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

Abstract

 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 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.