and green rice leafhopper, Nephotettix cincticeps Uhier in rice, Oryza sativa L.

H. YASuI and A. YOSHIMURA

Plant Breeding Laboratory, Faculty of Agriculture, Kyushu University Fukuoka, 812-8581 Japan

    Two Nephorettix species, Nephoteuix virescens Distant (Green leafhopper: GLH) and Nephotettix cincticeps Uhier (Green rice leafhopper: GRH), are serious insect pests in tropical and temperate Asian rice fields, respectively. Genes for antibiosis type of resistance to these insects were identified by RFLP mapping using two sets of recombinant inbred lines (RILs) derived from the crosses between resistant Indica and susceptible Japonica varieties in this study.

    Two sets of RILs, derived from Japonica / Indica crosses of Asominori / IR24 (Tsunematsu et al. 1996) and Kinmaze / DV85 (Ikeda et a!. 1998), were evaluated for antibiosis to GLH and GRH, respectively. GLH and GRH populations were developed from about 50-100 adult insects collected from Ishigaki, Okinawa, in 1997 and from Kasuya, Fukuoka, in 1991, respectively. They have been maintained at 25°C and a 16h light:8h dark photoregime. Five plants of each RIL were evaluated for antibiosis to the insects at seedling stage. The antibiosis score was computed as nymphal mortality at four days after infestation of 5 first- or second-instar nymphs. Quantitative trait locus (QTL) analyses for nymphal mortality were performed using composite interval mapping in QTL Cartographer vl. 12 model. The loci with LOD scores greater than 2.0 were considered as indicative of the presence of a QTL.

    When RILs derived from Asominori / IR24 were infested with GLH, there was no RIL resistant to GLH. Both the parents were also susceptible. When the same RILs were infested with GRH, 1R24 and F1 were highly resistant with above 80 % nymphal mortality while Asominori was susceptible with 0 % nymphal mortality. A major QTL on chromosome 5 (C309) and a minor QTL on chromosome 6 (C259C) were detected (Table 1). IR24 allele of chromosome 5 and Asominori allele of chromosome 6 positively contributed to the resistance to GRH. Tamura et al. (1999) identified a dominant gene for resistance to GRH on chromosome 5 and designated it as Grhl. A major resistance QTL identified in this study in variety IR24, which corresponds to Grhl, was mapped on rice chromosome 5 and it only confers resistance to not GLH but GRH.
    When RILs derived from Kinmaze / DV85 were infested with GLH, segregation for resistance was observed in the RILs. QTL analysis confirmed two major QTLs on chromosome 3 (XNpbl44) and on chromosome 11 (G1465), respectively (Table 1). Both DV85 alleles on chromosomes 3 and 11 positively contributed to the resistance to GLH. The plants having both DV85 alleles of the two QTLs showed high nymphal mortality, suggesting that a pair of complementary genes in DV85 govern the resistance to GLH. The same tendency was observed when infested with GRH (Table 1). It has been reported that a pair of dominant resistant genes with complementary expression to GRH were located on rice chromosomes 3 and 11 (Fukuta eta!, 1998; Yazawa eta!. 1998) and designated as Grh4 and Grh2, respectively. 1\vo major QTLs for resistance to GLH and GRH identified in variety DV85 may correspond to Grh4 and Grh2. These genes confer resistance to both of the Nephotettix species, GLH and GRH.

Table I. Quantitative trait loci detected by composite interval mapping (LOD>=2.0) affecting resistance to green leathopper (GLH), Nephotettix virescens Distan and green rice leafhopper (GRH), Nephotettix cincticeps Uhler in rice

a) The percentage of phenotypic variance explained.

b) Additive effects indicate the effects of Japonica alleles (Asominod and Kinmaze).