title: Major Histocompatibility Complex Genes Map to Two Chromosomes in an Evolutionarily Ancient Reptile, the Tuatara Sphenodon punctatus.
abstract: Major histocompatibility complex (MHC) genes are a central component of the vertebrate immune system and usually exist in a single genomic region. However, considerable differences in MHC organization and size exist between different vertebrate lineages. Reptiles occupy a key evolutionary position for understanding how variation in MHC structure evolved in vertebrates, but information on the structure of the MHC region in reptiles is limited. In this study, we investigate the organization and cytogenetic location of MHC genes in the tuatara (Sphenodon punctatus), the sole extant representative of the early-diverging reptilian order Rhynchocephalia. Sequencing and mapping of 12 clones containing class I and II MHC genes from a bacterial artificial chromosome library indicated that the core MHC region is located on chromosome 13q. However, duplication and translocation of MHC genes outside of the core region was evident, because additional class I MHC genes were located on chromosome 4p. We found a total of seven class I sequences and 11 class II β sequences, with evidence for duplication and pseudogenization of genes within the tuatara lineage. The tuatara MHC is characterized by high repeat content and low gene density compared with other species and we found no antigen processing or MHC framework genes on the MHC gene-containing clones. Our findings indicate substantial differences in MHC organization in tuatara compared with mammalian and avian MHCs and highlight the dynamic nature of the MHC. Further sequencing and annotation of tuatara and other reptile MHCs will determine if the tuatara MHC is representative of nonavian reptiles in general.
authors.affiliation: Allan Wilson Centre for Molecular Ecology and Evolution, School of Biological Sciences, Victoria University of Wellington, Wellington 6140, New Zealand Hilary.miller10@gmail.com.
authors.forename: Hilary C
authors.initials: HC
authors.lastname: Miller
authors.affiliation: Comparative Genomics Group, Research School of Biological Sciences, Australian National University, Canberra ACT 0200, Australia.
authors.forename: Denis
authors.initials: D
authors.lastname: O'Meally
authors.affiliation: Comparative Genomics Group, Research School of Biological Sciences, Australian National University, Canberra ACT 0200, Australia.
authors.forename: Tariq
authors.initials: T
authors.lastname: Ezaz
authors.affiliation: Molecular Genetics Program, Benaroya Research Institute at Virginia Mason, Seattle, Washington 98101.
authors.forename: Chris
authors.initials: C
authors.lastname: Amemiya
authors.affiliation: Comparative Genomics Group, Research School of Biological Sciences, Australian National University, Canberra ACT 0200, Australia.
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Marshall-Graves
authors.affiliation: Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138.
authors.forename: Scott
authors.initials: S
authors.lastname: Edwards
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002874:Chromosome Mapping; D002875:Chromosomes; D022202:Chromosomes, Artificial, Bacterial / Q000235:genetics; D055028:Comparative Genomic Hybridization; D008040:Genetic Linkage; D015395:Histocompatibility Antigens Class I / Q000145:classification / Q000235:genetics*; D000949:Histocompatibility Antigens Class II / Q000145:classification / Q000235:genetics*; D017404:In Situ Hybridization, Fluorescence; D010802:Phylogeny; D012104:Reptiles / Q000145:classification / Q000235:genetics*; D017422:Sequence Analysis, DNA
chemical_list: D015395:Histocompatibility Antigens Class I; D000949:Histocompatibility Antigens Class II
keywords: MHC class I; MHC class II; comparative genomics; reptilia