Female reed warblers in social pairs with low MHC dissimilarity achieve higher MHC dissimilarity through random extra-pair mating

Major Histocompatibility Complex (MHC) polymorphism is maintained by balancing selection through host-pathogen interactions and mate choice. MHC-based mate choice has been investigated across a wide range of vertebrates, and an established concept is that females should choose a mate with an MHC genotype that is dissimilar to her own to ensure high MHC divergence in her offspring. Here we present evidence from a population of reed warblers, Acrocephalus scirpaceus, that social pairs with extra pair young in their nest have significantly lower MHC dissimilarity than expected by random MHC-based mate choice. Moreover, social pairs with extra pair young in their nest have lower MHC dissimilarity than the potential pairs females could form with other males surrounding the social nest. Therefore, females in pairs with low MHC dissimilarity could improve the MHC divergence of her offspring through extra-pair mating. We propose that when the MHC dissimilarity in the social pair is low, any alternative male represents a better genetic prospect for the female in terms of MHC dissimilarity. This scenario generates a pattern of MHC-disassortative extra-pair mating without requiring active MHC-based mate choice.