The gut microbiota is a key determinant of insect physiology, influencing nutrition, immunity, and interactions with plants and pathogens. In Lepidoptera, larval gut communities are dynamic, but a core microbiota, often dominated by Enterococcus species, persists across instars. In Spodoptera littoralis, the enzyme kynurenine 3-monooxygenase (KMO) regulates gut bacterial composition via 8-hydroxyquinoline-2-carboxylic acid (8-HQA), a secreted iron-chelating compound. To investigate whether this mechanism is conserved in Noctuidae, we generated Spodoptera exigua kmo-/- mutants using CRISPR/Cas9 and analyzed bacterial communities in foregut, midgut, hindgut, and oral secretions by 16S metabarcoding, using RNA-derived cDNA for gut samples and DNA for oral secretions due to lower microbial biomass. The kmo deletion abolished 8-HQA production, reduced bacterial diversity, and collapsed compartment-specific bacterial communities in the gut, while also being associated with Enterococcus dominance in oral secretions. Fitness assays revealed that kmo-/- larvae exhibited reduced weight gain on artificial diet, and higher mortality and delayed growth when fed on pepper leaves. Moreover, kmo-/- larvae were threefold more susceptible to Bacillus thuringiensis, consistent with an interaction between host physiological state, gut microbial homeostasis, and pathogen susceptibility. Dietary supplementation with 8-HQA partially mitigated, but did not fully rescue, growth deficits. Our results demonstrate that the kynurenine pathway and 8-HQA production are crucial for maintaining gut microbial homeostasis, particularly within Enterococcus, thereby supporting larval development, dietary adaptation, and pathogen resilience. These findings reveal a conserved mechanism in noctuid moths linking host metabolism, microbiota regulation, and ecological performance, emphasizing the interplay between host genetics, microbiota composition, and environmental stressors.
Pinos et al. (Wed,) studied this question.