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 any system.

物种的地理分布范围边界通常并非由显性扩散障碍所划定。分布区边缘的低质生境，可通过阻碍物种向外扩散，或是降低由遗传多样性（genetic diversity）下降所引发的适应潜力（adaptive potential），来阻止分布范围扩张。本研究针对美国东部的特有钝口螈（Ambystoma barbouri）全地理分布范围内限制基因流（gene flow）的生境变量进行了识别，并检验了由这些变量所产生的阻力增强效应，是否在分布区边缘形成了隐蔽扩散障碍（cryptic dispersal barriers）。研究团队首先利用多态性微卫星位点（microsatellite loci），识别出了三个被俄亥俄河与肯塔基河分隔的遗传聚类群（genetic clusters）。随后结合景观遗传学分析（landscape genetic analyses）与广义相异性模型（generalized dissimilarity modeling），对两类变量进行了分类：一是在钝口螈全地理分布范围内，限制各遗传聚类群内基因流的变量；二是在分布区外围愈发常见的变量。石灰岩可利用性的下降与生长季降水量的增加，与全分布范围内的高基因流阻力呈显著相关，且这两类特征在物种分布区边缘的出现频率均有所提升。但其他景观变量对于解释分布区内不同区域的基因流速率差异更为关键，例如南部区域的年均温与无霜期升高，与北部区域的生长季降水量变化所发挥的调控作用各不相同。综合来看，上述结果表明，同时存在全分布范围尺度与区域特异性的隐蔽生境障碍，阻碍了物种地理分布范围的扩张。物种的地理分布边界大概率由一系列生态与进化因子共同调控，而本研究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，以获取更多针对性的研究见解。