The green rice leafhopper (GRH), Nephotettix cincticeps Uhler is a serious insect pest of rice in East Asia. The leafhopper causes either direct damage to the rice plant by sucking the sap or by transmitting several viral diseases. Germplasm resistant to GRH has been found in Indica varieties such as IR24 and DV85. To date, four resistance genes, Grh1, Grh2, Grh3 and Grh4, have been identified (Saka et al. 1997; Fukuta et al. 1998; Yazawa et al. 1998; Tamura et al. 1999).

To identify new resistance genes to GRH, four Indica varieties with GRH resistance were screened by antibiosis test. IR20965-26-1-2, Gadur and Garia were highly resistant and ASD7 was moderately resistant. The resistant varieties were crossed with a susceptible Japonica variety Taichung65. The F₂ individuals derived from the respective crosses were evaluated by antibiosis test at the seedling stage, to find QTLs responsible for resistance to GRH. Seedlings were infested with 7-10 first- or second-instar nymphs from an insect population collected in Fukuoka Prefecture in 1991. Antibiosis was scored as nymph mortality four days after infestation. After evaluation, the plants were transplanted in the field. Leaf samples were collected and the genomic DNA was extracted for RFLP genotyping. A total of 88 RFLP markers scattered in the rice genome were used for constructing RFLP linkage map of respective crosses. QTL analyses for resistance to GRH were conducted using MAPMAKER/QTL v1.1, with a threshold of LOD>2.0.

The resistant varieties, IR20965-26-1-2 and Gadur showed high nymph mortality, nearly 100%, whereas ASD7 showed unstable resistance, 50-100% nymph mortality. Taichung65 was susceptible, without nymph mortality. Frequency distribution of nymph mortality in each F₂ population was continuous and did not represent any kind of discrete segregation. A total of eight QTLs for antibiosis were detected on six different loci in four cross combinations (Table 1). Each QTL explained 25.2% to 70.0% of the phenotypic variance. All Indica alleles on the QTLs contributed to the increase in nymph mortality. Six of the eight QTLs corresponded with the genes resistant to GRH based on chromosome location. Four QTLs,

qGRH-5, qGRH-6, qGRH-11 and qGRH-3, appeared to coincide with Grh1, Grh3, Grh2 and Grh4, respectively (Saka et al. 1997; Fukuta et al. 1998; Yazawa et al. 1998; Tamura et al. 1999).

The four Indica varieties carried a few genes resistant to GRH. ASD7 had both Grh1 and Grh3. Although the resistance of ASD7 was moderate, the F₂ segregants carrying both Grh1 and Grh3 were highly resistant. The highly resistant varieties, Gadur and Garia, carried the same resistance genes, Grh2 and Grh4. The high level of resistance of IR20965-26-1-2 due to both qGRH-4 and qGRH-2 was noted because there has been no report on GRH resistance gene on chromosome 2. qGRH-4 was located near the gene resistant to green leafhopper Nephotettix virescens Distant (Sebastian et al. 1995). Development of isogenic lines for qGRH-4 and qGRH-2 is necessary to precisely analyze the genetic basis of GRH resistance of IR20965-26-1-2.

References

Fukuta, Y., K. Tamura, M. Hirae and S. Oya, 1998. Genetic analysis of Resistance to green rice leafhopper (Nephotettix cincticeps Uhler) in rice parental line, Norin-PL6, using RFLP markers. Breed. Sci. 48: 243-249.

Saka, N., T. Toyama, T. Tuji, H. Nakamae and T. Izawa, 1997. Fine mapping of green ricehopper resistant gene Grh-3 (t) and screening of Grh-3 (t) among green ricehopper resistant and green leafhopper resistant cultivars in rice. Breed. Sci. 47(suppl.1): 55.

Sebastian, L. S., R. Ikeda, N. Huang, T. Imbe, W. R. Coffman and S. R. McCouch, 1995. Molecular mapping of resistance genes to rice tungro spherical virus and green leafhopper in rice. Phytopathology 86: 25-30.

Tamura, K., Y. Fukuta, M. Hirae, S. Oya, I. Ashikawa and T. Yagi, 1999. Mapping of Grh1 locus for green rice leafhopper resistance in rice using RFLP markers. Breed. Sci. 49: 11-14.

Yazawa, S., H. Yasui, A. Yoshimura and N. Iwata, 1998. RFLP mapping of genes for resistance to green rice leafhopper (Nephotettix cincticeps Uhler) in rice cultivar DV85 using near isogenic lines. Sci. Bull. Fac. Agr. Kyushu Univ. 52: 169-175. (Japanese with English summary)