Scab is a prevalent disease of cucumber worldwide. Scab can cause serious losses for cucumber production, especially in protected culture such as high tunnel production (Lee et al., 1997; Mao et al., 2008; Rego, 1994). Resistance to cucumber scab is dominant and is controlled by a single gene, Ccu (Abul-Hayja et al., 1975; Andeweg, 1956; Bailey and Burgess, 1934; Vakalounakis, 1993; Wehner, 2005). Researchers have reported on methods of identification (Xu and Zhu, 1994), mechanisms of pathogenicity (Li et al., 2001, 2008), and techniques for control (Li et al., 1998; Yuan, 1989) of cucumber scab. Genetic linkage of Ccu with other traits has also been studied (Mao et al., 2008; Vakalounakis, 1993; Vakalounakis and Klironomou, 1994). However, there are only a few reports on the molecular biology of scab on cucumber. Zhang et al. (2006) screened the amplified fragment length polymorphism (AFLP) marker, E20M64, linked to the Ccu gene, and reported a genetic distance of 4.83 cM. Sun et al. (2006) reported a simple sequence repeat (SSR) primer, CSWCT02B, that was linked to the Ccu gene with genetic distance of 28.7 cM. Bradeen et al. (2001) identified one restricted fragment length polymorphism (RFLP) marker, CMTC51 (0.5 cM), and one AFLP marker, E14M49-F-158-P2 (1.9 cM), linked to the Ccu gene using wide-based, merged maps. Thus far, no linkage of SSR markers to the Ccu gene has been reported, and no SSR marker with genetic distance less than 1 cM to the Ccu gene in cucumber has been identified.
Scab can be efficiently controlled using resistant cultivars. Breeding for resistance was reported as early as 1934, but breeding work has continued throughout the world (Andeweg, 1956; Bailey and Burgess, 1934; Cavatorta et al., 2007; Li and Feng, 1994). However, phenotypic selection for scab resistance is difficult due to the large influence of environmental factors on development of the disease, and may require tests with multiple replications and years. Given these difficulties, cucumber breeders would benefit from a more efficient and more reliable scab resistance breeding procedure such as marker-assisted selection (MAS). With MAS, selection is on the marker rather than the disease rating.
An efficient MAS system requires markers having tight linkage to Ccu, but combined with an assay that is easy to run. In previous studies, the markers linked to Ccu were RFLP and AFLP types, which are expensive, time consuming to measure, and involve the use of radioactive materials. Those marker types are not cost-efficient to use in molecular breeding. However, SSR markers are polymerase chain reaction (PCR)-based markers with codominant inheritance that are effective and easy to use in a breeding program. Development of SSR markers requires an investment before they can be used because genomic sequences have to be determined before primers can be designed and then used for mapping. Fortunately, the Cucumber Genome Initiative (CUGI), aimed at sequencing the genome of cucumber, was sponsored in China in 2007. Over 2000 pairs of highly polymorphic SSR markers from whole genome shotgun sequences were developed (Ren et al., 2009). In this article, a study of genetic mapping of the Ccu gene was conducted and SSR markers for the Ccu gene were identified that had tight linkage. Those SSR markers have been used in the selection of resistant germplasm.
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Park, Y.H. Sensoy, S. Wye, C. Antonise, R. Peleman, J. Havey, M.J. 2000 A genetic map of cucumber composed of RAPDs, RFLPs, AFLPs, and loci conditioning resistance to papaya ringspot and zucchini yellow mosaic virusesGenome 43 1003 1010 10.1139/g00-075
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