Mitigation using a tandem construct containing a selectively unfit gene precludes establishment of Brassica napus transgenes in hybrids and backcrosses with weedy Brassica rapa

Summary Transgenic oilseed rape ( Brassica napus ) plants can interbreed with nearby weedy Brassica rapa , potentially enhancing the weediness and/or invasiveness of subsequent hybrid offspring. We have previously demonstrated that transgenic mitigation effectively reduces the fitness of the transgenic dwarf and herbicide‐resistant B. napus volunteers. We now report the efficacy of such a tandem construct, including a primary herbicide‐resistant gene and a dwarfing mitigator gene, to preclude the risks of gene establishment in the related weed B. rapa and its backcross progeny. The transgenically mitigated and non‐transgenic B. rapa  ×  B. napus interspecific hybrids and the backcrosses (BC 1 ) with B. rapa were grown alone and in competition with B. rapa weed. The reproductive fitness of hybrid offspring progressively decreased with increased B. rapa genes in the offspring, illustrating the efficacy of the concept. The fitness of F 2 interspecific non‐transgenic hybrids was between 50% and 80% of the competing weedy B. rapa , whereas the fitness of the comparable T 2 interspecific transgenic hybrids was never more than 2%. The reproductive fitness of the transgenic T 2 BC 1 mixed with B. rapa was further severely suppressed to 0.9% of that of the competing weed due to dwarfism. Clearly, the mitigation technology works efficiently in a rapeseed crop–weed system under biocontainment‐controlled environments, but field studies should further validate its utility for minimizing the risks of gene flow.