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 intermediate frequency, as would be expected under balancing selection.

解析物种种内与物种间表型多样化的核心机制，最终需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，因为在同一环境中，种群内的离散色斑模式往往仅由少数基因调控。本研究针对马斯卡林群岛留尼汪岛的留尼汪绣眼鸟（Zosterops borbonicus）——一种具备颜色多态性的鸣禽，所有高地种群均包含呈棕色或灰色羽色变型的个体——探究了其表型多样性的产生与维持机制及原因。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颜色。不同颜色变型间的差异似乎源于复杂的顺式调控变异：这类变异要么可阻止灰色羽毛中褐黑素的合成，要么可提升棕色羽毛中褐黑素的生成量。本研究通过溯祖分析证实，在以棕色为主体的祖先种群中，新突变产生的显性"灰色"等位基因一经出现便快速扩散。由于种群中始终存在两种颜色变型的混合分布，这意味着受选择的等位基因并未达到固定状态，而是维持在中间频率——这与平衡选择下的理论预期相符。