Coloration is evolutionarily labile and so provides an excellent trait for examining the repeatability of evolution. Here, we investigate the repeatability of the evolution of polymorphic variation in ventral plumage coloration in skuas (Stercorarius: Stercorariidae). In 2 species, arctic (S. parasiticus) and pomarine skuas (S. pomarinus), plumage polymorphism was previously shown to be associated with coding changes at the melanocortin-1 receptor (MC1R) locus. Here, we show that polymorphism in a third species, the south polar skua (S. maccormicki), is not associated with coding variation at MC1R or with variation at a Z-linked second candidate locus, tyrosinase-related protein 1 (TYRP1). Hence, convergent evolution of plumage polymorphisms in skuas is only partly repeatable at the level of the genetic locus involved. Interestingly, the pattern of repeatability in skuas is aligned not with phylogeny but with the nature of the phenotypic variation. In particular, south polar skuas show a strong sex bias to coloration that is absent in the other species, and it may be that this has a unique genetic architecture.

中文翻译：

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贼鸥 (Stercorarius) 羽毛多态性遗传机制的变异

着色在进化上是不稳定的，因此为检查进化的可重复性提供了极好的特征。在这里，我们研究了贼鸥（Stercorarius：Stercorariidae）腹侧羽毛颜色多态性变异演化的可重复性。在 2 个物种，北极（S. parasiticus）和深海贼鸥（S. pomarinus）中，先前已证明羽毛多态性与黑皮质素 1 受体 (MC1R) 基因座的编码变化有关。在这里，我们表明第三种物种南极贼鸥 (S. maccormicki) 的多态性与 MC1R 的编码变异或 Z 连锁的第二个候选基因座酪氨酸酶相关蛋白 1 (TYRP1) 的变异无关。因此，贼鸥羽毛多态性的趋同进化在所涉及的遗传基因座水平上只是部分可重复的。有趣的是，贼鸥的可重复性模式与系统发育无关，而是与表型变异的性质相一致。特别是，南极贼鸥对颜色表现出强烈的性别偏见，这在其他物种中是不存在的，这可能是因为它具有独特的遗传结构。