Data from: Hybridization between two gartersnake species (Thamnophis) of conservation concern: A threat or an important natural interaction?

Distinguishing between hybrid zones formed by secondary contact versus parapatric divergence-with-gene-flow is an important challenge for understanding the interplay of geographic isolation and local adaptation in the origin of species. Similarly, distinguishing between natural hybrid zones and those that formed as a consequence of recent human activities has important conservation implications. Recent work has demonstrated the existence of a narrow hybrid zone between the plains gartersnake (Thamnophis radix) and Butler’s gartersnake (T. butleri) in the Great Lakes region of North America, raising questions about the history and conservation value of genetically admixed populations. Both taxa are of conservation concern, and it is not clear whether to regard hybridization as a threat or a natural interaction. Here we use phylogeographic and population genetic methods to assess the timescales of divergence and hybridization, and test for evidence that the hybrid zone is of recent origin. We assayed AFLP markers and ND2 mitochondrial DNA (mtDNA) sequences from T. radix, T. butleri, and the closely related short-headed gartersnake (T. brachystoma) throughout their North American ranges. We find shallow mtDNA divergence overall and high levels of variation within the contact zone. These patterns are inconsistent with recent contact of long-diverged taxa. It is not possible to distinguish true divergence-with-gene-flow from a long-term secondary contact zone, but we infer that the hybrid zone is a long-standing, natural interaction.区分由次生接触形成的杂交带与邻域伴基因流分歧（parapatric divergence-with-gene-flow）所产生的杂交带，是理解物种起源过程中地理隔离与局部适应相互作用的重要课题。同理，区分自然杂交带与近期人类活动诱导形成的杂交带，对生物保护工作具有重要意义。近期研究已证实，在北美五大湖区域存在一条狭窄的平原束带蛇（Thamnophis radix）与巴特勒束带蛇（T. butleri）之间的杂交带，这引发了关于遗传混合种群的演化历史与保护价值的诸多疑问。这两个类群均需受到保护关注，目前尚不明确应将二者的杂交行为视为一种威胁，还是自然的相互作用。本研究采用系统地理学与种群遗传学方法，评估分歧与杂交的时间尺度，并检验该杂交带是否为近期形成的相关证据。我们对北美分布范围内的平原束带蛇、巴特勒束带蛇以及近缘的短头束带蛇（T. brachystoma）进行了扩增片段长度多态性（AFLP）标记与ND2线粒体DNA（mtDNA）序列分析。研究发现整体线粒体DNA分歧程度较浅，而接触带内的变异水平较高。该结果与长期分歧类群的近期接触模式不符。目前无法区分真正的邻域伴基因流分歧与长期次生接触杂交带，但本研究推断该杂交带是一种长期存在的自然相互作用。