Fitness of the first backcross generations from the second to the sixth progenies of glyphosate-resistant transgenic Brassica napus and wild Brassica juncea in absence of the herbicide

Successful introgression of a transgene from a transgenic crop into a wild or weedy relative is determined by the fitness of backcross generations carrying the transgene. To provide insight for ecological risk assessment of gene flow between transgenic Brassica napus and wild Brassica juncea, this study investigated the fitness of the first backcross generations from the second to the sixth progenies (BC1F2R-BC1F6R) between glyphosate-resistant transgenic B. napus and wild B. juncea at low density (5 plants/m(2)) and high density (10 plants/m(2)), and monoculture and mixed planting (wild B. juncea: BC1F2R-BC1F6R = 1:1). Correlations between the fitness components of backcross progeny, planting density and planting patterns were analyzed. In the monoculture at low density, compared with B. juncea, earlier generations BC1F2R and BC1F3R had low composite fitness, while later generations from BC1F4R to BC1F6R were more fit. At high density, whatever monoculture or mixed planting, all backcrossed generations had lower composite fitness than B. juncea. Correlation analysis indicated that both planting density and pattern significantly affected the fitness components of the first backcross generations from the second to the sixth progenies (BC1F2R-BC1F6R). These results indicate that the probability of transgene introgression from cultivated B. napus to weedy B. juncea is likely to be highly contingent on the specific growing conditions of their backcross descendants.