CONSTRUCTION OF A GENETIC LINKAGE MAP AND IDENTIFICATION OF A MAJOR QTL CONFERRING RESISTANCE TO POWDERY MILDEW IN THE MELON GENOTYPE TGR-1551

Abstract

Melon (Cucumis melo L.) is a valuable crop grown in temperate and tropical regions worldwide. Powdery mildew caused by Podosphaera xanthii is one of the most frequent diseases of melon. The genotype TGR-1551 is resistant to races 1, 2, and 5 of powdery mildew. We constructed a genetic linkage map using an F 2 population derived from a cross between TGR-1551 and the susceptible Spanish cultivar 'Bola de Oro'. The software JoinMap ® 4.0 was used to locate on the linkage map a total of 429 loci: 241 AFLP, 55 RAPD, 117 SSR, 14 SCAR/CAPS/dCAPS, and 2 phenotypic traits (Vat and a loci), which were distributed among 12 linkage groups with a LOD score ≥6.0. The map spans 1318.2 cM, with an average of 3.1 cM/ marker. Powdery mildew resistance was evaluated as a quantitative trait and then, QTL analyses were performed using interval mapping and multiple QTL model mapping with MapQTL ® 5.0 software. Both methods detected a dominant QTL for resistance to either 1, 2 or 5 races of powdery mildew, all located at the same genomic region of G4. Therefore, these QTL were considered as a unique QTL designated as Pm-R. This QTL was supported by LOD scores of 26.5, 33.3, and 36.2, explaining 53.6, 61.9, and 64.9% of the phenotypic variance in the mapping population for resistance to powdery mildew races 1, 2, and 5, respectively. In addition, four codominant markers, previously described as linked to the dominant powdery mildew resistance gene, were linked to the Pm-R QTL, which suggests the correspondence between this QTL and the dominant resistance gene. One of those codominant markers, PM3-CAPS, co-segregated with the Pm-R QTL, and the markers PM2-CAPS and PM4-dCAPS flanked this QTL. These markers could be used to select resistance to powdery mildew in melon breeding.