**7. Genetic mapping of clubroot resistance**

All the first generation of molecular markers such as restriction fragment length polymorphism (RFLP), amplified fragment length polymorphism (AFLP), random amplified polymorphic DNA (RAPD), and simple sequence repeats (SSR) are used to map clubroot resistance in various Brassica species [76–83]. Landry et al. [80] used RFLP markers detected two QTLs in *B. oleracea*. Figdore et al. [79] used RFLP markers and associated several linkage groups to clubroot resistance in *B. oleracea*. Grandclément and Thomas [82] used RAPD markers and analyzed QTL for clubroot resistance in broccoli and cauliflower, and identified some RAPD markers significantly linked to clubroot resistance. Moreover, Voorrips et al. [83] used RFLP and AFLP markers, detected two genes for clubroot resistance in *B. oleracea* doubled haploid lines. Rocherieux et al. [84] performed QTL analysis in *B. oleracea* and detected isolate-specific and broad spectrum QTLs, suggesting that the clubroot resistance in *B. oleracea* is genetically complicated and molecular marker-assisted selection might be not so effective. More recently, Nagaoka et al. [24] performed QTL mapping using a DH line population between resistant cabbage and susceptible broccoli and detected two major QTL for clubroot resistance on chromosomes O2 and O5 and minor QTLs on chromosome O2, O3, and O7. Similar to the conclusion drowned from conventional genetic analysis, clubroot resistance in *B. oleracea* is most likely due to the effects of multiple minor genes and molecular marker-assisted selection might be not as effective as that in Chinese cabbage which contains dominant resistance genes introduced from European turnips.

resistance was identified. For example, 71 accessions of cabbage, broccoli, and curly kale were tested and most of them showed some levels of resistance to clubroot, while all the F1 populations of these resistant and susceptible *B. oleracea* accessions were susceptible [62, 63]. Further analysis indicated that multiple loci are involved in the clubroot resistance in *B. oleracea*, but it was not easy to determine how many loci control clubroot resistance in the analysis of F1, F2, and backcross populations of *B. oleracea* [64]. Moreover, 44 landraces of Portuguese coles (*B. oleracea*) were tested to identify clubroot resistance and three accessions

In *B. napus*, rutabaga cultivars are identified to contain dominant clubroot resistance. In one report, the clubroot resistance in rutabaga was suggested to be controlled by one dominant resistance gene [66]. To investigate the diversity of clubroot pathogen (*P. brassicae*), the Williams differential set was suggested; [67] and in this set, there are two rutabaga accessions that show clubroot resistance in several reports [66, 68, 69]. Vigier et al. [70] tested 31 cultivars and breeding lines of spring canola under controlled environmental conditions and found that several Swedish accessions showed clubroot resistance, but the resistance was not recovered in the subsequent progenies. In another report, the clubroot resistance from rutabaga was transferred into cabbage through interspecific hybridization and results indicated that all the

Radish (*Raphanus sativus*) is a Brassica relative and there are several reports that focus on the identification and transfer of clubroot resistance to Brassica species. Rowe [72] tested 68 radish cultivars and breeding lines collected from several countries and found that all Japanese and most Dutch radish cultivars were completely resistant to clubroot. Akaba et al. [73] used *B. napus*–radish chromosome additional lines to analyze clubroot resistance and found that one chromosome additional line, the c-type, showed a high level of clubroot resistance. More recently, quantitative resistance loci (QTL) mapping for clubroot resistance in radish has been performed and one major gene on one linkage group was found to control the high level of

As discussed earlier, European turnips contain dominant clubroot resistance genes which makes gene mapping easier than in *B. oleracea* varieties. To control clubroot disease, Chinese cabbage hybrid cultivars were developed by introducing clubroot resistance from European turnips into Chinese cabbage in Japan [75]; and currently, clubroot-resistant Chinese cabbage cultivars containing turnip clubroot resistance genes are being used in Japan, South Korea,

All the first generation of molecular markers such as restriction fragment length polymorphism (RFLP), amplified fragment length polymorphism (AFLP), random amplified polymorphic DNA (RAPD), and simple sequence repeats (SSR) are used to map clubroot resistance in various Brassica species [76–83]. Landry et al. [80] used RFLP markers detected two QTLs in *B. oleracea*. Figdore et al. [79] used RFLP markers and associated several linkage groups to clubroot resistance in *B. oleracea*. Grandclément and Thomas [82] used RAPD markers and

showed high levels of clubroot resistance [65].

F1 hybrids were resistant to clubroot disease [71].

**7. Genetic mapping of clubroot resistance**

clubroot resistance in radish [74].

and China.

10 Plants for the Future

In Chinese cabbage, major clubroot resistance loci that are introduced from European turnips have been mapped (Table 1). These clubroot resistance loci are named as *Crr1*, *Crr2*, *Crr3*, and *Crr4* and also *CRa*, *CRb*, *CRc*, and *CRk* in a dozen of investigations, suggesting that there might be eight independent loci [81, 85–87]. Two clubroot resistance loci, *Crr1* and *Crr2* were mapped using SSR markers [75, 88] and a third locus, *Crr3* was identified using RAPD markers, which suggested that there are three independent clubroot resistance loci in Chinese cabbage [85, 87]. Using RFLP markers, a genetic map was constructed and a clubroot resistance locus, *CRa*, was mapped on linkage group 3 [77], and SCAR and CAPS markers were used to map another locus, *CRb*, on chromosome R3 [81]. Moreover, three clubroot resistance loci *CRa*, *CRc*, and *CRk* have been added in the list of clubroot resistance through molecular markerassisted selection [89].

Due to the complex genome structure of *B. napus*, QTL mapping for clubroot resistance is necessary in *B. napus* (Table 1). It is quite common to find the clubroot resistance in *B. napus* that does not segregate as a typical Mendelian trait as in *B. rapa*. Although one major locus was mapped on chromosome N3, two minor QTL on chromosomes N12 and N19 were identified for clubroot resistance in *B. napus* [90]. In another report, [91] a DH line population derived from a cross of clubroot-resistant synthetic *B. napus* and susceptible canola was used to perform QTL mapping for clubroot resistance. The synthetic *B. napus* contains dominant clubroot resistance from European turnip ECD4 and may be medium resistance from *B. oleracea*. They identified a total of nineteen QTLs on chromosomes N02, N03, N08, N13, N15, N16, and N19 for clubroot resistance, and surprisingly, there were four QTLs with LOD values of over 11, of which three were located on chromosome N3 and one on N19 and the proportion of the phenotypic variance explained by each QTL was over 40%. Their data suggested that the major QTLs might come from the C genome of *B. oleracea*, which is contradictory to previous reports where major clubroot resistance genes in *B. napus* come from the A genome of *B. rapa*.
