Evolutionary history of Scinax treefrogs on land-bridge islands in south-eastern Brazil

AIM: We investigated how Pleistocene refugia and recent (~12,000 years ago) sea level incursions shaped genetic differentiation in mainland and island populations of the Scinax perpusillus treefrog group. LOCATION: Brazilian Atlantic Forest, São Paulo state, southeastern Brazil. METHODS: Using mitochondrial and microsatellite loci, we surveyed population structure and genetic diversity in Scinax from five mainland populations and three land-bridge islands to understand the roles of Pleistocene forest fragmentation and sea level incursions on genetic differentiation. We calculated metrics of relatedness and genetic diversity to assess whether island populations exhibit signatures of genetic drift and isolation. Two of the three island populations in this study have previously been described as new species based on a combination of distinct morphological and behavioural characters, thus we used the molecular datasets to determine whether phenotypic change is consistent with genetic differentiation. RESULTS: Our analyses recovered three distinct lineages or demes comprised of northern mainland São Paulo populations, southern mainland São Paulo populations, and one divergent island population. The two remaining island populations clustered with samples from adjacent mainland populations. Estimates of allelic richness were significantly lower, and estimates of relatedness were significantly higher, in island populations relative to their mainland counterparts. MAIN CONCLUSIONS: Fine-scale genetic structure across mainland populations indicates the possible existence of local refugia within São Paulo state, underscoring the small geographic scale at which populations diverge in this species rich region of the Atlantic Coastal Forest. Variation in genetic signatures across the three islands indicates that the populations experienced different demographic processes after marine incursions fragmented the distribution of Scinax perpusillus. Genetic signatures of inbreeding and drift in some island populations indicate that small population sizes coupled with strong ecological selection may be important evolutionary forces driving speciation on land-bridge islands.

研究目的：本研究旨在探讨更新世避难所与约12000年前的海侵事件，如何塑造了斯温蟾属树蛙类群（Scinax perpusillus）的大陆与岛屿种群的遗传分化。研究区域：巴西东南部圣保罗州境内的巴西大西洋森林。研究方法：本研究采用线粒体基因位点与微卫星位点，对5个大陆种群及3个陆桥岛屿种群的斯温蟾属树蛙类群开展种群结构与遗传多样性调查，以明晰更新世森林片段化与海侵事件对遗传分化的作用。我们计算了亲缘关系与遗传多样性相关指标，以评估岛屿种群是否存在遗传漂变与遗传隔离的信号。本研究涉及的3个岛屿种群中，有2个此前曾基于独特的形态与行为特征组合被描述为新物种，因此我们借助分子数据集验证表型变化是否与遗传分化相符。研究结果：本研究分析共识别出3个显著分化的支系或局域种群单元，分别对应圣保罗州北部大陆种群、圣保罗州南部大陆种群，以及1个高度分化的岛屿种群。剩余的2个岛屿种群则与邻近大陆种群的样本聚为一类。相较于对应大陆种群，岛屿种群的等位基因丰富度显著更低，而亲缘关系估计值则显著更高。主要结论：大陆种群间的精细尺度遗传结构表明，圣保罗州境内可能存在局域避难所，这凸显了大西洋沿岸森林这一物种丰富区域内，种群分化的地理尺度极小。3个岛屿种群的遗传信号存在差异，表明在海侵事件导致斯温蟾属树蛙类群的分布范围碎片化后，各岛屿种群经历了不同的种群动态过程。部分岛屿种群中检测到近交与遗传漂变的遗传信号，提示小种群规模与强烈的生态选择，可能是驱动陆桥岛屿物种形成的重要进化力量。