Nutritional flexibility of the Euwallacea interjectus-Fusarium populicola mutualism: Fungal physiology, beetle development, and insulin signaling.

Ambrosia beetles thrive on nutritionally barren xylem through obligate mutualisms with fungi that translocate and concentrate essential elements from wood. However, the nutritional physiology of most beetle-fungus systems and the molecular responses of the beetle host remain poorly understood. Here, we investigated the Euwallacea interjectus-Fusarium populicola mutualism through an integrated approach combining fungal nutritional assays, insect developmental bioassays, and transcriptional profiling of the insulin/insulin-like signaling (IIS) pathway. F. Populicola exhibited pronounced metabolic flexibility: On organic nitrogen, soluble sugars supported maximal growth, whereas on inorganic nitrogen, cellulose and sawdust outperformed soluble carbohydrates-A shift indicating that the fungus efficiently degrades lignocellulose when nitrogen form changes. Sporulation strategies also varied, with soluble sugars favoring macroconidia resting spore production and recalcitrant polymers promoting high conidial yields. Larval feeding bioassays demonstrated that fungal nutritional quality directly determines survival, pupation dynamics, and pupal weight, with organic nitrogen being indispensable for successful development. Semi-artificial diet experiments revealed that 5% NaNO₃ maximized adult production, while higher concentrations arrested development, and phosphorus partially rescued nitrogen-induced growth suppression. Transcriptional profiling of 19 IIS genes showed stage- and tissue-specific expression patterns, with EiAKT upregulated only on optimal fungal diets, but these findings remain correlative due to the lack of protein-level validation. Collectively, our results demonstrate that F. Populicola acts as a 'translocator-accumulator' that converts woody substrates into a nutritionally complete diet, with beetle fitness tightly linked to fungal nutritional quality.