Genetic Analysis And Preliminary Mapping Of QTL Associated With Powdery Mildew Resistance In Cucumber

Cucumber (Cucumis sativus L) is one of the most important vegetable crops grown in China. Powdery mildew caused by Sphaerotheca fuliginea is one of the most serious diseases in cucumber (Cucumis sativus L.) which occured frequently in greenhouse and open filed. It would not only reduce the photosynthetic efficiency of plants, while impact the quality of fruits and the yield of plants. Therefore, powdery mildew resistance breeding of cucumber has become one of the main objectives. In order to obtain the genetics and molecular data concerning powdery mildew resistance of cucumber, we carried out the experiment.Based on a population of F2 family drived from a cross of the cultivar D8 (susceptible to powdery mildew) and JIN5 (resistant to powdery mildew) of cucumber, genetic analysis for powdery mildew resistance in cucumber has been carried out with the method of quantitive traits heredity mode by using the commix model of main gene and multiple genes. A molecular marker related to the gene of resistant to powdery mildew cucumber was screened with ISSR marker and BSA (bulked segregation analysis) method, and 97*92-2 could be identified as the near-isogenic line with powdery mildew resistance on the basis of this marker. A molecular linkage map was constructed with ISSR and SRAP markers. QTL ananlysis of the resistant to powdery mildew in cucumber was performed with composite interval mapping (CIM). The main results are as follows:1. Six generations of P1 P2 F1 BC1 BC2 and F2 from the cross of D8 (susceptible to powdery mildew) and JIN5 (resistant to powdery mildew) of cucumber were obtained to analyze the genetic effects of resistant to powdery mildew in cucumber by the method which mentioned above. Results indicated that the heredity of resistant to powdery mildew in cucumber could be explained by two additive-dominance-epistasis major genes and additive-dominance polygene model (E-1 model), and mainly by the main genes. The major genes'heritability of resistance to powdery mildew in BC1, BC2 and F2 generations is 84.36%, 80.08% and 90.87%, respectively, whereas the highest value is in F2 and the lowest in BC2. Heritability of polygenes in BC1 is the lowest, and in F2, BC2 is the highest, which was 5.04%, 6.25%, 10.06%, respectively.2. 170 F2 plants from a cross of the cultivar D8 (susceptible) and JIN5 (resistant) were generated to screen the powdery mildew resistance of cucumber. A combination of bulked segregant analysis (BSA) and ISSR methodology was employed to identify markers linked to powdery mildew resistance in cucumber. One ISSR marker UBC809 was identified as being tightly linked to the powdery mildew resistance. Based on the sequence information of the coloned flanking sequence of marker UBC809, one SCAR marker was developed and designated as S2F+S2R. Then, the SCAR marker was used to screen 14 resistant backcross lines which were produced by six generations of backcrosses between D8 and JIN5. Specific band amplified by S2F+S2R was found in each individual of the line 97*92-2, and the disease index of this line showed the lowest value among all 14 lines. Therefore, 97*92-2 could be identified as the near-isogenic line with powdery mildew resistance. The resistant near-isogenic line will be useful for the development of cucumber cultivars with powdery mildew resistance and facilitate the cloning of genes responsible for powdery mildew resistance.3. Based on a population of 188 F2 families drived from a cross of the cultivar D8 (suscepitable to powdery mildew) and JIN5 (resistant to powdery mildew) of cucumber, a genetic linkage map was constructed and mapped QTLs related to the powdery mildew. In this map, a total of 65 markers including ISSR and SRAP markers were clustered into 7 major linkage groups, spanning a total of 831.6cM, with an average distance of 12.7cM. The minimum distance of markers intervals is 4.8cM, and the maximum is 22.3cM. The association of genetic markers and cucumber powdery mildew resistance was investigated. By using of composite interval-mapping approach, 2 QTLs associated with powdery mildew resistance were found, which located on the third linkage group, and they explained 7.6% and 13.5% of the trait variation, respectively. The results would be useful for the marker assisted selection for the cucumber with high powdery mildew resistance.