Construction of a high-density genetic linkage map and identification of quantitative trait loci associated with clubroot resistance in radish (Raphanus sativus L.)

Clubroot is a devastating disease caused by Plasmodiophora brassicae, leading to substantial yield loss of cruciferous plants, especially radish (Raphanus sativus) in China. Thus, there is a need to elucidate the mechanisms underlying the pathogenicity of P. brassicae and to breed clubroot-resistant radish cultivars. In the present study, we constructed a high-density linkage map of R. sativus by restriction-site-associated DNA sequencing (RAD-Seq) using an F-2 population derived from a cross between the clubroot-resistant and clubroot-susceptible inbred lines BJJ and XNQ, respectively. The genetic map spans 794.3 cM and has 1148 SNPs distributed across nine linkage groups corresponding to the nine chromosomes of R. sativus. The average distance between the adjacent markers is 0.7 cM. A set of five QTLs (viz., RsCr1, RsCr2, RsCr3, RsCr4, and RsCr5) associated with resistance to clubroot was detected on chromosomes 8 and 9. The limit of detection values of quantitative trait loci (QTLs) ranged from 5.23 to 7.65, accounting for 7.26-31.38% of the observed phenotypic variance. Synteny analysis showed that RsCr1 is homologous to the clubroot resistance gene Crr1 in Brassica rapa. This high-density genetic map of R. sativus provides valuable information for clubroot resistance gene selection and resistant variety breeding, while the QTL mapping results provide a reference dataset for effective gene exploration and marker-assisted breeding programs.