Evolutionary genetics of neutral and immune related loci in North American thinhorn sheep (Ovis dalli spp)

The regulation of genetic variation in natural populations is via both neutral and
selective processes. Signatures of the neutral processes of drift and gene flow can be
found within non-coding regions of the genome, while natural selection acts on
variation within coding genes that confer changes in fitness. The affects of neutral
and selective processes are examined in thinhorn sheep (Ovis daW spp), a rare
example of a North American large mammal that occupies most of its native range.
There are currently two recognised thinhorn subspecies (0. d. daW and 0. d. stone i),
the validity of which remains uncertain.
Microsatellites reveal significant genetic structure throughout the thinhorn species
range (FsT=0.16). At least eight regional genetic groups can be defined, the limits of
which are delineated by mountain range topology. Strong isolation-by-distance is
evident (r=0.75, P<O.OOI), suggesting limited dispersal and philopatry within the
species. Analysis of mitochondrial DNA reveals that sheep survived Pleistocene
glaciations in four refugia. In addition to the well known refugia of eastern Beringia
and areas south of the Laurentide and Cordilleran ice sheets I show evidence of two
smaller refugia, providing support for the presence of the disputed 'ice-free corridor'
through eastern British Columbia. MtDNA also reveals a genetic signal confirming
the past hybridization of thinhorn and bighorn sheep.
Patterns of variation in linked microsatellite loci show no evidence of natural
selection acting on three genes involved in the thinhorn immune region. Although
Watterson tests suggest balancing selection is acting in all genes, evidence for
selection is confounded by popUlation structure. Concordantly, the translated coding
sequences of thinhorn interferon gamma, natural resistance associated macrophage
protein and prion protein have low genetic diversity. In contrast, the major
histocompatibility complex locus DRB3 shows significant evidence of
overdominance through both an excess of nonsynonymous substitution and transspecies
polymorphism.