Resistance to Leptosphaeria maculans is conserved in a specific region of the Brassica B genome

Offspring from asymmetric hybrids between Brassica napus and the three B-genome species Brassica nigra, Brassica juncea and Brassica carinata were analysed for the presence of B-genome markers and resistance to the fungus Leptosphaeria ia maculans, the causal agent of blackleg disease. Twenty five plants from each species combination were analysed in the first backcross (BC1) generation, 30 plants in BC2 and 60 plants in BC3. The plants were analysed by 46 RFLP markers detecting 85 loci dispersed throughout the B, nigra genome. The plants with additional B. carinata DNA had a decrease in the presence of RFLP markers ranging from 59% in BC1 to 36% in BC2 and down to 11% in BC3. Similar results were obtained in the lines with additional DNA from B. juncea where the 60% presence of RFLP markers in BC1 was reduced to 33% in BC2 and to 10% in BC3. However presence of the markers were significantly lower in the B. nigra-derived material where BC1 had 46%, BC2 25% and BC3 8%. Since at least two loci could be detected on each end of the eight linkage groups of the B genome, the degree of symmetry was estimated. After one back-cross between 0.5 and 1.25% intact chromosomes were retained, whereas in BC2 this frequency was 0.21% for all three B-genome donor species. The maintenance of half-chromosomes ranged from 2.63% to 5.38% in BC1 and between 0.73% and 1.15% in BC2. No chromosome arms were found in any of the BC3 plants. In total, four co-segregating markers for cotyledon and adult-leaf resistance to L. maculans were found which detected six loci located on linkage groups 2, 5 and 8. When the results from the three donor species were compared one triplicate region in the B genome had preserved the resistance loci in all three species.