近日刊登在PNAS上的一篇文章中,研究者对北极熊和棕熊的基因组进行了相关研究,揭示出了古代杂交繁殖很可能与过去的气候变化事件有关,这提示当前的气候变化可能会威胁北极熊作为一个独特物种的地位。
Webb Miller及其同事收集了当代北极熊、棕熊和美洲黑熊的血液和组织样本。再加上提取自大约13万年到11万年前的北极熊的DNA,这些样本被用于产生大量的基因组序列数据。对这些数据的分析表明北极熊和棕熊在大约400万年到500万年前分离。这组作者进一步确定了来自阿拉斯加的亚历山大列岛的棕熊和北极熊共享了5%到10%的DNA,发现了在历史上的气候变化事件期间古代杂交繁殖的可能性。
此外,这组作者提出,北极熊对棕熊的一个长期的种群瓶颈可能解释在现代北极熊身上观察到的较低的遗传多样性。北冰洋当前朝着不断变暖发展的趋势可能迫使北极熊在陆地上待更长的时间并且增加它们在交配季节与棕熊的接触。这组作者说,遗传多样性低和杂交繁殖因此可能让现代北极熊种群对未来的气候和其他环境扰动变得极端脆弱。(生物谷Bioon.com)
doi:10.1073/pnas.1210506109
PMC:
PMID:
Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change
Webb Millera,1, Stephan C. Schustera,b,1, Andreanna J. Welchc, Aakrosh Ratana, Oscar C. Bedoya-Reinaa, Fangqing Zhaob,d, Hie Lim Kima, Richard C. Burhansa, Daniela I. Drautzb, Nicola E. Wittekindtb, Lynn P. Tomshoa, Enrique Ibarra-Laclettee, Luis Herrera-Estrellae,2, Elizabeth Peacockf, Sean Farleyg, George K. Sagef, Karyn Rodeh, Martyn Obbardi, Rafael Montiele, Lutz Bachmannj, Ólafur Ingólfssonk,l, Jon Aarsm, Thomas Mailundn, Øystein Wiigj, Sandra L. Talbotf, and Charlotte Lindqvistc,1,2
Polar bears (PBs) are superbly adapted to the extreme Arctic environment and have become emblematic of the threat to biodiversity from global climate change. Their divergence from the lower-latitude brown bear provides a textbook example of rapid evolution of distinct phenotypes. However, limited mitochondrial and nuclear DNA evidence conflicts in the timing of PB origin as well as placement of the species within versus sister to the brown bear lineage. We gathered extensive genomic sequence data from contemporary polar, brown, and American black bear samples, in addition to a 130,000- to 110,000-y old PB, to examine this problem from a genome-wide perspective. Nuclear DNA markers reflect a species tree consistent with expectation, showing polar and brown bears to be sister species. However, for the enigmatic brown bears native to Alaska's Alexander Archipelago, we estimate that not only their mitochondrial genome, but also 5–10% of their nuclear genome, is most closely related to PBs, indicating ancient admixture between the two species. Explicit admixture analyses are consistent with ancient splits among PBs, brown bears and black bears that were later followed by occasional admixture. We also provide paleodemographic estimates that suggest bear evolution has tracked key climate events, and that PB in particular experienced a prolonged and dramatic decline in its effective population size during the last ca. 500,000 years. We demonstrate that brown bears and PBs have had sufficiently independent evolutionary histories over the last 4–5 million years to leave imprints in the PB nuclear genome that likely are associated with ecological adaptation to the Arctic environment.