Understanding how populations evolve under thermal stress and how this process shapes the response of other stress responses is an important research topic in the context of thermal adaptation and climate change. In a thermal experimental evolution study in Drosophila subobscura, Castañeda (2024) addressed the correlated responses to selection for increasing knockdown temperature in different resistance traits, either directly related to thermal stress (e.g. knockdown time at different temperatures and CTmax) or not (e.g. desiccation and starvation resistance). 

The author found that the evolution of higher knockdown temperature did in fact lead to correlated responses in other stress traits. While such correlations might be expected for the thermal stress traits measured (knockdown time and CTmax), it was perhaps less expectable for desiccation and starvation resistance. However, the general occurrence of correlated evolutionary responses between stressors has been previously described, namely in Drosophila (e.g. see Bubliy and Loeschcke 2005), pointing to a possible genetic link between distinct (thermal) stress traits. 

There are however some features that make the findings of this study rather appealing. First, the evidence that the correlated stress responses depend on the intensity of thermal selection (i.e. the warming rate) and on the sex of the organisms. Second, correlated patterns of both desiccation and starvation resistance highlight the possibility of the evolution of a cross-tolerance response, which might positively impact on population ability to evolve under sustained stressful environments (Rodgers and Gomez Izasa 2023). However, it is important to point out that the correlated patterns between these two resistance traits (desiccation and starvation) were not exactly consistent. In fact, the negative correlated response observed for female starvation resistance is thought provoking and argues again a general scenario of cross-tolerance. 

While these findings are a step forward for a more multifaceted understanding of thermal adaptation in the context of stressful environments, they also highlight the need for further studies of thermal adaptation namely 1) addressing the underlying physiological and genomic mechanisms that link male and female heat tolerance and the response to other stress resistance traits (namely starvation resistance); 2) testing the extent to which cross-resistance patterns can be generalized to different thermal selection contexts and populations. 

In addition, this study also opens new questions considering the scope of correlated evolution to other stress traits, that might be relevant in diverse ecological scenarios. For instance, does selection towards higher heat resistance lead to correlated evolution of cold resistance? And under which circumstances (e.g. different heat selection intensities)?  In fact, the occurrence of a positive (or negative) correlation cold and heat stress responses is a topic of high interest, with relevant ecological implications particularly considering the increased thermal fluctuations in natural environments because of climate warming. Cross-tolerance between cold and heat stress responses has been described (Singh 2022, Rodgers and Gomez Izasa 2023). On the other hand, negative correlations (i.e. trade-offs) between these stress traits (Stazione et al. 2020; Schou et al 2022) can impact negatively on populations’ ability to withstand thermal variability. 

As climatic changes proceed leading to increasing environmental variability, empirical studies such as that of Castañeda (2024) are critical in the pursue for a multivariate perspective on trait evolution in scenarios of climate change adaptation. Understanding how tolerance to different environmental stressors may evolve and which factors can act as drivers of that variation will ultimately enable better forecasts of climate change effects on biodiversity in nature.

References

Castañeda, LE. Cross-tolerance evolution is driven by selection on heat tolerance in Drosophila subobscura. Biorxiv, ver. 4 peer-reviewed and recommended by Peer Community in Evolutionary Biology (2024). https://www.doi.org/10.1101/2023.09.05.556367

Bubliy, OA, Loeschcke, V. Correlated responses to selection for stress resistance and longevity in a laboratory population of Drosophila melanogaster. J Evol Biol. 18(4):789-803 (2005). https://www.doi.org/10.1111/j.1420-9101.2005.00928.x. 

Rodgers, EM, Gomez Isaza, DF. The mechanistic basis and adaptive significance of cross-tolerance: a 'pre-adaptation' to a changing world? J Exp Biol. 226(11):jeb245644 (2023). https://www.doi.org/10.1242/jeb.245644. 

Schou, MF, Engelbrecht, A, Brand, Z, Svensson, EI, Cloete, S, Cornwallis, CK. Evolutionary trade-offs between heat and cold tolerance limit responses to fluctuating climates. Sci Adv. 8(21):eabn9580 (2022). https://www.doi.org/10.1126/sciadv.abn9580. 

Singh, K, Arun Samant, M, Prasad, NG. Evolution of cross-tolerance in Drosophila melanogaster as a result of increased resistance to cold stress. Sci Rep. 12(1):19536 (2022). https://www.doi.org/10.1038/s41598-022-23674-z.

Stazione, L, Norry, FM, Gomez, FH, Sambucetti, P. Heat knockdown resistance and chill-coma recovery as correlated responses to selection on mating success at high temperature in Drosophila buzzatii. Ecol Evol. 10(4):1998-2006 (2020). https://www.doi.org/10.1002/ece3.6032.