An InR/mir-9a/NlUbx regulatory cascade regulates wing diphenism in brown planthoppers

Wing polymorphism significantly contributes to the ecological success of some insect species. For example, the brown planthopper (BPH) Nilaparvata lugens, which is one of the most destructive rice pests in Asia, can develop into either highly mobile long-winged or highly fecund short-winged adult morphs. A recent study reported a highly provocative result that the Hox gene Ultrabithorax (Ubx) is expressed in BPH forewings and showed that this wing development gene is differentially expressed in nymphs that develop into long-winged versus short-winged morphs. Here, we found that Ubx may be a mir-9a target, and used dual luciferase reporter assays and injected micro RNA (miRNA) mimics and inhibitors to confirm the interactions between mir-9a and NlUbx. We measured the mir-9a and NlUbx expression profiles in nymphs and found that the expression of these two biomolecules was negatively correlated. By rearing BPH nymphs on host rice plants with different nutritional status, we were able to characterize a regulatory cascade between insulin receptor genes, mir-9a, and NlUbx that regulate wing length in BPHs. When host quality was low, NlInR1 expression in the nymph terga increased and NlInR2 expression decreased; this led to a higher mir-9a level, which in turn reduced the NlUbx transcript level and ultimately resulted in longer wing lengths. Beyond extending our understanding of the interplay between host plant status and genetic events that modulate polymorphism, we demonstrated both the upstream signal and miRNA-based regulatory mechanism that control Ubx expression in BPH forewings.