Fig. 1

a Distribution of chlorotypes in 26 populations of E. pleiosperma (China) and 10 populations of E. polyandra (Japan) (see Additional file 2: Table S1 for population codes). Chlorotypes shared among populations are denoted by colour, while population-specific ones (‘E’) are white. The red and black dashed lines represent, respectively, the Sino-Himalayan/Sino-Japanese Forest boundary and the species’ boundary across the East China Sea region. The black dotted lines delimitate the four phylogroups of E. pleiosperma (i.e. population groups that share closely related chlorotypes). The black dotted line in Japan marks the boundary between ‘southern’ and ‘central’ phylogroups in E. polyandra. Abbreviations: TGMR, Three Gorges Mountain Region. b Dark-shaded areas in the inset indicate current mainland and island configurations, and light-shaded areas indicate exposed coastal areas and sea basins of East Asia during Late Pleistocene sea-level alterations owing to glaciations (modified after Park) [125]. c tcs-derived network of genealogical relationships between the 35 chlorotypes of the two species. Each circle denotes a single chlorotype with size proportional to frequency. Small open circles represent missing chlorotypes. The four clades (phylogroups) of E. pleiosperma identified by tcs were denoted as C1, C2, C3, C4, while those two clades (phylogroups) of E. polyandra were represented by J1 and J2. The baseline map was created by us using ArcGIS v.10.2.2