Nephotettix cincticeps Uhler (Green rice leafhopper: GRH) is one
of the serious insect pests of rice in temperate Asia. We are attempting
to search the GRH resistance gene of wild relatives in rice and identified
the new resistance gene, Grh5 (Green rice leafhopper resistance
5), using O. rufipogon introgression lines (Fujita et al.
2003). In this study an accession of wild relatives, Oryza nivara
(IRGC105715) from Cambodia was found to be highly resistant to GRH by
antibiosis test. The GRH resistance transferred from O. nivara
was analyzed by using introgression lines of rice.
Two sets of introgression lines each carrying wild and cultivated cytoplasm
have been developed from reciprocal crosses between O. sativa cv.
Taichung 65 (T65) and O. nivara (Acc. IRGC105715). To develop O.
nivara introgression lines, the F1 plants obtained by reciprocal
crosses were continuously backcrossed with T65. To detect GRH resistance
gene, the BC4F1 population with O. nivara
cytoplasm was analyzed through whole genome survey using RFLP markers.
The BC3F1 plants with T65 cytoplasm were evaluated
for GRH resistance. The BC3F2 individuals derived
from a single resistant BC3F1 plant were analyzed
using SSR markers (McCouch et al. 2002) for linkage analysis. GRH
resistance was evaluated as the following; the leaf blades were excised
and infested with 7-10 first- or second-instar nymphs. Then, the nymph
mortality (NM) at 4 days after infestation was calculated. The plants
with more than 50% NM and those with less than 30% NM were classified
to resistant and susceptible, respectively. The insect population was
collected in Fukuoka Prefecture in 1991, and was maintained by continuously
rearing at 25C and 16h light: 8h dark condition.
Five plants out of BC4F1 plants showed resistance
to GRH. These carried a common segment from O. nivara in the short
arm of chromosome 4 based on RFLP analysis. This suggests that the resistance
gene introgressed from O. nivara would be located on the short
arm of chromosome 4. The NM of 185 BC3F2 individuals
showed a discrete distribution, segregating into 140 resistant and 45
susceptible plants (Fig. 1). The segregation ratio fitted to a 3:1 ratio
(chi2 = 0.83), indicating that the GRH resistance introgressed
from O. nivara was controlled by a single dominant gene. SSR analysis
revealed that the resistance gene was located between RM5414 and
C60248 and tightly linked to RM8213 on the short arm of
chromosome 4 (Fig. 2).
Tamura et al. (1999) reported a GRH resistance gene derived from
a cultivar SML 17
(from Surinam) tightly linked to Y3635R on the short arm of chromosome
4. Since the location of Y3635R closed to that of RM8213
used in this study, the map position is possibly identical with the gene
identified in the present study. Tamura et al. (1999) named the
resistance gene Grh(t) tentatively and they plan to designate the
gene as Grh6 (Green rice leafhopper resistance 6) (personal communication
with Tamura). In this study, the new allele of Grh6 was identified
in O. nivara and it was designated as Grh6-nivara to discriminate
from the allele of SML 17.
Fujita D., K. Doi, A. Yoshimura and H. Yasui, 2003. Mapping of a new resistance
gene for green rice leafhopper introgressed from Oryza rufipogon
Griff. into cultivated rice, Oryza sativa L. RGN 20: 79-80.
McCouch, S. R., L. Teytelman, Y. Xu, K.B. Lobos, K. Clare, M. Walton,
B. Fu, R. Maghirang, Z. Li, Y. Xing, Q. Zhang, I. Kono, M. Yano, R. Fjellstrom,
G. DeClerck, D. Schneider, S. Cartinhour, D. Ware and L. Stein, 2002.
Development and mapping of 2240 new SSR markers for rice (Oryza sativa
L.). DNA research 9: 199-207.
Tamura, K., Y. Fukuta, M. Hirae, S. Oya, I. Ashikawa and T. Yagi, 1999.
Genetic analysis of the green rice leafhopper (Nephotettix cincticeps
UHLER) resitance VI. RFLP mapping of the resistant gene in Kanto PL10.
Breed. Res. 1 (suppl. 2): 118 (in Japanese).