Data from: An approach for identifying cryptic barriers to gene flow that limit species’ geographic ranges

Species’ geographic range limits are most often not demarcated by obvious dispersal barriers. Poor quality habitat at the edge of a species’ range can prevent range expansion by preventing outward migration or through reducing adaptive potential resulting from decreased genetic diversity. We identified habitat variables that constrain gene flow across the entire geographic range of an endemic salamander (Ambystoma barbouri) in the eastern United States, and we tested whether increased resistance resulting from these variables provides cryptic dispersal barriers at the range edges. Using polymorphic microsatellite loci, we first identified three genetic clusters that are separated by the Ohio and Kentucky rivers. Through a combination of landscape genetic analyses and generalized dissimilarity modeling, we then classified variables that (1), restrict gene flow in each of the genetic clusters across the geographic distribution of A. barbouri and (2), become more common towards the peripheries of the distribution. A decrease in limestone availability and an increase in growing season precipitation were correlated with high resistance to gene flow across the range, and both became more common at the edges of the species’ distribution. However, other landscape variables were more important for explaining variation in gene flow rates in different portions of the range, such as increased mean annual temperature and frost-free period in the south versus growing season precipitation in the north. Taken together, these results suggest that there are both range-wide and regionally specific cryptic habitat barriers preventing geographic range expansion. Species’ geographic range limits are likely governed by a set of ecological and evolutionary factors, and our landscape genetic approach could be applied to gain additional insight in many systems.

物种的地理分布范围界限往往并非由明显的扩散屏障所划定。分布范围边缘的劣质生境，可通过阻碍向外迁移，或是降低因遗传多样性下降带来的适应潜力，从而阻止分布范围扩张。我们以美国东部特有蝾螈（endemic salamander）巴氏钝口螈（Ambystoma barbouri）为研究对象，针对其完整地理分布范围，识别了限制基因流（gene flow）的生境变量，并检验了这些变量所产生的抗性增强效应，是否在该物种的分布范围边缘形成了隐秘的扩散屏障。利用多态性微卫星位点（polymorphic microsatellite loci），我们首先鉴定出三个遗传聚类群，这些聚类群由俄亥俄河与肯塔基河分隔开来。随后，我们结合景观遗传学分析（landscape genetic analyses）与广义差异模型（generalized dissimilarity modeling），对两类变量进行了分类：其一为限制巴氏钝口螈地理分布范围内各遗传聚类群基因流的变量；其二为在物种分布范围边缘愈发常见的变量。石灰岩可利用性下降与生长季降水量增加，与整个分布范围内的高基因流抗性呈显著相关，且这两种生境特征在物种分布边缘均更为常见。不过，其他景观变量对于解释分布区域不同区段的基因流速率差异更为关键，例如南部区域的年均温上升与无霜期延长，其对基因流的调控作用相较于北部区域的生长季降水量变化更为显著。综上，这些结果表明，既存在覆盖全分布范围的通用型隐秘生境屏障，也存在区域特异性的此类屏障，共同阻碍了该物种的地理分布范围扩张。物种的地理分布界限可能由一系列生态与进化因子共同调控，而我们所采用的景观遗传学研究方法，可被推广应用于更多生态系统，以获取更多有价值的研究洞见。