Genetic structure of a montane perennial plant: the influence of landscape and flowering phenology

The way that genetic variation is distributed geographically has important conservation and evolutionary implications. Here, we examined the distribution of genetic variation within and among populations of the montane perennial Ipomopsis aggregata. We sampled plants in western Colorado and examined (1) population genetic structure over a geographic area that spanned 130 km, including genetic variation within disturbed and undisturbed sites; (2) the relationship between genetic differentiation and geographic distance; and (3) the relationship between flowering time and genetic differentiation among plants within and among geographic areas. F_IS was significantly higher (t test, P = 0.006), expected heterozygosity was significantly lower (t test, P = 0.04), and allelic richness was marginally significantly lower (t test, P = 0.078) among anthropogenically-disturbed sites compared to undisturbed sites. We found moderate genetic differentiation over the area sampled (average pairwise F_ST = 0.04; average pairwise F′_ST=0.19), but no association of genetic and geographic distance (Mantel test P values 0.44 for F_ST and 0.36 for F′_ST). We found a strong association of flowering time and genetic differentiation over small and large spatial scales. Genetic differentiation between early and late flowering plants within a focal site was statistically significant (genic test for population differentiation combined P value <0.001; F_ST = 0.05). There was a significant correlation between genetic distance (F′_ST) and distance in flowering time, when controlling for geographic distance, over the whole geographic area (Partial Mantel test R_xy = 0.32, P = 0.013). A multiple regression with randomization further supported the inference that flowering time, but not geographic distance or elevation, predicted F′_ST (geographic distance: β = −0.03, P = 0.89; elevation: β = 0.01, P = 0.96; phenological distance: β = 0.30, P = 0.05), but not F_st (geographic distance: β = −0.02, P = 0.92; elevation: β = 0.14, P = 0.38; phenological distance: β = 0.25, P = 0.11), unless elevation was left out of the model (geographic distance: β = −0.03, P = 0.9; phenological distance: β = 0.29, P = 0.03). The association of flowering time and genetic distance despite the lack of isolation by distance provides further evidence for the usefulness of incorporating this variable into plant landscape genetic studies when possible.