EcR-JHAMT3 axis regulates reproductive plasticity in the predatory beetle Harmonia axyridis

Ecdysone (20-hydroxyecdysone, 20E) is a master regulator of insect development, but its function in adult reproduction-particularly in predatory species-remains poorly understood. We employed Harmonia axyridis to explore 20E signaling in reproduction regulation through RNAi, transcriptomic analysis, and hormone rescue assays. The results demonstrated that both JH and 20E biosynthesis were activated in aphid-fed females, whereas injection of 1.0 μg 20E effectively promoted ovarian development in artificial diet-fed individuals. Expression of the 20E receptor EcR was significantly upregulated in aphid-fed H. axyridis females. EcR silencing severely arrested ovarian development and reduced Vg expression. Transcriptomic analysis revealed that the insect hormone biosynthesis pathway (ko00981) serves as the crucial downstream pathway of EcR-mediated reproductive regulation, with JHAMT3 identified as the core effector. JHAMT3 silencing phenocopied the reproductive suppression observed in EcR-silenced beetles, while exogenous JHA and JH III treatment effectively rescued ovarian development in both cases. Furthermore, multiple high-confidence EcR binding motifs were predicted in the JHAMT3 promoter, and two binding sites (5'-AACTACACTGCCATC-3', 5'-AAGATCCTTGAATTT-3') were verified through EMSA and dual-luciferase reporter assays. These findings establish an EcR-JHAMT3-reproduction regulatory axis in H. axyridis, offering a mechanistic framework for understanding reproductive plasticity in predatory insects.