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.