Balancing selection and genetic drift create unusual patterns of MHCII variation in Galapagos mockingbirds

The extracellular subunit of the major histocompatibility complex MHCII plays an important role in the recognition of pathogens and the initiation of the adaptive immune response of vertebrates. It is widely accepted that pathogen-mediated selection in combination with neutral micro-evolutionary forces (e.g. genetic drift) shape the diversity of MHCII, but it has proved difficult to determine the relative effects of these forces. We evaluated the effect of genetic drift and balancing selection on MHCII diversity in 12 small populations of Galapagos mockingbirds belonging to four different species, and one larger population of the Northern mockingbird from the continental USA. After genotyping MHCII loci by high-throughput sequencing, we applied a correlational approach to explore the relationships between MHCII diversity and population size by proxy of island size. As expected when drift predominates, we found a positive effect of population size on the number of MHCII alleles present in a population. However, the number of MHCII alleles per individual and number of supertypes were not correlated with population size. This discrepancy points to an interesting feature of MHCII diversity dynamics: some levels of diversity might be shaped by genetic drift while others are independent and possibly maintained by balancing selection.