Genetic and breeding research on the natural resistance system against a serious insect pest of rice, brown planthopper (BPH), Nilaparvata lugens Stal, begun in the mid-1960s. Twelve BPH resistance genes have so far been identified. A BPH resistance gene, Bph10, from Oryza australiensis was first assigned to rice chromosome 12 (Ishii et al. 1994). Later, Bph1 from an Indian variety, Mudgo, and bph2 from an IRRI breeding line, IR1154-243, were also mapped on chromosome 12 (Hirabayashi and Ogawa 1995; Murata et al. 1997, 1998). We have now determined the map position of a dominant BPH resistance gene, Bph9, which was first identified in a Sri Lankan variety, Pokkali (Nemoto et al. 1989).

A japonica breeding line, Norin-PL9, was used as a female parent in a cross with

Pokkali, because Norin-PL9 was known to be cross-compatible with indica varieties. Contrary to the expectation, however, this japonica-indica cross resulted in highly sterile F₁s and F₂s. Ten F₃ plants were grown from each of 98 F₂ individuals but only 65 F₃ families with enough numbers of F₄ seeds for bioassay were obtained. Genotypes of 62 F₃ families thus 62 F₂ individuals for BPH resistance/susceptibility were determined by bioassay of F₄ families according to the previously described method (Murata et al. 1998). The heterozygous F₃s had 70-75 % resistant F₄ progenies, agreeing with that Bph9 was dominant. The segregation ratio in 62 F₂s was 12 RR:33 RS:17 SS, which did not deviate from the expected single gene control of resistance.

DNA was extracted from an equal amount (8 g) of leaves of 10 F₃ plants derived from each F₂ individual. DNA bulks were further prepared by combining four F₃ families (each with 10 F₃ plants) with RR and SS genotypes, respectively. A total of 106 RFLP markers and eight restriction enzymes were used for the bulked segregant analysis. In addition, a total of 240 random 10-mer primers (Operon Technologies) were surveyed in RAPD-PCR analysis.

Segregation of seven RFLP and two RAPD markers in the 62 F₂ plants did not deviate from the expected 1:2:1 ratio or 1:3 ratio. The result indicated no apparent segregation distortion in the chromosomal region covering the Bph9 locus, despite the observed high sterility in this cross combination. Recombination values were calculated by MAPMAKER Version 2.0 with LOD scores greater than 3.0. These markers and Bph9 were located in the 53 cM segment, which was delimited by two RFLP markers (R617 and 1709) on the long arm of chromosome 12 (Fig. 1). A RAPD marker (OPR04) was found to be closest, with a map distance of 8.8 cM from the Bph9 locus. The map distance between the two RFLP markers (G2140 and S2545), however, was much greater than the corresponding distance on the standard Nipponbare/Kasalath map. This might be ascribed either to the use of different mapping populations or to the smaller population size and high sterility in the present cross.

References

Hirabayashi, H. and T. Ogawa, 1995. RFLP mapping of Bph-1 (brown planthopper resistance gene) in rice. Jpn. J. Breed. 45: 369-371.

Ishii, T., D.S. Brar, D.S. Multani and G.S. Khush, 1994. Molecular tagging of genes for brown planthopper resistance and earliness introgressed from Oryza australiensis into cultivated rice, O. sativa. Genome 37: 217-221.

Murata, K., C. Nakamura, M. Fujiwara, N. Mori and C. Kaneda, 1997. Tagging and mapping of brown planthopper resistance genes in rice. In: Proc. 5th Int. Symp. on Rice Molecular Biology, J-C Su, ed., Yi-Hsien Pub., Taipei, Taiwan, p. 217-231.

Murata, K., M. Fujiwara, C. Kaneda, S. Takumi, N. Mori and C. Nakamura, 1998. RFLP mapping of a brown planthopper (Nilaparvata lugens Stal) resistance gene bph2 of indica rice introgressed into a japonica breeding line 'Norin-PL4'. Genes Genet. Syst. 73: 359-364.

Nemoto, H., R. Ikeda and C. Kaneda, 1989. New genes for resistance to brown planthopper, Nilaparvata lugens Stal., in rice. Jpn. J. Breed. 39: 23-28.