Spätzle Regulates Developmental and Immune Trade-Offs Induced by Bacillus thuringiensis Priming in Rhynchophorus ferrugineus

The red palm weevil (RPW) is an invasive pest that causes devastating damage to a variety of palm plants, which exhibit specific immune priming to <i>Bacillus thuringiensis</i> (Bt). However, immune priming in RPW may incur a high fitness cost, and its molecular signaling pathways have not yet been reported. Here, we investigated the effect of Bt priming on RPW development and subsequently analyzed the hormonal and immune-related molecular pathways influencing the fitness cost induced by Bt priming. Bt priming delayed the body weight gain of fifth-instar larvae and prolonged their developmental duration. Bt priming significantly reduced the 20-hydroxyecdysone (20E) content in RPW hemolymph, and the expression levels of the 20E biosynthesis-related genes <i>SHADOW</i> and <i>SHADE</i> were significantly downregulated. Furthermore, we analyzed Toll pathway genes influencing Bt priming and found that only <i>Spätzle</i> (<i>SPZ</i>) transcription was significantly activated under Bt priming. After silencing <i>SPZ</i> expression, the negative effects of Bt priming on development, <i>SHADOW</i> expression, and 20E synthesis were eliminated, thereby suggesting that <i>SPZ</i> is a key molecular signal mediating developmental and immune trade-offs induced by Bt priming. Our results elucidate the molecular cascade pathway of immune priming and provide new targets for improving the efficiency of RPW biological controls.