Phylogenomics Reveals that Mitochondrial Capture and Nuclear Introgression Characterizes Skua Species Proposed to be of Hybrid Origin.

The skuas and jaegers (Stercorariidae) are an enigmatic family of seven seabird species that breed at Arctic and Antarctic latitudes. The phylogenetic relationships amongst the species have been controversial, with one of the biggest enigmas involving the Pomarine Jaeger (Stercorarius pomarinus), which has been proposed to represent a hybrid species originating from the merging of distant lineages within the complex. We inferred a phylogeny for the family using multispecies coalescent methods with whole-genome sequencing for all seven species of Stercorariidae, and document an evolutionary history rich in introgression. We uncover evidence for mitochondrial capture and nuclear introgression between S. pomarinus and S. skua, providing a potential avenue for adaptive introgression. One candidate for adaptive introgression is the MC1R plumage gene which appears to have introgressed from one of the large skuas into S. pomarinus, where it now forms the basis of the dark-morph colour polymorphism of that species. We further highlight a complex biogeographical history of interchange between the Arctic and Antarctic, with unexpected close ancestry between S. skua of the northern hemisphere and S. antarcticus of the southern hemisphere. These results highlight the dynamic history of introgression during a pelagic seabird radiation.