Bph9

Background

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, Bph1from 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).Bph9, is a dominant brown planthopper resistance gene, locating on the long arm of rice chromosome 12. Bph9 was first identified in a Sri Lankan variety by Nemoto (Nemoto et al. 1989). Then map position of BPH turned out to be on the long arm of rice chromosome 12, which is discovered by Murata in 2000 ( Murata et al. 2000).

Functions of BPH

The brown planthopper (BPH) is one of the most devastating insect pests of rice in Asia. BPH is a sap-feeding insect that causes "hopper-burn" and could also be a vector for rice grassy stunt virus and ragged stunt virus. Use of pesticides to combat this pest is costly and could also cause the decrease in the population of other beneficial insects in the field. Rice variety Shanyou 63, which was widely cultivated in China, has decreased in production due to its susceptibility to pests such as BPH.

The discovery of Bph9 map position

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 F2individuals 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.

Map position

Reference

1. K. MURATA, M. FUJIWARA, H. MURAI, S. TAKUMI, N. MORI and C. NAKAMURA. Bph9, a dominant brown planthopper resistance gene, is located on the long arm of rice chromosome 12, Rice Genetics Newsletters, 2000, 17(0): 84-86 2. Hiroshi Nemoto, Ryoichi Ikeda and Chukichi Kaneda, New genes for resistance to brown planthopper, Nilaparvata lugens Stål, in rice. Japanese Journal of Breeding, 1989, 39(0): 23-28