Relative effects of nocturnal vs diurnal pollinators and distance on gene flow in small Silene alba populations

Silene alba exists in natural metapopulations throughout its range and is visited by a suite of both diurnal and nocturnal pollinators. Pollen-mediated gene flow may help reduce genetic isolation of subpopulations. Here, we compared the relative effects of nocturnal vs diurnal pollinators on pollen-mediated gene flow in subpopulations separated by two distance treatments. We established populations consisting of genetically marked individuals in an old field in Tennessee (USA). Electrophoretic examination of seedlings produced by plants exposed to nocturnal, diurnal and control pollinator treatments and separated by either 20 or 80 m allowed us to directly measure pollen-mediated gene flow. Gene flow was more common between populations separated by only 20 m. Nocturnal pollinators were responsible for most gene flow between populations, regardless of distance. Diurnal pollinators played only a small role in pollen-mediated gene flow. The results suggest that nocturnal pollinators are better than diurnal pollinators at moving pollen between small S. alba subpopulations. However, their effectiveness declines as the distance between subpopulations increases, making them relatively ineffective at moving genes between isolated subpopulations.