Reproductive diapause in insects is a crucial seasonal adaptation, predominantly regulated by photoperiod, with the circadian clock providing the internal time reference to measure day or night length. Despite its importance, the role of core circadian clock genes in photoperiodic responses and reproductive diapause induction remains poorly understood. The green lacewing, Chrysoperla nipponensis, an important predatory natural enemy, undergoes reproductive diapause under short-day (SD) conditions. This study aimed to investigate the role of circadian clock gene period (per) in photoperiod-mediated diapause of C. nipponensis. Firstly, we identified and molecularly characterized per in C. nipponensis. The encoded PER protein contains conserved domains (PAS, PAC, Period_C) and clusters phylogenetically with neuropteran and coleopteran orthologs. Next, we examined the spatiotemporal expression pattern of per, revealing that its expression was significantly influenced by photoperiod. The daily expression pattern of per indicates that it measures night length. Functional RNAi assays demonstrated that per knockdown under diapause-inducing (short-day) conditions upregulated other core clock genes (timeless1, cycle, clock), abolished diapause phenotypes (ovarian arrest, lipid accumulation), and triggered precocious reproduction. Furthermore, transcriptional analysis revealed that per silencing upregulates the core circadian clock gene cry1 as well as 20-hydroxyecdysone (20E) signaling genes, and reprograms lipid synthesis and metabolism genes under short-day conditions in C. nipponensis. Overall, these findings establish per as a critical genetic switch linking circadian timing to diapause induction in C. nipponensis, offering new insights into the molecular mechanisms of circadian regulation in seasonal adaptations of natural enemies, with potential applications in biological control of agricultural pests.
