Summary 1. Thermal tolerance may limit and therefore predict ectotherm geographic distributions. However, which of the many metrics of thermal tolerance best predict distribution is often unclear, even for drosophilids, which constitute a popular and well-described animal model. 2. Five metrics of cold tolerance were measured for 14 Drosophila species to determine which metrics most strongly correlate with geographic distribution. The species represent tropical to temperate regions but all were reared under similar (common garden) conditions (20°C). The traits measured were: chill coma temperature (CT min ), lethal temperature (LTe 50 ), lethal time at low temperature (LTi 50 ), chill coma recovery time (CCRT) and supercooling point (SCP). 3. Measures of CT min , LTe 50 and LTi 50 proved to be the best predictors to describe the variation in realized latitudinal distributions (R 2 = 0Á699, R 2 = 0Á741 and 0Á550, respectively) and estimated environmental cold exposure (R 2 = 0Á633, R 2 = 0Á641 and 0Á511, respectively).Measures of CCRT also correlated significantly with estimated minimum temperature (R 2 = 0Á373), while the SCP did not. These results remained consistent after phylogenetically independent analysis or when applying nonlinear regression. Moreover, our findings were supported by a similar analysis based on existing data compiled from the Drosophila cold tolerance literature. 4. Trait correlations were strong between LTe 50 , LTi 50 and CT min , respectively (0Á83 > R 2 > 0Á55). However, surprisingly, there was only a weak correlation between the entrance into coma (CT min ) and the recovery from chill coma (CCRT) (R 2 = 0Á256). 5.Considering the findings of the present study, data from previous studies and the logistical constraints of each measure of cold tolerance, we conclude that CT min and LTe 50 are superior measures when estimating the ecologically relevant cold tolerance of drosophilids. Of these two traits, CT min requires less equipment, time and animals and thereby presents a relatively fast, simple and dynamic measure of cold tolerance.

The adaptability of organisms to novel environmental conditions depends on the amount of genetic variance present in the population as well as on the ability of individuals to adjust their phenotype through phenotypic plasticity. Here, we investigated the phenotypic plasticity induced by a single generation's exposure to three different temperature regimes with respect to several life-history and stress-resistance traits in a natural population of Drosophila simulans. We studied a constant as well as a predictably and an unpredictably fluctuating temperature regime. We found high levels of phenotypic plasticity among all temperature regimes, suggesting a strong influence of both temperature fluctuations and their predictability. Increased heat tolerance was observed for flies developed in both types of fluctuating thermal environments compared with flies developed in a constant environment. We suggest that this was due to beneficial hardening when developing in either fluctuating temperature environment. To our surprise, flies that developed in constant and predictably changing environments were similar to each other in most traits when compared to flies from the unpredictably fluctuating environment. The unpredictably changing thermal environment imposed the most stressful condition, resulting in the lowest performance for stress-related traits, even though the absolute temperature changes never exceeded that of the predictably fluctuating environment. The overall decreased stress resistance of flies in the unpredictably fluctuating environment may be the consequence of maladaptive phenotypic plasticity in this setting, indicating that the adaptive value of plasticity depends on the predictability of the environment.

The selective past of populations is presumed to affect the levels of phenotypic plasticity. Experimental evolution at constant temperatures is generally expected to lead to a decreased level of plasticity due to presumed costs associated with phenotypic plasticity when not needed. In this study, we investigated the effect of experimental evolution in constant, predictable and unpredictable daily fluctuating temperature regimes on the levels of phenotype plasticity in several life history and stress resistance traits in Drosophila simulans. Contrary to the expectation, evolution in the different regimes did not affect the levels of plasticity in any of the traits investigated even though the populations from the different thermal regimes had evolved different stress resistance and fitness trait means. Although costs associated with phenotypic plasticity are known, our results suggest that the maintenance of phenotypic plasticity might come at low and negligible costs, and thus, the potential of phenotypic plasticity to evolve in populations exposed to different environmental conditions might be limited.

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Few genetic and environmental correlations between life history and stress resistance traits affect adaptation to fluctuating thermal regimes T Manenti, JG Sørensen, NN Moghadam and V Loeschcke Laboratory selection in thermal regimes that differed in the amplitude and the predictability of daily fluctuations had a marked effect on stress resistance and life history traits in Drosophila simulans. The observed evolutionary changes are expected to be the result of both direct and correlated responses to selection. Thus, a given trait might not evolve independently from other traits because of genetic correlations among these traits. Moreover, different test environments can induce novel genetic correlations because of the activation of environmentally dependent genes. To test whether and how genetic correlations among stress resistance and life history traits constrain evolutionary adaptation, we used three populations of D. simulans selected for 20 generations in constant, predictable and unpredictable daily fluctuating thermal regimes and tested each of these selected populations in the same three thermal regimes. We explored the relationship between genetic correlations between traits and the evolutionary potential of D. simulans by comparing genetic correlation matrices in flies selected and tested in different thermal test regimes. We observed genetic correlations mainly between productivity, body size, starvation and desiccation tolerance, suggesting that adaptation to the three thermal regimes was affected by correlations between these traits. We also found that the correlations between some traits such as body size and productivity or starvation tolerance and productivity were determined by test regime rather than selection regime that is expected to limit genetic adaptation to thermal regimes in these traits. The results of this study suggest that several traits and several environments are needed to explore adaptive responses, as genetic and environmentally induced correlations between traits as results obtained in one environment cannot be used to predict the response of the same population in another environment.

Adaptation of natural populations to variable environmental conditions may occur by changes in trait means and/or in the levels of plasticity. Theory predicts that environmental heterogeneity favors plasticity of adaptive traits. Here we investigated the performance in several traits of three sympatric Drosophila species freshly collected in two environments that differ in the heterogeneity of environmental conditions. Differences in trait means within species were found in several traits, indicating that populations differed in their evolutionary response to the environmental conditions of their origin. Different species showed distinct adaptation with a very different role of plasticity across species for coping with environmental changes. However, geographically distinct populations of the same species generally displayed the same levels of plasticity as induced by fluctuating thermal regimes. This indicates a weak and trait‐specific effect of environmental heterogeneity on plasticity. Furthermore, similar levels of plasticity were found in a laboratory‐adapted population of Drosophila melanogaster with a common geographic origin but adapted to the laboratory conditions for more than 100 generations. Thus, this study does not confirm theoretical predictions on the degree of adaptive plasticity among populations in relation to environmental heterogeneity but shows a very distinct role of species‐specific plasticity.

Populations of Drosophila melanogaster, as well as of other Drosophila species, show body size differences according to their geographic origin. Thermal selection is considered to be the most likely cause explaining these differences. We investigated wing size, wing shape and their relationship in 3 different geographic populations of D. melanogaster, 1 population of D. simulans, and their interspecific hybrids grown at 18, 21, 25 and 28°C. The aim was to explore how the past adaptive history of 2 related species, or populations of the same species, modulates the plastic response to the environment. The wing size plasticity of hybrids between the temperate D. simulans (Bologna) and the 2 tropical D. melanogaster populations (Belém and Rio de Janeiro) was higher than that of parental species, probably as a side effect of a decanalized and compromised development at higher temperatures. The wing size plasticity of Bologna hybrids was the same as the parents, suggesting that the 2 species are subjected to the same plasticity selection. Wing shape was typical of each species, population and temperature. Shape differences increased in hybrids at higher temperatures as a consequence of developmental perturbation. The allometric relationship between size and shape changed among temperatures and among species, suggesting that the wing development is differently regulated in the 2 species and can be altered by natural selection. The allometry changed between populations of different geographic origin of D. melanogaster, but was similar in the 2 species that shared the same selective environment. Two species would have been subjected to the same plasticity selection, so the breakdown of the plastic response is avoided in hybrids. As a whole, these data suggest that thermal plasticity is a trait under selection and that similar mechanisms are at work in different species.

Organisms are exposed to temperatures that vary, for example on diurnal and seasonal time scales. Thus, the ability to behaviorally and/or physiologically respond to variation in temperatures is a fundamental requirement for long-term persistence. Studies on thermal biology in ectotherms are typically performed under constant laboratory conditions, which differ markedly from the variation in temperature across time and space in nature. Here, we investigate evolutionary adaptation and environmentally induced plastic responses of Drosophila simulans to no fluctuations (constant), predictable fluctuations or unpredictable fluctuations in temperature. We whole-genome sequenced populations exposed to 20 generations of experimental evolution under the three thermal regimes and examined the proteome after short-term exposure to the same three regimes. We find that unpredictable fluctuations cause the strongest response at both genome and proteome levels. The loci showing evolutionary responses were generally unique to each thermal regime, but a minor overlap suggests either common laboratory adaptation or that some loci were involved in the adaptation to multiple thermal regimes. The evolutionary response, i.e., loci under selection, did not coincide with induced responses of the proteome. Thus, genes under selection in fluctuating thermal environments are distinct from genes important for the adaptive plastic response observed within a generation. This information is key to obtain a better understanding and prediction of the effects of future increases in both mean and variability of temperatures.