Molecular mapping of resistance to Leptosphaeria maculans in Australian cultivars of Brassica napus.

Doubled haploid (DH) lines together with a cotyledon bioassay were employed for the molecular analysis of resistance to the blackleg fungus Leptosphaeria maculans in the Australian Brassica napus cultivars Shiralee and Maluka. We used bulked segregant analysis to identify 13 RAPD and two RFLP markers linked to the resistance phenotype and mapped these markers in the segregating DH population. Our data suggest the presence of a single major locus controlling resistance in the cultivar Shiralee, confirming our previous results obtained from Mendelian genetic analyses. In addition, preliminary mapping data for the cultivar Maluka also support a single locus model for resistance and indicate that the resistance genes from 'Shiralee' and 'Maluka' are either linked or possibly identical. The molecular markers identified in this study should be a useful tool for breeding blackleg resistant varieties using marker-assisted selection, and are the essential first step towards the map-based cloning of this resistance gene.