Introduction
Phylogeography focuses on the gene genealogy spatio-temporal pattern of related species or intraspecies as well as how the pattern was formed (Hickerson et al. , 2010; Bai & Zhang, 2014). Taking advantages of genetic diversity, genetic structure analysis and molecular time dating, temporal and spatial population divergence and expansion, evolution history can be inferred; the glacial refugia can also be identified combing the climate changes and glacial events in late Quaternary (Hickerson et al. , 2010). It is widely acknowledged that the glacial-interglacial oscillations in Quaternary glaciation, especially the LGM, had eminent effects on shaping the contemporary distribution of species and their genetic diversity (Chung et al., 2017).
The Sino-Japanese forest subkingdom, involved in the Sino-Japanese Floristic Region (SJFR) except the Himalayan forest region (Wu & Wu, 1996), covers the south and east of China, as well as the Korean Peninsula and the Japanese Archipelago. This region is embroiled in the subtropical and temperate climate zones in the northern hemisphere, mainly consisted of the subtropical and temperate China. There are remarkably rich flora in this region due to the high rainfall brought by the south-eastern monsoon, and they may have experienced dramatic distribution changes during the Quaternary climatic oscillations (Mitsui et al., 2007). Mountains are believed to play important roles as refugia and dispersal corridors for species during the Quaternary global climate changes. Nanling Mountain and Qinling Mountain acted dual roles as a dispersal corridor in east-west direction and as a glacial refugium in subtropical China during the late Quaternary (Tian et al., 2018;Guo et al., 2014; Wulufu, 1964; Zhou, 1997). The main Korean mountain range [the so-called “Baekdudaegan” (BDDG)] is also thought be a glacial refugium, mainly for the boreal and temperate flora of northeastern Asia (Chung et al. , 2017). In addition to large mountains, microrefugia such as smaller massifs or lowland sites are also of dominant importance for sustaining species. It was hypothesized that multiple microrefugia locate in the northern part of the southern refuge (24°N-33°N) (Wang & Ge, 2006).
Cerasus serrulata (Lindley) Loudon is one of the most widely spread species in genus Cerasus , distributing from the north in Heilongjiang Province to the south in Guangzhou Province (Li & Bartholomew, 2003), covering a widest latitude range (~ 24°N- 45°N) (Li et al., 2014); and from west in Guizhou Provincce to Japan (Li & Bartholomew, 2003), also across a broad longitude range. The distribution pattern is involved in the Sino-Japanese forest subkingdom, and is also within the subtropical and temperate monsoon climate zones. All the extensive coverage, various and special habitats as well as complex climate, indicating a strong adaptation, have bred high variants in C. serrulata , and highlighted a high research value of phylogeography to bring new clues to the history of geological events, as well as the development and utilization of cherries such as breeding new or improved ornamental cultivars since C. serrulataitself posses good ornamental characteristics and discrepancies of flower color, flower type, leaf color, tree shape, high resistance, etc. Nevertheless, the high variant level has as well caused some controversies on taxonomic relationships of related species, which needs further studies.
In this study, C. serrlata was selected as a typical model of cherry in subtropical and temperate China along with Korean peninsula. Three maternal-inherited chloroplast DNA and the bi-parentally inherited nuclear DNA ITS sequences were used together for phylogeographic analysis of C. serrulata for the first time to better understand the genetic diversity and genetic structure, as well as to clarify the temporal and spatial population divergence and expansion history ofC. serrulata , which can also provide new clues for cherry evolutionary history and identifying refugia of subtropical and temperate China in SJFR. In addition, controversial taxonomic groups within the species and of related species, seemingly caused by the different processes of speciation inner Cerasus serrulata , were also discussed to add more proofs for further clarifying their relationships.