Wild relatives are sources of genes for resistance to diseases and insect pests. The Nephotettix cincticeps Uhler (Green rice leafhopper: GRH) is one of the serious insect pests of rice in temperate Asia. An accession of wild relative, Oryza rufipogon Griff. (W1962) was found to be highly resistant to GRH by antibiosis test. In the present study, we found a new resistance gene for GRH introgressed from O. rufipogon.

The introgression lines derived from a cross between a resistant accession of O. rufipogon (W1962) and a susceptible Japonica variety Taichung 65 were used for the analysis. To evaluate nymph mortality as GRH resistance, leaf blades of 98 BC₄F₂ plants were excised and infested with 7-10 first- or second-instar nymphs. The insect population was collected in Fukuoka Prefecture in 1991, and was maintained by continuously rearing at 25C and 16h light: 8h dark condition. The nymph mortality in BC₄F₂ individuals showed a discrete distribution, segregating into 69 resistant and 29 susceptible plants. The segregation ratio fitted to a 3:1 ratio (chi²=1.35), indicating that the GRH resistance was controlled by a single dominant gene. To determine the chromosomal location of the resistance gene, BC₃F₁ plants were analyzed using 767 simple sequence repeats (SSR) markers (McCouch et al. 2002) by bulked segregant analysis. The linkage between the resistance gene and one SSR marker, RM4154 was detected. Further SSR analysis using BC₄F₂ individuals revealed that the resistance gene was

located between RM1615 and RM6845 and tightly linked to RM3754 and RM3761 on the distal region in long arm of chromosome 8 (Fig. 1). So far, five genes for resistance to GRH have been mapped on rice chromosomes using RFLP markers, Grh1 on chromosome 5 (Tamura et al. 1999a), Grh2 and Grh4 on chromosomes 11 and 3, respectively (Fukuta et al. 1998; Yazawa et al. 1998), Grh3(t) on chromosome 6 (Saka et al. 1997), and Grh(t) on chromosome 4 (Tamura et al. 1999b). There have been no genes for resistance to GRH on chromosome 8. Therefore, we designated the new resistance gene as Grh5 (Green rice leafhopper resistance 5). Nearly isogenic line (NIL) for Grh5 was selected in BC₄F₂ population. The nymph mortality in the NIL was more than 90%. It was higher than those of varieties each carrying single GRH resistance genes reported previously (unpublished data). Thus, Grh5 introgressed from O. rufipogon is a favorable gene resource for rice improvement.

References

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Harushima, Y., M. Yano, A. Shomura, M. Sato, T. Shimano, Y. Kuboki, T. Yamamoto, S. Y. Lin, B. A. Antonio, A. Parco, H. Kajiya, N. Huang, K. Yamamoto, Y. Nagamura, N. Kurata, G. S. Khush and T. Sasaki, 1998. A highdensity rice genetic linkage map with 2275 markers using a single F₂ population. Genetics 148: 479-494.

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