence. However, the genetic basis of melanism and the origin of color differentiation in this group are still unknown. Here, we used whole-genome resequencing to investigate the genetic basis of melanism by comparing the blue and black M. leucopterus subspecies to identify highly divergent genomic regions. We identified a well-known pigmentation gene ASIP and four candidate genes that may contribute to feather nanostructure development. Contrary to the prediction of convergent evolution of island melanism, we detected signatures of a selective sweep in genomic regions containing ASIP and SCUBE2 not in the black subspecies but in the blue subspecies, which possesses many derived SNPs in these regions, suggesting that the mainland subspecies has re-evolved a blue plumage from a black ancestor. This proposed re-evolution was likely driven by a pre-existing female preference. Our findings provide new insight into the evolution of plumage coloration in island versus continental populations, and, importantly, we identify candidate genes that likely play roles in the development and evolution of feather structural coloration.

中文翻译：

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澳大利亚鸣禽结构颜色多态性的遗传基础和进化

岛屿生物经常进化出与大陆生物不同的表型，这为研究差异选择下的适应提供了一个有用的系统。在白翅细尾鹩莺 (Malurus leucopterus) 中，两个岛屿上的亚种有黑色的结婚羽毛，而澳大利亚大陆上的亚种则有蓝色的结婚羽毛。黑色亚种具有羽毛纳米结构，原则上可以产生蓝色结构色，表明其祖先是蓝色的。早期的一项研究根据亚种分化的历史提出了岛屿上黑色素的独立进化。然而，该群体黑化症的遗传基础和颜色分化的起源仍然未知。在这里，我们使用全基因组重测序来研究黑色素病的遗传基础，通过比较蓝色和黑色白翅雉亚种来识别高度分歧的基因组区域。我们鉴定了一个著名的色素沉着基因 ASIP 和四个可能有助于羽毛纳米结构发育的候选基因。与岛屿黑化趋同进化的预测相反，我们在包含 ASIP 和 SCUBE2 的基因组区域中检测到选择性扫描的特征，但不是在黑色亚种中，而是在蓝色亚种中，该亚种在这些区域中拥有许多衍生的 SNP，这表明大陆亚种从黑色祖先那里重新进化出蓝色羽毛。这种重新进化的提议很可能是由先前存在的女性偏好驱动的。我们的研究结果为岛屿与大陆种群羽毛颜色的演变提供了新的见解，而且重要的是，我们确定了可能在羽毛结构颜色的发育和进化中发挥作用的候选基因。