Genetic introgression from distant provenances reduces fitness in local weed populations

1. Seed mixtures of wildflowers are used increasingly in schemes to restore biodiversity in intensively managed farmland. Usually, the seed mixtures are produced by commercial suppliers and they may be distributed over large geographical distances. It is therefore important to ask what problems may arise from using seed that is not of local origin. The aim of this study was to evaluate one potential problem, namely the effects of genetic introgression of foreign provenances on the fitness of local weed populations. 2. The problem was investigated using the arable weed species Agrostemma githago, Papaver rhoeas and Silene alba, all of which are commonly included in commercial seed mixtures in Switzerland. Hybrids (F-1 and F-2 backcrosses) were made between local Swiss plants and plants of English, German and Hungarian provenance (plus F-1 from one US source in Silene). In a field experiment the growth of the hybrid plants was compared with that of the parents. Above-ground dry matter after one growing season was taken as a measure of fitness. Additionally, survivorship and seed mass were determined for some of the hybrids. 3. The biomass data revealed negative outbreeding effects caused by epistasis in all four F-2 backcrosses of Papaver and in the F-2 of Agrostemma hybridized with plants of German provenance; no such effects were found in Silene. Survival was slightly lower in the F-1 hybrids of Papaver, and considerably reduced in the F-2 backcrosses. For Silene, a heterosis effect was evident in seed mass in the F-1 generation, while seed mass decreased in the F-2. The same trend, although less strong, was also observed in Agrostemma. 4. The results suggest that only plants of relatively local origin should be used in wildflower mixtures, although it is not possible to specify precisely over what distance seed can safely be transferred. The same recommendation is also valid for schemes to reinvigorate endangered plant populations. The relevance of the fitness components that were measured, and the long-term effects of genetic introgressions, are discussed.