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 a re 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 wa s to evaluate one potential problem, namely the effects of genetic introgre ssion of foreign provenances on the fitness of local weed populations. 2. The problem was investigated using the arable weed species Agrostemma gi thago, Papaver rhoeas and Silene alba, all of which are commonly included i n commercial seed mixtures in Switzerland. Hybrids (F-1 and F-2 backcrosses ) were made between local Swiss plants and plants of English, German and Hu ngarian provenance (plus F-1 from one US source in Silene). In a field expe riment the growth of the hybrid plants was compared with that of the parent s. Above-ground dry matter after one growing season was taken as a measure of fitness. Additionally, survivorship and seed mass were determined for so me of the hybrids. 3. The biomass data revealed negative outbreeding effects caused by epistas is in all four F-2 backcrosses of Papaver and in the F-2 of Agrostemma hybr idized with plants of German provenance; no such effects were found in Sile ne. Survival was slightly lower in the F-1 hybrids of Papaver, and consider ably reduced in the F-2 backcrosses. For Silene, a heterosis effect was evi dent 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 b e used in wildflower mixtures, although it is not possible to specify preci sely over what distance seed can safely be transferred. The same recommenda tion 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.