近日刊登在PNAS上的一篇文章中,，研究者對北極熊和棕熊的基因組進行了相關(guān)研究，揭示出了古代雜交繁殖很可能與過去的氣候變化事件有關(guān),，這提示當(dāng)前的氣候變化可能會威脅北極熊作為一個獨特物種的地位。
Webb Miller及其同事收集了當(dāng)代北極熊,、棕熊和美洲黑熊的血液和組織樣本,。再加上提取自大約13萬年到11萬年前的北極熊的DNA，這些樣本被用于產(chǎn)生大量的基因組序列數(shù)據(jù),。對這些數(shù)據(jù)的分析表明北極熊和棕熊在大約400萬年到500萬年前分離,。這組作者進一步確定了來自阿拉斯加的亞歷山大列島的棕熊和北極熊共享了5%到10%的DNA，發(fā)現(xiàn)了在歷史上的氣候變化事件期間古代雜交繁殖的可能性。
此外,，這組作者提出,，北極熊對棕熊的一個長期的種群瓶頸可能解釋在現(xiàn)代北極熊身上觀察到的較低的遺傳多樣性。北冰洋當(dāng)前朝著不斷變暖發(fā)展的趨勢可能迫使北極熊在陸地上待更長的時間并且增加它們在交配季節(jié)與棕熊的接觸,。這組作者說,，遺傳多樣性低和雜交繁殖因此可能讓現(xiàn)代北極熊種群對未來的氣候和其他環(huán)境擾動變得極端脆弱。（生物谷Bioon.com）
doi:10.1073/pnas.1210506109
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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.
