Data from: Genetic reconstruction of a bullfrog invasion to elucidate vectors of introduction and secondary spread

Reconstructing historical colonization pathways of an invasive species is critical for uncovering factors that determine invasion success and for designing management strategies. The American bullfrog (Lithobates catesbeianus) is endemic to eastern North America, but now has a global distribution and is considered to be one of the worst invaders in the world. In Montana, several introduced populations have been reported, but little is known of their sources and vectors of introduction and secondary spread. We evaluated the genetic composition of introduced populations at local (Yellowstone River floodplain) and regional (Montana and Wyoming) scales in contrast to native range populations. Our objectives were to (1) estimate the number of introductions, (2) identify probable native sources, (3) evaluate genetic variation relative to sources, and (4) characterize properties of local- and regional-scale spread. We sequenced 937 bp of the mitochondrial cytochrome b locus in 395 tadpoles collected along 100 km of the Yellowstone River, from three additional sites in MT and a proximate site in WY. Pairwise ΦST revealed high divergence among nonnative populations, suggesting at least four independent introductions into MT from diverse sources. Three cyt b haplotypes were identical to native haplotypes distributed across the Midwest and Great Lakes regions, and AMOVA confirmed the western native region as a likely source. While haplotype (Hd = 0.69) and nucleotide diversity (π = 0.005) were low in introduced bullfrogs, the levels of diversity did not differ significantly from source populations. In the Yellowstone, two identified haplotypes implied few introduction vectors and a significant relationship between genetic and river distance was found. Evidence for multiple invasions and lack of subsequent regional spread emphasizes the importance of enforcing legislation prohibiting bullfrog importation and the need for continuing public education to prevent transport of bullfrogs in MT. More broadly, this study demonstrates how genetic approaches can reveal key properties of a biological invasion to inform management strategies.

解析入侵物种的历史定殖路径，对于揭示决定其入侵成功的关键因子、制定科学防控策略均具有重要意义。美洲牛蛙（Lithobates catesbeianus）原分布于北美东部，现已扩散至全球范围，被列为全球最具危害性的入侵物种之一。美国蒙大拿州已有多例美洲牛蛙引入种群的报道，但目前对其引入来源、传播媒介以及二次扩散路径仍知之甚少。本研究以土著种群作为对照，分别在局域（黄石河泛滥平原）和区域（蒙大拿州与怀俄明州）尺度上对引入种群的遗传组成展开评估。本研究的目标为：（1）估算美洲牛蛙的引入次数；（2）确定其最可能的土著来源种群；（3）对比分析引入种群与来源种群的遗传变异情况；（4）解析局域与区域尺度下的扩散特征。本研究对采自黄石河沿岸100公里河段、蒙大拿州另外3个采样点以及怀俄明州邻近1个采样点的395只蝌蚪样本，进行了线粒体细胞色素b（cytochrome b）基因937个碱基对（bp）片段的测序。成对群体遗传分化指数（ΦST）分析显示，非土著种群间存在显著遗传分化，表明蒙大拿州的美洲牛蛙种群至少存在4次来自不同源地的独立引入事件。3个细胞色素b单倍型与分布于北美中西部及五大湖区域的土著单倍型完全一致；分子方差分析（AMOVA）结果证实，北美西部土著种群为本次引入事件的潜在来源。尽管引入种群的单倍型多样性指数（Hd = 0.69）与核苷酸多样性指数（π = 0.005）均处于较低水平，但其多样性水平与来源种群并无显著差异。在黄石河区域内，仅检测到2种单倍型，表明其引入媒介较为单一，且遗传距离与河流空间距离之间存在显著的相关性。研究证实蒙大拿州存在多次独立入侵事件且未发生后续的区域扩散，这凸显了严格执行禁止美洲牛蛙进口的相关立法，以及持续开展公众教育以防范牛蛙人为运输的必要性。从更广泛的意义上来说，本研究阐明了遗传分析方法如何揭示生物入侵的关键特征，从而为入侵物种防控策略的制定提供科学依据。