Data from: MHC class II supertypes affect survival and lifetime reproductive success in a migratory songbird

The major histocompatibility complex (MHC) plays a critical role in the
immune response against pathogens. Its high polymorphism is thought to be
mainly the consequence of host-pathogen co-evolution, but elucidating the
mechanism(s) driving MHC evolution remains challenging for natural
populations. We investigated the diversity of MHC class II genes in a wild
population of pied flycatchers Ficedula hypoleuca, and tested its
associations with two key components of individual fitness: lifetime
reproductive success and survival. Among 180 breeding adults in our study
population, we found 182 unique MHC class II exon 2 alleles. The alleles
showed a strong signal of positive selection and grouped into 9 functional
supertypes based on physicochemical properties at the inferred
antigen-binding sites. Three supertypes were found in > 98% of the
sampled individuals, indicating that they are nearly fixed in the
population. We found no rare supertypes in the population, as all
supertypes were present in >70% of individuals. Three supertypes
were related to different components of individual fitness: two were
associated with lower offspring production over time, while the third was
positively associated with survival. Overall, the substantial allelic and
functional diversity and the relationship between specific supertypes and
fitness is in accordance with the notion that balancing selection
maintains MHC class II diversity in the study population, possibly with
fluctuating selection as the underlying mechanism. The absence of rare
supertypes in the population suggests that the balancing selection is not
driven by rare-allele advantage.