Virulence adaptation in a rice leafhopper: Exposure to ineffective genes compromises pyramided resistance.

Pyramiding resistance genes is predicted to increase the durability of resistant rice varieties against phloem-feeding herbivores. We examined responses by the green leafhopper, (Hemiptera: Cicadellidae), to near-isogenic rice lines with zero, one and two resistance genes. The recurrent parent (T65) and monogenic lines (NIL and NIL) with genes for resistance to the green rice leafhopper, (Hemiptera: Cicadellidae), were susceptible to the green leafhopper, but the pyramided line (PYL) was highly resistant to the green leafhopper. We selected green leafhoppers, , from five sites in the Philippines for over 20 generations on each of the four lines. Populations selected on PYL gained partial virulence (feeding and development equal to that on T65) to the pyramided line within 10 generations and complete virulence (egg-laying equal to that on T65) within 20 generations. After 20 generations of rearing on the susceptible monogenic lines, green leafhoppers were also capable of developing and laying eggs on PYL. Furthermore, green leafhoppers reared on the susceptible NIL for 20 generations showed equal preferences for T65 and PYL in choice bioassays. Our results indicate that previous long-term exposure to ineffective genes (including unperceived resistance genes) could dramatically reduce the durability of pyramided resistance. We suggest that informed crop management and deployment strategies should be developed to accompany rice lines with pyramided resistance and avoid the build-up of virulent herbivore populations.