Identification and Fine-Mapping of Clubroot (Plasmodiophora brassicae) Resistant QTL in Brassica rapa

European fodder turnips (Brassica rapa ssp. rapifera) were identified as sources of clubroot resistance (CR) and have been widely used in Brassica resistance breeding. An F-2 population derived from a cross between a resistant turnip and a susceptible Chinese cabbage was used to determine the inheritance and locating the resistance Quantitative Trait Loci (QTLs). The parents showed to be very resistant/susceptible to the field isolates (pathotype 4) of clubroot from Henan in China. After inoculation, 27 very resistant or susceptible individuals were selected to construct bulks, respectively. Next-generation-sequencing-based Bulk Segregant Analysis Sequencing (BSA-Seq) was used and located resistance QTL on chromosome A03 (3.3-7.5 Mb) and A08 (0.01-6.5 Mb), named Bcr1 and Bcr2, respectively. Furthermore, an F-3 population including 180 families derived from F-2 individuals was phenotyped and used to verify and narrow candidate regions. Ten and seven Kompetitive Allele-Specific PCR (KASP) markers narrowed the target regions to 4.3-4.78 Mb (A03) and 0.02-0.79 Mb (A08), respectively. The phenotypic variation explained (PVE) of the two QTLs were 33.3% and 13.3% respectively. The two candidate regions contained 99 and 109 genes. In the A03 candidate region, there were three candidate R genes, namely Bra006630, Bra006631 and Bra006632. In the A08 candidate region, there were two candidate R genes, namely Bra030815 and Bra030846.

Automated summary

European fodder turnips were used as sources of clubroot resistance and two QTLs were identified using a hybrid F-2 population. The resistance QTLs were further confirmed and narrowed down using next-generation sequencing and molecular markers. Candidate resistance genes were also identified within the candidate regions.