Selection of stable Brassica napus-B. juncea recombinant lines resistant to blackleg (Leptosphaeria maculans). 1. Identification of molecular markers, chromosomal and genomic origin of the introgression1. Identification of molecular markers, chromosomal and genomic origin of the introgression

 A scheme of selection combining selfing and backcross was applied to a B. napus line with the blackleg resistance from B. juncea in order to transfer this resistance to a winter oilseed rape variety. Cytogenetic analyses combined with cotyledon blackleg resistance tests at each generation allowed us to obtain a recombinant line showing regular meiotic behavior. The resistance is monogenic and is highly efficient under field conditions. Four-hundred RAPD primers were tested on two segregating populations by bulk segregant analysis. Three markers totally linked to the introgression were identified. The analysis of these markers on both sets of B. napus-B. nigra and B. oleracea-B. nigra addition lines revealed that they are not located on the B4 chromosome of B. nigra, which has already been shown to carry a blackleg resistance gene, but rather on the B8 chromosome. We confirmed that the resistance gene is carried by the B genome of B. juncea. Based on these data, two hypotheses, one involving chromosome rearrangements between the two B genomes of B. nigra and B. juncea, and the other based on a more probable digenic control of the resistance within B. juncea, are discussed.