Data from: Molecular population genetics of the melanic plumage polymorphism in arctic skuas (Stercorarius parasiticus): evidence for divergent selection on plumage colour

The arctic skua (Stercorarius parasiticus) is a classic example of an avian plumage polymorphism, with variation in melanin-based ventral plumage coloration defining pale, intermediate and dark morphs in adults of both sexes. However, despite several decades of field research, there is an incomplete understanding of how the polymorphism in ventral plumage colour is maintained and the selective forces involved. Here we investigate selection on a locus (MC1R) that is strongly associated with plumage colour variation in arctic skuas using patterns of nucleotide variation and comparison to neutral loci (nuclear introns and mtDNA). We find that three linked non-synonymous mutations in MC1R, including the single mutation described previously, are associated with plumage colour in the arctic skua. The position of non-synonymous mutations on a MC1R haplotype network implies that divergent selection drove the initial evolution of the colour morphs. Comparisons of FSTs of MC1R vs. nuclear introns among five skua populations differing in proportion of dark morphs along an approximate north-south cline reveals a signature of divergent selection on MC1R. In contrast, we find limited evidence for balancing selection on MC1R within populations, although the power is low. Our results provide strong evidence for both past and ongoing selection on MC1R, and, by implication, plumage colour in arctic skuas. The results suggest that a fruitful avenue for future ecological studies will be analysis of selection on morphs in colonies at the extremes along the morph ratio cline.

北极贼鸥（Stercorarius parasiticus）是鸟类羽色多态性的经典类群，其基于黑色素的腹部羽色变异可将成年雌雄个体划分为浅色、中间型与深色3种色型。尽管已有数十年的野外研究积累，但学界对腹部羽色多态性的维持机制及所涉及的选择压力仍缺乏完整认知。本研究基于核苷酸变异模式，并与中性位点（核内含子与线粒体DNA（mtDNA））进行比较，探究了与北极贼鸥羽色变异显著相关的MC1R基因座所受的选择作用。研究发现，MC1R基因座上存在3个连锁的非同义突变，其中包含此前已报道的单个突变，这些突变均与北极贼鸥的羽色表型显著相关。MC1R单倍型网络上非同义突变的分布位置表明，歧化选择驱动了羽色型的初始演化。对沿近似南北梯度分布的5个贼鸥种群（其深色色型占比存在显著差异）的MC1R与核内含子的FST值进行比较，结果显示MC1R基因座存在歧化选择的遗传信号。与之相对，本研究未在种群内发现MC1R基因座受平衡选择的有力证据，不过统计检验效力较低。本研究结果为MC1R基因座（以及由此推及的北极贼鸥羽色）所受的历史与当前选择作用提供了强有力的实证支持。研究结果表明，未来生态学研究的一个富有前景的方向，便是对色型比例梯度两端的种群的色型选择压力开展分析。