The most serious pests of rice (Oryza sativa L.) in the Asian rice growing areas are the sucking insects, including brown planthopper (Nilaparvata lugens Stal), small brown planthopper (Laodelphax striatellus Fallen), and green leafhopper (Nephotettix virescens Distant). Brown planthopper (BPH) causes direct damage by sucking plant sap, and it also transmits several viral diseases such as rice grassy stunt (Rivera et al. 1966) and rugged stunt (Ling et al. 1978). Small brown planthopper (SBPH) transmits rice stripe virus (RSV) causal organism of the rice stripe disease, one of the most serious viral diseases of rice (Toriyama et al. 1986). We report here the molecular marker-based genetic analyses conducted to find the common genetic factors for resistance to the sucking insect pest BPH and RSV in the resistant cv DV85.

The genetic materials were 81 recombinant inbred lines (RILs) derived from a cross between Japonica rice variety Kinmaze and Indica DV85 (Ikeda et al. 1998). The BPH population used for infestation was biotype 2, first collected from a rice field at Hangzhou, China, has been maintained on Mudgo in the greenhouse for ten generations. In this experiment, insects were maintained on a susceptible cultivar TN1 under natural condition of the summer season in the greenhouse of Nanjing Agriculture University, Nanjing, China. The small brown planthopper population, first collected from a rice field at Yancheng, China, have been maintained a susceptible cultivar Wuyujing3 under natural condition of the summer season in the greenhouse of Nanjing Agriculture University.

To evaluate BPH in RIL population, approximately sixty-five seeds of each RIL and parent were sown in a 10cm-diameter plastic pot with a hole in the bottom. Twenty-eight pots, together with a pot each of parental varieties, were placed in a 68 × 42 × 16cm plastic seed-box. Approximately 2-cm of water was maintained in the bottom of the seed-box. At the secondleaf stage, 15 days after sown, the seedlings were infested with 2nd to 3rd-instar BPH nymphs at six insects per seedling. The plants were given a score of 0, 1, 3, 5, 7 or 9 according to criteria based on Athwal et al. (1971). The BPH resistance level of each entry was then calculated based on the weighted average of the seedlings examined. On the other hand, each RIL and parent was used to evaluate the reactions to virus by inoculating small brown planthopper with rice stripe virus. The rate of viruliferous SBPH was estimated around 40% by dotimmunobinding assay. Twenty-five to thirty seedlings of each RIL were planted within 9cm × 9cm space in a plastic tray filled with nutrient soil. When the rice plants reached 1.5-2.1 leaf stage, a plastic cylinder 9 cm in diameter covered with gauze was put on each RIL. Inoculation was conducted by releasing 2nd to 3rd instar nymphs of vector insects into the plastic cylinder at the rate of five insects per plant. The RIL were evaluated at 3-4 weeks after inoculation by surveying the diseased percentage and type of the symptom following the criteria of Washio et al. (1968).

The results of evaluations indicated that DV85 was resistant to the BPH and RSV while Kinmaze was highly susceptible (Fig. 1 and 2). The BPH and RSV resistance scores of the RIL families showed continuous distributions with valleys in the distribution curves. Such distributions indicated the involvement of major genes controlling the segregation of resistances in this population. Therefore, QTL analyses were carried out using composite interval mapping (CIM) with QTL Cartographer (Zeng, 1994). A relatively low threshold of LOD (log₁₀ of the likelihood odds) score of 2.0 was set up to detect all the possible QTLs and epistasis in this study because it is difficult to detect QTLs using a high threshold in smaller population (Yano, 1997). One major QTL (Qbph11) associated with BPH resistance with a LOD score of 10.1 was detected near RFLP marker C1172 on chromosome 11, of which the resistance alleles

originated from DV85 (Table 1, Fig. 3). This QTL explained 68.4% of the phenotypic variance of BPH resistance, thus it should be classified as a QTL with major effects. Meanwhile, three QTLs underlying resistance to RSV were detected on chromosomes 1, 7 and 11, respectively (Table 2, Fig. 3). The resistant allele of the QTLs on chromosomes 7 and 11 originated from DV85, while the QTL on chromosome 1 was from Kinmaze. Both the major QTLs of Qstv11 and Qbph11 were detected near C1172 on chromosome 11 and all the resistance alleles of them originated from DV85. No epistasis among these resistance QTLs could be detected with two way ANOVAs for epistatic interaction due to the limited population size. Interestingly, a pair of dominant resistance genes of DV85 with complementary expression to green rice leafhopper (GRH) and green leafhopper (GLH) has already been mapped on chromosomes 3 and 11, which were designated as Grh4 and Grh2, respectively (Yasui et al. 1999). Grh2, was tightly linked to marker G1465. On the linkage map used in the present study, G1465 was only 9.5 cM distance to C1172, which in turn was found to be near to Qbph11 and Qstv11 on chromosome 11 (Fig. 3). Therefore, chromosome regions around G1465 might be related to rice resistance to sucking insects, or some sucking insects resistance genes might cluster in this chromosome region of DV85.

Acknowledgement

We are indebted to Professors H. Yasui and A. Yoshimura (Plant Breeding Laboratory, Agricultural Faculty of Kyushu University, Japan) for kindly providing the genetic materials, marker data and important suggestions. This study was sponsored by 863 project (2003AA207020) and SRF for ROCS, SEM (16073), China.

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