Data from: Divergence before and after the isolation of islands: phylogeography of the Bradybaena land snails on the Ryukyu Islands of Japan

Aim: Vicariance events have been proposed as a major source of lineage divergence on continental islands, whereas dispersal events followed by isolation have been proposed as the major cause on oceanic islands. However, organisms on continental islands may include taxa with characteristics similar to those on oceanic islands. Lineage divergence unassociated with the geological events that separated islands may also have occurred. This study addresses these possibilities through morphological and molecular phylogeographic analyses of land snails (Bradybaena) on continental islands. Location: Samples were mainly collected on the Ryukyu and Izu Islands in Japan Methods: Molecular phylogenetic analyses using mtDNA (16S rRNA + COI) and nDNA (18S rRNA + ITS1 + 5.8S rRNA + ITS2 + 28S rRNA) were conducted for 225 individuals comprising B. circulus, B. phaeogramma, and four of its subspecies endemic to the Ryukyu Islands. Shell and genital morphologies were also examined using qualitative and statistical methods. Results: The Bradybaena populations examined were phylogenetically classified into six major clades whose divergence occurred during either Miocene-Pliocene or Pliocene-Pleistocene. Although phylogeographic patterns estimated on the basis of the 16S rRNA gene support the vicariance hypothesis for the origin of the major clades of Bradybaena, the phylogeographic patterns estimated by COI showed that these major clades resulted from diversification before the archipelagos separated from the continent. Both of these phylogeographic patterns suggest that genetic divergence occurred by dispersal from the southern to northern islands after divergence of major clades. The inferred phylogenies document parallel morphological evolution, yielding phylogenetic, morphological, and taxonomic incongruences that created mosaic geographical patterns in the distribution of nominal taxa. Main conclusions: Lineage diversification in endemic species on continental islands can occur before archipelago separation or subsequently via dispersal, as is seen in species on oceanic islands. Mechanisms other than vicariance should also be considered when estimating processes that created geographical patterns of genetic variations on continental islands.

研究目标：学界曾提出，隔离分化（vicariance）事件是大陆岛（continental island）类群谱系分化的主要驱动因素，而远洋岛（oceanic island）的类群谱系分化则主要由扩散事件后发生隔离所致。然而，大陆岛上的生物类群可能具备与远洋岛类群相似的特征，且可能存在与岛屿分离的地质事件无关的谱系分化。本研究以大陆岛陆生蜗牛巴蜗牛属（Bradybaena）为研究对象，通过形态学与分子系统地理学分析，对上述可能性展开探讨。
研究区域：样本主要采集自日本的琉球群岛与伊豆诸岛。
研究方法：针对225份样本（涵盖琉球群岛特有物种圆顶巴蜗牛（Bradybaena circulus）、暗纹巴蜗牛（B. phaeogramma）及其4个亚种），开展基于线粒体DNA（mtDNA：16S rRNA + COI）与核DNA（nDNA：18S rRNA + ITS1 + 5.8S rRNA + ITS2 + 28S rRNA）的分子系统发育分析。同时采用定性与统计方法，对贝壳与生殖系统的形态特征进行检视。
研究结果：所分析的巴蜗牛种群在系统发育上可划分为6个主要演化支（clade），其分化时间集中于中新世-上新世或上新世-更新世时期。尽管基于16S rRNA基因推导的系统地理学格局支持巴蜗牛主要演化支起源于隔离分化假说，但基于COI基因的分析结果显示，这些主要演化支的分化发生在群岛与大陆分离之前。两类系统地理学分析结果均表明，在主要演化支分化完成后，遗传分化通过从南岛向北岛的扩散事件得以形成。系统发育推断结果证实存在平行形态演化现象，进而产生了系统发育、形态与分类学之间的不一致性，最终导致有效类群的分布呈现镶嵌式地理格局。
主要结论：大陆岛特有物种的谱系分化，既可能发生在群岛与大陆分离之前，也可如同远洋岛物种一般，在后续通过扩散事件完成。在解析大陆岛遗传变异地理格局的形成过程时，除隔离分化机制外，其他驱动机制也应纳入考量范畴。