Genetic Map Construction And Disease Resistance Gene Analysis Of Cucumis Melo L.

Xinjiang is one of the second origin center of melon where there are many excellent melon varieties, but these varieties have poor resistance to diseases such as powdery mildew, downy mildew, so these diseases cause great loss to melon production. Breeding and using the resistance cultivars is an effective way to solve the problem. The genetic analysis and cloning of the functional gene especially the disease resistance gene was an important content of the melon genomic research, and it's the pivotal problem of molecular breeding of melon. In this paper, the disease resistance and other important traits of melon were studied by genetic analysis with molecular markers. The study consisted of four parts: (1) construcion of a genetic map of melon with molecular markers and mapping of QTLs involved in melon fruit and seed traits. (2) the resistance gene analogs(RGA) of melon were isolated by PCR, in which degenerate primers designed according to conservative domain of resistance genes were used, and the RGAs'genomic distribution, involution and the relation with resistance genes were studied by the bioinformation and Comparative Genomics method. (3) the genes that confer resistance to powdery mildew and downy mildew were mapped in two populations, and layed foundation for the cloning of disease resistance gene and resistance breeing of melon. (4) elite and powdery mildew and wilt resistance lines were breeded by marker assisted selection.A genetic map of melon was constructed using 143 F2 population developed from a cross between two distant line Annong2 of Japan and Hami melon K413. The map contains 12 linkage groups and 142 markers, including 121 AFLPs, 16 SSRs, 3 STSs, 2 trait markers and covers 1014cM. Composite Interval Mapping (CIM) method was used to detect QTLs involved in melon fruit and seed trait: fruit length (FL), fruit width (FW), fruit shape (FS), flesh color (FLESH), centre sugar (CS), edge sugar (ES), flesh texture (FT), seed length (SL), seed width (SW), seed shape (SS) and seed weight (SW). The result showed that FLESH was located in between AFLP markers NDAA and NCFA on C9. A total of 25 QTLs were detected involved in other traits and some co-located with each other. The QTLs Sl5.1, Sw5.1 and Swt5.1 located on linkage C5 between NCA and N73C explained a significant portion of the associated phenotypic variation (R2=17%, 19%, 23%). The allele from Annong2 obviously suppressed the length,width and weight of melon seed; QTLs between N73A and NFDA on C8 were involved in seed width, shape and weight; QTL Fs8.1 on C8 was detected using both F2 and F3 fruit data and explained a significant portion of phenotypic variation 25% and 19%, Fs8.1 showed partly dominant, the allele from Annong2 suppressed elongation of fruit and so formed round melon. QTLs related to centre sugar, edge sugar and fruit texture were detected in our research.To isolate resistance gene analogs(RGA) of melon, degenerate primers designed according to NBS-LRR conservative domain of resistance genes were used to amplify with melon genomic DNA. Seven RGAs were cloned and sequenced, out of which 641 and 324 were newly found in this research. All the RGAs were compared to discard the redundant sequence and incorporate the useful information. Different distributing status between CC-NBS-LRRs and TIR-NBS-LRRs were found by The homologous RGA analysis, and also relation of different RGA was found, such as MRGA10 and MRGH5 might be originated from common ancestor. Bioinformation and comparativegenomics method were used to discover the relation between RGAs and disease resistance genes. The Fom-1 gene conferring resistance to Fusarium oxysporum f.sp. melonis race 2 and the Prv conferring resistance to papaya ring spot virus were located between RGAs nbs47-3 (MRGH11) and MRGH21, so it was deduced that candidate gene MRGH8 and the Fom-1 were the same gene, while Prv might be same with one of MRGH9 and MRGH10.Powdery mildew is one of the most important melon pathogens all over the world. So far, many genes conferring resistance to powdery mildew of melon have been described, but few of these have been finely mapped or cloned. Two F₂ populations derived from Annong2×Hami413 and Annong2×Queen were used to map the powdery mildew resistance gene by methods of Bulked Segregation Analysis (BSA), comparative genomics and Resistance Gene Analogues (RGA) mapping. It was found that the resistance to powdery mildew in Annong2 was conferred by a dominant gene, and the gene was named Pm-AN. The genetic analysis revealed that Pm-AN located between two codominant markers RPW and MRGH63B in linkage groupV. The genetic distances between Pm-AN and these two markers were 1.4-1.8cM and 1.6-2cM. No recombination was found between Pm-AN and markers ME/E1, SRAP23. Pm-AN was located in a RGA-rich region and cosegragated with the RGA marker MRGH5 and the resistance gene Vat. Synteny analysis showed that markers in this region were collinear between melon and cucumber. Segregation distortion was found in this region using both Annong2×Hami413 and Annong2×Queen F₂ populations, and the distortion was more distinct in Annong2×Hami413 F₂ population. The center of segregation distortion was located in the RGA rich region harboring Pm-AN.Base on the mapping of resistance to powdery mildew melon, marker assisted selection was adopted to help breeding elite resistance melon cultivars. Two homozygous lines resistant to both diseases were selected because their excellent traits such as high sugar content and the beautiful appearance. Other elite lines with improved disease resistance were obtained in the experiment, too.