Brown planthopper (BPH), Nilaparvata lugens (Stal), is one of the most serious insect pests of rice throughout Asia. The use of resistant varieties is the most economical and effective way of controlling damage by BPH. It has long been proposed that varieties with moderate level of resistance or varieties carrying polygenes provide a more durable resistance than those carrying single major genes. The use of molecular-marker techniques in quantitative trait locus (QTL) analysis opened new opportunities to work with quantitative traits.

In screening germplasm resistant to brown planthopper, we found that Kasalath was moderately resistant (Table 1).

A mapping population of 98 BC₁F₉ lines (Backcross inbred lines: BIL), derived from a backcross of Nipponbare (japonica)/Kasalath (indica)//Nipponbare by single-seed descent method, was used to characterize QTL resistant to BPH. In this report, about 35 seedlings of each BIL line and parent line were used to evaluate the reactions to insect infestation by bulk seedling test. To ensure that all seedlings were at the same growth stage at the tome of insect infestation, seeds were first germinated in a net bag, and 20 germinating seeds were spaced in a plastic pot (8 cm in diameter) with two replications for each line or variety. BPH population, mixed with biotype 1 and biotype 2, were collected from farmer's fields and maintained on Nanjing 11. At one-leaf stage, seedlings were infested with second- and third-instar nymphs at the rate of 5-7 per seedling. When the susceptible checks (TN1 and Nanjing 11) were damaged, the whole BIL population and parents were scored for resistant and susceptible reactions.

QTL of BPH-resistance were mapped with MAPMAKER/QTL1.1b. A total of three QTL controlling BPH resistance were detected on chromosomes 2, 10 and 12. Individual QTL accounted for 10.4 to 16.6% of the phenotypic variance. The resistance of all three QTLs came from Kasalath (Table 2 and Fig. 1).

Bph1, bph2 and Bph9 had already been mapped on Chromosome 12 (Hirabayashi 1995,

Tooyama 1995, Huang 1997, Murata 1998). In this report, we identified a new QTL, qBPH-12, on Chromosome 12 (Fig. 2). Bph10(t) has also been mapped on this chromosome (Ishii, 1994), however, its allelic relationship to Bph1, bph2 and Bph9 needs to be determined. So, we postulated that genes on Chromosome 12 are greatly involved in resistance to brown planthopper during evolution.

Acknowledgment

Thanks to Dr. M. Yano (NIAR, Japan) for his kindly providing as materials.

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