BmS6K Regulates Growth and Survival Traits of Silkworm, Bombyx mori

Ribosomal protein S6 kinase (S6K) is a conserved effector of the mTOR signaling pathway that regulates cell growth and metabolism. The organism-level function of S6K in the silkworm Bombyx mori has not yet been examined by direct genetic manipulation. Here, we combined CRISPR/Cas9-mediated knockout, piggyBac-based transgenic re-expression and overexpression, and multi-tissue RNA-seq with weighted gene co-expression network analysis (WGCNA) to characterize the role of BmS6K in vivo. BmS6K-null mutants were viable but showed 26.5% and 28.6% reductions in pupal weight in males and females, respectively. Furthermore, mutant survival rates were 44.2% (males) and 28.5% (females) lower than those of controls, with mortality concentrated at the late fifth larval stage. Targeted transgenic re-expression rescued these phenotypic defects. Conversely, overexpressing BmS6K in a wild-type background increased the larval growth rate by 16.9% and significantly raised both whole cocoon weight and pupal weights. WGCNA of the three tissue types identified a core co-expression module (MEred) that was significantly enriched for genes involved in proton-transporting V-type ATPase activity and vacuolar complex assembly, mTOR signaling, and oxidative phosphorylation coupled to cellular redox homeostasis, including BmV1-ATPase, BmGabaT, and BmNadDac, whose orthologs represent core components or mediate critical cross-talk in the TOR-S6K pathway, a function conserved in Drosophila and mammals. Since the loss and gain of BmS6K produced opposite effects on growth and viability, it can be regarded as a candidate target for increasing cocoon yield in sericulture, as well as a target for RNAi-based control of lepidopteran pests through the suppression of its orthologs.