Two planthopper species, whitebacked planthopper (WBPH), Sogatella furcifera Horvath and brown planthopper (BPH), Nilaparvata lugens Stal are serious insect pests of rice (Oryza sativa L.). The rice ovicidal response to WBPH and BPH is characterized by the formation of watery lesion, which results in the death of their eggs at ovipositional site (Suzuki et al. 1996, Kiyonaga et al. 1997). In this study, quantitative trait loci (QTLs) for rice ovicidal response to WBPH and BPH were mapped on an RFLP linkage map.

A set of 98 backcrossed inbred lines (BILs) derived from Nipponbare(Japonica)/Kasalath (Indica)//Nipponbare (Lin et al. 1998) was used to phenotype the rice ovicidal response to WBPH and BPH. The WBPH and BPH populations originated from adults collected from the paddy fields of Kyushu National Agricultural Experiment Station in Chikugo, Fukuoka, in 1989. These populations were maintained and used for infestation. Eight weeks after seeding, a single plant of each BIL in a 545ml plastic cup was infested with 10 to 12 reared gravid females for two days. At three days after removing the insects, each individual was phenotyped for grade of watery lesions (GWL) and egg mortality (EM) to evaluate ovicidal response following Yamasaki et al. (2000). The QTL analyses for GWL and EM were conducted by composite interval mapping as recombinant inbred line mode in QTL Cartographer v1.13 model 6 (Basten et al. 1998). To reduce type II errors, we set a LOD score of 1.6 as the threshold for declaring a QTL present (Yamasaki et al. 2000).

Nipponbare showed high ovicidal response whereas Kasalath showed low ovicidal response. The BILs for the ovicidal response to WBPH showed continuous distributions (Fig. 1). Transgressive resistant individuals for EM beyond the value of Nipponbare were observed. A total of six QTLs for the ovicidal response to WBPH were detected on chromosomes 3, 6, 7, 8 and 10 (Table 1). A major QTL was identified on chromosome 6 and accounted for 33.8% (GWL) and 46.4% (EM) of the phenotypic variance. The alleles from Nipponbare on chromosomes 6, 7, 8 and 10 increased values of the ovicidal response whereas the alleles from Kasalath on chromosomes 3 and 7 positively contributed to the ovicidal response. The accumulation of the positive QTLs alleles from both parents might explain the genetic basis of the transgressive segregation for EM.

The frequency distributions of the ovicidal response to BPH seemed to be bimodal (Fig. 1). Three QTLs were detected on chromosomes 2, 5 and 6 (Table 1). The chromosome 6 QTL explained 84.7% of phenotypic variance in GWL and EM, suggesting that this QTL may be responsible for the bimodal distributions. The alleles from Nipponbare at three QTLs contributed the positive values of the ovicidal response.

It was concluded that the Nipponbare allele at the chromosome 6 QTL was most important for the ovicidal response to both planthoppers. This QTL corresponds to ovicidal gene (Ovc), which was identified using a nearly isogenic population derived from a Japonica-Indica cross of Asominori/IR24 (Yamasaki et al. 2003). The Asominori allele at Ovc was essential for the ovicidal response to BPH as well as to WBPH (Yamasaki et al. 2000). Thus, Japonica alleles at Ovc in Asominori and Nipponbare were crucial for the ovicidal response to both planthoppers. Except Ovc, no common QTL was found between the reports using recombi-

nant inbred lines derived from Asominori/IR24 (Yamasaki et al. 1999, 2000) and present study using BILs derived from Nipponbare/Kasalath//Nipponbare. There might be differences among genetic effect of parental alleles at the QTLs.

Acknowledgements

We are grateful to Drs. K. Sogawa and Y. Suzuki for providing insects and helpful comments. We also thank Dr. M. Yano for providing the plant materials.

References

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Kiyonaga, T., T. Watanabe, K. Miyamoto and Y. Suzuki, 1997. Varietal differences in the brown planthopper egg mortality caused by antibiotic response of rice plants. Kyushu Agric. Res. 59: 75 (in Japanese).

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