Effect of RNAi‐mediated silencing of two Knickkopf family genes (LmKnk2 and LmKnk3) on cuticle formation and insecticide susceptibility in Locusta migratoria

Abstract

Knickkopf (Knk) proteins play crucial roles in the formation of insect cuticle. Recent studies in the holometabolous insect red flour beetle (Tribolium castaneum) have shown that three Knk genes encoding TcKnk, TcKnk2 and TcKnk3 play different but essential roles at different developmental stages and in different tissues. However, the functions of Knk genes had not been fully explored in hemimetabolous insects such as the migratory locust Locusta migratoria. We identified three transcripts of LmKnk-like genes LmKnk2 and LmKnk3 with the full-length cDNA sequences, which were named as LmKnk2, LmKnk3-FL and LmKnk3-5'. These three transcripts were highly expressed before molting and mainly expressed in the integument. Among these genes, silencing only LmKnk3-5' by RNA interference (RNAi) caused molting defects and high mortality of the locusts. Injection of dsLmKnk3-5' dramatically decreased chitin content, but did not affect cuticle laminar ultra-structures in the integument. After the knockdown of LmKnk3-5' transcript, lipid deposition on the cuticle surface was impeded, and locusts exhibited increased susceptibility to each of four insecticides in three different classes. However, no visible phenotypic changes were observed after LmKnk2 or LmKnk3-FL was silenced by RNAi. We demonstrate that LmKnk3-5' is essential for cuticle formation in L. migratoria. This contrasts the findings that the cognate protein in T. castaneum TcKnk3-5' is dispensable for cuticle formation and survival. Hence, we provide some evidence that the function of Knk-type proteins may be species-specific. We therefore think that LmKnk3-5' may be a good target for the application of RNAi-based technologies for species-specific insect pest management. © 2020 Society of Chemical Industry.