Using thermal death time models to analyze cold stress resistance across Drosophila species

Chill susceptible insects such as Drosophila are vulnerable to progressive disruption of homeostasis during cold stress, and low‐temperature exposure is a key factor affecting their physiology and distribution. Comparative studies of cold tolerance traditionally use single‐condition assays for interspecific comparisons, but the emergence of thermal death time (TDT) models offers a comprehensive framework to assess cold tolerance across different stress intensities and durations. Here we construct TDT curves for six Drosophila species, spanning boreal to tropical habitats, using Lt 50 estimates across a range of stressful low temperatures (Lt 50 —time to reach 50% mortality—ranging from ~20 min to 2 days). For all species, the TDT curves provided good fits to the log(Lt 50 ) vs. temperature data ( R 2 = 0.87–0.99). TDT curves from all species had steep slopes demonstrating that cold injury rate has a high thermal sensitivity (high Q 10 ) such that small changes in temperature have profound effects on survival duration. The similarity of TDT slopes among most species indicates that a similar physiological dysfunction could underlie cold injury across most Drosophila species. Further, additive accumulation of cold‐induced injury in split‐dose experiments suggests that acute and moderate cold damage could represent the same physiological failure occurring at different rates. TDT curve intercepts (species‐specific tolerance thresholds) differed markedly between boreal, temperate and tropical species and correlated strongly with their habitat temperature. Data from the present study and meta‐analysis of published data find that basal species cold tolerance increases by ~0.45°C per°C lowering of the species winter temperature. When also considering cold acclimation cues in cold climates, we argue that the experienced level of cold stress intensity is largely similar across environments inhabited by the Drosophila genus. This reaffirms that cold tolerance is an important trait shaping the fundamental niche of both boreal and tropical species. Overall, the TDT analysis of Drosophila at low temperature provides a powerful and predictive tool for quantifying insect cold resistance. This approach enables detailed cross‐species comparisons that allow for both ecological and physiological inference. On a general level, we argue that TDT analysis offers relevant approximations of insect cold resistance that could help predict insect responses to climatic change. Read the free Plain Language Summary for this article on the Journal blog.