Costs of transgenic herbicide resistance introgressed from Brassica napus into weedy B-rapa

Wild relatives of genetically engineered crops can acquire transgenic trait s such as herbicide resistance via spontaneous crop-wild hybridization. In agricultural weeds, resistance to herbicides is often a beneficial trait, b ut little is known about possible costs that could affect the persistence o f this trait when herbicides are not used. We tested for costs associated w ith transgenic resistance to glufosinate when introgressed into weedy Brass ica rapa. Crosses were made between transgenic B. napus and wild B. rapa fr om Denmark. F-1 progeny were backcrossed to B. rapa and BC1 plants were sel ected for chromosome numbers similar to B. rapa. Further backcrossing resul ted in a BC2 generation that was hemizygous for herbicide resistance. We qu antified the reproductive success of 457 BC3 progeny representing six full- sib families raised in growth rooms (plants were pollinated by captive bumb lebees). Pollen fertility and seed production of BC3 plants were as great a s those of B. rapa raised in the same growth rooms. Segregation for herbici de resistance in BC3 plants was 1:1 overall, but the frequency of resistant progeny was lower than expected in one family and higher than expected in another. There were no significant differences between transgenic and nontr ansgenic plants in survival or the number of seeds per plant, indicating th at costs associated with the transgene are probably negligible. Results fro m this growth-chamber study suggest that transgenic resistance to glufosina te is capable of introgressing into populations of B. rapa and persisting, even in the absence of selection due to herbicide application.