The polar bear ( Ursus maritimus ) has become a symbol of the threat to biodiversity from climate change. Understanding polar bear evolutionary history may provide insights into apex carnivore responses and prospects during periods of extreme environmental perturbations. In recent years, genomic studies have examined bear speciation and population history, including evidence for ancient admixture between polar bears and brown bears ( Ursus arctos ). Here, we extend our earlier studies of a 130,000- to 115,000-y-old polar bear from the Svalbard Archipelago using a 10× coverage genome sequence and 10 new genomes of polar and brown bears from contemporary zones of overlap in northern Alaska. We demonstrate a dramatic decline in effective population size for this ancient polar bear’s lineage, followed by a modest increase just before its demise. A slightly higher genetic diversity in the ancient polar bear suggests a severe genetic erosion over a prolonged bottleneck in modern polar bears. Statistical fitting of data to alternative admixture graph scenarios favors at least one ancient introgression event from brown bears into the ancestor of polar bears, possibly dating back over 150,000 y. Gene flow was likely bidirectional, but allelic transfer from brown into polar bear is the strongest detected signal, which contrasts with other published work. These findings may have implications for our understanding of climate change impacts: Polar bears, a specialist Arctic lineage, may not only have undergone severe genetic bottlenecks but also been the recipient of generalist, boreal genetic variants from brown bears during critical phases of Northern Hemisphere glacial oscillations.

中文翻译：

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从更新世北极熊基因组深入了解熊的进化

北极熊（滨熊) 已成为气候变化对生物多样性威胁的象征。了解北极熊的进化史可能有助于了解极端环境扰动期间顶级食肉动物的反应和前景。近年来，基因组研究检查了熊的物种形成和种群历史，包括北极熊和棕熊之间古老杂交的证据（熊属). 在这里，我们使用 10 倍覆盖基因组序列和 10 个来自阿拉斯加北部当代重叠区的北极熊和棕熊的新基因组，扩展了我们早期对斯瓦尔巴群岛一只 130,000 至 115,000 岁北极熊的研究。我们证明了这种古老北极熊谱系的有效种群规模急剧下降，随后在其消亡前适度增加。古代北极熊的遗传多样性略高表明现代北极熊的长期瓶颈存在严重的遗传侵蚀。数据与替代混合图情景的统计拟合有利于至少一次从棕熊到北极熊祖先的古老基因渗入事件，可能追溯到 150,000 多年前。基因流动可能是双向的，但是从棕色到北极熊的等位基因转移是检测到的最强信号，这与其他已发表的工作形成鲜明对比。这些发现可能对我们理解气候变化的影响具有重要意义：北极熊是一种专门的北极血统，它可能不仅经历了严重的遗传瓶颈，而且在北半球冰川的关键阶段接受了棕熊的通才、北方遗传变异。振荡。