Fine-scale genetic structure and clinal variation in Silene acaulis despite high gene flow

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Abstract
We investigated whether the distribution of genes reflects the patchy distribution of individuals of Silene acaulis on Pennsylvania Mountain in central Colorado. Five polymorphic protein loci were analysed using both F-statistics and spatial autocorrelation. Low thetaPOP (FST) indicated little genetic differentiation between populations approximately 1 km apart. This indicates high gene flow within our study site, perhaps as a result of long-distance pollen dispersal. Despite little differentiation between populations, there was clinal variation at the 6-Pgd-1 locus and significant within-population genetic structure (indicated by both thetaPATCH and spatial autocorrelation). We infer that this fine-scale genetic structure is the result of limited seed dispersal combined with genetic drift. The level of genetic structure varied markedly among populations, with the greatest genetic structure (highest Moran's I and thetaPATCH values) in two low-altitude, small, low-density populations. Intensive sampling such as used in this study may reveal similar patterns of fine-scale genetic differentiation in other patchily distributed plant species, particularly those with limited seed dispersal.