Genotyping-by-sequencing reveals three QTL for clubroot resistance to six pathotypes of Plasmodiophora brassicae in Brassica rapa

Clubroot, caused by Plasmodiophora brassicae , is an important disease of Brassica crops worldwide. F 1 progeny from the Brassica rapa lines T19 (resistant) × ACDC (susceptible) were backcrossed with ACDC, then self-pollinated to produce BC 1 S 1 lines, From genotyping-by-sequencing (GBS) of the parental lines and BC 1 plants, about 1.32 M sequences from T19 were aligned into the reference genome of B . rapa with 0.4-fold coverage, and 1.77 M sequences with 0.5-fold coverage in ACDC. The number of aligned short reads per plant in the BC 1 ranged from 0.07 to 1.41 M sequences with 0.1-fold coverage. A total of 1584 high quality SNP loci were obtained, distributed on 10 chromosomes. A single co-localized QTL, designated as Rcr4 on chromosome A03, conferred resistance to pathotypes 2, 3, 5, 6 and 8. The peak was at SNP locus A03_23710236, where LOD values were 30.3 to 38.8, with phenotypic variation explained (PVE) of 85–95%. Two QTLs for resistance to a novel P . brassicae pathotype 5x, designated Rcr8 on chromosome A02 and Rcr9 on A08, were detected with 15.0 LOD and 15.8 LOD, and PVE of 36% and 39%, respectively. Bulked segregant analysis was performed to examine TIR-NBS-LRR proteins in the regions harboring the QTL.