Persistence of the bar transgene during recurrent backcrossing between GM Brassica napus and Brassica rapa subsp. narinosa

Interspecific hybridization between cultivated crops and their wild or closely related species is a major route for transgene escape and remains an important concern in environmental biosafety assessment of genetically modified (GM) crops. In this study, we evaluated the potential for bar transgene introgression from GM Brassica napus line TG#39 into Brassica rapa subsp. narinosa ‘Dachae’ (DC) through reciprocal hybridization and recurrent backcrossing. Although both reciprocal crosses produced viable F₁ hybrids, reproductive performance declined markedly in the F₂ generation and varied between the two backcross lineages; nevertheless, viable BC₁ and BC₂ progenies were obtained. Morphological analyses showed that F₁ hybrids displayed paternal- or intermediate-like characteristics, while BC₁ progenies showed substantial phenotypic variation. BC₂ progenies showed reduced within-group dispersion for some traits, although the direction and magnitude of variation differed among traits and lineages. Morphological and SSR analyses provided complementary evidence: morphological traits revealed generation- and lineage-dependent segregation, whereas SSR markers showed lineage-specific similarity to the corresponding recurrent parents. In the DC × TG#39 lineage, the bar transgene remained detectable in tested BC₂ progenies despite increased SSR-based similarity to the DC recurrent parent. These findings show that the bar transgene remained detectable through two successive backcross generations in this controlled crossing system, supporting consideration of subsequent backcross generations in GM Brassica biosafety assessment.