title: Fine mapping of the clubroot resistance gene CRb and development of a useful selectable marker in Brassica rapa.
abstract: In Chinese cabbage (Brassica rapa), the clubroot resistance (CR) gene CRb is effective against Plasmodiophora brassicae isolate No. 14, which is classified as pathotype group 3. Although markers linked to CRb have been reported, an accurate position in the genome and the gene structure are unknown. To determine the genomic location and estimate the structure of CRb, we developed 28 markers (average distance, 20.4 kb) around CRb and constructed a high-density partial map. The precise position of CRb was determined by using a population of 2,032 F2 plants generated by selfing B. rapa 'CR Shinki.' We determined that CRb is located in the 140-kb genomic region between markers KB59N07 and B1005 and found candidate resistance genes. Among other CR genes on chromosome R3, a genotype of CRa closest marker clearly matched those of CRb and Crr3 did not confer resistance to isolate No. 14. Based on the genotypes of 11 markers developed near CRb and resistance to isolate No. 14, 82 of 108 cultivars showed a strong correlation between genotypes and phenotypes. The results of this study will be useful for isolating CRb and breeding cultivars with resistance to pathotype group 3 by introducing CRb into susceptible cultivars through marker-assisted selection.
authors: Graduate School of Bioresources, Mie University , 1577 Kurima-machiya, Tsu, Mie 514-8507, Japan ; NARO Institute of Vegetable and Tea Science , 360 Kusawa, Ano, Tsu, Mie 514-2392, Japan.
authors: Takeyuki
authors: T
authors: Kato
authors: Katsunori
authors: K
authors: Hatakeyama
authors: Nobuko
authors: N
authors: Fukino
authors: Satoru
authors: S
authors: Matsumoto
keywords: Brassica rapa; CRb; Plasmodiophora brassicae; clubroot; fine mapping; marker-assisted selection
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