Structure and Evolution of a New Avian MHC Class II B Gene in a Sub-Antarctic Seabird,the Thin-Billed Prion (Procellariiformes: <Emphasis Type="Italic">Pachyptila belcheri</Emphasis>) |
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Authors: | Mónica C Silva Scott V Edwards |
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Institution: | (1) Department of Biology, University of Washington, Box 351800, Seattle, WA 98195, USA;(2) Departamento de Biologia Animal, Centro de Biologia Ambiental, Faculdade de Ciências, Universidade de Lisboa, Bloco C2, 3o Piso, Campo Grande, Lisboa, 1749-016, Portugal;(3) Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA |
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Abstract: | The major histocompatibility complex encodes molecules that present foreign peptides to T cells of the immune system. The
peptide binding region (PBR) of these molecules is among the most polymorphic regions found in vertebrate taxa. Genomic cloning
approaches are improving our understanding of the evolution of this multigene family in nonmodel avian groups. By building
a cosmid library, a new MHC class II B gene, Pabe-DAB1, was isolated and characterized at the genomic level in a sub-Antarctic seabird, the thin-billed prion (Pachyptila belcheri). Pabe-DAB1 exhibits the hallmark structural features of functional MHC class II loci. Direct sequencing of the PBR encoding exon in
a panel of prions revealed significantly higher levels of genetic diversity compared to two noncoding neutral loci, with most
alleles differing by at least one replacement substitution in the peptide binding codons. We estimated evolutionary dynamics
for Pabe-DAB1 using a variety of Bayesian and other approaches. Evidence for balancing selection comes from a spatially variable ratio
of nonsynonymous-to-synonymous substitutions (mean d
N/d
S = 2.87) in the PBR, with sites predicted to be functionally relevant exhibiting the highest ω values. We estimate the population recombination rate to be approximately 0.3 per site per generation, indicating an important
role for recombination in generating polymorphism at this locus. Pabe-DAB1 is among the few avian class II loci characterized at the genomic level and with a known intron-exon structure, a feature
that greatly facilitated the amplification and sequencing of a single MHC locus in this species. |
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Keywords: | Major histocompatibility complex Peptide binding region Polymorphism Balancing selection Procellariiformes |
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