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Title: QTL mapping for downy mildew resistance in WI7120B (PI 330628) cucumber

item WANG, Y - University Of Wisconsin
item VANDEN LANGENBERG, K - North Carolina State University
item WEHNER, T - North Carolina State University
item KRAAN, P - Nunhems Seeds
item SUELMANN, J - Nunhems Seeds
item ZHENG, X - Magnum Seeds Inc
item OWENS, K - Magnum Seeds Inc
item Weng, Yiqun

Submitted to: Pickle Packers International Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/14/2015
Publication Date: 10/29/2015
Citation: Wang, Y.H., Vanden Langenberg, K., Wehner, T.C., Kraan, P., Suelmann, J., Zheng, X.Y., Owens, K., Weng, Y. 2015. QTL mapping for downy mildew resistance in WI7120B (PI 330628) cucumber [abstract]. Pickle Packers International Meeting Proceedings.

Interpretive Summary:

Technical Abstract: Background: Downy mildew (DM) is the most devastating fungal diseases of cucumber worldwide. Several plant introduction (PI) lines have been identified to be highly resistant to the post-2004 DM strain in the US including PI 197088 and PI 330628 (WI7120B). However, the genetic basis of resistance in these lines is less well understood; molecular markers for the DM resistance amendable for marker-assisted selection in breeding for DM resistance are not widely available. In the present study, we investigated the inheritance of DM resistance in WI7120B. We also conducted QTL mapping to identify molecular markers associated with DM resistance in this line. Genotyping: A cross was made between the DM resistant inbred line WI7120B and a north China type, DM susceptible inbred line ‘9930’. A linkage map was developed using 91 F2 plants from this cross. Refined map in target QTL regions was based on 243 F2:3 families of the same population. The resulting genetic map contained 347 marker loci including 271 SSRs and 76 SNPs spanning 674.7 cM in 7 linkage groups with an average marker interval of 2.0 cM and a maximum interval of 12.9 cM in Chr6. This map seemed to physically cover the majority of the cucumber genome. The marker orders were also highly consistent with their physical locations in the Gy14 and 9930 scaffold and genome assemblies. Phenotyping: DM screening on 243 F2:3 families were conducted in replicated field trials in two years (2013 and 2014) at three locations: NCSU (2013, 2014), the Netherlands (NL, 2013), and Italy (IT, 2013). Rating of disease severity was based on chlorotic lesions, necrotic lesions, and lesion size of the entire plants. Statistical analysis of data indicated high correlation of mean disease scores across the 4 environments. The normal distribution of data suggested quantitative nature of DM resistance in WI7120B. QTL analysis: Family means and BLUPs of disease scores of each environment were used in QTL analysis. MQM model identified 4 QTL regions underlying DM resistance with three QTL, dm2.1, dm4.1 and dm5.1 being reproducibly detected in 4 environments that together explained 61.8- 75.5% phenotypically variations. Among them, dm4.1 in Chr4 was a major-effect QTL (R2=20.9-50.7%); dm5.1 (Chr5) showed moderate effect on DM resistance (R2=9.5-22.4%) and dm2.1 in Chr2 was a minor-effect QTL (R2=5.5-15.7%). All resistance alleles were contributed by WI7120B with additive effects. Dominant effects of the 3 QTL were weak. The chromosome locations of the three detected QTL were refined with the large F2:3 population. Interactions among the three QTLs were also investigated. Results were compared with those from another resistance source PI 197088.