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.