Nuclear receptor <scp>E75</scp> regulates moulting and ovarian development in <i>Nilaparvata lugens</i>

E75, a member of the nuclear receptor gene family, regulates diverse biological processes, including ovarian development and fat body remodelling. The brown planthopper (Nilaparvata lugens Stål) (Hemiptera:Delphacidae), a major rice pest, poses a significant threat to rice production. However, the role of Nilaparvata lugens E75 (NlE75) in development, particularly ovarian development and its underlying mechanisms, remains largely unknown. In this study, we found that downregulation of NlE75 expression significantly reduced the eclosion rate, especially in the third-, fourth- and fifth-instar nymphs. Moreover, decreased NlE75 levels affected fat body morphology as well as ovarian and testicular development, leading to delayed ovarian maturation and reduced egg production. NlE75 knockdown drastically reduced the total number of eggs laid. We also found that NlE75 may participate in the transcriptional regulation of Vitellogenin (Vg), Vitellogenin-like1 (Vg-like1) and Vitellogenin-like2 (Vg-like2) and that downregulation of NlE75 significantly reduced Vg protein expression. In summary, we found that NlE75 is required for lipid droplet formation in the fat body, as well as for ovary and vas deferens development. Furthermore, downregulation of NlE75 altered fat body morphology and reduced lipid content, suggesting decreased energy reserves. Importantly, NlE75 was found to be involved in the transcriptional regulation of cell cycle-related genes, influencing DNA replication and the expression of genes controlling cell cycle progression. Flow cytometry analysis further revealed significant disruption of ovarian cell cycles following NlE75 downregulation, suggesting that E75 may function in cell cycle progression to coordinate development and reproduction in N. lugens. The involvement of NlE75 in regulating cell cycle-related genes and cell cycle progression in N. lugens represents a novel finding. This study provides a valuable reference for related research in other species, offering new insights into the mechanisms of NlE75 function and its interaction with both JH and ecdysone signalling.