Warm acclimation reduces the sensitivity of Drosophila species to heat stress at ecologically relevant scales

Thermal acclimation is presumed to affect heat tolerance, though it is unclear how this should impact populations under realistic natural conditions. In this study, we quantified how thermal acclimation affect heat tolerance landscapes in Drosophila and, as a consequence, their predicted mortality in the field based on simulations with the dynamic landscape. We measured the thermal tolerance of four Drosophila species (D. repleta, D. hydei, D. simulans, and D. virilis) acclimated to five constant temperatures along a gradient. We then combined this information with field temperatures to construct dynamic tolerance landscapes for these species and examine how survival varies over the course of a year. Our analyses reveal the effect of acclimation on an ecologically relevant scale, specifically through the study of cumulative mortality under natural thermal regimes. We show that different species exhibit a common strategy in response to thermal challenges during acclimation, resulting in a trade-off between increasing critical temperature (CTmax) and thermal sensitivity (z). Furthermore, we show that while acclimation presents a relatively modest improvement in thermal tolerance during short ramping laboratory trials, this response becomes stronger when tolerance estimates are translated into ecologically relevant timescales, such as annual survival. Our results indicate that acclimation to warm conditions can substantially increase their thermal tolerance, contradicting the idea that thermal acclimation in ectotherms has only a minor effect. Our work applies novel approaches to studying thermal tolerance and aims to highlight the role of acclimation in ameliorating the impact of global warming.

Methods
Adult Drosophila were collected in Santiago, Chile, and identified based on morphology. Laboratory lines of four species (D. hydei, D. repleta, D. virilis, and D. simulans) were established and maintained under controlled conditions at 21°C. Flies were acclimated to five different temperatures (18, 21, 24, 27, and 30°C), and their development was closely monitored.

From each acclimation group, a random selection of adult flies was made. After briefly anesthetizing and sexing them, the flies were allowed to recover for 1-2 days before heat tolerance experiments. For each species and acclimation temperature, heat tolerance assays were performed with 10 males and 10 females. The flies were placed in vials and submerged in water baths at four critical temperatures (36, 38, 40, and 42°C), with knockdown times recorded. A total of 1,482 individuals were tested across 59 assays.

Thermal tolerance landscapes for each species were estimated using thermal death time (TDT) curves. These landscapes were then applied to a dynamic model to predict survival under field conditions. The model calculates instantaneous survival as temperatures vary over time, converting constant temperature survival probabilities to variable conditions. Hourly field temperature data (2014-2018) from Santiago was interpolated to 1-minute intervals, and the annual cycle (July to June) was analyzed to evaluate the effect of Austral summer temperatures on survival.