Mapping clubroot resistance of Brassica rapa introgressed into Brassica napus and development of molecular markers for the resistance

Abstract To date, more than 20 clubroot resistance (CR) loci have been reported in the A‐genome of Brassica rapa ; however, only a few of them has been introgressed into B. napus canola. The introgression of additional CR loci will broaden the genetic base of resistance of this crop. In this paper, we report the genetic basis of CR of B. rapa var. pekinensis cultivar ‘Bilko’ introgressed into B. napus , mapping this resistance using a recombinant inbred line (RIL) population developed from B. napus × Bilko‐CR interspecific cross. Evaluation of the F 2 and F 3 populations of Bilko revealed that a single gene controls resistance to pathotype 3 in this cultivar. Quantitative trait loci‐seq approach using whole‐genome resequencing identified a genomic region of chromosome A03 associated with this resistance in the RIL population. Single nucleotide polymorphism (SNP)‐based allele‐specific markers from the TIR‐NB‐LRR (TNL) gene Bra012688 co‐segregated with this resistance, however, with 0.4–0.8% recombination. Bra012688 is located at 23,877,250–23,883,169 bp of B. rapa cultivar Chiifu‐401 whole‐genome assembly v.3.0, and at 378 bp downstream of another TNL gene Bra012689 . Molecular markers, linked to previously reported CR loci of A03, did not co‐segregate with the resistance in the RIL population; this demonstrates the need for the development of new markers for the CR loci following introgression into the recipient species. The knowledge and the SNP allele‐specific markers developed in this study could be used in breeding for clubroot resistance in B. napus .