Reproductive Outbreaks of Sogatella furcifera Mediated by Overexpression of the Nuclear Receptor USP under Pressure from Triflumezopyrim

Long-term pesticide-driven selection pressure is one of the main causes of insect outbreaks. In this study, we found that low doses of triflumezopyrim could increase the fecundity of white-backed planthoppers (Sogatella furcifera). By continuously screening 20 generations with a low dose of triflumezopyrim, a triflumezopyrim-resistant strain (Tri-strain, resistance ratio = 20.9-fold) was obtained. The average oviposition quantity and longevity of the Tri-strain (208.77 eggs and 21.31 days, respectively) were significantly higher than those of the susceptible strain (Sus-strain) (164.62 eggs and 17.85 days, respectively). To better understand the mechanism underlying the effects on reproduction, we detected the expression levels of several reproduction-related transcription factors in both the Tri- and Sus-strains. Ultraspiracle (USP) was significantly overexpressed in the Tri-strain. Knockdown of USP by RNAi severely inhibited the moulting process of S. furcifera and disrupted the development of female adult ovaries. Among the potential downstream target genes of USP, Kr-h1 (0.19-fold), Cht8 (0.56-fold) and GPCR A22 (0.31-fold) showed downregulated expression after USP-RNAi. In contrast, the expression of EcR (2.55-fold), which forms heterodimers with USP, was significantly upregulated. Furthermore, RNAi was performed on Kr-h1 in the Tri-strain, and the results show that larval moulting and the development of female adult ovaries were inhibited, consistent with the USP-RNAi results in S. furcifera. These results suggest that the transcription factors USP and Kr-h1 play important roles in the reproductive development of S. furcifera, and overexpression of USP and Kr-h1 in the Tri-resistant strain may result in reproductive outbreaks of pests.