Mapping quantitative trait loci (QTL) for clubroot resistance in Brassica rapa L.

Abstract

SummaryThe genes related to clubroot resistance (CR) from a new genetic resource of European turnip (Brassica rapa ssp. rapifera) were identified, and their chromosomal locations were determined using quantitative trait loci (QTL) analysis. One hundred and ninety F2 plants derived from a cross between a clubroot-resistant inbred turnip line (IT033820) and a susceptible inbred Chinese cabbage (B. rapa ssp. pekinensis) line (BP079) were used to construct a genetic linkage map. The map contained 477 markers, including 401 amplified fragment length polymorphism (AFLP) and 56 simple sequence repeat (SSR) markers, in ten linkage groups (LGs). It covered a total length of 1,149.1 cM, with an average interval of 2.53 cM between markers. The majority of individual F2 plants were found to be highly resistant to clubroot disease based on phenotypic evaluations following inoculation with race 4 of Plasmodiophora brassicae. Using composite interval mapping analysis, two major QTLs (Pb-Br3 and Pb-Br8) and one minor QTL associated with resistance to race 4 of P. brassicae were detected. Pb-Br3 and the minor QTL were located on LG3, while Pb-Br8 was on LG8. Most of the markers identified by bulk segregant analysis (BSA) were located in the region of the two major QTLs. Along with the markers developed in the present study, the Pb-Br3 region included the previously published markers HC352b-SCAR and TCR08, for the loci CRa and CRb, respectively. Five markers, including OPW06_297 identified by BSA, were located in the Pb-Br8 region, spanning 5.9 cM. The QTL Pb-Br8 differed from previously reported CR genes. The two major QTLs explained 85.4% of the total phenotypic variation. The QTLs Pb-Br3 and Pb-Br8 were marked by two convenient co-dominant sequence characterised amplified region (SCAR) and cleaved amplified polymorphic sequence (CAPS) markers, CRW06_297 and CRU12_SspI, respectively. These markers will assist in future marker-assisted selection to accelerate the breeding of clubroot-resistant Brassica crops.