Divergence between genes but limited allelic polymorphism in two MHC class II A genes in Leach’s storm-petrels Oceanodroma leucorhoa
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The major histocompatibility complex (MHC) is critical to host-pathogen interactions. Class II MHC is a heterodimer, with α and β subunits encoded by different genes. The peptide-binding groove is formed by the first domain of both subunits (α1 and β1), but studies of class II variation or natural selection focus primarily on the β subunit and II B genes. We explored MHC II A in Leach’s storm-petrel, a seabird with two expressed, polymorphic II B genes. We found two II A genes, Ocle-DAA and Ocle-DBA, in contrast to the single II A gene in chicken and duck. In exon 2 which encodes the α1 domain, the storm-petrel II A genes differed strongly from each other but showed little within-gene polymorphism in 30 individuals: just one Ocle-DAA allele, and three Ocle-DBA alleles differing from each other by single non-synonymous substitutions. In a comparable sample, the two II B genes had nine markedly diverged alleles each. Differences between the α1 domains of Ocle-DAA and Ocle-DBA showed signatures of positive selection, but mainly at non-peptide-binding site (PBS) positions. In contrast, positive selection within and between the II B genes corresponded to putative PBS codons. Phylogenetic analysis of the conserved α2 domain did not reveal deep or well-supported lineages of II A genes in birds, in contrast to the pronounced differentiation of DQA, DPA, and DRA isotypes in mammals. This uncertain homology complicates efforts to compare levels of functional variation and modes of evolution of II A genes across taxa.
KeywordsMHC class II A Polymorphism Selection Heterodimer Seabird
We thank Bob Mauck, Frank Hailer, Erin Voss, Sabine Berzins, and Hillary Chavez for help with this project. Funding and equipment were provided by the Arthur Vining Davis Foundations and Institutional Development Awards (P20GM0103423) from the National Institute of General Medical Sciences of the National Institutes of Health. This is contribution number 277 from the Bowdoin Scientific Station on Kent Island.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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