Discussion
I. Genetic structure of B. gargarizans
We contribute to understanding the evolutionary history of B.
gargarizans by combining broad geographic sampling with a multilocus
genetic dataset in a single study. Our mtDNA (ND2 ) data analysis
results support B. gargarizans being divided into two groups that
are further divided into eight clades (Group I: Clades A, B, C, D, H;
Group II: Clades E, F, G) (Fig. 2 ). The genetic status of seven
clades (A–G) was identified in the previous study (Zhan & Fu, 2011),
while an additional clade (H) was newly identified in our study
(Fig. 2 ). We uncovered weak phylogeographic pattern for B.
gargarizans , where the genetic differentiation did not strongly match
any geographic pattern. As Zhan and Fu (2011) suggested, for such a
genetic pattern to appear, the geographic distribution of B.
gargarizans would have expanded rapidly after genetic differentiation
occurred.
We propose two hypotheses regarding the weak phylogeographic pattern ofB. gargarizans related to their adaptations to environmental
stressors. First, B. gargarizans is large-bodied and has a dry,
tough skin, allowing it to survive in xeric conditions and to disperse
long distances across land relative to other amphibian species. These
features of B. gargarizans likely played a role in its wide
distribution across China. If the range expansion involved many
individuals and occurred soon after genetic differentiation, such an
undifferentiated phylogeographic pattern could result.
Second, we suggest that anthropogenic effects contributed to the lack of
a clear phylogeographic pattern in B. gargarizans . In China, a
traditional Chinese medicine (hua chan su, 華蟾素) extracted from skin
secretions of toads (including B. gargarizans ) has been used as
medicine for thousands of years (Su & Nu, 2001; Meng et al., 2012;
Cheng et al., 2019). A current online search for toad farms identifies
locations operating in various regions in China (Table S7 ). Any
escape or release of translocated individuals, followed by reproduction
with native individuals would contribute to obscuring phylogeographic
patterns.