Probing metabolic integration in obligate, intracellular symbioses

Microbial symbionts can contribute crucial capabilities to insect physiology, such as limited nutrients, digestive or detoxifying enzymes, or bioactive specialized metabolites. While symbiont identity, localization, and genomic capabilities are comparatively well understood due to the rise of sequencing technologies, the molecular and metabolic integration of intracellular symbionts is less well characterized. Due to the obligate nature of most of these symbioses, the study of the host or symbiont in isolation is rarely possible. Instead, in situ techniques are required to decipher intertwined physiological processes. Temporally and spatially resolved structural analyses of symbiotic associations, omics approaches, and integrated metabolic models allow for a better understanding of host-symbiont interaction and provide testable hypotheses. Gene expression modulation of both host and symbiont, pharmacological perturbation of enzyme activities, and isotope tracing, then allow for testing these hypotheses experimentally. Here, we provide a conceptual overview of these approaches and emphasize how integrated workflows can yield causal insights.