Dismissing-then-recruiting: a unique mechanism for symbiont recruitment and lignocellulose degradation in gut of black soldier fly larvae.

Black soldier fly larvae (Hermetia illucens L., BSFL) efficiently degrade lignocellulosic waste despite lacking endogenous lignocellulolytic enzymes, indicating a critical dependence on gut-associated microbiota. However, how BSFL are associated with the enrichment and spatial organization of lignocellulose-degrading symbionts within the gut remains poorly understood. We show that BSFL establish a spatially structured microbiome through gut compartment-specific immune regulation. Axenic larvae showed negligible lignocellulose degradation, whereas microbiota-associated larvae achieved ~ 33.3%, confirming microbial dependence. Antimicrobial peptides, including cecropin and defensin, were highly expressed in the anterior midgut, forming a selective barrier. In contrast, the posterior midgut showed reduced immune activity via peptidoglycan recognition proteins (PGRP-LB, PGRP-SC). Concurrently, host glycosylation-related genes (C1galt1, GlcAT-P, FUT8/11) were significantly upregulated in the posterior midgut (17-19 TPM), representing a 4-fivefold increase relative to the anterior region (p < 0.01). This region was correspondingly enriched in carbohydrate-active symbionts producing glycoside hydrolases, including galactosidases, fucosidases, and mannosidases, suggesting that upregulated glycan biosynthesis may provide sustained nutrient availability for microbial taxa with corresponding carbohydrate-utilizing capacities. Such compartmentalized immune-metabolic coordination is consistent with a potentially conserved strategy for modulating immune tolerance and fostering symbiont recruitment. Collectively, our findings define a compartmentalized "Dismissing-then-Recruiting" strategy for microbiome assembly in BSFL, which is associated with the structuring of functional symbiont communities for efficient lignocellulose bioconversion, thereby elucidating novel principles for sustainable waste management. Video Abstract.