title: The evolution of highly variable immunity genes across a passerine bird radiation.
abstract: To survive, individuals must be able to recognize and eliminate pathogens. The genes of the major histocompatibility complex (MHC) play an essential role in this process in vertebrates as their diversity affects the repertoire of pathogens that can be recognized by the immune system. Emerging evidence suggests that birds within the parvorder Passerida possess an exceptionally high number of MHC genes. However, this has yet to be directly investigated using a consistent framework, and the question of how this MHC diversity has evolved has not been addressed. We used next-generation sequencing to investigate how MHC class I gene copy number and sequence diversity varies across the Passerida radiation using twelve species chosen to represent the phylogenetic range of this group. Additionally, we performed phylogenetic analyses on this data to identify, for the first time, the evolutionary model that best describes how MHC class I gene diversity has evolved within Passerida. We found evidence of multiple MHC class I genes in every family tested, with an extremely broad range in gene copy number across Passerida. There was a strong phylogenetic signal in MHC gene copy number and diversity, and these traits appear to have evolved through a process of Brownian motion in the species studied, that is following the pattern of genetic drift or fluctuating selection, as opposed to towards a single optimal value or through evolutionary 'bursts'. By characterizing MHC class I gene diversity across Passerida in a systematic framework, this study provides a first step towards understanding this huge variation.
authors.affiliation: Molecular Ecology and Evolution Lab, Lund University, Ecology building, 223 62, Lund, Sweden.
authors.forename: E A
authors.initials: EA
authors.lastname: O'Connor
authors.affiliation: Molecular Ecology and Evolution Lab, Lund University, Ecology building, 223 62, Lund, Sweden.
authors.forename: M
authors.initials: M
authors.lastname: Strandh
authors.affiliation: Molecular Ecology and Evolution Lab, Lund University, Ecology building, 223 62, Lund, Sweden.
authors.forename: D
authors.initials: D
authors.lastname: Hasselquist
authors.affiliation: Molecular Ecology and Evolution Lab, Lund University, Ecology building, 223 62, Lund, Sweden.
authors.forename: J-Å
authors.initials: JÅ
authors.lastname: Nilsson
authors.affiliation: Molecular Ecology and Evolution Lab, Lund University, Ecology building, 223 62, Lund, Sweden.
authors.forename: H
authors.initials: H
authors.lastname: Westerdahl
mesh_terms: D000483:Alleles; D000818:Animals; D005075:Biological Evolution; D018628:Gene Dosage; D005802:Genes, MHC Class II; D040961:Genetic Drift; D014644:Genetic Variation; D010802:Phylogeny; D012641:Selection, Genetic; D017422:Sequence Analysis, DNA; D046370:Sparrows / Q000145:classification* / Q000235:genetics
keywords: birds; class I; diversity; evolutionary model; major histocompatibility complex