title: New data from basal Australian songbird lineages show that complex structure of MHC class II β genes has early evolutionary origins within passerines.
abstract: The major histocompatibility complex (MHC) plays a crucial role in the adaptive immune system and has been extensively studied across vertebrate taxa. Although the function of MHC genes appears to be conserved across taxa, there is great variation in the number and organisation of these genes. Among avian species, for instance, there are notable differences in MHC structure between passerine and non-passerine lineages: passerines typically have a high number of highly polymorphic MHC paralogs whereas non-passerines have fewer loci and lower levels of polymorphism. Although the occurrence of highly polymorphic MHC paralogs in passerines is well documented, their evolutionary origins are relatively unexplored. The majority of studies have focussed on the more derived passerine lineages and there is very little empirical information on the diversity of the MHC in basal passerine lineages. We undertook a study of MHC diversity and evolutionary relationships across seven species from four families (Climacteridae, Maluridae, Pardalotidae, Meliphagidae) that comprise a prominent component of the basal passerine lineages. We aimed to determine if highly polymorphic MHC paralogs have an early evolutionary origin within passerines or are a more derived feature of the infraorder Passerida.
  
  
  
  We identified 177 alleles of the MHC class II β exon 2 in seven basal passerine species, with variation in numbers of alleles across individuals and species. Overall, we found evidence of multiple gene loci, pseudoalleles, trans-species polymorphism and high allelic diversity in these basal lineages. Phylogenetic reconstruction of avian lineages based on MHC class II β exon 2 sequences strongly supported the monophyletic grouping of basal and derived passerine species.
  
  
  
  Our study provides evidence of a large number of highly polymorphic MHC paralogs in seven basal passerine species, with strong similarities to the MHC described in more derived passerine lineages rather than the simpler MHC in non-passerine lineages. These findings indicate an early evolutionary origin of highly polymorphic MHC paralogs in passerines and shed light on the evolutionary forces shaping the avian MHC.
authors.affiliation: Department of Sciences, Museum Victoria, Melbourne, VIC, 3001, Australia. sbalasubramaniam@museum.vic.gov.au.|School of BioSciences, The University of Melbourne, Melbourne, VIC, 3010, Australia. sbalasubramaniam@museum.vic.gov.au.
authors.forename: Shandiya
authors.initials: S
authors.lastname: Balasubramaniam
authors.affiliation: Terrestrial Vertebrates, Western Australian Museum, Perth, WA, 6986, Australia.
authors.forename: Rebecca D
authors.initials: RD
authors.lastname: Bray
authors.affiliation: School of BioSciences, The University of Melbourne, Melbourne, VIC, 3010, Australia.
authors.forename: Raoul A
authors.initials: RA
authors.lastname: Mulder
authors.affiliation: School of Biological Sciences, Monash University, Melbourne, VIC, 3800, Australia.
authors.forename: Paul
authors.initials: P
authors.lastname: Sunnucks
authors.affiliation: School of Biological Sciences, Monash University, Melbourne, VIC, 3800, Australia.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Pavlova
authors.affiliation: Department of Sciences, Museum Victoria, Melbourne, VIC, 3001, Australia.
authors.forename: Jane
authors.initials: J
authors.lastname: Melville
mesh_terms: D000483:Alleles; D000818:Animals; D001315:Australia; D001483:Base Sequence; D019143:Evolution, Molecular; D005091:Exons / Q000235:genetics; D005802:Genes, MHC Class II; D056426:Genetic Loci; D000949:Histocompatibility Antigens Class II / Q000235:genetics*; D046109:Passeriformes / Q000235:genetics*; D010802:Phylogeny; D011110:Polymorphism, Genetic; D020308:Songbirds / Q000235:genetics*
chemical_list: D000949:Histocompatibility Antigens Class II
keywords: Accordion model; Birth-and-death model; Concerted evolution; Convergent evolution; Corvida; Gene duplication; Passerida; Trans-species polymorphism