Timeframe of speciation inferred from secondary contact zones in the European tree frog radiation (Hyla arborea group)

Background: Hybridization between incipient species is expected to become progressively limited as their genetic divergence increases and reproductive isolation proceeds. Amphibian radiations and their secondary contact zones are useful models to infer the timeframes of speciation, but empirical data from natural systems remains extremely scarce. Here we follow this approach in the European radiation of tree frogs (Hyla arborea group). We investigated a natural hybrid zone between two lineages (Hyla arborea and Hyla orientalis) of Mio-Pliocene divergence (~5 My) for comparison with other hybrid systems from this group. Results: We found concordant geographic distributions of nuclear and mitochondrial gene pools, and replicated narrow transitions (~30 km) across two independent transects, indicating an advanced state of reproductive isolation and potential local barriers to dispersal. This result parallels the situation between H. arborea and H. intermedia, which share the same amount of...研究背景：随着初期物种（incipient species）间的遗传分化（genetic divergence）不断增加、生殖隔离（reproductive isolation）逐步建立，二者之间的杂交（hybridization）预期会逐渐受到限制。两栖动物适应性辐射类群（amphibian radiations）及其次生接触带（secondary contact zones）是推断物种形成（speciation）时间框架的理想模型，但来自自然系统的实证数据（empirical data）仍极为匮乏。本研究针对欧洲树蛙（tree frogs）适应性辐射类群——树蛙属arborea群（Hyla arborea group）——采用该研究思路。我们对两个具有中新世-上新世（Mio-Pliocene）分化时间（约500万年）的支系（lineages）——欧洲树蛙（Hyla arborea）与东方雨蛙（Hyla orientalis）——之间的天然杂交带（hybrid zone）开展调查，以期与该类群的其他杂交系统进行对比分析。研究结果：本研究发现核基因库（nuclear gene pools）与线粒体基因库（mitochondrial gene pools）的地理分布具有一致性，且在两条独立样带（transects）上均重复观测到宽度约30公里的狭窄遗传渐变带，这表明二者的生殖隔离已处于较高水平，且存在潜在的局部扩散（dispersal）障碍。该结果与欧洲树蛙（Hyla arborea）与中间雨蛙（Hyla intermedia）之间的情况相似，二者具有相同程度的……