Adaptation of the brown planthopper Nilaparvata lugens (Stål) to resistant rice varieties is mediated by its microbial endosymbionts

Brown planthopper (BPH) populations collected from the Philippines were artificially selected on 2 resistant rice varieties, IR62 and PTB33. These varieties have been used extensively in breeding programs aimed at increasing rice resistance to biotic stresses. The source of resistance of IR62 is the presence of a dominant gene, Bph3. Although the source of resistance in PTB33 is still not fully understood, it appears that the variety may contain several resistance genes (bph2, Bph3, Bph17), thus contributing to its durability.  We examined the resistance of IR62 and PTB33 on 6 BPH populations. Planthopper weight-gain, longevity and egg-laying were lower on IR62 and PTB33 than on the susceptible standard TN1. We then artificially selected for virulence in all six populations by rearing planthoppers on the resistant varieties over 20 generations. Adapted planthoppers had increased survival, longevity and reproductive output when compared to non-adapted colonies.  We also monitored sequentially the abundance of endosymbionts in the pre-adapted and adapted colonies to examine possible relations between endosymbionts and planthopper virulence. Our results showed that the number of yeast-like symbionts (YLS) followed a gradual increase over successive generations of selective rearing on the resistant varieties when the planthoppers themselves ultimately adapting to the novel resistance after several generations. However, we observed that as planthoppers adapted to the resistance of the selected varieties, the abundance of YLS  eventually was reduced possibly representing a cost to the insect after several generations of feeding on the same host variety.