Variation in Genetic Mechanisms for Plumage Polymorphism in Skuas (Stercorarius)

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.

Automated summary

Coloration is evolutionarily labile, providing a useful trait for studying the repeatability of evolution. Polymorphic variation in ventral plumage coloration in skuas is not entirely associated with genetic coding changes at MC1R, suggesting that convergent evolution of plumage polymorphisms in skuas is only partially repeatable at the genetic locus level. The pattern of repeatability in skuas is not phylogenetically aligned, but rather with the nature of the phenotypic variation, with south polar skuas showing a unique sex bias in coloration.