Genetic structure in the species-pair Silene vulgaris and S. uniflora (Caryophyllaceae) on the Baltic island of Öland

Allozyme data were used to assess the genetic structure between 37 sympatric populations of the species-pair Stlene vulgaris and S uniflora ssp petraea, and to infer levels of intra- and interspecific gene flow in the two species Silene vulgaris is a geographically widespread weed of disturbed habitats whereas S uniflora ssp petraea is endemic to the Baltic islands of Oland and Gotland On Oland, Silene vulgaris forms extensive linear populations along roads while S uniflora ssp petraea occurs in sparse and spatially-separated populations in open limestone habitats Despite the differences in population size and structure between the two species, both species show extremely low levels of between-population differentiation Between-site differences account for <2% of the total allozyme diversity within Oland in S vulgaris, and < 1% in S uniflora ssp petraea Indirect estimates of gene flow are high for both species (Nm = 11 and 27, respectively) There is no relationship between genetic distance and geographic distance within either species, and the lack of genetic structure is consistent with the pollination biology of the species - both of which are predominantly moth-pollinated The two species hybridize in intermediate habitats, and the geographic distribution of species-characteristic alleles indicates a potential for spatially extensive interspecific gene flow Nevertheless, there are significant differences in allele frequencies between the two species and multivariate analyses show no overlap between populations of the two species The species are ecologically separated by their different habitat preferences and by differences in their flowering phenology There is no evidence that the endemic S uniflora ssp petraea is threatened by genetic contamination or assimilation by the widespread weed, S vulgaris