Genetics and comparative mapping of resistance to Leptosphaeria maculans in Brassica napus
Blackleg, caused by 'Leptosphaeria maculans', is a serious disease of 'Brassica' species. This disease is most effectively and economically controlled by the use of resistant cultivars. Genetic analysis of resistance to 'L. maculans' was conducted in the ' B. napus' accessions, RB87-62 and DH88-752. The two resistant accessions and a susceptible 'B. napus' cultivar Westar were crossed in a complete diallel and their progenies were evaluated for disease reaction by cotyledon and stem inoculation with 'L. maculans', isolate P186-12 (PG2). For crosses between Westar and either resistant parents, the segregation of F2 populations fit 3:1 (resistant:susceptible) both for cotyledon and adult plant reaction, indicating single dominant gene models. Analysis of the relationship between the cotyledon and adult plant resistance showed that genes controlling these two traits were different but linked. These results were confirmed by segregation data from F3 families. The segregation in the cross between two resistant accessions demonstrated that the pairs of resistance genes from the two different sources were non-allelic and independently assorting. DH lines developed from single F1 plants of the crosses between Westar and RB87-62, DH88-752 or Cresor (a 'B. napus' cultivar), were employed for comparative mapping for cotyledon and adult plant resistance to 'L. maculans'. The cotyledon and adult plant resistance genes were mapped 6.1 cM apart in RB87-62 and 10.0 cM apart in DH88-752 respectively. In Cresor, only the adult plant resistance gene was mapped. Although the resistance genes carried by the three accessions were derived from different sources, comparison of the arrangement and map distances of closely linked RAPD, AFLP and RFLP markers on the three resistance linkage maps demonstrated that there is a conserved chromosomal region that carries the resistance genes. The conserved regions covered 41.4 cM in R1387-62, 36.5 cM in DH88-752 and at least 32.0 cM in Cresor. RAPD1, a RAPD marker, was closely linked with cotyledon and adult plant resistance in RB87-62 and DH88-752. For marker-assisted selection (MAS), RAPD1 was converted to a sequence characterized amplified region (SCAR) marker by sequencing the targeted RAPD products. The SCAR marker was tested in two doubled haploid (DH) populations developed from Westar x RB87-62 and Westar x DH88-752 and the F2 plants developed from RB87-62 x DH88-752 and in all cases amplified the locus corresponding to RAPD1. This marker could provide an efficient and reliable screening tool for selection of blackleg resistant plants in breeding programs.