GENE FLOW AND POLLINATOR BEHAVIOR IN SILENE-DIOICA POPULATIONS

Silene dioica (L.) Clairv. (Caryophyllaceae) is an insect pollinated a nd obligately cross-fertilising herb commonly found on serpentine, i.e . soils rich in heavy metals, and on cultivated meadows. Groups of Sil ene populations were studied in two areas in the Scandianvian mountain s. In a northern area Silene populations grow in an open pine and birc h forest while in the south, populations are surrounded by a continuou s and dense spruce forest. Gene flow, estimated on the basis of allele frequencies, is highly restricted in the south in comparison to the n orth. This has led to an extensive genetic differentiation among popul ations. The pollinator guilds (Thricops flies, syrphid flies and bumbl ebees) are similar in the northern and southern populations and can th erefore not account for the difference in gene flow. The flowering in nearby populations overlaps broadly and is also not the reason for the genetic differentiation in the south. The differentiation is more lik ely due to vegetation that limits pollinator movement. The level of ge ne flow differs in the two habitats both in the north and in the south . The gene flow is restricted among serpentine populations but common among meadow populations. Pollen dispersal, and in particular, anthrop ogenic seed dispersal mediate gene flow among meadows. Serpentine popu lations are unaffeced by human activity. Pollinators are therefore the main agent of gene flow on serpentine. This study shows the importanc e of the surrounding vegetation in influencing gene flow patterns amon g populations as well as that habitat fragmentation affects the geneti c properties of plant populations. In addition, we have also observed a restricted gene flow within populations. Enzyme allele frequencies s how an excess of homozygotes within most of the populations. This can be explained through limited pollen dispersal and differences in male and female flowering density.