Divergence between genes but limited allelic polymorphism in two MHC class II A genes in Leach ’s storm-petrels Oceanodroma leucorhoa

AbstractThe 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 andOcle-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 oneOcle-DAA allele, and threeOcle-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 ofOcle-DAA andOcle-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 m...

Source: Immunogenetics - September 9, 2019 Category: Genetics & Stem Cells Source Type: research