Chengdu Institute of Biology, Chinese Academy of Sciences, #9 of Section 4, Ren-Min-Nan Road, Wuhou District, Chengdu, 610041, Sichuan Province, China.
Key Laboratory of Southwest China Wildlife Resources Conservation (Ministry of Education), China West Normal University, Nanchong, 637009, Sichuan Province, China.
BMC Evol Biol. 2020 Aug 28;20(1):111. doi: 10.1186/s12862-020-01675-8.
The distribution of genetic diversity and the underlying processes are important for conservation planning but are unknown for most species and have not been well studied in many regions. In East Asia, the Sichuan Basin and surrounding mountains constitute an understudied region that exhibits a "ring" of high species richness overlapping the eastern edge of the global biodiversity hotspot Mountains of Southwest China. We examine the distributional history and genetic diversification of the Emei mustache toad Leptobrachium boringii, a typical "ring" element characterized by disjunct ranges in the mountains, by integrating time-calibrated gene tree, genetic variability, individual-level clustering, inference of population splitting and mixing from allele frequencies, and paleoclimatic suitability modeling.
The results reveal extensive range dynamics, including secondary contact after long-term isolation via westward dispersal accompanied by variability loss. They allow the proposal of a model that combines recurrent contractions caused by Quaternary climatic changes and some failed expansions under suitable conditions for explaining the shared disjunct distribution pattern. Providing exceptional low-elevation habitats in the hotspot area, the eastern edge harbors both long-term refugial and young immigrant populations. This finding and a synthesis of evidence from other taxa demonstrate that a certain contributor to biodiversity, one that preserves and receives low-elevation elements of the east in this case, can be significant for only a particular part of a hotspot. By clarifying the low variability of these refugial populations, we show that discordant mitochondrial estimates of diversity can be obtained for populations that experienced admixture, which would have unlikely left proportional immigrant alleles for each locus.
Dispersal after long-term isolation can explain much of the spatial distribution of genetic diversity in this species, while secondary contact and long-term persistence do not guarantee a large variation. The model for the formation of disjunct ranges may apply to many other taxa isolated in the mountains surrounding the Sichuan Basin. Furthermore, this study provides insights into the heterogeneous nature of hotspots and discordant variability obtained from genome-wide and mitochondrial data.
遗传多样性的分布及其潜在过程对于保护规划至关重要，但对于大多数物种而言，这些信息尚不清楚，并且在许多地区也没有得到充分研究。东亚的四川盆地及其周边山脉是一个研究不足的地区，这里存在一个“环”，其中高物种丰富度与全球生物多样性热点之一的中国西南山地的东缘重叠。我们通过整合时间校准的基因树、遗传变异性、个体水平聚类、基于等位基因频率的种群分裂和混合推断以及古气候适宜性建模，研究了四川湍蛙 Leptobrachium boringii 的分布历史和遗传多样化，四川湍蛙是一种典型的“环”元素，其特征是在山脉中断续分布。
研究结果揭示了广泛的分布动态，包括通过向西扩散进行长期隔离后的再次接触，同时伴随着遗传变异的丧失。这些结果提出了一个模型，该模型将由第四纪气候变化引起的周期性收缩与在适宜条件下某些未能扩张的因素相结合，用于解释共享的间断分布模式。热点地区东部边缘提供了特殊的低海拔栖息地，既包含长期避难所，也包含年轻的移民种群。这一发现以及来自其他分类群的证据综合表明，对于热点地区的一个特定部分，可能有一个特定的生物多样性贡献者，在这种情况下，它可以保护和接收东部的低海拔元素。通过阐明这些避难所种群的低变异性，我们表明，对于经历过混合的种群，可能会得到与线粒体多样性估计不一致的结果，因为每个基因座都不太可能留下成比例的移民等位基因。
长期隔离后的扩散可以解释该物种遗传多样性的大部分空间分布，而二次接触和长期持续存在并不能保证有大量的变异。这种间断分布范围形成的模型可能适用于四川盆地周边山脉中许多其他被隔离的分类群。此外，本研究还深入了解了热点地区的异质性以及全基因组和线粒体数据所获得的不一致变异性。
