Analysis of molecular variance and isolation by distance
A hierarchical AMOVA, as implemented in poppr with method = ‘ade4’, was
carried out using population within host (Table 4). Consistent with
previous studies in O. crenata (Romàn et al. 2001) the majority
of variation is found within individuals, suggesting very little
population differentiation. However, there is some evidence for
separation by host (phi = 0.031, p = 0.004) and population within host
(phi = 0.028, p=0.001). The observed increased individual variation
among O. crenata populations results from a significant excess of
heterozygosity (FIS = -0.22, Bartlett’s K-squared =
1969, df = 1, p-value < 2.2e-16).
To test the role of isolation by distance (IBD), a Mantel test was
conducted using Edward’s genetic distance and a geographic distance
matrix (latitude, longitude). Overall, we find no support for IBD (R =
0.55, p = 0.10) across populations. We do find however, two distinct
patches in the kernel density estimates for IBD (Figure 3). This
patchiness appears to be driven by the outlying population, AT, which
exhibits moderate genetic differentiation, with mean FST= 0.075 (Table 2) and geographic distance, mean = 1.94. This outlying
patch drives a significant linear relationship between genetic distance
and geographic distance (dashed line Figure 3, R2 =
0.283, p = 0.0001)