Thermal fluctuations affect the transcriptome through mechanisms independent of average temperature

Transcriptomics of differences in thermal plasticity associated with selection for an exaggerated male sexual trait

The information about the magnitude of differences in thermal plasticity both between and within populations, as well as identification of the underlying molecular mechanisms are key to understanding the evolution of thermal plasticity. In particular, genes underlying variation in the physiological response to temperature can provide raw material for selection acting on plastic traits. Using RNAseq, we investigate the transcriptional response to temperature in males and females from bulb mite populations selected for the increased frequency of one of two discrete male morphs (fighter- and scrambler-selected populations) that differ in relative fitness depending on temperature. We show that different mechanisms underlie the divergence in thermal response between fighter- and scrambler-selected populations at decreased vs. increased temperature. Temperature decrease to 18 °C was associated with higher transcriptomic plasticity of males with more elaborate armaments, as indicated by a significant selection-by-temperature interaction effect on the expression of 40 genes, 38 of which were upregulated in fighter-selected populations in response to temperature decrease. In response to 28 °C, no selection-by-temperature interaction in gene expression was detected. Hence, differences in phenotypic response to temperature increase likely depended on genes associated with their distinct morph-specific thermal tolerance. Selection of males also drove gene expression patterns in females. These patterns could be associated with temperature-dependent fitness differences between females from fighter- vs. scrambler-selected populations reported in previous studies. Our study shows that selection for divergent male sexually selected morphologies and behaviors has a potential to drive divergence in metabolic pathways underlying plastic response to temperature in both sexes.

Strong acclimation effect of temperature and humidity on heat tolerance of the Arctic collembolan Megaphorura arctica

The Arctic is a highly variable environment in which extreme daily and seasonal temperature fluctuations can occur. With climate change, an increase in the occurrence of extreme high temperatures and drought events is expected. While the effects of cold and dehydration stress on polar arthropods are well studied in combination, little is known about how these species respond to the combined effects of heat and dehydration stress. In this paper, we investigated how the heat tolerance of the Arctic collembola Megaphorura arctica is affected by combinations of different temperature and humidity acclimation regimes under controlled laboratory conditions. The effect of acclimation temperature was complex and highly dependent on both acclimation time and temperature, and was found to have a positive, negative or no effect depending on experimental conditions. Further, we found marked effects of the interaction between temperature and humidity on heat tolerance, with lower humidity severely decreasing heat tolerance when the acclimation temperature was increased. This effect was more pronounced with increasing acclimation time. Lastly, the effect of acclimation on heat tolerance under a fluctuating temperature regime was dependent on acclimation temperature and time, as well as humidity levels. Together, these results show that thermal acclimation alone has moderate or no effect on heat tolerance, but that drought events, likely to be more frequent in the future, in combination with high temperature stress can have large negative impacts on heat tolerance of some Arctic arthropods.

The time course of acclimation of critical thermal maxima is modulated by the magnitude of temperature change and thermal daily fluctuations

Plasticity in the critical thermal maximum (CT_max) helps ectotherms survive in variable thermal conditions. Yet, little is known about the environmental mechanisms modulating its time course. We used the larvae of three neotropical anurans (Boana platanera, Engystomops pustulosus and Rhinella horribilis) to test whether the magnitude of temperature changes and the existence of fluctuations in the thermal environment affected both the amount of change in CT_max and its acclimation rate (i.e., its time course). For that, we transferred tadpoles from a pre-treatment temperature (23 °C, constant) to two different water temperatures: mean (28 °C) and hot (33 °C), crossed with constant and daily fluctuating thermal regimes, and recorded CT_max values, daily during six days. We modeled changes in CT_max as an asymptotic function of time, temperature, and the daily thermal fluctuation. The fitted function provided the asymptotic CT_max value (CT_max∞) and CT_max acclimation rate (k). Tadpoles achieved their CT_max∞ between one and three days. Transferring tadpoles to the hot treatment generated higher CT_max∞ at earlier times, inducing faster acclimation rates in tadpoles. In contrast, thermal fluctuations equally led to higher CT_max∞ values but tadpoles required longer times to achieve CT_max∞ (i.e., slower acclimation rates). These thermal treatments interacted differently with the studied species. In general, the thermal generalist Rhinella horribilis showed the most plastic acclimation rates whereas the ephemeral-pond breeder Engystomops pustulosus, more exposed to heat peaks during larval development, showed less plastic (i.e., canalized) acclimation rates. Further comparative studies of the time course of CT_max acclimation should help to disentangle the complex interplay between the thermal environment and species ecology, to understand how tadpoles acclimate to heat stress.

Diapause-Linked Gene Expression Pattern and Related Candidate Duplicated Genes of the Mountain Butterfly Parnassius glacialis (Lepidoptera: Papilionidae) Revealed by Comprehensive Transcriptome Profiling

The mountain butterfly Parnassius glacialis is a representative species of the genus Parnassius, which probably originated in the high-altitude Qinhai–Tibet Plateau in the Miocene and later dispersed eastward into relatively low-altitude regions of central to eastern China. However, little is known about the molecular mechanisms underlying the long-term evolutionary adaptation to heterogeneous environmental conditions of this butterfly species. In this study, we obtained the high-throughput RNA-Seq data from twenty-four adult individuals in eight localities, covering nearly all known distributional areas in China, and firstly identified the diapause-linked gene expression pattern that is likely to correlate with local adaptation in adult P. glacialis populations. Secondly, we found a series of pathways responsible for hormone biosynthesis, energy metabolism and immune defense that also exhibited unique enrichment patterns in each group that are probably related to habitat-specific adaptability. Furthermore, we also identified a suite of duplicated genes (including two transposable elements) that are mostly co-expressed to promote the plastic responses to different environmental conditions. Together, these findings can help us to better understand this species’ successful colonization to distinct geographic areas from the western to eastern areas of China, and also provide us with some insights into the evolution of diapause in mountain Parnassius butterfly species.

From perplexing to predictive: are we ready to forecast insect disease susceptibility in a warming world?

Insects are critical to our ecosystems, but we do not fully understand their future in our warming world. Rising temperatures are affecting insect physiology in myriad ways, including changes to their immune systems and the ability to fight infection. Whether predicted changes in temperature will contribute to insect mortality or success, and the role of disease in their future survival, remains unclear. Although heat can enhance immunity by activating the integrated defense system (e.g. via the production of protective molecules such as heat-shock proteins) and accelerating enzyme activity, heat can also compromise the immune system through energetic-resource trade-offs and damage. The responses to heat are highly variable among species. The reasons for this variability are poorly known, and we are lagging in our understanding of how and why the immune system responds to changes in temperature. In this Commentary, we highlight the variation in insect immune responses to heat and the likely underlying mechanisms. We suggest that we are currently limited in our ability to predict the effects of rising temperatures on insect immunity and disease susceptibility, largely owing to incomplete information, coupled with a lack of tools for data integration. Moreover, existing data are concentrated on a relatively small number of insect Orders. We provide suggestions for a path towards making more accurate predictions, which will require studies with realistic temperature exposures and housing design, and a greater understanding of both the thermal biology of the immune system and connections between immunity and the physiological responses to heat.

Signaling through the dystrophin glycoprotein complex affects the stress-dependent transcriptome in Drosophila

Deficiencies in the human dystrophin glycoprotein complex (DGC), which links the extracellular matrix with the intracellular cytoskeleton, cause muscular dystrophies, a group of incurable disorders associated with heterogeneous muscle, brain and eye abnormalities. Stresses such as nutrient deprivation and aging cause muscle wasting, which can be exacerbated by reduced levels of the DGC in membranes, the integrity of which is vital for muscle health and function. Moreover, the DGC operates in multiple signaling pathways, demonstrating an important function in gene expression regulation. To advance disease diagnostics and treatment strategies, we strive to understand the genetic pathways that are perturbed by DGC mutations. Here, we utilized a Drosophila model to investigate the transcriptomic changes in mutants of four DGC components under temperature and metabolic stress. We identified DGC-dependent genes, stress-dependent genes and genes dependent on the DGC for a proper stress response, confirming a novel function of the DGC in stress-response signaling. This perspective yields new insights into the etiology of muscular dystrophy symptoms, possible treatment directions and a better understanding of DGC signaling and regulation under normal and stress conditions.

The effect of a temperature-sensitive prophage on the evolution of virulence in an opportunistic bacterial pathogen

Viruses are key actors of ecosystems and have major impacts on global biogeochemical cycles. Prophages deserve particular attention as they are ubiquitous in bacterial genomes and can enter a lytic cycle when triggered by environmental conditions. We explored how temperature affects the interactions between prophages and other biological levels using an opportunistic pathogen, the bacterium Serratia marcescens, which harbours several prophages and that had undergone an evolution experiment under several temperature regimes. We found that the release of one of the prophages was temperature-sensitive and malleable to evolutionary changes. We further discovered that the virulence of the bacterium in an insect model also evolved and was positively correlated with phage release rates. We determined through analysis of genetic and epigenetic data that changes in the bacterial outer cell wall structure possibly explain this phenomenon. We hypothezise that the temperature-dependent phage release rate acted as a selection pressure on S. marcescens and that it resulted in modified bacterial virulence in the insect host. Our study system illustrates how viruses can mediate the influence of abiotic environmental changes to other biological levels and thus be involved in ecosystem feedback loops.

The effect of thermal microenvironment in upper thermal tolerance plasticity in tropical tadpoles. Implications for vulnerability to climate warming

Current climate change is generating accelerated increase in extreme heat events and organismal plastic adjustments in upper thermal tolerances, (critical thermal maximum -CT_max ) are recognized as the quicker mitigating mechanisms. However, current research casts doubt on the actual mitigating role of thermal acclimation to face heat impacts, due to its low magnitude and weak environmental signal. Here, we examined these drawbacks by first estimating maximum extent of thermal acclimation by examining known sources of variation affecting CT_max expression, such as daily thermal fluctuation and heating rates. Second, we examined whether the magnitude and pattern of CT_max plasticity is dependent of the thermal environment by comparing the acclimation responses of six species of tropical amphibian tadpoles inhabiting thermally contrasting open and shade habitats and, finally, estimating their warming tolerances (WT = CT_max  - maximum temperatures) as estimator of heating risk. We found that plastic CT_max responses are improved in tadpoles exposed to fluctuating daily regimens. Slow heating rates implying longer duration assays determined a contrasting pattern in CT_max plastic expression, depending on species environment. Shade habitat species suffer a decline in CT_max whereas open habitat tadpoles greatly increase it, suggesting an adaptive differential ability of hot exposed species to quick hardening adjustments. Open habitat tadpoles although overall acclimate more than shade habitat species, cannot capitalize this beneficial increase in CT_max, because the maximum ambient temperatures are very close to their critical limits, and this increase may not be large enough to reduce acute heat stress under the ongoing global warming.

Molecular mechanisms underlying plasticity in a thermally varying environment

Adaptation to environmental variability is a prerequisite for species’ persistence in their natural environments. With climate change predicted to increase the frequency and severity of temperature fluctuations, ectothermic organisms may increasingly depend on acclimation capacity to accommodate thermal variability. To elucidate the molecular basis of fluctuating temperature‐induced phenotypic plasticity, we investigated heat tolerance and the mechanisms induced by acclimation to thermal variability as compared to those seen at constant temperature. We ran genome‐wide transcriptomic analysis on Drosophila melanogaster subjected to acclimation at constant (19 ± 0°C) and fluctuating (19 ± 8°C) temperatures and contrasted the induction of molecular mechanisms in adult males, adult females and larvae. We found life stage‐ and sex‐specific dynamics of the acclimation responses to fluctuating temperatures. Adult flies exposed to temperature fluctuations showed a constitutive improvement in heat tolerance while heat tolerance of larvae tracked thermal fluctuations. A constitutive down‐regulation of gene expression was observed for several genes in the larvae exposed to fluctuations. Our results for adult females showed that, for several genes, fluctuating temperature acclimation resulted in canalization of gene expression. Both transcriptional and post‐transcriptional machinery were greatly affected by fluctuations in adult males. Gene ontology analysis showed enrichment of the heat stress response involving several major heat shock proteins in both larvae and adults exposed to fluctuating temperatures, even though fluctuations were in a benign range of temperatures. Finally, molecular mechanisms related to environmental sensing seem to be an important component of insect responses to thermal variability.

Transcriptome analysis uncovers different avenues for manipulating cold performance in Chrysomya megacephala (Diptera, Calliphoridae)

Temperature strongly impacts the rates of physiological and biochemical processes, which in turn can determine the survival and population size of insects. At low temperatures performance is limited, however, cold tolerance and performance at low temperature can be improved after short- or long-term acclimation in many insect species. To understand mechanisms underlying acclimation, we sequenced and compared the transcriptome of the blowfly Chrysomya megacephala under rapid cold hardening (RCH) and long-term cold acclimation (LCA) conditions. The RCH response was dominated by genes related to immune response, spliceosome, and protein processing in endoplasmic reticulum with up-regulation during recovery from RCH. In contrast, LCA was associated with genes related to carbohydrate metabolism and cytoskeleton branching and stabilizing. Meanwhile, mRNA levels of genes related to glycerophospholipid metabolism, and some heat shock proteins (Hsps) were collectively up-regulated by both RCH and LCA. There were more genes and pathway adjustments associated with LCA than RCH. Overall, the transcriptome data provide basic information of molecular mechanisms underpinning the RCH and LCA response. The partly independent molecular responses to RCH and LCA suggest that several avenues for manipulating cold performance exist and RCH might be more effective as it only triggers fewer genes and affects the general metabolisms less. These observations provide some appropriate methods to improve cold tolerance of C. megacephala, and hold promise for developing an extended use of mass-reared C. megacephala with better cold performance as a pollinator of crops at low temperatures.

Effects of warming rates on physiological and molecular components of response to CTMax heat stress in the Antarctic fish Harpagifer antarcticus

Maximum and minimum Critical thermal limits (CTMax and CTMin) have been studied extensively to assess thermal tolerance in ectotherms by means of ramping assays. Notothenioid fish have been proposed as particularly sensitive to temperature increases related to global climate change. However, there are large gaps in our understanding of the thermal responses of these extreme cold-adapted fish in assays with heating rates. We evaluated the effects of two commonly used heating rates (0.3 and 1 °C/min) on the cellular stress responses in the intertidal Antarctic fish Harpagifer antarcticus immediately after CTMax was reached, and at 2 and 4 h of recovery time in ambient water. We compared CTMax values, the relative transcript expression of genes relvant to heat shock response (Hsc70, Hsp70, Grp78), hypoxia (Hif1-α, LDHa, GR), ubiquitination (Ube2), and apoptosis (SMAC/DIABLO), and five plasma parameters - glucose, lactate, total protein, osmolality and cortisol. CTMax values between the two heating rates are not significantly different, and both rates elicited a similar stress response at molecular and physiological levels. We found a lack of up-regulated response of heat shock proteins, consistent with other Antarctic notothenioids. The general transcriptional pattern trended to downregulation, which was more evident in the slower 0.3 °C/min rate, and instances of upregulation were mainly related to ubiquitination. The faster 1 °C/min rate, rarely used for Antarctic fish, can be suitable for studying cold-adapted stenothermic fish without overestimating thermal tolerance or inducing damage from longer heat exposure.

Wide Diurnal Temperature Amplitude and High Population Density Can Positively Affect the Life History of Sitobion avenae (Hemiptera: Aphididae)

Diurnal temperature amplitude is known to have a large influence on insect life history. Population density affects intraspecific competition and many other aspects of insect life history. However, there is limited information on the interactive effects of these factors on insects. Here, we tested the interactive effects of three diurnal temperature amplitudes (22 ± 0°C, 22 ± 6°C, and 22 ± 12°C) and three population densities on the development, survival, longevity, and fecundity of the English grain aphid Sitobion avenae (Fabricius) (Homoptera: Aphididae). At a constant temperature, increasing population density reduced the growth and survival of early-instar nymphs, increased longevity, and reduced fecundity. At a low population density, increasing temperature amplitude inhibited nymph development. However, even at a high temperature amplitude, nymph survival rate was higher than expected, and reproduction was possible because the recovery of the lower night-temperatures eliminated thermal stress. Increasing the population density reduced, and even reversed, the negative effects of the wide temperature amplitude. This may reflect synergistic interactions between population density and wide temperature amplitude as these stressors each incur energetic costs. These findings emphasize the importance of temperature amplitude and population density for improving prediction accuracy and damage assessment during pest control modeling.

Dealing with predictable and unpredictable temperatures in a climate change context: the case of parasitoids and their hosts

The Earth's climate is changing at a rapid pace. To survive in increasingly fluctuating and unpredictable environments, species can either migrate or evolve through rapid local adaptation, plasticity and/or bet-hedging. For small ectotherm insects, like parasitoids and their hosts, phenotypic plasticity and bet-hedging could be critical strategies for population and species persistence in response to immediate, intense and unpredictable temperature changes. Here, we focus on studies evaluating phenotypic responses to variable predictable thermal conditions (for which phenotypic plasticity is favoured) and unpredictable thermal environments (for which bet-hedging is favoured), both within and between host and parasitoid generations. We then address the effects of fluctuating temperatures on host-parasitoid interactions, potential cascading effects on the food web, as well as biological control services. We conclude our review by proposing a road map for designing experiments to assess if plasticity and bet-hedging can be adaptive strategies, and to disentangle how fluctuating temperatures can affect the evolution of these two strategies in parasitoids and their hosts.

Different amplitudes of temperature fluctuation induce distinct transcriptomic and metabolomic responses in the dung beetle Phanaeus vindex

Most studies exploring molecular and physiological responses to temperature have focused on constant temperature treatments. To gain a better understanding of the impact of fluctuating temperatures, we investigated the effects of increased temperature variation on Phanaeus vindex dung beetles across levels of biological organization. Specifically, we hypothesized that increased temperature variation is energetically demanding. We predicted that thermal sensitivity of metabolic rate and energetic reserves would be reduced with increasing fluctuation. To test this, we examined the responses of dung beetles to constant (20°C), low fluctuation (20±5°C), or high fluctuation (20±12°C) temperature treatments using respirometry, assessment of energetic reserves and HPLC-MS-based metabolomics. We found no significant differences in metabolic rate or energetic reserves, suggesting increased fluctuations were not energetically demanding. To understand why there was no effect of increased amplitude of temperature fluctuation on energetics, we assembled and annotated a de novo transcriptome, finding non-overlapping transcriptomic and metabolomic responses of beetles exposed to different fluctuations. We found that 58 metabolites increased in abundance in both fluctuation treatments, but 15 only did so in response to high-amplitude fluctuations. We found that 120 transcripts were significantly upregulated following acclimation to any fluctuation, but 174 were upregulated only in beetles from the high-amplitude fluctuation treatment. Several differentially expressed transcripts were associated with post-translational modifications to histones that support a more open chromatin structure. Our results demonstrate that acclimation to different temperature fluctuations is distinct and may be supported by increasing transcriptional plasticity. Our results indicate for the first time that histone modifications may underlie rapid acclimation to temperature variation.

Analysis of the transcription of genes encoding heat shock proteins (hsp) in Aedes aegypti Linnaeus, 1762 (Diptera: Culicidae), maintained under climatic conditions provided by the IPCC (Intergovernmental Panel On Climate Change) for the year 2100

Human actions intensify the greenhouse effect, aggravating climate changes in the Amazon and elsewhere in the world. The Intergovernmental Panel on Climate Change (IPCC) foresees a global increase of up to 4.5 °C and 850 ppm CO₂ (above current levels) by 2100. This will impact the biology of the Aedes aegypti mosquito, vector of Dengue, Zika, urban Yellow Fever and Chikungunya. Heat shock proteins are associated with adaptations to anthropic environments and the interaction of some viruses with the vector. The transcription of the hsp26, hsp83 and hsc70 genes of an A. aegypti population, maintained for more than forty-eight generations, in the Current, Intermediate and Extreme climatic scenario predicted by the IPCC was evaluated with qPCR. In females, highest levels of hsp26, hsp83 and hsc70 expression occurred in the Intermediate scenario, while in males, levels were high only for hsp26 gene in Current and Extreme scenarios. Expression of hsp83 and hsc70 genes in males was low under all climatic scenarios, while in the Extreme scenario females had lower expression than in the Current scenario. The data suggest compensatory or adaptive processes acting on heat shock proteins, which can lead to changes in the mosquito's biology, altering vectorial competence.

Impacts of thermal fluctuations on heat tolerance and its metabolomic basis in Arabidopsis thaliana, Drosophila melanogaster, and Orchesella cincta

Temperature varies on a daily and seasonal scale and thermal fluctuations are predicted to become even more pronounced under future climate changes. Studies suggest that plastic responses are crucial for species’ ability to cope with thermal stress including variability in temperature, but most often laboratory studies on thermal adaptation in plant and ectotherm organisms are performed at constant temperatures and few species included. Recent studies using fluctuating thermal regimes find that thermal performance is affected by both temperature mean and fluctuations, and that plastic responses likely will differ between species according to life strategy and selective past. Here we investigate how acclimation to fluctuating or constant temperature regimes, but with the same mean temperature, impact on heat stress tolerance across a plant (Arabidopsis thaliana) and two arthropod species (Orchesella cincta and Drosophila melanogaster) inhabiting widely different thermal microhabitats and with varying capability for behavioral stress avoidance. Moreover, we investigate the underlying metabolic responses of acclimation using NMR metabolomics. We find increased heat tolerance for D. melanogaster and A. thaliana exposed to fluctuating acclimation temperatures, but not for O. cincta. The response was most pronounced for A. thaliana, which also showed a stronger metabolome response to thermal fluctuations than both arthropods. Generally, sugars were more abundant across A. thaliana and D. melanogaster when exposed to fluctuating compared to constant temperature, whereas amino acids were less abundant. This pattern was not evident for O. cincta, and generally we do not find much evidence for similar metabolomics responses to fluctuating temperature acclimation across species. Differences between the investigated species’ ecology and different ability to behaviorally thermoregulate may have shaped their physiological responses to thermal fluctuations.

Pronounced Plastic and Evolutionary Responses to Unpredictable Thermal Fluctuations in Drosophila simulans

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.

Genetic Constraints, Transcriptome Plasticity, and the Evolutionary Response to Climate Change

In situ adaptation to climate change will be critical for the persistence of many ectotherm species due to their relative lack of dispersal capacity. Climate change is causing increases in both the mean and the variance of environmental temperature, each of which may act as agents of selection on different traits. Importantly, these traits may not be heritable or have the capacity to evolve independently from one another. When genetic constraints prevent the "baseline" values of thermal performance traits from evolving rapidly, phenotypic plasticity driven by gene expression might become critical. We review the literature for evidence that thermal performance traits in ectotherms are heritable and have genetic architectures that permit their unconstrained evolution. Next, we examine the relationship between gene expression and both the magnitude and duration of thermal stress. Finally, we identify genes that are likely to be important for adaptation to a changing climate and determine whether they show patterns consistent with thermal adaptation. Although few studies have measured narrow-sense heritabilities of thermal performance traits, current evidence suggests that the end points of thermal reaction norms (tolerance limits) are moderately heritable and have the potential to evolve rapidly. By contrast, performance at intermediate temperatures has substantially lower evolutionary potential. Moreover, evolution in many species appears to be constrained by genetic correlations such that populations can adapt to either increases in mean temperature or temperature variability, but not both. Finally, many species have the capacity for plastic expression of the transcriptome in response to temperature shifts, with the number of differentially expressed genes increasing with the magnitude, but not the duration, of thermal stress. We use these observations to develop a conceptual model that describes the likely trajectory of genome evolution in response to changes in environmental temperature. Our results indicate that extreme weather events, rather than gradual increases in mean temperature, are more likely to drive genetic and phenotypic change in wild ectotherms.

Evolution and Plasticity of the Transcriptome Under Temperature Fluctuations in the Fungal Plant Pathogen Zymoseptoria tritici

Most species live in a variable environment in nature. Yet understanding the evolutionary processes underlying molecular adaptation to fluctuations remains a challenge. In this study we investigate the transcriptome of the fungal wheat pathogen Zymoseptoria tritici after experimental evolution under stable or fluctuating temperature, by comparing ancestral and evolved populations simultaneously. We found that temperature regimes could have a large and pervasive effect on the transcriptome evolution, with as much as 38% of the genes being differentially expressed between selection regimes. Although evolved lineages showed different changes of gene expression based on ancestral genotypes, we identified a set of genes responding specifically to fluctuation. We found that transcriptome evolution in fluctuating conditions was repeatable between parallel lineages initiated from the same genotype for about 60% of the differentially expressed genes. Further, we detected several hotspots of significantly differentially expressed genes in the genome, in regions known to be enriched in repetitive elements, including accessory chromosomes. Our findings also evidenced gene expression evolution toward a gain of robustness (loss of phenotypic plasticity) associated with the fluctuating regime, suggesting robustness is adaptive in changing environment. This work provides valuable insight into the role of transcriptional rewiring for rapid adaptation to abiotic changes in filamentous plant pathogens.

Rapid within- and transgenerational changes in thermal tolerance and fitness in variable thermal landscapes

Phenotypic plasticity may increase the performance and fitness and allow organisms to cope with variable environmental conditions. We studied within-generation plasticity and transgenerational effects of thermal conditions on temperature tolerance and demographic parameters in Drosophila melanogaster. We employed a fully factorial design, in which both parental (P) and offspring generations (F1) were reared in a constant or a variable thermal environment. Thermal variability during ontogeny increased heat tolerance in P, but with demographic cost as this treatment resulted in substantially lower survival, fecundity, and net reproductive rate. The adverse effects of thermal variability (V) on demographic parameters were less drastic in flies from the F1, which exhibited higher net reproductive rates than their parents. These compensatory responses could not totally overcome the challenges of the thermally variable regime, contrasting with the offspring of flies raised in a constant temperature (C) that showed no reduction in fitness with thermal variation. Thus, the parental thermal environment had effects on thermal tolerance and demographic parameters in fruit fly. These results demonstrate how transgenerational effects of environmental conditions on heat tolerance, as well as their potential costs on other fitness components, can have a major impact on populations' resilience to warming temperatures and more frequent thermal extremes.

Drawing the line: Linear or non-linear reaction norms in response to adult acclimation on lower thermal limits

Estimates of lower thermal limits are widely used to infer sensitivity to climate variability, local adaptation and adaptive acclimation responses in ectotherms. These inferences build on the ecological relevance of the tolerance estimates and assume that estimates can be extrapolated to relevant conditions. Methodological effects for upper thermal limits have been extensively investigated, with different ramping rates and acclimation regimes giving rise to varying, and even disparate, conclusions. However, methodological effects have received much less attention for lower thermal limits. In this study, we explicitly test whether methodology could affect estimates of lower thermal limits in interaction with acclimation temperature and thermal variability, by acclimating adult Drosophila melanogaster to different constant and fluctuating temperature regimes and generating reaction norms at different ramping rates. We find that ramping rates have no significant effect on the lower thermal limits. Constant temperature acclimation resulted in non-linear reaction norms, while the introduction of thermal variability during adult life result in linear reaction norms. Thus, applying ecologically relevant conditions (here thermal variability) potentially impacts the results and conclusions of insect low temperature tolerance and acclimation capacity.

Integrating GWAS and Transcriptomics to Identify the Molecular Underpinnings of Thermal Stress Responses in Drosophila melanogaster

Thermal tolerance of an organism depends on both the ability to dynamically adjust to a thermal stress and preparatory developmental processes that enhance thermal resistance. However, the extent to which standing genetic variation in thermal tolerance alleles influence dynamic stress responses vs. preparatory processes is unknown. Here, using the model species Drosophila melanogaster, we used a combination of Genome Wide Association mapping (GWAS) and transcriptomic profiling to characterize whether genes associated with thermal tolerance are primarily involved in dynamic stress responses or preparatory processes that influence physiological condition at the time of thermal stress. To test our hypotheses, we measured the critical thermal minimum (CTmin) and critical thermal maximum (CTmax) of 100 lines of the Drosophila Genetic Reference Panel (DGRP) and used GWAS to identify loci that explain variation in thermal limits. We observed greater variation in lower thermal limits, with CTmin ranging from 1.81 to 8.60°C, while CTmax ranged from 38.74 to 40.64°C. We identified 151 and 99 distinct genes associated with CTmin and CTmax, respectively, and there was strong support that these genes are involved in both dynamic responses to thermal stress and preparatory processes that increase thermal resistance. Many of the genes identified by GWAS were involved in the direct transcriptional response to thermal stress (72/151 for cold; 59/99 for heat), and overall GWAS candidates were more likely to be differentially expressed than other genes. Further, several GWAS candidates were regulatory genes that may participate in the regulation of stress responses, and gene ontologies related to development and morphogenesis were enriched, suggesting many of these genes influence thermal tolerance through effects on development and physiological status. Overall, our results suggest that thermal tolerance alleles can influence both dynamic plastic responses to thermal stress and preparatory processes that improve thermal resistance. These results also have utility for directly comparing GWAS and transcriptomic approaches for identifying candidate genes associated with thermal tolerance.

The ghost of temperature past: interactive effects of previous and current thermal conditions on gene expression in Manduca sexta

High temperatures can negatively impact the performance and survival of organisms, particularly ectotherms. While an organism's response to high temperature stress clearly depends on current thermal conditions, its response may also be affected by the temporal pattern and duration of past temperature exposures. We used RNA sequencing of Manduca sexta larvae fat body tissue to evaluate how diurnal temperature fluctuations during development affected gene expression both independently and in conjunction with subsequent heat stress. Additionally, we compared gene expression between two M. sexta populations, a lab colony and a genetically related field population that have been separated for >300 generations and differ in their thermal sensitivities. Lab-adapted larvae were predicted to show increased expression responses to both single and repeated thermal stress, whereas recurrent exposure could decrease later stress responses for field individuals. We found large differences in overall gene expression patterns between the two populations across all treatments, as well as population-specific transcriptomic responses to temperature; more differentially expressed genes were upregulated in the field compared with lab larvae. Developmental temperature fluctuations alone had minimal effects on long-term gene expression patterns, with the exception of a somewhat elevated stress response in the lab population. Fluctuating rearing conditions did alter gene expression during exposure to later heat stress, but this effect depended on both the population and the particular temperature conditions. This study contributes to increased knowledge of molecular mechanisms underlying physiological responses of organisms to temperature fluctuations, which is needed for the development of more accurate thermal performance models.

Life-History Evolution and the Genetics of Fitness Components in Drosophila melanogaster

Life-history traits or "fitness components"-such as age and size at maturity, fecundity and fertility, age-specific rates of survival, and life span-are the major phenotypic determinants of Darwinian fitness. Analyzing the evolution and genetics of these phenotypic targets of selection is central to our understanding of adaptation. Due to its simple and rapid life cycle, cosmopolitan distribution, ease of maintenance in the laboratory, well-understood evolutionary genetics, and its versatile genetic toolbox, the "vinegar fly" Drosophila melanogaster is one of the most powerful, experimentally tractable model systems for studying "life-history evolution." Here, I review what has been learned about the evolution and genetics of life-history variation in D. melanogaster by drawing on numerous sources spanning population and quantitative genetics, genomics, experimental evolution, evolutionary ecology, and physiology. This body of work has contributed greatly to our knowledge of several fundamental problems in evolutionary biology, including the amount and maintenance of genetic variation, the evolution of body size, clines and climate adaptation, the evolution of senescence, phenotypic plasticity, the nature of life-history trade-offs, and so forth. While major progress has been made, important facets of these and other questions remain open, and the D. melanogaster system will undoubtedly continue to deliver key insights into central issues of life-history evolution and the genetics of adaptation.

Life stages differ in plasticity to temperature fluctuations and uniquely contribute to adult phenotype

Adaptive thermal plasticity allows organisms to adjust their physiology to cope with fluctuating environments. However, thermal plasticity is rarely studied in response to thermal variability and is often measured in a single life stage. Plasticity in response to thermal variability likely differs from responses to constant temperatures or acute stress. In addition, life stages likely differ in their plasticity and responses in one stage may be affected by the experiences in a previous stage. Increasing the resolution with which we understand thermal plasticity in response to thermal variation across ontogeny is crucial to understanding how organisms cope with the thermal variation in their environment and to estimating the capacity of plasticity to mitigate costs of rapid environmental change. We wanted to know if life stages differ in their capacity for thermal plasticity under temperature fluctuations. We reared Onthophagus taurus dung beetles in either low or high temperature fluctuation treatments and quantified thermal plasticity of metabolism of pupae and adults. We found that adults were thermally plastic and pupae were not. Next, we wanted to know if the plasticity observed in the adult life stage was affected by the thermal conditions during development. We again used low and high temperature fluctuation treatments and reared individuals in one condition through all egg to pupal stages. At eclosion, we switched half of the individuals in each treatment to the opposite fluctuation condition and, later, measured thermal plasticity of metabolism on adults. We found that temperature conditions experienced during the adult stage, but not egg to pupal stages, affects adult thermal plasticity. However, temperature fluctuations during development affect adult body size, suggesting that some aspects of the adult phenotype are decoupled from previous life stages and others are not. Our data demonstrate that life stages mount different responses to temperature variability and uniquely contribute to the adult phenotype. These findings emphasize the need to broadly integrate the life cycle into studies of phenotypic plasticity and physiology; doing so should enhance our ability to predict organismal responses to rapid global change and inform conservation efforts.

Using natural laboratories to study evolution to global warming: contrasting altitudinal, latitudinal, and urbanization gradients

Demonstrating the likelihood of evolution in response to global warming is important, yet challenging. We discuss how three spatial thermal gradients (latitudinal, altitudinal, and urbanization) can be used as natural laboratories to inform about the gradual thermal evolution of populations by applying a space-for-time substitution (SFTS) approach. We compare thermal variables and confounding non-thermal abiotic variables, methodological approaches and evolutionary aspects associated with each type of gradient. On the basis of an overview of recent insect studies, we show that a key assumption of SFTS, local thermal adaptation along these gradients, is often but not always met, requiring explicit validation. To increase realism when applying SFTS, we highlight the importance of integrating daily temperature fluctuations, multiple stressors and multiple interacting species. Finally, comparative studies, especially across gradient types, are important to provide more robust inferences of evolution under gradual global warming. Integrating these research directions will further strengthen the still underused, yet powerful SFTS approach to infer gradual evolution under global warming.

Testing the thermal limits: Non-linear reaction norms drive disparate thermal acclimation responses in Drosophila melanogaster

Critical thermal limits are important ecological parameters for studying thermal biology and for modelling species' distributions under current and changing climatic conditions (including predicting the risk of extinction for species from future warming). However, estimates of the critical thermal limits are biased by the choice of assay and assay conditions, which differ among studies. Furthermore, estimates of the potential for phenotypic plasticity (thermal acclimation) to buffer against thermal variability are usually based on single assay conditions and (usually linear) extrapolation from a few acclimation temperatures. We produced high resolution estimates of adult acclimation capacity for upper tolerance limits at different assay conditions (ramping rates and knock-down temperatures) using CTmax (dynamic) and knock-down (static) thermal assays in the model species Drosophila melanogaster. We found the reaction norms to be highly dependent on assay conditions. We confirmed that progressively lower ramping rates or higher knock-down temperatures led to overall lower tolerance estimates. More surprisingly, extended assays (lower ramping rates or lower knock-down temperatures) also led to increasingly non-linear reaction norms for upper thermal tolerance across adult acclimation temperatures. Our results suggest that the magnitude (capacity) and direction (beneficial or detrimental) of acclimation responses are highly sensitive to assay conditions. The results offer a framework for comparison of acclimation responses between different assay conditions and a potential for explaining disparate acclimation capacity theories. We advocate cautious interpretation of acclimation capacities and careful consideration of assay conditions, which should represent realistic environmental conditions based on species' ecological niches.

Comparative studies of critical physiological limits and vulnerability to environmental extremes in small ectotherms: How much environmental control is needed?

Researchers and practitioners are increasingly using comparative assessments of critical thermal and physiological limits to assess the relative vulnerability of ectothermic species to extreme thermal and aridity conditions occurring under climate change. In most assessments of vulnerability, critical limits are compared across taxa exposed to different environmental and developmental conditions. However, many aspects of vulnerability should ideally be compared when species are exposed to the same environmental conditions, allowing a partitioning of sources of variation such as used in quantitative genetics. This is particularly important when assessing the importance of different types of plasticity to critical limits, using phylogenetic analyses to test for evolutionary constraints, isolating genetic variants that contribute to limits, characterizing evolutionary interactions among traits limiting adaptive responses, and when assessing the role of cross generation effects. However, vulnerability assessments based on critical thermal/physiological limits also need to take place within a context that is relevant to field conditions, which is not easily provided under controlled environmental conditions where behavior, microhabitat, stress exposure rates and other factors will differ from field conditions. There are ways of reconciling these requirements, such as by taking organisms from controlled environments and then testing their performance under field conditions (or vice versa). While comparisons under controlled environments are challenging for many taxa, assessments of critical thermal limits and vulnerability will always be incomplete unless environmental effects within and across generations are considered, and where the ecological relevance of assays measuring critical limits can be established.

Independent and combined effects of daytime heat stress and night-time recovery determine thermal performance

Organisms often experience adverse high temperatures during the daytime, but they may also recover or repair themselves during the night-time when temperatures are more moderate. Thermal effects of daily fluctuating temperatures may thus be divided into two opposite processes (i.e. negative effects of daytime heat stress and positive effects of night-time recovery). Despite recent progress on the consequences of increased daily temperature variability, the independent and combined effects of daytime and night-time temperatures on organism performance remain unclear. By independently manipulating daily maximum and minimum temperatures, we tested how changes in daytime heat stress and night-time recovery affect development, survival and heat tolerance of the lady beetle species Propylea japonica Thermal effects on development and survival differed between daytime and night-time. Daytime high temperatures had negative effects whereas night-time mild temperatures had positive effects. The extent of daytime heat stress and night-time recovery also affected development and critical thermal maximum, which indicates that there were both independent and combined effects of daytime and night-time temperatures on thermal performances. Our findings provide insight into the thermal effect of day-to-night temperature variability and have important implications for predicting the impacts of diel asymmetric warming under climate change.

Trait variation in extreme thermal environments under constant and fluctuating temperatures

Climate change is increasingly exposing populations to rare and novel environmental conditions. Theory suggests that extreme conditions will expose cryptic phenotypes, with a concomitant increase in trait variation. Although some empirical support for this exists, it is also well established that physiological mechanisms (e.g. heat shock protein expression) change when organisms are exposed to constant versus fluctuating temperatures. To determine the effect of common, rare and novel temperatures on the release of hidden variation, we exposed fathead minnows, Pimephales promelas, to five fluctuating and four constant temperature regimes (constant treatments: 23.5, 25, 28.5 and 31°C; all fluctuating treatments shared a minimum temperature of 22°C at 00.00 and a maximum of 25, 28, 31, 34 or 37°C at 12.00). We measured each individual's length weekly over 60 days, critical thermal maximum (CTmax), five morphometric traits (eye anterior-posterior distance, pelvic fin length, pectoral fin length, pelvic fin ray count and pectoral fin ray count) and fluctuating asymmetry (FA, absolute difference between left and right morphometric measurements; FA is typically associated with stress). Length-at-age in both constant and fluctuating conditions decreased with temperature, and this trait's variance decreased with temperature under fluctuating conditions but increased and then decreased in constant temperatures. CTmax in both treatments increased with increasing water temperature, while its variance decreased in warmer waters. No consistent pattern in mean or variance was found across morphometric traits or FA. Our results suggest that, for fathead minnows, variance can decrease in important traits (e.g. length-at-age and CTmax) as the environment becomes more stressful, so it may be difficult to establish comprehensive rules for the effects of rarer or stressful environments on trait variation. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

Effects of adult temperature on gene expression in a butterfly: identifying pathways associated with thermal acclimation

BACKGROUND

Phenotypic plasticity is a pervasive property of all organisms and considered to be of key importance for dealing with environmental variation. Plastic responses to temperature, which is one of the most important ecological factors, have received much attention over recent decades. A recurrent pattern of temperature-induced adaptive plasticity includes increased heat tolerance after exposure to warmer temperatures and increased cold tolerance after exposure to cooler temperatures. However, the mechanisms underlying these plastic responses are hitherto not well understood. Therefore, we here investigate effects of adult acclimation on gene expression in the tropical butterfly Bicyclus anynana, using an RNAseq approach.

RESULTS

We show that several antioxidant markers (e.g. peroxidase, cytochrome P450) were up-regulated at a higher temperature compared with a lower adult temperature, which might play an important role in the acclamatory responses subsequently providing increased heat tolerance. Furthermore, several metabolic pathways were up-regulated at the higher temperature, likely reflecting increased metabolic rates. In contrast, we found no evidence for a decisive role of the heat shock response.

CONCLUSIONS

Although the important role of antioxidant defence mechanisms in alleviating detrimental effects of oxidative stress is firmly established, we speculate that its potentially important role in mediating heat tolerance and survival under stress has been underestimated thus far and thus deserves more attention.

Adaptation to environmental stress at different timescales

Environments are changing rapidly, and to cope with these changes, organisms have to adapt. Adaptation can take many shapes and occur at different speeds, depending on the type of response, the trait, the population, and the environmental conditions. The biodiversity crisis that we are currently facing illustrates that numerous species and populations are not capable of adapting with sufficient speed to ongoing environmental changes. Here, we discuss current knowledge on the ability of animals and plants to adapt to environmental stress on different timescales, mainly focusing on thermal stress and ectotherms. We discuss within-generation responses that can be fast and induced within minutes or hours, evolutionary adaptations that are often slow and take several generations, and mechanisms that lay somewhere in between and that include epigenetic transgenerational effects. To understand and predict the impacts of environmental change and stress on biodiversity, we suggest that future studies should (1) have an increased focus on understanding the type and speed of responses to fast environmental changes; (2) focus on the importance of environmental fluctuations and the predictability of environmental conditions on adaptive capabilities, preferably in field studies encompassing several fitness components; and (3) look at ecosystem responses to environmental stress and their resilience when disturbed.

A transcriptomics assessment of oxygen-temperature interactions reveals novel candidate genes underlying variation in thermal tolerance and survival

While single stress responses are fairly well researched, multiple, interactive stress responses are not-despite the obvious importance thereof. Here, using D. melanogaster, we investigated the effects of simultaneous exposures to low O₂ (hypoxia) and varying thermal conditions on mortality rates, estimates of thermal tolerance and the transcriptome. We used combinations of 21 (normoxia), 10 or 5kPa O₂ with control (23°C), cold (4°C) or hot (31°C) temperature exposures before assaying chill coma recovery time (CCRT) and heat knock down time (HKDT) as measures of cold and heat tolerance respectively. We found that mortality was significantly affected by temperature, oxygen partial pressure (PO₂) and the interaction between the two. Cold treatments resulted in low mortality (<5%), regardless of PO₂ treatment; while hot treatments resulted in higher mortality (∼20%), especially at 5kPa O₂ which was lethal for most flies (∼80%). Both CCRT and HKDT were significantly affected by temperature, but not PO₂, of the treatments, and the interaction of temperature and PO₂ was non-significant. Hot treatments led to significantly longer CCRT, and shorter HKDT in comparison to cold treatments. Global gene expression profiling provided the first transcriptome level response to the combined stress of PO₂ and temperature, showing that stressful treatments resulted in higher mortality and induced transcripts that were associated with protein kinases, catabolic processes (proteases, hydrolases, peptidases) and membrane function. Several genes and pathways that may be responsible for the protective effects of combined PO₂ and cold treatments were identified. We found that urate oxidase was upregulated in all three cold treatments, regardless of the PO₂. Small heat shock proteins Hsp22 and Hsp23 were upregulated after both 10 and 21kPa O₂-hot treatments. Collectively, the data from PO₂-hot treatments suggests that hypoxia does exacerbate heat stress, through an as yet unidentified mechanism. Hsp70B and an unannotated transcript (CG6733) were significantly differentially expressed after 5kPa O₂-cold and 10kPa O₂-hot treatments relative to their controls. Downregulation of these transcripts was correlated with reduced thermal tolerance (longer CCRT and shorter HKDT), suggesting that these genes may be important candidates for future research.

Constitutive up-regulation of Turandot genes rather than changes in acclimation ability is associated with the evolutionary adaptation to temperature fluctuations in Drosophila simulans

Most research on thermal adaptation of ectotherms is based on experiments performed at constant temperatures. However, for short-lived insects daily fluctuations of temperature could be an important environmental parameter involved in evolutionary adaptation to thermal heterogeneity. In this study we investigated the mechanisms underlying evolutionary adaptation to daily fluctuating temperatures. We studied replicated selection lines of Drosophila simulans evolved in a constant or a daily fluctuating thermal regime. Previous studies of these lines have shown clear acclimation benefits to heat tolerance induced by the fluctuating regime. First, we tested the existence of an evolved circadian controlled adjustment of heat resistance in selected flies. This was done by investigating the daily variation in time to heat knockdown in flies from both selection regimes when exposed to either a constant or a daily fluctuating thermal regime for a single generation. While daily variation in heat resistance was found, the results suggest that there was neither an evolved adaptive circadian controlled adjustment of heat resistance nor a continuous acclimation response induced by fluctuating temperatures in these lines. Second, in order to reveal functional candidates for adaptation to the fluctuating thermal regime, we investigated the global transcriptomic response to a high temperature exposure in flies from both regimes. We found that flies selected both in constant and fluctuating thermal regimes responded similarly to increasing temperature. However, we found that evolutionary adaptation to the fluctuating thermal regime led to transcriptional enrichment of the GO terms eggshell chorion assembly and cellular response to heat. The latter category was constituted by a constitutive up-regulation of four Turandot genes and not heat shock protein genes, suggesting that Turandot genes could play a prominent role for adaptation to daily fluctuating thermal conditions.

Strong responses of Drosophila melanogaster microbiota to developmental temperature

Physiological responses to changes in environmental conditions such as temperature may partly arise from the resident microbial community that integrates a wide range of bio-physiological aspects of the host. In the present study, we assessed the effect of developmental temperature on the thermal tolerance and microbial community of Drosophila melanogaster. We also developed a bacterial transplantation protocol in order to examine the possibility of reshaping the host bacterial composition and assessed its influence on the thermotolerance phenotype. We found that the temperature during development affected thermal tolerance and the microbial composition of male D. melanogaster. Flies that developed at low temperature (13°C) were the most cold resistant and showed the highest abundance of Wolbachia, while flies that developed at high temperature (31°C) were the most heat tolerant and had the highest abundance of Acetobacter. In addition, feeding newly eclosed flies with bacterial suspensions from intestines of flies developed at low temperatures changed the heat tolerance of recipient flies. However, we were not able to link this directly to a change in the host bacterial composition.

Environmental heterogeneity does not affect levels of phenotypic plasticity in natural populations of three Drosophila species

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.

Critical thermal limits affected differently by developmental and adult thermal fluctuations

Means and variances of the environmental thermal regime play an important role in determining the fitness of terrestrial ectotherms. Adaptive phenotypic responses induced by heterogeneous temperatures have been shown to be mediated by molecular pathways independent of the classic heat shock responses, however, an in-depth understanding of plasticity induced by fluctuating temperatures is still lacking. We investigated high and low temperature acclimation induced by fluctuating thermal regimes at two different mean temperatures, at two different amplitudes of fluctuation and across the developmental and adult life stages. For developmental acclimation, we found mildly detrimental effects of high amplitude fluctuations for critical thermal minima, while the critical thermal maxima showed a beneficial response to higher amplitude fluctuations. For adult acclimation involving shifts between fluctuating and constant regimes, cold tolerance was shown to be dictated by developmental temperature conditions irrespective of the adult treatments, while the acquired heat tolerance was readily lost when flies developed at fluctuating temperature were shifted to a constant regime as adults. Interestingly, we also found that effect of fluctuations at any life stage was gradually lost with prolonged adult maintenance suggesting a more prominent effect of fluctuations during developmental compared to adult acclimation in Drosophila melanogaster.