The Roles of E93 and Kr-h1 in Metamorphosis of Nilaparvata lugens

Metamorphosis is a crucial process in insect development. Ecdysone-induced protein 93 (E93) is a determinant that promotes adult metamorphosis in both hemimetabolous and holometabolous insects. Kruppel-homolog 1 (Kr-h1), an early juvenile hormone (JH)-inducible gene, participates in JH signaling pathway controlling insect metamorphosis. In the current study, an E93 cDNA (NlE93) and two Kr-h1 cDNA variants (NlKr-h1-a and NlKr-h1-b) were cloned from Nilaparvata lugens (Stal), one of the most destructive hemimetabolous insect pests on rice. Multiple sequence alignment showed that both NlE93 and NlKr-h1 share high identity with their orthologs from other insects. The expression patterns revealed that decreasing NlKr-h1 mRNA levels were correlated with increasing NlE93 mRNA levels and vice versa. Moreover, RNA interference (RNAi) assays showed that the knockdown of one of the two genes resulted in significantly upregulated expression of the other. Correspondingly, phenotypical observation of the RNAi insects revealed that depletion of NlE93 prevented nymph-adult transition (causing a supernumerary nymphal instar), while depletion of NlKr-h1 triggered precocious formation of incomplete adult features. The results suggest that Nlkr-h1 and NlE93 are mutual repressors, fitting into the MEKRE93 pathway. The balance between these two genes plays a critical role in the metamorphosis of N. lugens determining the proper timing for activating metamorphosis during the nymphal stage.