Genetic Mapping, QTL Analysis and Marker-Assisted Selection for Disease Resistance Loci in Grapes

Genetic linkage maps were created from a segregating population (58 seedlings) of the cross of two grapevine genotypes, ‘Horizon’ (‘Seyval’ x ‘Schuyler’) and Illinois 547-1 (V. rupestris x V. cinerea), using the pseudotestcross strategy. Maps were based on 277 Random Amplified Polymorphic DNA (RAPD) markers plus 30 microsatellites, 4 Cleaved Amplified Polymorphic Sequences (CAPS), and 11 Amplified Fragment Length Polymorphism (AFLP) markers. The 'Horizon' map has 157 markers covering 1199 cM and that of Illinois 547-1 map has 181 markers covering 1470 cM. Both maps have 20 linkage groups. The average map distance between adjacent markers is 7.6 cM for 'Horizon' and 8.1 cM for Ill. 547-1. Segregating resistance to some fungal diseases and two morphological traits were subjected to quantitative trait loci (QTL) analysis. Fourteen QTLs exceeding the threshold of LOD 2.0 were detected. The strongest QTL (LOD 16.4) was found for flower type, indicating a single gene form of inheritance. A major QTL (LOD 6.6) was found for powdery mildew resistance in the Ill. 547-1 (resistant parent) map and two other QTLs with a smaller effect were found in the 'Horizon' map. For black rot resistance, four QTLs were detected, two in each parent. The three most important QTLs were located in the same linkage groups as the ones for powdery mildew. One was also associated with a QTL for production of the phytoalexin resveratrol. The results indicate some form of association among these traits and the presence of major genes or gene clusters for disease resistance in grapes. The possibility of using the major QTL for powdery mildew resistance located on Ill. 547-1 map for marker-assisted selection was also studied. Two markers (a RAPD and an AFLP) linked to this QTL were obtained by bulked segregant analysis and then converted into CAPS markers for testing in four different crosses. Segregation ratio distortions were observed in some crosses. In all cases, the markers were strongly associated with resistance to powdery mildew.