Nephotettix cincticeps Uhler (Green rice leafhopper: GRH) is one of the serious insect pests of rice in temperate Asia. We are attempting to search the GRH resistance gene of wild relatives in rice and identified the new resistance gene, Grh5 (Green rice leafhopper resistance 5), using O. rufipogon introgression lines (Fujita et al. 2003). In this study an accession of wild relatives, Oryza nivara (IRGC105715) from Cambodia was found to be highly resistant to GRH by antibiosis test. The GRH resistance transferred from O. nivara was analyzed by using introgression lines of rice.

Two sets of introgression lines each carrying wild and cultivated cytoplasm have been developed from reciprocal crosses between O. sativa cv. Taichung 65 (T65) and O. nivara (Acc. IRGC105715). To develop O. nivara introgression lines, the F₁ plants obtained by reciprocal crosses were continuously backcrossed with T65. To detect GRH resistance gene, the BC₄F₁ population with O. nivara cytoplasm was analyzed through whole genome survey using RFLP markers. The BC₃F₁ plants with T65 cytoplasm were evaluated for GRH resistance. The BC₃F₂ individuals derived from a single resistant BC₃F₁ plant were analyzed using SSR markers (McCouch et al. 2002) for linkage analysis. GRH resistance was evaluated as the following; the leaf blades were excised and infested with 7-10 first- or second-instar nymphs. Then, the nymph mortality (NM) at 4 days after infestation was calculated. The plants with more than 50% NM and those with less than 30% NM were classified to resistant and susceptible, respectively. The insect population was collected in Fukuoka Prefecture in 1991, and was maintained by continuously rearing at 25C and 16h light: 8h dark condition.

Five plants out of BC₄F₁ plants showed resistance to GRH. These carried a common segment from O. nivara in the short arm of chromosome 4 based on RFLP analysis. This suggests that the resistance gene introgressed from O. nivara would be located on the short arm of chromosome 4. The NM of 185 BC₃F₂ individuals showed a discrete distribution, segregating into 140 resistant and 45 susceptible plants (Fig. 1). The segregation ratio fitted to a 3:1 ratio (chi² = 0.83), indicating that the GRH resistance introgressed from O. nivara was controlled by a single dominant gene. SSR analysis revealed that the resistance gene was located between RM5414 and C60248 and tightly linked to RM8213 on the short arm of chromosome 4 (Fig. 2).

Tamura et al. (1999) reported a GRH resistance gene derived from a cultivar SML 17

(from Surinam) tightly linked to Y3635R on the short arm of chromosome 4. Since the location of Y3635R closed to that of RM8213 used in this study, the map position is possibly identical with the gene identified in the present study. Tamura et al. (1999) named the resistance gene Grh(t) tentatively and they plan to designate the gene as Grh6 (Green rice leafhopper resistance 6) (personal communication with Tamura). In this study, the new allele of Grh6 was identified in O. nivara and it was designated as Grh6-nivara to discriminate from the allele of SML 17.

References

Fujita D., K. Doi, A. Yoshimura and H. Yasui, 2003. Mapping of a new resistance gene for green rice leafhopper introgressed from Oryza rufipogon Griff. into cultivated rice, Oryza sativa L. RGN 20: 79-80.

McCouch, S. R., L. Teytelman, Y. Xu, K.B. Lobos, K. Clare, M. Walton, B. Fu, R. Maghirang, Z. Li, Y. Xing, Q. Zhang, I. Kono, M. Yano, R. Fjellstrom, G. DeClerck, D. Schneider, S. Cartinhour, D. Ware and L. Stein, 2002. Development and mapping of 2240 new SSR markers for rice (Oryza sativa L.). DNA research 9: 199-207.

Tamura, K., Y. Fukuta, M. Hirae, S. Oya, I. Ashikawa and T. Yagi, 1999. Genetic analysis of the green rice leafhopper (Nephotettix cincticeps UHLER) resitance VI. RFLP mapping of the resistant gene in Kanto PL10. Breed. Res. 1 (suppl. 2): 118 (in Japanese).