Reconstructing hotspots of genetic diversity from glacial refugia and subsequent dispersal in Italian common toads (Bufo bufo)

Genetic diversity feeds the evolutionary process and allows populations to adapt to environmental changes. However, we still lack a thorough understanding of why hotspots of genetic diversity are so 'hot'. Here, we analysed the relative contribution of bioclimatic stability and genetic admixture between divergent lineages in shaping spatial patterns of genetic diversity in the common toad Bufo bufo along the Italian peninsula. We combined population genetic, phylogeographic and species distribution modelling (SDM) approaches to map ancestral areas, glacial refugia, and secondary contact zones. We consistently identified three phylogeographic lineages, distributed in northern, central and southern Italy. These lineages expanded from their ancestral areas and established secondary contact zones, before the last interglacial. SDM identified widespread glacial refugia in peninsular Italy, sometimes located under the present-day sea-level. Generalized linear models indicated genetic admixture as the only significant predictor of the levels of population genetic diversity. Our results show that glacial refugia contributed to preserving both levels and patterns of genetic diversity across glacial-interglacial cycles, but not to their formation, and highlight a general principle emerging in Mediterranean species: higher levels of genetic diversity mark populations with substantial contributions from multiple genetic lineages, irrespective of the location of glacial refugia.

遗传多样性支撑进化过程，可使种群适应环境变化。然而，我们仍未透彻理解遗传多样性热点何以如此“热门”。本研究以意大利半岛的普通蟾蜍（Bufo bufo）为研究对象，剖析了生物气候稳定性与分化支系间的遗传混合对其遗传多样性空间格局的相对贡献。我们结合种群遗传学、系统地理学与物种分布模型（SDM）方法，绘制了祖先分布区、冰期避难所及次生接触带的空间分布格局。研究一致鉴定出三个系统地理支系，分别分布于意大利北部、中部与南部。这些支系从各自的祖先分布区扩散，并在末次间冰期前形成次生接触带。物种分布模型（SDM）分析显示，意大利半岛广泛分布冰期避难所，部分避难所位于当前海平面之下。广义线性模型结果表明，遗传混合是唯一可显著预测种群遗传多样性水平的因子。本研究结果显示，冰期避难所仅在冰期-间冰期循环中维持了遗传多样性的水平与格局，而非促成其形成；同时揭示了地中海物种共通的演化规律：无论冰期避难所的位置如何，接收多个遗传支系显著贡献的种群，其遗传多样性水平更高。