A semi-fertile interspecific hybrid of Brassica rapa and B. nigra and the cytogenetic analysis of its progeny

Black rot is a bacterial disease of Brassica rapa caused by Xanthomonas campestris pv. campestris (Xcc.). Sources of resistance to this disease within B. rapa are insufficient and control measures are limited, making the development of resistant breeding lines extremely important. Certain lines of B. nigra exhibit very high resistance to Xcc. For this study, an interspecific cross between Brassica rapa and B. nigra was performed, and a total of 6 F₁ hybrids were obtained through ovary culture. Five plants (H_1–5) were relatively slow-growing, entirely sterile, and had 18 chromosomes in the majority of pollen mother cells (PMCs). GISH analysis showed that most of the PMCs had 8 B. nigra chromosomes, which indicated the expected AB genomic constitution. The last plant (H₆) was partially fertile and the majority of PMCs contained 10 chromosomes of B. rapa and 16 chromosomes of B. nigra, indicating an ABB genomic constitution. Ovary culture techniques were not necessary for the development of the first- or second-backcross generation. Sequence-related amplified polymorphism analysis of F₁, BC₁, and BC₂ plants indicated that some fragments from B. nigra were lost, particularly in the genome of BC₁/BC₂ in successive generation(s). The BC₂ plants expressing good resistance to Xcc. were observed.