Data from: A novel locus on chromosome 1 underlies the evolution of a melanic plumage polymorphism in a wild songbird

Understanding the mechanisms responsible for phenotypic diversification within and among species ultimately rests with linking naturally occurring mutations to functionally and ecologically significant traits. Colour polymorphisms are of great interest in this context because discrete colour patterns within a population are often controlled by just a few genes in a common environment. We investigated how and why phenotypic diversity arose and persists in the Zosterops borbonicus white-eye of Reunion (Mascarene archipelago), a colour polymorphic songbird in which all highland populations contain individuals belonging to either a brown or a grey plumage morph. Using extensive phenotypic and genomic data, we demonstrate that this melanin-based colour polymorphism is controlled by a single locus on chromosome 1 with two large-effect alleles, which was not previously described as affecting hair or feather colour. Differences between colour morphs appear to rely upon complex cis-regulatory variation that either prevents the synthesis of pheomelanin in grey feathers, or increases its production in brown ones. We used coalescent analyses to show that, from a ‘brown’ ancestral population, the dominant ‘grey’ allele spread quickly once it arose from a new mutation. Since colour morphs are always found in mixture, this implies that the selected allele does not go to fixation, but instead reaches an intermediate frequency, as would be expected under balancing selection.

阐明物种内部及物种间表型分化的潜在机制，最终需要将自然发生的突变与具备功能及生态重要性的性状建立关联。在此研究框架下，颜色多态性备受关注，因为在相同环境背景下，种群内离散的色彩模式通常仅由少数几个基因调控。本研究以马斯卡林群岛留尼汪岛的留尼汪绣眼鸟（Zosterops borbonicus）为研究对象——这是一种具有颜色多态性的鸣禽，其所有高地种群均包含棕羽型与灰羽型两种个体——并探讨了该物种种表型多样性的产生与维持机制及成因。本研究借助大规模表型与基因组数据，证实这种基于黑色素的颜色多态性由1号染色体上的单个基因座调控，该基因座携带两种大效应等位基因，此前尚无研究表明该基因座会影响毛发或羽毛的颜色。两种颜色形态间的差异似乎源于复杂的顺式调控变异：这类变异可抑制灰羽中褐黑素的合成，或是提升棕羽中褐黑素的生成量。我们通过溯祖分析证实：该物种的祖先种群为棕羽型，一旦优势灰羽等位基因通过新发突变产生，便会快速在种群中扩散。由于种群中始终存在两种颜色形态的混合个体，这意味着受选择的等位基因并未达到固定频率，而是维持在中间频率水平，这与平衡选择下的理论预期相符。