Variation In Mitochondrial DNA and Microsatellite DNA in Caribou (Rangifer Tarandus) in North America
Matthew A. Cronin, Michael D. Macneil, and John C. Patton. 2005. Variation In Mitochondrial DNA and Microsatellite DNA in Caribou (Rangifer Tarandus) in North America. Journal of Mammalogy, 86(3):495–505, 2005.
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ABSTRACT
Genetic variation of caribou (Rangifer tarandus) at 18 microsatellite DNA loci and the cytochrome-b gene of mitochondrial DNA (mtDNA) was quantified in 11 herds of 3 North American subspecies: Alaskan barren ground caribou (R. t. granti), Canadian barren ground caribou (R. t. groenlandicus), and woodland caribou (R. t. caribou). Phylogenetic analysis of 1,194 nucleotides of cytochrome-b sequence resulted in a clade of 52 genotypes in R. t. granti, R. t. groenlandicus, and in 1 herd of R. t. caribou, and a clade of 7 genotypes in R. t. caribou. mtDNA sequence divergence is approximately 1% between these clades and 0.3–0.6% within these clades. The subspecies do not have monophyletic mtDNA, but do have different frequencies of mtDNA genotypes. Microsatellite allele frequencies also are differentiated between the woodland (R. t. caribou) and barren ground (R. t. granti and R. t. groenlandicus) subspecies. An exception is the George River herd in Labrador, which is classified as R. t. caribou but has mtDNA and microsatellite allele frequencies intermediate between the other herds of R. t. caribou and R. t. groenlandicus. Within subspecies, there is relatively low differentiation of microsatellite allele frequencies and mtDNA genotypes among herds of R. t. granti and R. t. groenlandicus, and relatively high differentiation of microsatellite alleles and mtDNA genotypes among herds of R. t. caribou in 4 geographically separate areas in Canada. The extent of differentiation of mtDNA genotype frequencies and microsatellite allele frequencies within and among each subspecies reflects past and present gene flow among herds. Issues related to subspecies, populations, ecotypes, and herds are discussed.
Four extant subspecies of caribou (Rangifer tarandus) commonly are recognized in North America (Fig. 1): Alaskan barren ground caribou (R. t granti), Canadian barren ground caribou (R. t. groenlandicus), Peary caribou (R. t. pearyi), and woodland caribou (R. t. caribou—Banfield 1961; Bergerud 2000; Røed et al. 1991), although other classifications have been suggested (e.g., Geist 1998). Subspecies of caribou have been designated based on variation in morphology, habitat use, and behavior that may reflect adaptation to local conditions, sexual selection, or nongenetic environmental influences on phenotype (Bergerud 2000; Courtois et al. 2003; Cronin et al. 2003a; Geist 1987, 1998; Klein et al. 1987; Reimers 1993). However, it is generally agreed that subspecies designations should be based on phylogenetic relatedness (Avise and Ball 1990; Cronin et al. 2003b), and the phylogenetic relationships of the North American subspecies are not definitive. …
Within subspecies, caribou occur in herds or populations that have temporally and spatially variable levels of mixing and gene flow. Caribou herds generally are considered groups that share calving or winter ranges (Bergerud 2000; Skoog 1968; Zittlau et al. 2000), and populations are interbreeding groups that have limited gene flow with other groups (Courtois et al. 2003; Cronin et al. 2003b). …
Genetic relationships of caribou herds and populations have been used to identify management and conservation units, and to estimate the extent of immigration and emigration (Courtois et al. 2003; Zittlau et al. 2000). In some cases, there is differentiation of microsatellite DNA allele frequencies over small geographic scales (45–200 km) of herds of woodland caribou in Quebec (Courtois et al. 2003) and the Yukon Territory (Zittlau et al. 2000), and reindeer (R. t. platyrhynchus) on Svalbard Island, Norway (Coˆte´ et al. 2002). In contrast, there is limited differentiation of microsatellite allele frequencies of barren ground caribou (R. t. granti) herds across 1,000 km of the North Slope of the Brooks Range of Alaska (Cronin et al. 2003b).
In this paper, we further quantify the genetic relationships of 3 caribou subspecies (R. t. granti, R. t. groenlandicus, and R. t. caribou) including 11 herds in North America with 18 microsatellite loci and sequences of the mtDNA cytochromeb gene. mtDNA is maternally inherited and reflects female mediated gene flow and phylogeny, and microsatellites are biparentally inherited in the nuclear genome and reflect male and female-mediated gene flow. Our objectives were to assess the mtDNA phylogeny of the 3 subspecies and to compare frequencies of mtDNA genotypes and microsatellite alleles among the 3 subspecies and among herds within each subspecies in Alaska and Canada.
DISCUSSION
Several patterns of differentiation of mtDNA cytochrome b and microsatellite loci are apparent in North American caribou, and consistent with studies of other loci (Courtois et al. 2003; Flagstad and Røed 2003; Gravlund et al. 1998; Røed et al. 1991; Zittlau et al. 2000). First, there is limited genetic differentiation of the 4 herds of R. t. granti in northern Alaska. The Central Arctic herd and Porcupine River herd have a particularly low level of differentiation for both mtDNA and microsatellites (Fig. 3). …
This suggests that there is gene flow among these herds (Cronin et al. 2003b; Skoog 1968; Whitten and Cameron 1983). The Alaskan herds have different calving ranges but their breeding and winter ranges may overlap and there may be movement of individuals between herds (Bergerud et al. 1984; Skoog 1968). Telemetry data indicate high fidelity of female caribou to herds (Whitten and Cameron 1983), but there is little information on movements of males. Males of other mammal species have larger home ranges and disperse more than females, so male mediated gene flow may be substantial in the Alaskan caribou herds. It is important to note that molecular genetic data such as ours give indirect long-term estimates of gene flow, whereas field observations of movements give direct, short-term estimates of gene flow (Avise 2000:78; Slatkin 1987).
An additional observation is the occurrence in R. t. granti of an mtDNA genotype that is characteristic of domestic reindeer (R. t. tarandus) in Alaska (genotype R1). Reindeer were introduced to Alaska from Eurasia and the occurrence of this genotype in caribou probably reflects introgressive hybridization from domestic reindeer into wild caribou (Cronin et al. 1995, 2003b). …
The patterns of genetic differentiation of Rangifer we present provide some insights for intraspecific taxonomy of Rangifer. First, R. t. granti and R. t. groenlandicus have somewhat differentiated microsatellite and mtDNA genotype frequencies (Fig. 3), but do not have phylogenetically distinct mtDNA (Fig. 2). Both of these subspecies are considerably more differentiated from the woodland subspecies (R. t. caribou) for both microsatellite allele and mtDNA genotype frequencies (Fig. 3) and mtDNA sequence phylogeny (Fig. 2). This suggests that R. t. granti and R. t. groenlandicus appropriately could be placed into 1 subspecies, and R. t. caribou could be maintained as a separate subspecies. This is consistent with the classification of Geist (1998), based on morphology. In addition, as described previously, the sharing of an mtDNA genotype and similar microsatellite allele frequencies between the R. t. caribou in Labrador and R. t. groenlandicus indicate that there is not absolute differentiation of these subspecies as currently designated. …
