Variation in Silene vulgaris and S. uniflora (Caryophyllaceae): genetic diversity, gene flow and habitat selection

Abstract: Genetic and morphometric variation was studies in the three closely-related taxa Silene vulgaris (a widespread weed), S. uniflora ssp. uniflora (restricted to coastal habitats) and S. uniflora ssp. petraea (endemic to Sweden and confined to open limestone habitats). The majority of alleles at the investigated allozyme loci are common to the three taxa, but the taxa show significant differences in allele frequencies and are clearly separated in terms of their overall morphology and their habitat requirements. The between-population component of genetic diversity is low in each of the taxa and indirect estimates suggest extensive intraspecific gene flow. Although S. vulgaris and S. uniflora hybridize, and there may be spatially extensive gene flow between taxa, multivariate analyses of allozymes and morphological characters confirm the distinctions between the taxa. All results support the present taxonomic treatment of S. uniflora ssp. petraea as a subspecies of S. uniflora. On a smaller scale, allozyme variation in a hybrid zone between S. vulgaris and S. uniflora ssp. petraea on the Baltic island of Öland show evidence for long-distance interspecific gene flow. The distribution of individual allozymes is significantly associated with vegetation characteristics in the hybrid zone. However, morphometric variation suggests restricted gene flow and strong selection on the overall phenotype in the hybrid zone. The natural habitats of S. vulgaris and S. uniflora ssp. petraea have few characteristics in common and habitat selection is expected to play an important role in keeping the species apart. The exchange of genes between S. vulgaris and S. uniflora ssp. petraea is not expected to have any impact on the survival or integrity of the endemic S. uniflora ssp. petraea.