RGA- and RAPD-derived SCAR markers for a Brassica B-genome introgression conferring resistance to blackleg in oilseed rape.

An introgression derived from the B genome of Brassica juncea in spring-type oilseed rape (B. napus) conferring recessively inherited cotyledon resistance against several pathotypes of the blackleg fungus Leptosphaeria maculans was mapped using PCR-based molecular markers. Resistance-associated B-genome-specific randomly amplified (RAPD) and resistance gene analog (RGA) DNA polymorphisms were converted into three sequence-specific markers (SCARs; B5-1520, C5-1000, RGALm). The flanking sequence of the RGALm locus was determined by genomic walking, leading to a 1,610-bp EcoRV fragment which showed extensive homology to known and putative resistance genes of a cluster on Arabidopsis chromosome 5. Partial sequence analysis of the genomic RAPD segment OPC-05-1700 revealed strong homology to the gibberellin 2-oxidase gene of Arabidopsis. The SCAR markers were analyzed in two segregating populations and were found to be linked in coupling to each other, and in repulsion to the resistance locus. In both populations, markers deviated significantly from a monogenic 3:1 segregation ratio, with plants lacking the markers being more frequent than expected. Although the mode of introgression is yet unknown, the recombinant individuals observed among susceptible progeny suggest homeology between the B-genome-specific segment and its B. napus counterpart. This would offer prospects for reducing the size of the introgression and further fine mapping of the resistance locus.