Abstract
The mountains of southern China are an excellent system for investigating the processes driving the geographic distribution of biodiversity and radiation of plant populations in response to Pleistocene climate fluctuations. How the key mountain ranges in southern China have affected the evolution of narrowly distributed species is less studied than more widespread species. Here we focused onCercis chuniana, a woody species endemic to the southern mountain ranges in subtropical China, to elucidate its population dynamics. We used genotyping by sequencing (GBS) to investigate the spatial pattern of genetic variation among 11 populations. Bayesian time estimation revealed that population divergence occurred in the middle Pleistocene, when populations in the Nanling Mts. separated from those to the east. Geographical isolation was detected between the populations located in adjacent mountain ranges, thought to function as geographical barriers due to their complex physiography. As inferred by ecological niche modeling and coalescent simulations, secondary contact occurred during the warm Lushan-Tali interglacial period in China, with intensified East Asia summer monsoon and continuous habitat available for occupation. Complex physiography plus long-term stable ecological conditions across glacial cycles facilitated the demographic expansion in the Nanling Mts., from which contemporary migration began. Our work shows that population genomic approaches are effective in detecting the population dynamics of narrowly distributed species. This study advances our understanding how glacial cycles have affected the evolutionary history of plant species in southern China montane ecosystems.
KEYWORDS: Cercis chuniana , demographic modeling, geographic isolation, Pleistocene glacial cycles, secondary contact, Southern China mountainous ranges
INTRODUCTION
High physiographical heterogeneity is suggested to prompt rapid diversification in montane habitats because of the increased ecological opportunities afforded by frequent episodes of geographic isolation (Colin & Ruth, 2006; Muellner-Riehl, 2019; Simpson, 1964). The high biodiversity of southern China is due in large part to the extreme physiographical heterogeneity of its mountain ranges (Fan et al. , 2018; Xu et al., 2017; Yang, Dick, Yao, & Huang, 2016). Often oriented in either north-south or northeast-southwest directions (Hou, 1983; Wang, 1992a; Wang, 1992b; Ying, 2001), these topographically diverse ranges have been suggested to serve as either geographical barriers or colonization corridors for various plant species (Gong et al., 2016; Tian et al., 2018; Xiong, Wu, & Zhang, 2019). Their local habitat uniqueness has been attributed to complex topography correlated with longitudinal or steep elevational gradients (Qiu, Zeng, Chen, Zhang, & Zhong, 2013; Wang, Fang, Tang, & Shi, 2012). The primary vegetation type of these mountains is subtropical evergreen broadleaved forest (STEBF), one of the largest continuous such forests in the world and well known for harbouring ancient relictual elements of the north-temperate biota (Qiu, Fu, & Comes, 2011; Wang, Fang, Tang, & Shi, 2012). Many of their plant species, predominantly endemics, exhibit high rates of local and rapid radiation (Hou et al., 2017; López-Pujol, Zhang, Sun, Ying, & Ge, 2011) presumably arising within the last 5 million years, in line with both orogenic events and Pleistocene glacial cycles (Li et al. , 1979; Liu et al. , 2013; Shi, Li, & Li, 1998; Wang et al. , 2010). These characteristics make southern China an excellent system for investigating the processes driving the geographic distribution of biodiversity and radiation of plant populations in response to Pleistocene climate fluctuations.
Climatic oscillations associated with glacial cycles during the Pleistocene are also considered as an important factor driving the distribution pattern of biodiversity and shaping the demographic history of populations, particularly in montane regions (Hewitt, 2004; Li et al., 2019; Mesquita, Tillmann, Bernad, Rosemond, & Suding, 2018; Svenningm, Normand, & Skov, 2009). Although still under debate, considerable data are now available to support four glacial periods in eastern China (east to 105ºE) during the Pleistocene, i.e. the Poyang, Dagu, Lushan and Tali Glacials (Duan, Pu, & Wu, 1980). In southern China, the degree of habitat connectivity is thought to have decreased during glacial periods, with vegetation belts lowering in elevation and contracting in geographic range, providing the opportunity for both geographical isolation and genetic divergence to occur (Shi, Cui, & Su, 2006). Multiple glacial refugia correlated with centres of genetic diversity have been identified in southern China, out of which subsequent localized or rapid range expansions have been inferred (Chen et al., 2012; Gong, Chen, Dobes, Fu, & Koch, 2008; Li, Shao, Lu, Zhang, & Qiu, 2012; Qiu, Fu, & Comes, 2011; Tian et al., 2015). Previous research has elucidated the scenarios involved with glacial and postglacial evolutionary history of plant species in southern China (Gong et al., 2016; Liu et al., 2012; Tian, Ye, Wang, Bao, & Wang, 2020). However, most of this research has focused on widespread species and ignored how key mountain ranges might affect the evolution of narrowly distributed species.
Cercis chuniana F. P. Metcalf (Fabaceae: Cercidoideae; Azani et al., 2017) is a small tree or shrub endemic to the STEBF of southern China. It has a narrow geographical distribution, extending from the Wuyi and Eastern China Mountains westwards to the Nanling Mountains with increasingly larger population sizes and densities. Unique amongCercis species, it has an asymmetrical leaf blade (Chen, Zhang, Larsen, & Vincent, 2010; Metcalf, 1940), which makes it easily identifiable morphologically. The species is resolved near the base of the Cercis phylogenetic tree, with an estimated age of 2.40 Ma (Fritsch et al. , 2018; Liu et al., unpublished data, 2020). As with its congeners in China, it exhibits an adaptation to mesic environments through its acuminate leaf blade apex (Fritsch et al. , 2018; Isely, 1975; Wunderlin Larsen, & Larsen, 1981).
Genotyping by sequencing (GBS) has been widely used as a genomic approach for investigating genetic diversity and population structure (Chen, Hou, Zhang, Pang, & Li, 2017; Metzker, 2010; Niu et al. , 2019). Because it is based on genomic reduction with restriction enzymes, GBS does not require a reference genome to detect single nucleotide polymorphisms (SNPs). In combination with marker discovery and genotyping, GBS provides a rapid, high-throughput, and cost-effective tool for a genome-wide analysis for nonmodel species (Andrews, Good, Miller, Luikart, & Hohenlohe, 2016; Davey et al. , 2011; Scheben, Batley, & Edwards, 2017). Here, we used GBS and collected genome-wide SNPs for population genetic analyses ofC. chuniana . We aimed to 1) investigate genetic diversity and population structure of the species, 2) elucidate its demographic history, and 3) use the data to infer the roles of mountain ranges in southern China and Pleistocene climatic fluctuations in driving its diversification and geographic distribution. We thereby hoped to gain a better understanding of the impact of this environment on the evolutionary history of narrowly distributed species.