Genetic Differentiation, Isolation-by-Distance, and Metapopulation Dynamics of the Arizona Treefrog (Hyla wrightorum) in an Isolated Portion of Its Range

Population attributes such as diversity, connectivity, and structure are important components of understanding species persistence and vulnerability to extinction. Hyla wrightorum, the Arizona treefrog, is native to the southwestern United States and Mexico, and an isolated group of populations exists in the Huachuca Mountains and Canelo Hills (HMCH) of southeastern Arizona, USA. Due to concerns about declining observations of the species within the isolated HMCH portion of its range, the HMCH group is currently a candidate for federal protection under the U.S. Endangered Species Act. We present results of a genetic study examining population diversity, structure, and connectivity within the HMCH region. We sampled DNA from H. wrightorum larvae and adults from ten distinct locations, 8 of which were breeding sites and 4 of which were previously undescribed localities for the species. We developed and genotyped 17 polymorphic microsatellite loci and quantified genetic diversity, population differentiation, and landscape influences on population genetic structure. We found evidence of larger than expected effective population sizes, significant genetic differentiation between populations, and evidence of distance being the primary driver of genetic structure of populations with some influence of slope and canopy cover. We found little evidence of recent genetic bottlenecks, and individual-based analyses indicate admixture between populations despite significant genetic differentiation. These patterns may indicate that the breeding sites within the Huachuca Mountains constitute a metapopulation. We suggest that the HMCH region may contain larger and more connected breeding populations than previously understood, but the dynamics of this system and the limited geographic extent of the HMCH group justify current concern for the persistence of the species in this region. Efforts to ensure availability of high-quality breeding habitats and control for local threats such as effects of invasive predators may be critical to the persistence of these unique populations of H. wrightorum.

种群的多样性、连通性与结构等种群属性，是理解物种存续能力及其灭绝脆弱性的核心要素。亚利桑那树蛙（Hyla wrightorum）原产于美国西南部与墨西哥，其一处孤立种群分布于美国亚利桑那州东南部的瓦楚卡山脉与卡内洛山地区（以下简称HMCH）。鉴于该物种在其分布范围的孤立HMCH区域内的观测记录呈下降趋势，HMCH种群目前已被纳入美国《濒危物种法案》下的联邦保护候选名录。本研究围绕HMCH区域内的种群多样性、种群结构及连通性展开遗传研究，现将相关结果报告如下：我们从10个不同采样点的亚利桑那树蛙幼体与成体中获取了DNA样本，其中8个采样点为繁殖位点，4个为该物种此前未被记录的分布地。我们开发并完成了17个多态性微卫星（microsatellite）位点的基因分型，并量化了种群遗传多样性、种群分化程度以及景观因子对种群遗传结构的影响。研究发现，该类群的有效种群规模大于预期值，种群间存在显著遗传分化；同时证实地理距离是塑造种群遗传结构的核心驱动因子，坡度与冠层覆盖度亦存在一定影响。未检测到近期遗传瓶颈的相关证据；基于个体的分析结果显示，尽管种群间存在显著遗传分化，但仍存在种群间的基因混合（admixture）现象。上述结果表明，瓦楚卡山脉内的各繁殖位点共同构成了一个集合种群（metapopulation）。我们认为，HMCH区域内的繁殖种群规模可能较此前认知的更大，且种群连通性更强，但该系统的种群动态特征以及HMCH种群有限的地理分布范围，仍需引发对该区域内该物种存续状况的持续关注。为保障高质量繁殖栖息地的可用性，并防控入侵捕食者等本地威胁所采取的相关措施，对该独特的亚利桑那树蛙种群的存续至关重要。