Consequences of gene flow and transgene introgression in hybrids between transgenic <i>Brassica napus</i> and its weedy wild relative <i>Brassica rapa</i>

The adventitious presence of transgenes and their potential impact on the environment has been a topic of concern for many years. To address these concerns the following chapters discuss past and current research of gene flow and introgression, methods for transgene detection and monitoring, and the results from field-level experiments using artificially introgressed advanced generation hybrids. The field studies were designed to be a worst-case scenario where hybrids were produced by hand-crossing transgenic Brassica napus (AACC, 2n = 38) and its weedy wild relative Brassica rapa (AA, 2n = 20). B. napus was transgenic for the green fluorescent protein [m-GFP-5ER (GFP)] and the insecticidal protein Bt cry1Ac (Bt). GFP was used as a visual marker to track the presence of Bt, and Bt served as a model fitness enhancing transgene. Hybrids were repeatedly backcrossed to B. rapa to mimic a transgenic introgressed population with a weedy background. Advanced hybrid generations (BC₂F₂, BC₃F₂, and BC₄F₂) were used in productivity and competitive fitness studies over a two-year period. Productivity was found to be variable in each study. Hybrids were equally productive as both parental species in the first year, and in the second overall hybrid productivity was less than both parents. These findings might have differed because rainfall in the first year was below normal and plants suffered from water stress. No water stress was observed in the second year. Additionally, productivity was comparable between transgenic and nontransgenic hybrid genotypes. In the competitive fitness study, hybrids were found to be as competitive, but no more competitive than B. rapa. In both the productivity and competitive fitness studies, transgenic hybrids were at times comparable to their parents, but were not found to be more productive or fit. Therefore, there was no evidence that Bt offered enhanced productivity or fitness. These results suggest that if hybrids survive to advanced generations they will not outcompete their parents, but the percentage of hybrid individuals within the population should remain stable.