The influence of population size and isolation on gene flow by pollen in Silene alba

In a series of experiments conducted over two seasons, we used arrays of ex perimental populations to examine the effects of flower number and distance between patches on gene flow by pollen. For this study we used the dioecio us, short-lived perennial plant Silene alba (Caryophyllaceae). This species lives in disturbed roadside and agricultural habitats and displays a weedy population dynamic with high colonization and extinction rates. The motiva tion for the study was to understand what factors may be influencing geneti c connectedness among newly colonized populations within a regional metapop ulation. By using experimental populations composed of genotypes homozygous at a diagnostic locus, it was possible to identify explicitly pollen movem ent into a focal patch as a function of flower number and distance to the n earest neighboring patch. Overall, the mean immigration rate (measured as t he fraction of seeds sired by males outside the focal patch) at 20 m was ju st over 47%, whereas at 80 m immigration rates were less than 6%. In additi on, by knowing the context in which each of these gene-flow events occurred , it was possible to understand some of the factors that influenced the exc hange of genes. Both the number of flowers in the focal population (target) and in the neighboring populations (source) had a significant effect on th e frequency of gene flow. Our experimental data also demonstrate that facto rs that influence gene flow at one spatial scale may not act in the same wa y at another. Specifically, the influence of target size and the relative s ize of the target and source patches on rates of gene flow depended on whet her the patches were separated by 20 m or 80 m. These data suggest that the patterns of gene flow within a metapopulation system can be complex and ma y vary within a growing season.