DISSERTATION RESEARCH: A hypothesis test for cryptic northern refugia in bitternut and shagbark hickory, with implications for migration and adaptation

在冰河时代的鼎盛时期，冰川覆盖了北美的大部分地区。温带森林在被称为冰川避难所的地区持续存在，比如美国南部的沿海平原。一些物种也可能存在于比以前认为的更北的地方，但这些神秘的北部避难所的证据是有争议的。这项拟议中的研究将测试两种山核桃物种是否在可能位于阿巴拉契亚山脉南部或奥扎克地区的神秘避难所中幸存下来。遗传变异将在美国东部两个物种的种群中进行评估，变异模式将与根据各种历史情景在计算机模拟下产生的模式进行统计比较。这将首次为神秘的北部难民存在的可能性提供信心。森林历史的这一方面对于理解过去树木种群如何迁移和适应气候变化非常重要。检测到的遗传变异模式将对林业和保护生物学产生影响，并作为未来研究的资源。该研究本身通过结合遗传分析和古地理重建促进了科学的进步，并且考虑到气候变化的潜在影响，它对国家福利具有重要意义；该项目还支持教育和多样性。末次冰期极大期（21.5 Ka），温带物种在南部避难所的生存是一个经典的系统地理学范式，然而，这一范式受到了新的但不确定的证据的挑战，这些证据表明，隐藏的北部避难所也存在。该研究将使用两个北美东部山核桃物种，利用计算机模拟和近似贝叶斯计算来分析单个核苷酸多态性（snp）的大型遗传数据集，以测试包含隐藏的北部避难所和南部避难所的空间显式人口统计学模型是否比仅包含传统南部避难所的模型更好地解释数据。广泛的SNP数据集也将允许对当地适应模式进行探索性分析。这个项目将是第一个评估任何物种的神秘北方避难所的统计支持，并将为未来的研究建立一个基因组资源。结果将有助于更好地理解过去对气候变化的反应，包括历史迁移率、适应模式和遗传多样性的地理分布。将在向通过实地工作建立的广泛的林务员和养护生物学家网络分发的小册子中强调与应用生物学的相关性。密歇根大学现有的一个项目也将作为一个平台，与当地高中生讨论该项目及其与气候变化的相关性。

During the height of the ice age, glaciers covered large portions of North America. Temperate forests persisted in areas known as glacial refugia, such as the coastal plains of the southern United States. Some species may also have existed much farther north than previously thought possible, but evidence for these cryptic northern refugia is controversial. The proposed research will test whether two hickory species survived glaciation in cryptic refugia, possibly located in the southern Appalachians or Ozarks. Genetic variation will be assessed in populations of the two species from across the eastern United States, and patterns of variation will be statistically compared to patterns generated under computer simulations according to a variety of historical scenarios. For the first time, this will provide a measure of confidence in how likely it is that cryptic northern refugia existed. This aspect of forest history is important for understanding how tree populations have migrated and adapted to climate change in the past. The patterns of genetic variation detected will have implications for forestry and conservation biology, and as a resource for future research. The research itself promotes the progress of science by combining genetic analysis and paleogeographical reconstruction, and it has implications for national welfare, given the potential impacts a changing climate; the project also supports education and diversity.The survival of temperate species in southern refugia during the Last Glacial Maximum (21.5 Ka) is a classic phylogeographic paradigm, yet this paradigm has been challenged by new but inconclusive evidence suggesting the additional existence of cryptic northern refugia. Using two eastern North American hickory species, the research will analyze a large genetic dataset of single nucleotide polymorphisms (SNPs) using computer simulations and Approximate Bayesian Computation to test whether spatially-explicit demographic models containing both cryptic northern refugia and southern refugia better explain the data than models containing only traditional southern refugia. The rangewide SNP dataset will also allow exploratory analysis of patterns of local adaptation. This project will be the first to assess statistical support for cryptic northern refugia in any species, and will establish a genomic resource for future studies. Results will aid in better understanding past responses to climate change, including historical migration rates, patterns of adaptation, and geographic distributions of genetic diversity. The relevance to applied biology will be highlighted in a brochure distributed to a wide network of foresters and conservation biologists established through fieldwork. An existing University of Michigan program will also serve as a platform for discussing the project and its relevance to climate change with local high school students.