3.3 Population structure at different scales
M. alternatus population structure was compared at intermediate
and fine scales. As shown in the phylogenetic tree, the population in
the two regions was divided into two branches at the intermediate scale
(Figure 2A). Principal component analysis confirmed this result, having
the populations from the two regions clearly separated (Figure 2B).
ADMIXTURE was also used to analyze the population genetic structure
between the two regions. In the ADMIXTURE figure (Figure 2C), each
column represents an individual, with the length of the different
colored segments representing the proportion of an ancestor in the
individual genome. Because the CV error was the smallest at K = 2 (CV
error = 0.42066), the population genetic structure revealed two main
differentiated gene pools (Figure 2C). Thus, the population in Shunchang
was originated from one ancestor and the one from Xiapu was originated
from a different ancestor, which is a conclusion consistent with the
results of the phylogenetic tree and the PCA. In conclusion, the
populations were clearly differentiated at the intermediate scale
(>250 km), which clearly indicated the importance of
geographic barriers against natural dispersal and gene flow of M.
alternatus over long distances.
To reveal whether there were differences in population genetic structure
at the fine scale, ADMIXTURE was also used to analyze the population
genetic structures within Shunchang and Xiapu. According to the smallest
CV error in Shunchang and Xiapu at K = 2 (CV error = 0.49264 and
0.47180, respectively; Supporting information Figure S1), the
populations at the fine scale formed two clusters. Thus, the populations
in SC included two clusters, and those XP included two other clusters
(Figure 3). Although the fine-scale genetic structure detected by
ADMIXTURE was weak, with an increase in the K value, seven meaningful
clusters were obtained (Supporting information Figure S2).