Introgressive hybridization: brown bears as vectors for polar bear alleles

Abstract

The dynamics and consequences of introgression can inform about numerous evolutionary processes. Biologists have therefore long been interested in hybridization. One challenge, however, lies in the identification of nonadmixed genotypes that can serve as a baseline for accurate quantification of admixture. In this issue of Molecular Ecology, Cahill et al. (2015) analyse a genomic data set of 28 polar bears, eight brown bears and one American black bear. Polar bear alleles are found to be introgressed into brown bears not only near a previously identified admixture zone on the Alaskan Admiralty, Baranof and Chichagof (ABC) Islands, but also far into the North American mainland. Elegantly contrasting admixture levels at autosomal and X chromosomal markers, Cahill and colleagues infer that male-biased dispersal has spread these introgressed alleles away from the Late Pleistocene contact zone. Compared to a previous study on the ABC Island population in which an Alaskan brown bear served as a putatively admixture-free reference, Cahill et al. (2015) utilize a newly sequenced Swedish brown bear as admixture baseline. This approach reveals that brown bears have been impacted by introgression from polar bears to a larger extent (up to 8.8% of their genome), than previously known, including the bear that had previously served as admixture baseline. No evidence for introgression of brown bear into polar bear is found, which the authors argue could be a consequence of selection. Besides adding new exciting pieces to the puzzle of polar/brown bear evolutionary history, the study by Cahill and colleagues highlights that wildlife genomics is moving from analysing single genomes towards a landscape genomics approach.

Lions and brown bears colonized North America in multiple synchronous waves of dispersal across the Bering Land Bridge

“…The same substitution model settings were used as described above, but the alignments were combined in a single analysis, with a separate tree estimated simultaneously for each taxon. Clade 2 brown bears were excluded from the analysis due to lack of sampling, and the introgressed nature with polar bears resulting in a complicated evolutionary history of the clade (Cahill et al, 2013(Cahill et al, , 2015(Cahill et al, , 2018Edwards et al, 2011;Hailer, 2015;Hailer & Welch, 2016;Miller et al, 2012). Each tip was assigned a binary phylogeographical character (Eurasia vs. North America), and the rate of evolution of this character was estimated directly from the data.…”

Ancient genomes reveal hybridisation between extinct short-faced bears and the extant spectacled bear (Tremarctos ornatus)

“…15,[16][17][18][19]. Importantly, discordance between mitochondrial and nuclear loci has been previously noted in bears, and has been attributed to a combination of stochastic processes and the rapid evolution of bears [13], as well as hybridisation between species [13,14,[20][21][22][23][24][25]. To further resolve the evolutionary history of short-faced bears, we applied ancient DNA techniques to retrieve and analyse whole genome data from both Arctodus and University of Kansas; KU 31956).…”

Evolutionary History of Polar and Brown Bears

“…Male gene flow has subsequently diluted the original insular polar bear alleles, with higher swamping (lower percentage of polar bear alleles remaining) at loci that show a malebiased inheritance (autosomal > X > mtDNA) (Cahill et al 2013). As subsequently shown by Cahill et al (2015) who sequenced two additional ABC-Island and one European brown bear, brown bears have effectively acted as vectors for polar bear alleles (Hailer 2015): malebiased dispersal by brown bears has carried introgressed polar bear alleles far away from the original zone of admixture into brown bear populations resident on the adjacent North American mainland Liu et al (2014) reported the deep sequencing and de novo assembly of another polar bear genome. The authors used allele frequency and linkage information from 89 sequenced bear genomes for an in-depth demographic modelling of polar/brown bear evolutionary history [see next section for details on aspects of genomic adaptation signals].…”

“…The observed pattern of (1) mtDNA paraphyly of brown bears, contrasted by (2) autosomal multilocus species trees (despite introgression at some autosomal loci; see above) and Y-chromosomal phylogenies showing reciprocal monophyly of polar and brown bears, is still lacking a definite explanation (Edwards et al 2011;Hailer 2015). There are at least three potential explanations for the overall pattern.…”