No abstract

Wild relatives of genetically engineered crops can acquire transgenic traits such as herbicide resistance via spontaneous crop–wild hybridization. In agricultural weeds, resistance to herbicides is often a beneficial trait, but little is known about possible costs that could affect the persistence of this trait when herbicides are not used. We tested for costs associated with transgenic resistance to glufosinate when introgressed into weedy Brassica rapa. Crosses were made between transgenic B. napus and wild B. rapa from Denmark. F1 progeny were backcrossed to B. rapa and BC1 plants were selected for chromosome numbers similar to B. rapa. Further backcrossing resulted in a BC2 generation that was hemizygous for herbicide resistance. We quantified the reproductive success of 457 BC3 progeny representing six full‐sib families raised in growth rooms (plants were pollinated by captive bumblebees). Pollen fertility and seed production of BC3 plants were as great as those of B. rapa raised in the same growth rooms. Segregation for herbicide 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 nontransgenic plants in survival or the number of seeds per plant, indicating that costs associated with the transgene are probably negligible. Results from this growth‐chamber study suggest that transgenic resistance to glufosinate is capable of introgressing into populations of B. rapa and persisting, even in the absence of selection due to herbicide application.

No abstract

Feral oilseed rape is not a relevant source of macroscopic impurity at its present density in the landscape but provides opportunity for genetic recombination, stacking of transgenes and the evolution of genotypes that under strong selection pressure could increase and re-occupy fields to constitute an economic weed burden and impurity in future crops.

Fitness of interspecific hybrids is sometimes high relative to their parents, despite the conventional belief that they are mostly unfit. F(1) hybrids between oilseed rape (Brassica napus) and weedy B. rapa can be significantly more fit than their weedy parents under some conditions; however, under other conditions they are less fit. To understand the reasons, we measured the seed production of B. napus, B. rapa, and different generations of hybrid plants at three different densities and in mixtures of different frequencies (including pure stands). Brassica napus, B. rapa, and backcross plants (F(1) ♀ × B. rapa) produced many more seeds per plant in pure plots than in mixtures and more seeds in plots when each was present at high frequency. The opposite was true for F(1) plants that produced many more seeds than B. rapa in mixtures, but fewer in pure stands. Both vegetative and reproductive interactions may be responsible for these effects. Our results show that the fitness of both parents and hybrids is strongly frequency-dependent and that the likelihood of introgression of genes between the species thus may depend on the numbers and densities of parents and their various hybrid offspring in the population.