Microbial contributions to aromatic amino acid acquisition in insects

Aromatic amino acids (AAA) are essential for insect development and survival, serving as precursors for cuticle formation, immune defenses, and neurotransmitter synthesis. Because insect genomes lack the shikimate pathway, they must obtain AAA from external sources, raising the question of how microorganisms contribute to host AAA availability. Here, we review evidence spanning the continuum of insect-microbe associations, from obligate intracellular symbioses to diverse gut microbial communities. Comparative genomics reveals the repeated retention of shikimate and AAA biosynthetic pathways in intracellular symbionts despite extensive genome reduction, while experimental studies demonstrate that symbiont-derived AAA directly support cuticle maturation, desiccation resistance, and overall host performance. Similar nutritional roles are increasingly suspected in specialized extracellular gut symbioses, suggesting functional convergence between distinct organizational frameworks. Beyond these highly integrated associations, growing evidence indicates that gut microbiota may also influence host AAA homeostasis. Although direct microbial AAA provisioning remains difficult to demonstrate, microbiota disruption consistently affects AAA-dependent traits, including cuticle melanization and neurotransmitter-mediated behaviors. These effects are reinforced by experimental inhibition of microbial AAA biosynthesis. Together, these observations support the view that AAA are a central metabolic currency between insects and their microbial partners, with important consequences for insect physiology, fitness, and adaptation.