Meeting Abstract
Genes of the major histocompatibility complex (MHC) defend against disease by helping the immune system recognize pathogens. MHC heterozygotes have higher fitness than homozygotes, which in turn should favor MHC-disassortative mating, but a surprising number of species mate randomly with respect to MHC. The explanation may lie in evolutionary processes following gene duplication: if two duplicated MHC genes become functionally diverged from each other, offspring will inherit diverse multilocus genotypes even under random mating. We used locus-specific primers for high throughput sequencing of two expressed MHC Class II B genes in Leach’s storm-petrels, Oceanodroma leucorhoa, and found that exon 2 alleles fall into two gene-specific monophyletic clades. Analysis of mated pairs consistently showed no evidence of disassortative mating, despite good statistical power and multiple analytical approaches. Even with random mating, though, birds had MHC genotypes with functionally diverged alleles, averaging 13 amino acid differences in pairwise comparisons of exon 2 alleles within individuals. We built a phylogenetic permutation model to test whether this high MHC diversity in individuals is driven by evolutionary divergence of the two duplicated genes. The model showed that genotypic diversity was strongly impacted by sequence divergence between the most common allele of each gene, with a smaller additional impact of monophyly of the two genes. This divergence of allele sequences between genes may reduce the importance of actively seeking MHC-dissimilar mates, in which case the evolutionary history of duplicated genes is shaping the adaptive landscape of sexual selection.
