Introgression of Herbicide-Resistant Gene from Genetically Modified Brassica napus L. to Brassica rapa through Backcrossing

Interspecific hybridization between two different Brassicaceae species, namely <i>Brassica rapa</i> ssp. <i>pekinensis</i> (♀) (AA, 2n = 2x = 20) and genetically modified <i>Brassica napus</i> (♂) (AACC, 2n = 4x = 38), was performed to study the transmission of a herbicide resistance gene from a tetraploid to a diploid <i>Brassica</i> species. Initially, four different GM <i>B. napus</i> lines were used for hybridization with <i>B. rapa</i> via hand pollination. Among the F1 hybrids, the cross involving the <i>B. rapa</i> (♀) × GM <i>B. napus</i> (♂) TG#39 line exhibited the highest recorded crossability index of 14.7 ± 5.7. However, subsequent backcross progenies (BC1, BC2, and BC3) displayed notably lower crossability indices. The F1 plants displayed morphological characteristics more aligned with the male parent <i>B. napus</i>, with significant segregation observed in the BC1 generation upon backcrossing with the recurrent parent <i>B. rapa</i>. By the BC2 and BC3 generations, the progeny stabilized, manifesting traits from both parents to varying degrees. Cytogenetic analysis revealed a substantial reduction in chromosome numbers, particularly in backcrossing progenies. BC1 plants typically exhibited 21-25 chromosomes, while BC2 progenies showed 21-22 chromosomes, and by the BC3 generation, stability was achieved with an average of 20 chromosomes. SSR marker analysis confirmed the progressive reduction of C-genome regions, retaining minimal C-genome-specific bands throughout successive backcrossing. Despite the extensive elimination of C-genome-specific genomic regions, the glyphosate resistance gene from the male parent <i>B. napus</i> was introgressed into BC3 progenies, suggesting that the glyphosate resistance gene located and introgressed in A-chromosome/genome regions of the Brassica plants.