Durability and genetic bases of the resistance of rice variety IR64 to brown planthopper Nilaparvata lugens (Stal)

Against Nilaparvata lugens populations collected from Central Luzon [Philippines], IR64 showed a moderate level of resistance. It showed slight levels of antibiosis although the main mechanism of resistance is antixenosis, and tolerance relative to the cultivars IR22 and Azucena, which contain no major genes for N. lugens resistance. IR64 was also more resistant than IR26 although both varieties have the same major gene for N. lugens resistance, Bph1. This indicates that IR64 contains one or more additional, apparently minor genes, for brown planthopper resistance. IR64 has more durable resistance than three other varieties - IR22. IR26, and IR72, when tested with three populations of BPH collected from different locations with different histories of varietal use. After 15 generations of selection, BPH from Central Luzon and IRRI [International Rice Research Inst., College, Laguna, Philippines] showed complete adaptation to IR64 in most tests of resistance. However, IR64 remained partially resistant to the BPH from Banaue (where no insect-or disease-resistant varieties have yet been cultivated) even after the fifteenth generation. The other resistant varieties tested broke down within 2 (IR26) or 9-11 generations (IR72), depending on the measure of resistance and the BPH populations. IR64 was highly compatible with two major predators of N. lugens, the spider, Pardosa pseudoannulata and the mirid bug Cyrtorhinus lividipennis, tested in an interaction experiment, in comparison with the highly resistant and antibiotic IR72, moderately resistant and tolerant Trivani, and susceptible IR22. On IR64, the predation rate of the spider and mirid bugs and the mirid bug population growth were always significantly higher. A quantitative trait loci (QTL) mapping study showed that the resistance of IR64 to the Central Luzon and IRRI BPH populations is mostly governed by a complex of minor genes. The previously identified major gene, Bph 1, had already lost its effectiveness due to its long time of exposure to N. lugens of Central Luzon and IRRI, but the minor gene complex still conferred resistance. From one to five QTLs were identified for six different traits, located on 8 of the 12 rice chromosomes. Most putative QTL activity was concentrated in fairly compact regions on chromosomes 1, 2, 3, 4, 6, and 8. Individual QTLs accounted for between 5.1 and 16.6 percent of phenotypic variation. Several QTLs were identified in tests with both BPH populations and/or in more than one type of resistance test, indicating that they may be particularly useful in further breeding for BPH resistance. A complex of minor genes conferring tolerance and antixenosis, along with the major gene Bph 1 and excellent compatibility with natural enemies, appears to account for the durable resistance of IR64 to BPH populations