Molecular Mapping Of Brown Planthopper Resistance Gene And Marker-Assisted Selection Breeding In Rice

The brown planthopper (BPH), Nilaparvata lugens Stal, is one of the most serious and destructive pests of rice, and can be found throughout the rice-growing areas of Asia. It attches by preference to the stem, from where it penetrates the phloem and sucks plant sap. thereby causing the plant desiccation. Heavy infestations known as "hopper burn"causes significant yield losses every year. Chemical control of BPH depends on spraying various insecticides, which is commonly practiced for a long time. However, this method is both a threat to the ecological balance and harmful to the environment. Besides, it enhances insect pest resistance to insecticides and affects grain quality. The use of genetically resistant cultivars is an effective environment friendly strategy to control this pest. However, the brown planthopper can overcome resistance genes by changing biotypes. There is thus an urgent need to seek BPH-resistant germplasm from various sources and to utilize the resistance genes for rice improvement. Because the phenotypic identification of brown planthopper resistance is very complex, it is difficult to utilize BPH-resistance genes effectively only by traditional breeding methods. Molecular Marker Assisted Selection (MAS) is considered a highly efficient breeding method because it offers rapid and precise selection of the targeted gene by the close linkage or co-segregated molecular marks.This study contains two research topics, genetic analysis of a BPH resistance indica cultivar LV-4and molecular marker-assisted selection of Bph3and Bph27(t). The results were as followings:1. The Vietnamese landrace LV-4was found by screening new sources for BPH resistance in our laboratory. It showed high level resistance to brown planthopper both at the seedling and mature stage. LV-4was crossed to susceptible cultivar02428and C418respectively. The BPH resistance of parental plants、LV-4/C418F1and LV-4/02428F2:3lines were evaluated through mass screening using seedbox test. The result showed that a single dominance gene dominated the resistance in LV-4. The LV-4/02428F2population containing159individuals was used for genetic analysis and gene mapping. Well, the phenotype of each F2plant was inferred from the phenotype of corresponding F2:3lines. Then linkage analysis was carried out. As a result, a main resistance gene named as Qbph4was detected between RH0078and S4-2on the short arm of chromosome4, with a genetic distance of1.3cM. It had a LOD value of33.76and explained62.4%of the phenotype variations. The mapping of Qbph4would provide gene source and molecular marks for molecular marker assisted selection breeding.2. Based on fine mapping of two BPH-resistance genes Bph3and Bph27(t), MAS was carried out by using the close linkage or co-segregated molecular marks RH0078、RH007、 Q13and Q31. Bph3and Bph27(t) were transferred into elite cultivar Ningjing3via continuous backcross, then a series of stable genetic Bph3-introgressed lines and Bph27(t)-introgressed lines were developed.8Bph3-introgressed and4Bph27(t)-introgressed lines were chosen according to their agronomic trait performance for comprehensive assessment. Results from BPH resistance identification showed that all the introgressed lines were highly resistant to BPH; Analysis of agronomic traits revealed that there were no significant differences in most agronomic traits between introgressed lines and Ningjing3expect spikelet fertility. Genetic background detection of3introgressed lines was carried on using376molecular markers which were evenly distributed on12chromosomes.The result showed that the background recovery rate was from92.80%to95.75%. We found that a large linked chromosome fragment together with targeted gene was introgressed into receptor parent Ningjing3. In order to break the bad linkage, we futher amplified the BC2F2population and used more linkage marks to choose small fragment introgressed lines. The Bph3and Bph27(t) introgressed lines created in this study provided a solid foundation in breeding materials for the establishment of the rice resistant breeding platform.