Isolated populations of a rare alpine plant show high genetic diversity and considerable population differentiation

Background and Aims Gene flow and genetic variability within and among alpine plant populations can be greatly influenced by the steep environmental gradients and heterogeneous topography of alpine landscapes. In this study, we examined the effect of natural isolation of alpine habitats on genetic diversity and geographic structure in populations of C. thyrsoides, a rare and isolated European Alpine monocarpic perennial with limited seed dispersal capacity.  Methods We analyzed molecular diversity of 736 individuals from 32 populations in the Swiss Alps and adjacent Jura mountains using five polymorphic microsatellite loci. We further estimated pollen flow using pollen grain sized fluorescent powder. Moreover, we applied individual-based Bayesian approaches to examine population structure.  Key Results We found high within-population genetic diversity (HE = 0.76) and a relatively low inbreeding coefficient (FIS = 0.022). Genetic differentiation among populations measured with a standardized measure was considerable (G`ST = 0.53). We found a significant isolation-by-distance relationship (R2 = 0.39, P> 0.001) and a significant geographic sub-structure coinciding with proposed postglacial migration patterns. Altitudinal location and size of populations did not influence molecular variation. Direct measures of pollen flow revealed that insect-mediated pollen dispersal was restricted to short distances within a population.  Conclusions The natural isolation of suitable habitats for C. thyrsoides restricts gene flow among the populations as expected for a monocarpic species with very limited seed dispersal capacities. The observed high within-population genetic diversity in this rare monocarpic perennial is best explained by its outcrossing behaviour, long-lived individuals and overlapping generations. Despite the high within-population genetic diversity, the considerable genetic differentiation and the clear western–eastern differentiation in this species merits consideration in future conservation efforts.