Avian MHC copy number variation is associated with helminth richness

Genes of the Major Histocompatibility Complex (MHC) play a key role in the
adaptive immunity of vertebrates, as they encode receptors responsible for
recognition of antigens. Evolutionary history of the MHC proceeded through
numerous gene duplications, which increases the spectrum of pathogens
recognized by individuals. Although pathogen-mediated selection is
believed to be a primary driver of MHC expansion over evolutionary times,
empirical evidence for this association is virtually lacking. Here, we
used an extensive dataset on MHC class II copy number variation in
non-passerine birds to test for an evolutionary correlation with helminth
parasite richness. As expected, our phylogenetically-informed modelling
revealed a positive association between MHC copy number and total helminth
richness, even after controlling for a broad spectrum of ecological and
life-history traits. Thus, total helminth richness appears to be the most
important correlate of MHC copy number, supporting a leading role of
pathogen-mediated selection in the evolution of MHC in birds. Our results
provide some of the first, although correlative, evidence linking
parasitism to inter-specific variation in MHC copy number among birds.