• A Comparative Analysis Of Mhc Genetic Diversity At The Class Ii Loci In Some Arctic Mammalian Species

      Wei, Zhengyu; Happ, George (2002)
      The genetic diversity at the Major Histocompatibility Complex (MHC) class II loci in some arctic mammalian species, musk ox, moose, caribou, and bears, have been characterized. The general objective of this study was to broaden the knowledge of the MHC polymorphism, selection, evolution and function in natural populations of arctic mammals. Allelic variation was assessed by analysis of MHC class II DR and DQ loci at exon 2 region. Sequences were amplified via the polymerase chain reaction (PCR), followed by either DNA sequencing after cloning of the PCR products or single-stranded conformation polymorphism (SSCP) analysis and sequencing. Monomorphism was observed at DRA, DRB, and DQA loci in both musk ox and moose, but relatively high polymorphism was observed at DQB locus. For the first time, four DQB alleles and one DQA allele were found identical in these two distantly related species which split approximately 23 million years ago, indicating stringent trans-species polymorphism. Both DRB and DQB seem to be functional by analyzing their cDNA expression. An intermediate level of MHC polymorphism at DRB locus was found in caribou and reindeer. Phylogenetic analysis of cervid DRB alleles indicated that all reindeer and caribou DRB alleles were from a monophyletic lineage, implying an ancient bottleneck in R. tarandus. High polymorphism at the DRB locus in polar bear was also observed. Four DRB alleles were found to be shared by polar bear and dog. The trans-species polymorphism of the shared alleles may have been persistent for 10 to 15 million years. Nine DQB alleles rather than two DRB alleles were also found in a pure domestic dog lineage of Doberman pinschers. These data imply that selection pressure may vary among MHC loci. In summary, the general level of MHC polymorphism at the class II loci is lower in herbivores (musk ox, moose, and caribou) than carnivores (polar bear). Biased selection may be applied on DQB locus. Stringent trans-species polymorphism between two distantly related species may be the result of persistent selection by shared parasites in the same environment.
    • The takin and muskox: A molecular and ecological evaluation of relationship

      Groves, Pamela; White, Robert G. (1995)
      This research clarifies the classification of and relationship between the takin (Budorcas taxicolor) and muskox (Ovibos moschatus). Although both are social ungulates of similar body size, takins live in dense mountainous habitats at temperate latitudes in Asia, and muskoxen live in open arctic habitats in Alaska, Canada and Greenland. Morphological, paleontological and chromosome comparisons have supported a close relationship between these species and their classification within the tribe Ovibovini in the subfamily Caprinae. Previous studies, however, have not defined the genetic relationship of the takin and muskox. This project used ecological and molecular comparisons to evaluate these proposed relationships. Ecological studies in Shaanxi, China showed takins are generalists in their use of habitat and forage, but live in dense habitat in groups, ostensibly to avoid predation. Likewise, muskoxen live in groups and are generalists, but inhabit open landscapes. Some ecological and behavioral similarities appear to support the hypothesis of close relationship. In contrast, molecular studies using cytochrome b sequences from mitochondrial DNA (mtDNA) clearly separate the takin and muskox into distinct clades. Takins are more closely related to sheep (Ovis spp.) and muskoxen to the Asian goral (Nemorhaedus caudatus). Similarities between the takin and muskox appear due to convergent evolution as an adaptation to large body size. A broader comparison of cytochrome b sequences from 11 species of Caprinae supported the separation of takins and muskoxen. Unequal rates of evolution among the species precluded complete resolution of Caprinae relationships. To define differences between muskox subspecies and populations, sequences of the highly variable control region of mtDNA from 37 muskoxen were compared. Delineation of muskox subspecies is a critical issue due to the potential for interbreeding of indigenous Canadian (O. m. moschatus) and introduced Alaskan (O. m. wardi) populations of muskoxen as range expansion occurs. Variability between these populations was so low differences could not be detected with this comparison. I suggest a history of genetic bottlenecks has reduced genetic variability of muskoxen to low levels and neither populations nor subspecies can be defined from control region sequences.