HiHR4 regulates chitin metabolism during molting of Hermetia illucens

The black soldier fly (Hermetia illucens) as a model bio-converter insect, is increasingly used in feed formulation and as a fertilizer source. Its growth rate and biomass yield are influenced by molting, a process involving the remodeling of chitinous structures. In this study, we identified a HiHR4 gene in Hermetia illucens and report its function in larval growth and molting. RT-qPCR results show that HiHR4 expression is stable during the third instar larval stage but peaks sharply prior to molting. Knockdown of HiHR4 by RNA interference (RNAi) in larvae results in molting delay. The dsHiHR4-injected larvae display slow growth. Consistently, integument sectioning and chitin staining showed that HiHR4 knockdown inhibits both the degradation of the old cuticle and the formation of new cuticle. In addition, HiHR4 and chitin metabolism-related genes are induced by the steroid hormone 20-hydroxyecdysone (20E), indicating their responsiveness to 20E signaling. Moreover, HiHR4 knockdown significantly suppresses the expression of chitin synthesis genes (HiHex, HiG6PI, HiCHS1) and chitin degradation genes (HiCht10). RNAi against HiCHS1 or HiCht10 disrupts larval molting and attenuates growth. Taken together, these findings suggest that HiHR4 is a key regulator of chitin metabolism, ensuring proper molting and promoting biomass production in H. illucens.