Abstract
Grizzly bears have recently become more common on the Arctic Islands in the Inuvialuit Settlement Region, concurrently with a period of environmental change. Over the last decade, grizzly bear – polar bear hybrids have been confirmed within this region, triggering extensive discussion and speculation regarding the impact of hybridization on the parent species. Through harvests, sightings, and captures, we document an increase in the presence of grizzly bears and combine field observations of hybrids with genetic analysis and parentage analysis to identify four first-generation (F1) hybrids and four offspring of F1 hybrids and grizzly bears (backcross-to-grizzly-bear individuals). We trace these eight hybrid individuals to a single female polar bear who mated with two grizzly bears. We sampled one of her mates on the sea ice in the High Arctic and deduced the genotype of the other from his five offspring. The two male grizzly bears are sires of both the F1 generation and the backcross-to-grizzly-bear generation. So what initially appeared to be a sudden spate of hybridization in the western Canadian Arctic originated with the unusual mating between three non-hybrid parents. The breakdown of species barriers may start with atypical mating preferences of select individuals; however, the story we present can be traced to a single female polar bear who, along with three of her known F1 offspring, has been killed.
Highlights
The close and complex evolutionary relationship between polar bears (Ursus maritimus) and brown bears (Ursus arctos) has been investigated intensively over the past decade (Hailer et al., 2012; Miller et al, 2012; Cahill et al, 2013, 2015; Cronin et al, 2013; Liu et al, 2014)
The second sighting was in the Wynniatt Bay area of Victoria Island, where the grizzly bear was on the sea ice with a bear that appeared to be a hybrid but was not sampled (Figs. 2, 3)
Comparing the 10 genotyped bears in the pedigree (Fig. 4) and the deduced genotype of BearX to the distributions produced by random sampling, we found that the presumed F1s and GBC1s all sat comfortably within the ranges expected for those ancestry states, while 10960 and X32854, the mating pair that gave rise to the original Sachs F1 (6505), were grouped with their parental species, outside the ranges of values observed for F1 and GBC1 genotypes (Fig. 5)
Summary
The close and complex evolutionary relationship between polar bears (Ursus maritimus) and brown bears (Ursus arctos) (hereafter referred to as grizzly bears) has been investigated intensively over the past decade (Hailer et al., 2012; Miller et al, 2012; Cahill et al, 2013, 2015; Cronin et al, 2013; Liu et al, 2014). Whereas no evidence has been found for the flow of grizzly bear genes into the polar bear genome on the time scale of 10 000 to 100 000 years, certain island populations of grizzly bears in southeastern Alaska have substantially introgressed genomes, with the polar bear component approaching 10% (Cahill et al, 2015). These studies by Hailer et al (2012) and Cahill et al (2013, 2015) provide strong evidence for introgression on an evolutionary time scale, but evidence for contemporary hybridization in the wild is lacking (Cronin and MacNeil, 2012, microsatellites; Cronin et al, 2013, amplified fragment length polymorphism and mtDNA; and Cronin et al, 2014, single nucleotide polymorphisms)
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