Metabolic and immune functions of the hemolymph and fat body in Hermetia illucens (Diptera: Stratiomyidae) under pathogen challenge.

The hemolymph and fat body tissues are essential for energy storage, metabolism, and immune defense in larval Hermetia illucens (L.). We analyzed the diverse proteins and genes expressed in the hemolymph and fat body of larval H. illucens in both naïve and pathogen-challenged conditions using the entomopathogenic fungus, Beauveria bassiana (Siemaszko). Notably, odorant-binding-like and cuticle-related proteins were abundant, with pathogen exposure leading to significant regulatory changes, highlighting their potential roles in immune defense. We examined the genes associated with key metabolic pathways in H. illucens, such as lipid storage, protein storage, fatty acid metabolism, and gluconeogenesis. Interestingly, eight different storage protein-encoding genes (hexamerins/larval serum proteins) with considerable sequence differences were expressed in the fat body. Exposure to B. bassiana resulted in significant downregulation of most of these storage protein-encoding genes, which was correlated with reduced larval body weight and probably fewer energy reserves for pupal development. These findings deepen our understanding of the physiological mechanisms by which larval H. illucens responds to pathogens and highlight the vulnerability of key metabolic pathways to stress. This study underscores the importance of the fat body and hemolymph in the metabolic and immune functions of insects, paving the way for future research into the molecular pathways governing their adaptation and resilience.