Testing the geographical dimensions of genetic diversity following range expansion in a North American snake

Spatial and demographic expansion can alter patterns of genetic variation and have predictable spatial and temporal consequences. Two-dimensional range expansion should result in genetic variation that is correlated with the geographical axis of expansion. Notably, populations across the range of a geographically widespread species may asynchronously experience expansion and contraction events. We tested for the genetic consequences of range expansion in the flat-headed snake, Tantilla gracilis, which inhabits the North American Great Plains, which has few barriers to terrestrial dispersal. We sequenced mitochondrial (mtDNA) and nuclear DNA from across the range of T. gracilis, inferred phylogenies, and constructed haplotype networks. We observed multiple geographically circumscribed mtDNA clades that varied greatly in their spatial extent, but little range-wide variation in nuclear DNA. Mitochondrial sequence variation was negatively associated with latitude across their geographic range, the expected pattern of northward expansion. Finally, the three largest mtDNA clades exhibited varying demographic patterns, with only one consistent with recent expansion. Despite substantial mtDNA cladogenesis, the predicted signature of northern range expansion was still detectable in T. gracilis. Our results bolster the notion that Post-Pleistocene range expansion was a dominant force shaping the genetic diversity of vertebrates in central North America.

空间扩张与种群扩张可改变遗传变异模式，并产生可预测的空间与时间维度效应。二维范围扩张应会产生与扩张地理轴相关的遗传变异模式。值得注意的是，地理分布广泛的物种种群，其扩张与收缩事件可能异步发生。我们以栖息于北美大平原（该区域陆地扩散障碍极少）的细尾平头蛇（Tantilla gracilis）为研究对象，探究其分布范围扩张带来的遗传效应。我们对细尾平头蛇整个分布范围内的线粒体DNA（mtDNA）与核DNA进行了测序，构建系统发育树并搭建单倍型网络。研究发现多个受地理范围限定的线粒体DNA进化枝，其空间分布范围差异显著，但核DNA在全分布范围内的变异程度极低。在其整个地理分布范围内，线粒体序列变异与纬度呈负相关，这符合向北扩张的预期演化模式。最后，三个最大的线粒体DNA进化枝呈现出不同的种群动态模式，其中仅一个符合近期种群扩张的特征。尽管线粒体DNA存在显著的进化枝分化，细尾平头蛇中仍可检测到北部范围扩张的预期遗传信号。本研究结果支持‘更新世后的范围扩张是塑造北美中部脊椎动物遗传多样性的主导力量’这一学术观点。