Chromosomal Mapping and QTL Analysis of Resistance to Downy Mildew in Cucumis sativus

Downy mildew of cucumber (Cucumis sativus), caused by Pseudoperonospora cubensis, is a major foliar disease worldwide. The cucumber inbred lines K8 (resistant to downy mildew) and K18 (susceptible) were used to study the inheritance of resistance to downy mildew. Chromosomal mapping of the resistance genes was completed to provide a theoretical basis for the resistance mechanisms and for marker assisted selection (MAS). Inoculation was used to test the level of resistance to P. cubensis in the F₂ and F_2:3 families derived from the cross K8 × K18. Simple sequence repeat (SSR) analysis, combined with bulked segregation analysis (BSA), was done with the DNA of F₂ plants using 2,360 pairs of SSR primers. JoinMap Version 3.0 and MapInspect were used to construct SSR linkages and to verify the relationships between these SSR linkages and cucumber chromosomes. Quantitative trait locus (QTL) analysis of downy mildew resistance was done using MapQTL Version 4.0. Inheritance of resistance to downy mildew in K8 was quantitative. Five QTLs for resistance to downy mildew were detected: dm1.1, dm5.1, dm5.2, dm5.3, and dm6.1. The loci of dm1.1 and dm6.1 were on chromosomes 1 and 6, respectively. The loci of dm5.1, dm5.2, and dm5.3 were on chromosome 5, and were linked. Six linked SSR markers for these five QTLs were identified: SSR31116, SSR20705, SSR00772, SSR11012, SSR16882, and SSR16110. Six and four nucleotide binding site (NBS)-type resistance gene analogs (RGAs) were predicted in the region of dm5.2 and dm5.3, respectively. These results will be of benefit for fine-mapping the major QTLs for downy mildew resistance, and for MAS in cucumber.