Howard Hughes Medical Institute, University of California Santa Cruz, Santa Cruz, CA, USA.
Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, CA, USA.
Nat Ecol Evol. 2022 Jul;6(7):936-944. doi: 10.1038/s41559-022-01753-8. Epub 2022 Jun 16.
Polar bears (Ursus maritimus) and brown bears (Ursus arctos) are sister species possessing distinct physiological and behavioural adaptations that evolved over the last 500,000 years. However, comparative and population genomics analyses have revealed that several extant and extinct brown bear populations have relatively recent polar bear ancestry, probably as the result of geographically localized instances of gene flow from polar bears into brown bears. Here, we generate and analyse an approximate 20X paleogenome from an approximately 100,000-year-old polar bear that reveals a massive prehistoric admixture event, which is evident in the genomes of all living brown bears. This ancient admixture event was not visible from genomic data derived from living polar bears. Like more recent events, this massive admixture event mainly involved unidirectional gene flow from polar bears into brown bears and occurred as climate changes caused overlap in the ranges of the two species. These findings highlight the complex reticulate paths that evolution can take within a regime of radically shifting climate.
北极熊（Ursus maritimus）和棕熊（Ursus arctos）是姐妹物种，拥有独特的生理和行为适应性，这些适应性在过去的 50 万年中进化而来。然而，比较和群体基因组学分析表明，一些现存和已灭绝的棕熊种群具有相对较近的北极熊祖先，这可能是由于北极熊在地理上局部向棕熊的基因流动所致。在这里，我们从一只大约 10 万年前的北极熊中生成并分析了一个近似 20X 的古基因组，揭示了一个大规模的史前混合事件，这在所有现存的棕熊基因组中都很明显。这种古老的混合事件在来自现存北极熊的基因组数据中是不可见的。与更近的事件一样，这种大规模的混合事件主要涉及从北极熊到棕熊的单向基因流动，并且发生在气候变化导致两种物种的范围重叠的时候。这些发现强调了在急剧变化的气候条件下，进化可以采取的复杂网状路径。
