Regional climates shape the biogeographic history of a broadly distributed freshwater crab species complex

Aim: The evolutionary importance of paleoclimate regimes has been noted in
biogeographic studies. However, little is known about how paleoclimate
differences shaped the biogeographic pattern and diversification history
of the freshwater fauna in important zoogeographical boundary regions.
Here, we aim to investigate how past regional climatic differences have
shaped the biogeographic history of the inland aquatic fauna in China
using an endemic freshwater crab species complex found on both sides of
the Qinling Mountains–Huaihe River Line (QHL), a critical ecological
boundary in eastern China, as a model system. Location: Eastern China, the
Qinling Mountains–Huaihe River Line. Taxon: The Sinopotamon yangtsekiense
species complex. Methods: A total of 482 individuals of Sinopotamon
yangtsekiense sensu lato were collected from 34 localities throughout its
entire distributional range. The phylogeographic analyses of population
structure, morphological and genetic variations, and demographic dynamics
were made based on multiple mtDNA and nuDNA loci and on morphological
traits. Fine-tuned ecological niche modeling was used to reconstruct the
location of climatically suitable areas that existed during the Last
Glacial Maximum. Results: The divergence of two freshwater crab lineages
across the QHL correlated with significant past variations in monsoon
intensity and with the location of multiple refuges. The divergence time
was broadly consistent with the timing of the critical paleoclimate
transition event in the mid-Pleistocene (95% HPD, 0.48–1.06 Ma). Each
freshwater crab lineage has evolved distinct male genital traits
associated with their isolation in areas with different precipitation
rates and temperatures in the past. The patterns of crab distribution
observed today reflect past contractions of the two lineages in response
to glacial and interglacial cycles during the Pleistocene, followed by
their subsequent rapid expansion after the Last Glacial Maximum (~15 kya).
Main conclusions: Populations of the widespread species Sinopotamon
yangtsekiense s.l. experienced a deep division in the past that led to the
phylogeographical isolation observed today. The two main drivers of
genetic isolation in this taxon were (a) differences in the intensity of
the monsoons on each side of the QHL boundary during
the mid-Pleistocene, and (b) isolation of different populations
of S. yangtsekiense s.l. in a number of separate refuges during the LGM.

研究目的：古气候格局的演化意义已在生物地理学研究中得到论述，但针对重要动物地理分界区域内，古气候差异如何塑造淡水动物区系的生物地理格局与分化历史，目前相关认知仍较为匮乏。本研究以横跨中国东部关键生态分界——秦岭-淮河一线（Qinling Mountains–Huaihe River Line, QHL）两侧的特有淡水蟹类物种复合体为研究模型，旨在探讨过去区域气候差异如何塑造中国内陆水生动物的生物地理历史。
研究区域：中国东部，秦岭-淮河一线（QHL）。
研究类群：扬子江华溪蟹（Sinopotamon yangtsekiense）物种复合体。
研究方法：在该物种复合体的整个分布范围内，于34个采样点共采集到482个广义扬子江华溪蟹（Sinopotamon yangtsekiense sensu lato）个体。基于多个线粒体DNA（mtDNA）和核DNA（nuDNA）位点以及形态性状，开展种群结构、形态与遗传变异及种群动态的系统地理学分析。采用精细化调参的生态位模型，重建末次冰盛期（Last Glacial Maximum, LGM）时期的气候适宜分布区范围。
研究结果：秦岭-淮河一线两侧的两支淡水蟹类支系的分化，与过去季风强度的显著变化以及多处避难所的分布密切相关。该分化时间与中更新世关键古气候转型事件的时间尺度大致吻合（95%最高后验密度区间，0.48–1.06 Ma）。两支淡水蟹类支系均演化出了与过去各自分布区的降水、温度差异相关的特有雄性生殖器性状。当前观测到的蟹类分布格局，反映了更新世时期两支系响应冰期-间冰期旋回发生的分布收缩，以及末次冰盛期（约15千年前）后快速扩张的历史过程。
主要结论：广布物种广义扬子江华溪蟹（Sinopotamon yangtsekiense sensu lato）的种群在历史上曾发生深度分化，造就了当前观测到的系统地理隔离格局。导致该类群遗传隔离的两大主要驱动因素为：（a）中更新世时期秦岭-淮河一线两侧的季风强度差异；（b）末次冰盛期期间，广义扬子江华溪蟹的不同种群在多处独立避难所中发生隔离。