Unexpectedly Low Rangewide Population Genetic Structure of the Imperiled Eastern Box Turtle Terrapene c. carolina

Rangewide studies of genetic parameters can elucidate patterns and processes that operate only over large geographic scales. Herein, we present a rangewide population genetic assessment of the eastern box turtle Terrapene c. carolina, a species that is in steep decline across its range. To inform conservation planning for this species, we address the hypothesis that disruptions to demographic and movement parameters associated with the decline of the eastern box turtle has resulted in distinctive genetic signatures in the form of low genetic diversity, high population structuring, and decreased gene flow. We used microsatellite genotype data from (n = 799) individuals from across the species range to perform two Bayesian population assignment approaches, two methods for comparing historical and contemporary migration among populations, an evaluation of isolation by distance, and a method for detecting barriers to gene flow. Both Bayesian methods of population assignment indicated that there are two populations rangewide, both of which have maintained high levels of genetic diversity (HO = 0.756). Evidence of isolation by distance was detected in this species at a spatial scale of 300 – 500 km, and the Appalachian Mountains were identified as the primary barrier to gene flow across the species range. We also found evidence for historical but not contemporary migration between populations. Our prediction of many, highly structured populations across the range was not supported. This may point to cryptic contemporary gene flow, which might in turn be explained by the presence of rare transients in populations. However these data may be influenced by historical signatures of genetic connectivity because individuals of this species can be long-lived.全分布区范围的遗传参数研究，可阐明仅在大地理尺度下才会显现的种群格局与演化过程。本研究针对全分布区范围内的东部箱龟（Terrapene c. carolina）开展种群遗传学评估，该物种在其整个分布区内种群数量正急剧下降。为给该物种的保护规划提供科学依据，我们验证了如下假说：东部箱龟种群衰退相关的种群统计与移动参数遭到破坏，已导致其出现以遗传多样性偏低、种群结构分化显著以及基因流减弱为特征的独特遗传信号。我们利用覆盖该物种全分布区的799个个体（n=799）的微卫星（microsatellite）基因型数据，应用了两类贝叶斯种群归属分析方法、两种比较种群间历史与当代基因流的方法、距离隔离效应评估方法，以及一种基因流障碍检测方法。两种贝叶斯种群归属分析方法均显示，该物种全分布区范围内存在两个种群，且二者均维持了较高的遗传多样性（观测杂合度HO=0.756）。该物种在300-500公里的空间尺度上检测到距离隔离效应的证据，且阿巴拉契亚山脉被确定为横跨其全分布区的主要基因流障碍。我们同时发现了种群间存在历史基因流的证据，但未检测到当代基因流。我们此前提出的“全分布区存在大量高度分化种群”的预测并未得到支持。这一结果或暗示存在隐秘的当代基因流，而该现象可通过种群中存在稀有迁徙个体得到解释。不过，由于该物种个体寿命较长，其遗传连通性的历史信号可能会对当前数据产生干扰。