Genetic Relationships of Extant Brown Bears (Ursus arctos) and Polar Bears (Ursus maritimus)

Cronin, Matthew A.; MacNeil, M. D.

doi:10.1093/jhered/ess090

Cited by 16 publications

(19 citation statements)

References 53 publications

(78 reference statements)

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“…From analysis of molecular variance (AMOVA), we obtained estimates of the proportion of variation among all populations of 0.28 for brown and 0.16 for polar bears (supplementary tables S4 and S5, Supplementary Material online). This is consistent with results from autosomal microsatellite markers which show stronger population differentiation in brown than in polar bears (Cronin and MacNeil 2012).…”

Section: Y Chromosome Phylogeography Of Bearssupporting

confidence: 91%

“…Current hybridization levels, however, appear to be low (Cronin and MacNeil 2012;Hailer et al 2012). Our findings of species-specific groups of Y chromosome haplotypes and a lack of haplotype sharing among species revealed no signal of patrilineal introgression.…”

Section: Speciation and Introgressionmentioning

confidence: 47%

“…Polar bears exhibit low levels of population differentiation at biparentally inherited and mitochondrial markers throughout their range (Paetkau et al 1999;Cronin and MacNeil 2012;Miller et al 2012;Campagna et al 2013). Brown bears, in contrast, show considerable phylogeographic structuring at mitochondrial markers (Davison et al 2011;Edwards et al 2011;Hirata et al 2013;Keis et al 2013), and population structuring can also be discerned at biparentally inherited microsatellites (Paetkau et al 1997;Tammeleht et al 2010;Kopatz et al 2012).…”

Section: Introductionmentioning

confidence: 99%

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Brown and Polar Bear Y Chromosomes Reveal Extensive Male-Biased Gene Flow within Brother Lineages

Bidon

Janke

Fain

et al. 2014

Molecular Biology and Evolution

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Brown and polar bears have become prominent examples in phylogeography, but previous phylogeographic studies relied largely on maternally inherited mitochondrial DNA (mtDNA) or were geographically restricted. The male-specific Y chromosome, a natural counterpart to mtDNA, has remained underexplored. Although this paternally inherited chromosome is indispensable for comprehensive analyses of phylogeographic patterns, technical difficulties and low variability have hampered its application in most mammals. We developed 13 novel Y-chromosomal sequence and microsatellite markers from the polar bear genome and screened these in a broad geographic sample of 130 brown and polar bears. We also analyzed a 390-kb-long Y-chromosomal scaffold using sequencing data from published male ursine genomes. Y chromosome evidence support the emerging understanding that brown and polar bears started to diverge no later than the Middle Pleistocene. Contrary to mtDNA patterns, we found 1) brown and polar bears to be reciprocally monophyletic sister (or rather brother) lineages, without signals of introgression, 2) male-biased gene flow across continents and on phylogeographic time scales, and 3) male dispersal that links the Alaskan ABC islands population to mainland brown bears. Due to female philopatry, mtDNA provides a highly structured estimate of population differentiation, while male-biased gene flow is a homogenizing force for nuclear genetic variation. Our findings highlight the importance of analyzing both maternally and paternally inherited loci for a comprehensive view of phylogeographic history, and that mtDNA-based phylogeographic studies of many mammals should be reevaluated. Recent advances in sequencing technology render the analysis of Y-chromosomal variation feasible, even in nonmodel organisms.

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Section: Y Chromosome Phylogeography Of Bearssupporting

confidence: 91%

Section: Speciation and Introgressionmentioning

confidence: 47%

Section: Introductionmentioning

confidence: 99%

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Brown and Polar Bear Y Chromosomes Reveal Extensive Male-Biased Gene Flow within Brother Lineages

Bidon

Janke

Fain

et al. 2014

Molecular Biology and Evolution

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“…Still, this alone does not exclude the possibility of an introgression event. A similar case, where results from mtDNA and microsatellites depict different patterns, has been documented between two other carnivore sister species, the polar bear (Ursus maritimus) and the brown bear (Ursus arctos) (Cronin and MacNeil 2012;Hailer et al 2012). Despite each species having currently independent gene pools, interbreeding in the wild has been reported locally, and it is advocated that a past introgression event could have occurred owing to climatic oscillations and changes in their ranges (Cronin and MacNeil 2012;Hailer et al 2012).…”

Section: Assessment Of Phylogenetic Relationships Amongst North Africmentioning

confidence: 59%

Differentiation of North African foxes and population genetic dynamics in the desert—insights into the evolutionary history of two sister taxa, Vulpes rueppellii and Vulpes vulpes

et al. 2015

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The effects of Pleistocene glaciations on moulding biodiversity have been extensively investigated within temperate biomes, yet arid ecosystems are largely neglected. A clear example comes from North Africa and the successive range of expansion/contraction cycles of the Sahara desert. This study focuses on North African foxes (genus Vulpes), in particular two sister taxa, Vulpes rueppellii and Vulpes vulpes, but also Vulpes zerda and Vulpes pallida. A set of two mitochondrial markers (Cyt-b and D-loop) and 31-33 autosomal microsatellites were used to conduct phylogenetic and population analyses, as well as to investigate the possible occurrence of hybridisation events. Phylogenetic analysis revealed V. rueppellii to be more closely related to North African V. vulpes than the latter with Eurasian V. vulpes, along with the occurrence of two sub-clades of V. vulpes within the Maghreb. In contrast, microsatellite analysis identified V. rueppellii and V. vulpes as clearly separate entities, and no sign of population structure was observed for both species within North Africa. Both mitochondrial and nuclear markers separated North African and Eurasian V. vulpes in two distinct groups. We propose two explanatory scenarios, both influenced by past climatic shifts: (1) past introgression of V. vulpes mitochondrial genome into V. rueppellii and (2) V. rueppellii represents an arid ecotype of V. vulpes trapped in the Sahara during a humid/arid transition. The successive expansions/contractions of the Sahara were also likely responsible for the mitochondrial structure of North African V. vulpes. We unveil intriguing insights on the genetic structure of carnivore species in North Africa, suggesting that further integrative research is needed.

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“…A number of additional studies utilizing multilocus data from the nuclear genome have recently been conducted, all recovering a sister species relationship (monophyly) of polar and brown bears. Intron sequences, autosomal microsatellites, amplified fragment length polymorphism (AFLP) and single nucleotide polymorphism (SNP) markers analyzed in samples from across the ranges of polar and brown bears support the conclusion that the two species are not hybridizing extensively at present, and that their genomes are largely differentiated from each other (Hailer et al 2012;Miller et al 2012;Cronin & MacNeil 2012;Cronin et al 2013Cronin et al , 2014Liu et al 2014). A notable exception are brown bears from the Alaskan ABC islands, in which prominent signals of introgression are found (see below).…”

Section: Discovery Of a Pleistocene Polar Bear Jawbone Stimulates Newmentioning

confidence: 81%

Evolutionary History of Polar and Brown Bears

Hailer

Welch

2016

Encyclopedia of Life Sciences

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Taxonomists have long recognised polar and brown bears as separate species with distinct ecological niches and largely nonoverlapping ranges. Surprisingly, phylogenetic studies of maternally inherited mitochondrial DNA (mtDNA) found polar bears nested within brown bears, with an estimated divergence time of <170 000 years. This indicated an unusually rapid speciation and adaptation of polar bears. However, several recent studies of autosomal and Y‐chromosomal DNA have revisited these findings, giving independent perspectives of bear evolutionary history. Results show that polar bears cluster separately from brown bears, and divergence time estimates are older than those based on mtDNA, ranging from >300 000 to 4–5 million years. These studies confirm uniqueness of the polar bear lineage, provide more time for speciation and adaptation, and have uncovered numerous candidate genes for evolutionary adaptations. Several instances of introgressive hybridisation between polar and brown bears have been inferred, revealing trans‐species transmission of mtDNA and some nuclear loci. Key Concepts DNA sequences in conjunction with a calibration of the mutation rate (e.g. inclusion of a previously estimated mutation rate, geological information or inclusion of a radiocarbon‐dated ancient sample) can be used to estimate the timing of speciation between species. Differentially inherited loci can reveal different aspects of evolutionary history. The genome of polar bears contains a wealth of alleles that are not found in brown bears, and vice versa. Polar bears have passed through bottlenecks during their evolutionary history, leaving them much less genetically variable than brown bears. When analysing DNA sequences that have passed through the species boundary due to introgressive hybridisation, studies will obtain information about the hybridisation event rather than the (earlier) speciation event. Brown bears have acted as vectors for polar bear alleles, transporting introgressed genetic material far beyond the species' contact zones. Incomplete lineage sorting (see ‘Glossary’) complicates phylogenetic inferences among rapidly and/or recently diverged taxa. Lineage sorting takes on average four N e generations, which for many taxa can span at least several hundred thousand years (N e being the effective population size). The time needed for lineages to be reciprocally monophyletic can therefore take very long, even under complete reproductive isolation. Molecular studies have found signals of positive selection in polar bear genes that are involved in fat metabolism, energy production and cardiovascular function. These genes are exciting candidates that may help us better understand the genetic basis of polar bear adaptations to Arctic conditions.

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Genetic Relationships of Extant Brown Bears (Ursus arctos) and Polar Bears (Ursus maritimus)

Cited by 16 publications

References 53 publications

Brown and Polar Bear Y Chromosomes Reveal Extensive Male-Biased Gene Flow within Brother Lineages

Brown and Polar Bear Y Chromosomes Reveal Extensive Male-Biased Gene Flow within Brother Lineages

Differentiation of North African foxes and population genetic dynamics in the desert—insights into the evolutionary history of two sister taxa, Vulpes rueppellii and Vulpes vulpes

Evolutionary History of Polar and Brown Bears

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