Demographic dynamic and potential glacial refugia
Collectively, two genealogies of C. serrulata were detected: the northweast (NW) genealogy and the other genealogy consisted of the left groups involving the northeast (NE), southeast (SE) and southwest (SW) groups in our sampling areas, located mainly in the south and east of China within the subtropical and temperate monsoon climate zones. But the phylogeographic pattern did not compare the geographic pattern or climate zones.
The most recent common ancestor of C. serrulata appeared around 10.6 Mya in the mid-late of Miocene, which was also the divergence time of the two genealogies, calculated in the molecular dating analysis. To be detailed, the northwest group (NW) which formed one lineage began to divided out firstly in 10.6 Mya; afterwards, the southwest (SW) and the east (E, containing NE+SE) groups in the other lineages started to differentiate away from each other during 8.02 Mya (Fig. 7, Table 4). In other species, adjacent time was also detected of C. dielsianaancestor (6.28 Mya) (Zhu et al., 2019), Polystichumglaciale ancestor (6.89 Mya) (Dong et al. , 2018) and the divergence of Ixiolirion tataricum and I. songaricum(~7 Mya) (Li et al., 2019). It seems that the divergence event of many species was concentrated during such a period of time, and this time overlapped with that of the Himalayan movement and that of the Arctic ice-sheet to begin developing while the global climate was cooling down (Liu et al., 1998) and drier (Zhao et al., 2018; Ma & Tian, 2015). The strong landform and climate changes might lead to such a concentration of species divergence. In this, the uplifting of Qinling Mountains might took place accompanied by the Himalayan movement (Xue et al., 2004), and gradually isolated the north groups from the other, finally resulting in the divergence of the two geographical lineages.
Demographic expansion was recognized both in neutrality test and mismatch distribution analysis, except the northeast group (NE). The expansion time of group southwest (SW), southeast (SE) and northwest (NW) were dated back to 0.719 Mya, 0.665 Mya and 0.586 Mya, respectively (Table 3). All the time was before the the Last Glaciation Maximum (LGM, from 0.0265 Mya to 0.019-0.02 Mya) during the Last Glaciation of Quaternary glacial period, indicating that the distribution center and pattern of C. serrulata was formed before the LGM, which was accordant to that of genus Cerasus (Cao, 2006; Li, 2009). Comparing these time with the global temperature change curve of the Earth since 5 Mya (Cao, 2006; Li, 2009; Wang, 2014), it was found that these time was all sited in the uprising temperature periods in the frequent cooling-warming climate oscillations (Lisiecki & Raymo, 2005). The warmed-up temperature which also brought humidity should have enabled the expansions. Besides, within all these populations, CPL possesses the highest diversity which was located in Guizhou Province in the northwest group (NW). Regarding the divergence time and genetic diversity level together, two lineage colonization events were inferred. One was confined to Qingling Mountain and Taihang Mt. The other comprised two separate steps: firstly from the Wuling Mt. to Jiangnan Hilly; and then northeast to the coast of Asia. Glacial refugia which protected species to survive are usually considered to be large mountains. Nevertheless, importance of microrefugia such as smaller massifs or lowland sites is rising more and more attentions (Tomasz et al., 2019; Zhu et al., 2019). The broad land of the southeast of Asia, linked together by Bohai Sea and the coast of Asia today as sea level declined resulted from the glaciation (Guo et al., 2014), as well as valleys in east-west mountains such as Qinling Mt, provided refuges and expansion accesses for many species (Wulufu, 1964; Zhou, 1997). In this, the large mountain of Qinling, Wuling and Taibai Mts in Korea, as well as some microrefugia in Jiangnan Hilly especially the Huang Mt., Lu Mt. and Tianmu Mt. were inferred to be glacial refugia for C. serrulata , also having regard to their higher genetic diversity. In addition, the consistent distribution pattern in cpDNA and SSR analysis (Chen, 2016; Yi et al., 2018) of the northeast (NE) and southeast (NW) groups which are isolated by the Bohai Sea is also elucidated by the conjunction of Korea and eastern China during glacial period. All the lineage colonizations and potential refugia are generally coincident with the result of phylogeographic analysis of Cerasus (Li, 2009), indicating that the wide spread C. serrulta can be a representative species of the genus Cerasus in the evolutionary process.