Balancing selection and genetic drift create unusual patterns of MHCIIß variation in Galápagos mockingbirds.

The extracellular subunit of the major histocompatibility complex MHCIIß 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 MHCIIß, but it has proved difficult to determine the relative effects of these forces. We evaluated the effect of genetic drift and balancing selection on MHCIIß diversity in 12 small populations of Galápagos mockingbirds belonging to four different species, and one larger population of the Northern mockingbird from the continental USA. After genotyping MHCIIß loci by high-throughput sequencing, we applied a correlational approach to explore the relationships between MHCIIß 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 MHCIIß alleles present in a population. However, the number of MHCIIß alleles per individual and number of supertypes were not correlated with population size. This discrepancy points to an interesting feature of MHCIIß diversity dynamics: some levels of diversity might be shaped by genetic drift while others are independent and possibly maintained by balancing selection.