Molecular and phenotypic identification of B-genome introgression linked to Leptosphaeria maculans resistant gene Rlm6 in Brassica napus × B. juncea interspecific hybrids

Blackleg is a devastating disease in canola worldwide, except in China, caused by the fungal pathogen Leptosphaeria maculans. The B-genome Brassica species were reported to have a strong resistance to the blackleg pathogen L. maculans. Backcross (BC) generations, BC1F1 to BC4F1, were derived from a cross B. napus × B. juncea. Phenotype of L. maculans isolate J20 showed that 49% of BC1F1, 27% of BC2F1, 15% of BC3F1, and 10% of BC4F1 plants were resistant to the isolate J20. Offspring from the interspecific hybridization were also analysed for the presence of dominant type SCAR markers detecting loci linked to the B. juncea genome. The plants with B. juncea introgression had a decrease in the presence of SCAR markers ranging from 47% in BC1F1 to 30% in BC2F1 and further down to 18% in BC3F1 and 11% in BC4F1 with respect to the marker B5Rlm6_1. A similar trend of loci reduction was also observed for the marker B5-1520. In contrast, the progression of the B. napus genome correlated with an incremental increase in the presence of the two markers with the advancement of the generations. However, segregation of SCAR markers and phenotypes for the blackleg resistance in BC1F1 plants had an acceptable fit to a Mendelian ratio of resistant versus susceptible, supporting the assumption that the genetic control of resistance is governed by a single dominant gene. The BC generations developed in this study, which show introgression of the B. juncea genome linked to the L. maculans resistance gene Rlm6, would facilitate breeding a B. napus variety resistant to blackleg in the future.