Abstract: 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/m²) and high density (10 plants/m²), 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.

Key words: wild Brassica juncea, glyphosate-resistant transgenic Brassica napus, backcross generations, fitness, gene flow