title: MHC class II pseudogene and genomic signature of a 32-kb cosmid in the house finch (Carpodacus mexicanus).
abstract: Large-scale sequencing studies in vertebrates have thus far focused primarily on the genomes of a few model organisms. Birds are of interest to genomics because of their much smaller and highly streamlined genomes compared to mammals. However, large-scale genetic work has been confined almost exclusively to the chicken; we know little about general aspects of genomes in nongame birds. This study examines the organization of a genomic region containing an Mhc class II B gene in a representative of another important lineage of the avian tree, the songbirds (Passeriformes). We used a shotgun sequencing approach to determine the sequence of a 32-kb cosmid insert containing a strongly hybridizing Mhc fragment from house finches (Carpodacus mexicanus). There were a total of three genes found on the cosmid clone, about the gene density expected for the mammalian Mhc: a class II Mhc beta-chain gene (Came-DAB1), a serine-threonine kinase, and a zinc finger motif. Frameshift mutations in both the second and third exons of Came-DAB1 and the unalignability of the gene after the third exon suggest that it is a nonfunctional pseudogene. In addition, the identifiable introns of Came-DAB1 are more than twice as large as those of chickens. Nucleotide diversity in the peptide-binding region of Came-DAB1 (Pi = 0.03) was much lower than polymorphic chicken and other functional Mhc genes but higher than the expected diversity for a neutral locus in birds, perhaps because of hitchhiking on a selected Mhc locus close by. The serine-threonine kinase gene is likely functional, whereas the zinc finger motif is likely nonfunctional. A paucity of long simple-sequence repeats and retroelements is consistent with emerging rules of chicken genomics, and a pictorial analysis of the "genomic signature" of this sequence, the first of its kind for birds, bears strong similarity to mammalian signatures, suggesting common higher-order structures in these homeothermic genomes. The house finch sequence is among a very few of its kind from nonmodel vertebrates and provides insight into the evolution of the avian Mhc and of avian genomes generally.
authors.affiliation: Department of Zoology, University of Washington, Seattle, Washington 98195 USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Hess
authors.forename: J
authors.initials: J
authors.lastname: Gasper
authors.forename: H E
authors.initials: HE
authors.lastname: Hoekstra
authors.forename: C E
authors.initials: CE
authors.lastname: Hill
authors.forename: S V
authors.initials: SV
authors.lastname: Edwards
mesh_terms: D000818:Animals; D030161:Avian Proteins; D001482:Base Composition; D001483:Base Sequence; D001717:Birds; D002645:Chickens; D003360:Cosmids / Q000235:genetics*; D019143:Evolution, Molecular; D005802:Genes, MHC Class II / Q000235:genetics*; D014644:Genetic Variation; D008969:Molecular Sequence Data; D005810:Multigene Family; D010802:Phylogeny; D011506:Proteins / Q000235:genetics; D011544:Pseudogenes / Q000235:genetics*; D012091:Repetitive Sequences, Nucleic Acid / Q000235:genetics; D016415:Sequence Alignment; D012689:Sequence Homology, Nucleic Acid; D020308:Songbirds / Q000235:genetics*
chemical_list: D030161:Avian Proteins; C491745:Came-DAB1 protein, Carpodacus mexicanus; D011506:Proteins