4.2 Population structure of M. alternatus at different scales
The analysis of population structure at different scales revealed that there was obvious differentiation among populations at the intermediate scale (>250 km); whereas the difference at the fine-scale (<10 km) was weak but still had certain influence (Figure 2, 3). The phylogenetic tree, PCA, and ADMIXTURE all showed that the two areas had different populations of M. alternatus , indicating that natural dispersal and gene flow at the intermediate scale were difficult. In the study of other species, the geographical distance and the genetic distance also are strongly correlated, and the geographical distance can also affect the genome-wide variation (Bay et al., 2018). This result demonstrates that the natural spread of M. alternatuscan be isolated at a distance of more than 250 km, which also implies that the PWD can not be transmitted naturally at this distance. At the same time, further study are required to collect some populations at the scale of 10-250 km , so as to analyze the influence of this range on the spread of M. alternatus .
In the analysis of genetic structure and diversity at the fine-scale, the results indicated that the populations in Shunchang and Xiapu were composed of two subpopulations, although the difference level within each subpopulation was not high (Figure 3; Table 2). According to the Mantel test, there was no significant correlation between genetic distance and IBD at the fine scale (Figure 4A). In Monochamusspecies, the flight distance is up to 22 km in the field, and based on a combination of flight mill and mark–release–recapture experiments, the cumulative flying distance averages 63 km by the end of the adult stage in these species (Takasu et al., 2000; David, Giffard, Piou, & Jactel, 2014; Robinet et al., 2019). As a result, strong inbreeding occurs in populations, leading to weak local genetic structure (Kawai et al., 2006; Shoda-Kagaya, 2007; Haran et al., 2015). Indeed, in this study, the genetic structure of the populations was weak at the fine-scale. The distribution of the host may also lead to the weakening of the population genetic structure. According to other studies, as long as the landscape type contains sufficient hosts for a species to inhabit and forage, the genetic structure may be insignificant at a distance of more than 120 km (McCulloch et al., 2013). In Shunchang and Xiapu, the landscape type was primarily forest, and there were many P. massoniana and P. elliottii distributed in the forest landscape. Because these tree species are important hosts of M. alternatus , the two species likely facilitate the dispersal of the beetle, and as a result, the genetic structure was poorly defined at the fine-scale. Although the geographic distance can have an important effect on the genetic structure of a species at large scales, differences in the genetic structure and obstruction of gene flow at a fine scale may be more affected by landscape types than by the geographic distance alone (Aurélie et al., 2017). Thus, with the increase in landscape scale, the influence of the geographical distance on the natural dispersal ofM. alternatus was also increased, whereas at a fine scale, the influence of landscape types was greater than that of distance.