II. Northeast Asian B. gargarizans
Our study confirmed the presence of two major genetic clusters in Northeast Asia (Fig. 4 ). Cluster A includes individuals exclusive to Northeast Asia (Clade H), while Cluster B includes individuals from across China (Clades A, C, and D) (Fig. 2 ). Our multilocus haplotype network suggests that these two clusters have different origins, with Cluster A likely originating from southeast China, and Cluster B from western and central China (Fig. 3 ). The genetic break between these two clusters seems to occur somewhere between eastern (Heilongjiang Province) and western (Liaoning and Jilin Provinces) regions of Northeast China (Fu et al., 2005; Hu et al., 2007; Tong & Wo, 2017). A similar pattern was found in the study of another widespread frog species (P. nigromaculatus )—a significant subdivision between Northeast China and other regions of Mainland China (Zhang et al., 2008).
Plant communities also mirror this pattern—mixed conifer-hardwood forest (Heilongjiang and Eastern Jilin Province), steppe (Western Jilin province and Inner Mongolia Autonomous Region), and deciduous forest (Liaoning and Hebei Provinces and Beijing) (Liu, 1988; Stebich et al., 2009). Zhang et al. (2008) suggested this genetic pattern was the result of two independent refugia during the last interglacial period in the late Pleistocene. As the divergence of the major B. gargarizansgroups is older than the Pleistocene, we suggest that the situation is a bit more complex for B. gargarizans , with the genetic pattern being shaped by habitat (biogeographic regions), older geologic events (e.g. formation of the Yellow Sea), and multiple glacial refugia. Northeast Asia, although it contains relatively low biodiversity, has had complex geologic and climatic history that deserves additional attention. Finer-scale sampling from Northeast Asia for B. gargarizans is needed to sort out the evolutionary history of the species, which will in turn help elucidate the geologic history of the region.