Collaborative Research: A Paleogenetic Survey of Late Quaternary Mammal Biodiversity in Southeast Alaska

更新世大约比现在早180万年，一直持续到大约1200万年前，也就是最后一个冰河时代结束的时候。许多大型哺乳动物在这一时期的恶劣气候条件下死亡，而其他大型哺乳动物则被限制在适宜居住的无冰地区或避难所。在上一次冰河时代的巅峰时期，冰层将新旧大陆分开，数千年来限制了生物穿越白令大陆桥的活动。然而，地质和生物学证据表明，北太平洋沿岸的避难所可能发挥了至关重要的作用，成为物种在新旧大陆之间迁徙的“垫脚石”，形成了北美重新殖民的早期冰后走廊。该项目以阿拉斯加东南部石灰岩洞穴中出土的无与伦比的脊椎动物骨骼为基础，将对过去50,000年来的黑熊和棕熊标本进行基因分析和放射性碳测年，以直接测试这些洞穴在上一个冰河时代的顶峰是否被熊占据。通过确保阿拉斯加东南部这些关键哺乳动物物种的准确物种确定，并提供与现有种群的进化联系，这项研究将阐明物种对气候变化的反应，并帮助限制阿拉斯加东南部可行的海岸避难所的范围和时间。因此，该项目的结果将为更深入地理解晚更新世期间跨柏林纪生物群的动态变化、该地区的多样性和生物地理历史以及其作为最后一次冰河时代期间和之后大陆之间交流走廊的重要性提供重要数据。该研究的一个重要成果是从威斯康星州冰川晚期和全新世占领阿拉斯加东南部的熊标本中恢复基因信息，并为人口统计事件的精确计时提供校准。该项目聚焦于棕熊(Ursus Arctos)和黑熊(Ursus Americanus)，这两种熊已经发展成为北半球系统地理研究的成熟系统，该项目将测试相同的熊种群是否在过去5万年里持续生活在该地区，或者它们是否在最后一次冰盛期之后重新定居，以及它们是否对冰后(现代)大陆种群做出了贡献。该项目将采用一种高度综合的方法，利用最先进的古代DNA分析(包括使用定制的生物素化RNA诱饵从整个线粒体基因组序列设计的定向杂交富集法)、放射性碳和分子测年、重建古代食谱和系统发育重建。这项研究将对了解北美过去和现在的哺乳动物多样性以及评估晚更新世气候变化对该地区生物多样性、殖民化和历史生物地理的影响具有广泛的意义。

The Pleistocene Epoch began approximately 1.8 million years before present and lasted until about 12 milennia ago, when the last Ice Age ended. Many large mammals succumbed to the harsh climatic conditions of this period, while others were confined to habitable ice-free areas, or refugia. During the peak of the last Ice Age, sheets of ice divided the Old and New Worlds, restricting movement of organisms across the Bering Land Bridge for thousands of years. Geological and biological evidence have suggested, however, that refugia along the North Pacific Coast may have played crucial roles as "stepping stones" for movements of species between the Old and New Worlds, forming an early postglacial corridor for the recolonization of North America. Building on an unparalleled vertebrate bone collection excavated from limestone caves in Southeast (SE) Alaska, this project will perform genetic analyses and radiocarbon dating of black and brown bear specimens from throughout the last 50,000 years to directly test if these caves were occupied by bears at the peak of the last Ice Age. By ensuring precise species determinations of these keystone mammal species in SE Alaska, and providing evolutionary links to extant populations, this research will elucidate species' responses to climate change and help circumscribe the extent and timing of viable coastal refugia in SE Alaska. As such, results from this project will provide important data for a deeper comprehension of the dynamic changes in trans-Beringian biota during the Late Pleistocene, the diversity and biogeographic history of the region, and its importance as a corridor for exchange between continents during and after the last Ice Age.An important outcome of this research is to recover genetic information from bear specimens that occupied SE Alaska during the late Wisconsin glaciation and into the Holocene, and provide calibrations for precise timings of demographic events. Focusing on brown bears (Ursus arctos) and black bears (Ursus americanus), which have developed into well-established systems for studies of Northern Hemisphere phylogeography, the project will test if the same bear populations inhabited this region continuously for the last 50,000 years, or if they were recolonized following the Last Glacial Maximum, and if they contributed to postglacial (modern) mainland populations. The project will take a highly integrative approach that leverages state-of-the-art ancient DNA analysis (including a targeted hybridization enrichment approach using custom-made biotinylated RNA baits designed from whole mitochondrial genome sequences), radiocarbon and molecular dating, reconstruction of paleodiets, and phylogenetic reconstruction. This research will have broad significance toward understanding past and present North American mammal diversity and for assessing the impact of late Pleistocene climate change on biotic diversification, colonization, and the historical biogeography of the region.