4.3 Secondary contact, demographic expansion and contemporary west-to-east migrations
FSC2 analyses indicate a best-fit model of isolation followed by secondary contact and demographic expansion (Table 3, Figure 5). The estimated time of secondary contact from our analysis (0.10 Ma) coincides with the Lushan-Tali interglacial period in China (0.10‒0.20 Ma) (Duan, Pu, & Wu, 1980), when a continuous geographic distribution of C. chuniana along the mountain ranges in southern China was detected by ENM (Figure 2a). Although the Lushan-Tali interglacial period is younger than the LIG (0.12~0.14 Ma), it still lies within the interglacial period in China when temperature increased and was estimated to be even higher than the present (Duan, Pu, & Wu, 1980; Zhu et al., 2004). This suggests that the secondary contact may occur during this warmer time. Moreover, it is thought that the East Asia summer monsoon intensified then due to increased temperature with more precipitation during the interglacial period (Liu et al. , 2018; Meng et al. , 2018; Wang et al., 1999; Wang et al., 2007; Wang et al., 2012), thus providing more suitable habitat, especially in considering that C. chuniana is adapted to mesic environments and most influenced by precipitation (Table S1). In the same model, demographic expansion in the Nanling Mts. was inferred with notably increased effective population size (Table 3, Figure 5), indicating high local population diversification (Figure 3). The Nanling Mts., which are composed of five distinct ridges, has a long history of STEBF in southern China (Fan et al. , 2018; Xu et al. , 2017). Its vegetation is characterized by highly varied elevational or longitudinal shifts (Shen et al. , 2019; Zhu et al. , 2017), which confers relatively stable ecological conditions to these mountains during glacial periods and has served as a buffer from climatic change. It had almost the same annual precipitation during the last glacial period as current (Tian & Jiang, 2016). Therefore, complex physiography plus long-term stable ecological conditions in the Nanling Mts. across glacial cycles are thought to preserve population diversity, finally resulting in population size increase and demographic expansion. Similar cases have been documented in widespread species in subtropical China, such as Eurycorymbus cavaleriei , Loropetalum chinense andEomecon chionantha (Gong et al., 2016; Tian et al., 2018; Wang, Gao, Kang, Lowe, & Huang, 2009). Most of the previous research is conducted using the traditional molecular markers and data analysis methods, with more intensive population sampling, compared with the current study. We consider population genomic approaches based on the next generation sequencing are more effective ways to detect the population dynamics, even for the species with relatively narrow distribution.
Additionally, FSC2 analysis indicated bidirectional migrations occurring after NL and ES divergence, with M NL-ES (2.14) higher than M ES-NL (0.33) (Table 3, Figure 5). The migrations in C. chuniana seem to proceed primarily from the Nanling Mts. to the east. Many examples of plant species in East Asia exhibit a similar distribution pattern and migration route, such asTetrastigma hemsleyanum and Eomecon chionantha (Tian et al., 2018; Wang, 1992a; 1992b; Wang et al. , 2015). The question arises as to why contemporary migration direction is inferred from the Nanling Mts. towards the east, whereas the former diverged more recently than the latter. The Nanling Mts., with distinct phytogeography and long-term stable ecological condition, is thought to be one of the glacial refugia for C. chuniana . Populations of C. chuniana are present at relatively higher elevations in the Nanling Mts. than the eastern ones, which may facilitate the west-to-east migration from higher elevations to lower ones via closely adjacent stepping-stone areas across the mountain ranges.