The effects of inbreeding, outbreeding and long‐distance gene flow on survivorship in North American populations of Silene vulgaris

Summary Silene vulgaris was introduced from Europe to North America prior to 1800. We evaluated the influence of the resulting population structure on traits related to fitness in a two generation series of glasshouse crosses. Plants were either self‐fertilized, outcrossed within local populations, outcrossed between local populations or outcrossed between three geographical regions separated by > 150 km. Inbreeding depression following self‐fertilization occurred in nearly every maternal lineage studied but its magnitude varied among geographical regions, especially when the fitness component compared was seed germination. The consequences of long‐distance gene flow varied among geographical regions: crosses between some pairs of regions resulted in fitter offspring than within region outcrosses (i.e. heterosis), whereas other pairs of regions showed outbreeding depression (offspring of more distant crosses were less fit). Individuals derived from self‐fertilization of F1 individuals were fitter if the first crosses had been made between populations or regions rather than within populations, both in terms of seed germination and seedling survivorship. The benefits of gene flow therefore persist for at least one generation beyond that created by the gene flow event. The fitness of offspring of crosses between F1 individuals and their maternal population did not depend on the relatedness of the parents of the F1. Thus, there was no evidence of outbreeding depression following recombination within F1 individuals whose creation involved gene flow. Our results imply that alteration of the rate of self‐fertilization or the rate of gene flow between populations of S. vulgaris could affect the demographic characters that influence population establishment and persistence in this species.