Metabolomic analysis of seasonal cold acclimation in the goldenrod gall fly, Eurosta solidaginis

The goldenrod gall fly, Eurosta solidaginis, is a well-studied model of freeze-tolerance in insects. Third instar larvae of E. solidganis overwinter inside stem galls on goldenrod plants and can survive freezing well below -20°C. Over the course of the fall, third instar larvae undergo a number of physiological and biochemical changes to increase their cold tolerance, such as the accumulation of cryoprotectants including glycerol and sorbitol. In this study, we used GC-MS-based metabolomics techniques to monitor metabolic changes during seasonal cold acclimation. Larvae from a local goldenrod plot were sampled every two to three weeks from September to January. Using our metabolomics platform, we were able to identify and track approximately 50 different polar metabolites. Some of the major findings of this study are 1) an increase in glycerol and sorbitol over the course of the fall, 2) a marked increase in inositol, suggesting that this polyol also serves a cryoprotective role in E. solidaginis, 3) the presence of many other polyols in low levels, which likely function in a multiple component cryoprotective system, 4) an apparent cryoprotective role for proline, 5) a steady decrease in trehalose over the course of the fall in favor of other cryoprotective compounds, and 6) a marked increase in free phosphate levels late in the season, which may be related to the presence of ice-nucleating calcium phosphate crystals present in the hemolymph. Overall, this study provides a broad snapshot of the metabolic state of E. solidaginis as it prepares for overwintering.