Stability of QTL for field resistance to blackleg across two genetic backgrounds in oilseed rape

Blackleg, caused by Leptosphaeria maculans (Desm,) Ces, ct de Not., is a ma jor disease of oilseed rape (Brassica napus L,) worldwide, Molecular marker s would be useful tools to assist breeding for blackleg resistance, The obj ective of this study was (i) to map and characterize quantitative trait loc i (QTL) for field blackleg resistance in doubled haploid (DH) and F, popula tions from the cross 'Darmor' (resistant) X 'Samourai' (susceptible) and (i i) to compare QTL with those previously identified in the cross 'Darmor-bzh ' X 'Yudal', A total of 134 DH lines and 185 F-2:3 families were genotyped with random amplified polymorphic DNA (RAPD) and restriction fragment lengt h polymorphism (RFLP) markers and assessed for a disease index of resistanc e in 1998 and/or 1999 in one location. Genetic maps derived from the two po pulations included a total of 257 and 81 markers, respectively. Up to 30% o f these markers were common to the Darmor-bzh X Yudal map previously used. A total of six and four genomic regions were associated with resistance in the DH and F-2:3 populations, respectively. They collectively explained 36 to 42% of the variation within each year and population. Three of them were consistent across the two populations derived from Darmor X Samourai cross and expressed dominant or overdominant effects, Four favorable alleles wer e derived from the susceptible parent. A total of 16 genomic regions were r evealed for blackleg resistance in the two crosses Darmor-bzh x Yudal and D armor x Samourai studied, Four of them were consistent over the two crosses . The inconsistencies observed between populations and crosses can be expla ined by different genetic backgrounds and disease infestation levels. For m arker-assisted selection, these results suggest that QTL mapping must be ca rried out separately for each population.