A conserved cold stress syndrome defines insect chill tolerance
The ability to withstand cold is arguably one of the most important traits defining the biogeographical distribution of insects. For most species, those classified as chill susceptible, cold tolerance is limited by the ability to maintain vital physiological functions at low temperature. However, there is currently no unified understanding of how physiological failure develops during cold stress or how this process explains variation in cold sensitivity within and among species. Although disruption of ion homeostasis has been implicated in chilling injury, it remains unclear whether it represents a common physiological limit across cold stress intensities and degrees of chill tolerance. Here, we tested whether ion balance disruption represents a unifying physiological syndrome of chill susceptibility by simultaneously quantifying cold mortality and hemolymph [K] in Drosophila. We examined this relationship across i) a broad range of cold stress intensities, ii) 25 species spanning extensive variation in cold sensitivity, biogeography and phylogeny, and iii) acclimation groups of D. melanogaster developed between 12 and 27{degrees}C. Across all comparisons, mortality was associated with the same cold-stress response, characterized by progressive hyperkalemia and mortality following a ~150% increase in hemolymph [K] above baseline ([K]crit). In contrast, adaptive and plastic variation in cold sensitivity was explained by differences in the critical temperature at which ion homeostasis failed (Tc). Extending this framework to published data from other insect orders revealed a remarkably similar [K]crit, suggesting that ionoregulatory collapse represents a conserved physiological constraint defining insect cold survival.