Maintenance of the species boundary between Silene dioica and S-latifolia (red and white campion)

The relative importance of floral versus ecological isolation in preventing hybridisation in plant species remains unknown, primarily due to a paucity of detailed data from a range of systems. We examined floral isolation between Silene dioica and S. latifolia (Caryophyllaceae) in southern England by measuring gene flow across the species boundary using allozymes, and by assessing interspecific transfer of flourescent dye powders to simulate pollination. Allozyme studies of wild populations demonstrated that gene flow between S. dioica and S. latifolia is considerable since the two species could not be distinguished at the loci studied, in sharp contrast to their distinct morphologies. Pollination studies using fluorescent dye powders and direct observation of insect behaviour concurred in that although there was a degree of assortative mating it was insufficient in itself to prevent introgression. Fluorescent dye studies also suggest that pollination rates of hybrids are similar to parental types and that they provide a bridge for gene flow since they are visited freely by the main pollinators of both S. dioica (bumblebees) and S. latifolia (moths). We conclude that although floral isolation and spatial segregation may be important contributory factors, morphological differences between species are probably maintained primarily by strong selective forces associated with habitat (ecological isolation).