The whitebacked planthopper (WBPH), Sogatella furcifera Horvath is a serious insect pest of rice (Oryza sativa L.) in Asia. The rice ovicidal response to WBPH is characterized by watery lesions which result in the death of the eggs at the ovipositional sites (Suzuki et al. 1996). This factor affects the suppression of WBPH multiplication in Japan.

A total of 15 putative QTLs for the ovicidal response have been detected using recombinant inbred lines derived from a cross between an ovicidal variety Asominori and a non-ovicidal variety IR24 (hereafter referred to as "RIA") (Yamasaki et al. 1999a). A gene ovicidal to WBPH designated as Ovc responsible for the ovicidal response was found and it was mapped on chromosome 6 using near isogenic line (NIL) in IR24 genetic background (Yamasaki et al. 1999b). Subsequently, NILs of the putative QTLs were developed through marker-assisted selection based on a set of Asominori chromosomal segment substitution lines in IR24 genetic background (Aida et al. 1997). One QTL on chromosome 1 and two QTLs on chromosome 5 were designated as qOVA-1-3, qOVA-5-1 and qOVA-5-2, respectively. The letters "qOV" refer to ovicidal QTL and "A" is derived from RIA. The populations derived from the three plants heterozygous for the respective QTLs affecting the ovicidal response (Fig. 1) were transplanted in the experimental paddy field of Kyushu University. Twelve weeks after seeding, the rice plants with WBPH eggs laid by migrant population in the paddy field were phenotyped for the egg mortality (EM) to map QTLs for EM. QTL mapping was carried out by MAPMAKER/QTL v1.1 (Lincoln et al. 1993).

The frequency distributions in the three populations exhibited wide variation with high EM (Fig. 1). There were segregants which showed higher EM than the NIL with Ovc. The Asominori alleles at qOVA-1-3, qOVA-5-1 and qOVA-5-2 increased the EM in

the presence of Ovc. QTL analysis revealed that qOVA-1-3 was located between XNpb346 and C112 on chromosome 1 (Fig. 2A). qOVA-5-1 was located between XNpb251 and R3313, and qOVA-5-2 was tightly linked to C1268 on chromosome 5 (Fig. 2B). Dominance effect was almost the same as additive effect in the respective QTLs, suggesting that the Asominori alleles at three QTLs showed complete dominance effect. The QTLs with small genetic effect enhanced the ovicidal response.

We are grateful to Drs. K. Sogawa and Y. Suzuki for providing insects and helpful comments.

References

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