Allele-specific expression of MHC IIB in the Amphilophus species complex in Nicaragua
Diversification by natural selection is a major source of biodiversity. Parasites impose selective pressures on their hosts, increasing diversity at immunologically relevant genes. The genes of the major histocompatibility complex (MHC) are an important target of parasite-mediated selection, which frequently leads to genetic differentiation and local adaptation among populations at these genes. We studied differential allele-specific MHC IIB expression in wild cichlid populations to assess whether this can serve as a substrate for parasite-mediated selection. We explored MHC expression patterns, their relationship with genetic variant frequency and their variability among Midas cichlid populations and species. We found a few highly abundant alleles with consistently low expression, and many relatively rare alleles with high expression levels that were specific to different host populations. MHC alleles belonging to the same functional supertypes had similar expression patterns, which was consistent among populations for lowly expressed supertypes, but highly variable for highly expressed supertypes. Our findings suggest that rare alleles might be immunologically relevant and that different alleles could provide similar functionality in different individuals. The common but lowly expressed alleles may have attained different functions (e.g. non-antigen binding). Our study shows that MHC expression is highly variable and population-specific or species-specific for supertypes and loci potentially interacting with parasites, providing an additional substrate for selection.