Fitness of backcross and F2 hybrids between weedy Brassica rapa and oilseed rape (B-napus)

With the cultivation of genetically modified crops, transgenes may spread by introgression from crops into weedy and wild populations of related species. The likelihood of this depends in part on the fitness of first and later generation hybrids. We here present results on the fitness of F-2 and backcross hybrids between oilseed rape (Brassica napus) and weedy B. rapa Two populations of B. rapa, two varieties of B. napus, and their F-1 hybrids were used for controlled crosses, and seed development, survival in the held, pollen viability, pod- and seed-set were estimated for the offspring. Offspring from F-2 and backcrosses had a reduced fitness relative to their parents for most of the fitness components and for a combined estimate of fitness, with F-2 offspring suffering the lowest fitness. Despite their lower fitness on average, some of the hybrids were as fit as the parents. Significant fitness differences were detected between backcross and F-2 offspring from different B. rapa populations, B. napus varieties, and parental plants. Our results suggest that introgression of transgenes from oilseed rape to B. rapa will be slowed down, but not hindered, by the low fitness of second generation hybrids.