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.