Indirect selection of thermal tolerance during experimental evolution of Drosophila melanogaster

Correlated responses to selection across diverse environments during experimental evolution of Tetrahymena thermophila

Correlated responses to selection have long been observed and studied; however, it remains unclear when they will arise, and in what direction. To contribute to a growing understanding of correlated responses to selection, we used experimental evolution of the ciliate Tetrahymena thermophila to study direct and correlated responses in a variety of different environmental conditions. One experiment focused on adaptation to two different temperatures and the correlated responses across temperatures. Another experiment used inhibitory concentrations of a variety of compounds to test direct and correlated responses to selection. We found that all populations adapted to the environments in which they evolved. We also found many cases of correlated evolution across environments; few conditions resulted in trade-offs and many resulted in a positive correlated response. Surprisingly, in many instances, the correlated response was of a larger magnitude than the direct response. We find that ancestral fitness predicts the extent of adaptation, consistent with diminishing returns epistasis. Unexpectedly, we also find that this pattern of diminishing returns holds across environments regardless of the environment in which evolution occurs. We also found that the correlated response is asymmetric across environments, that is, the fitness of a population evolved in one environment and assayed in a second was inversely related to the fitness of a population evolved in the second environment and assayed in the first. These results support the notion that positive correlated responses to selection across environments are frequent, and worth further study.

Parental exposure to heat waves improves offspring reproductive investment in Tetranychus urticae (Acari: Tetranychidae), but not in its predator, Phytoseiulus persimilis (Acari: Phytoseiidae)

The more frequent and intense occurrence of heat waves is a challenge for arthropods because their unpredictable incidence requires fast adaptations by the exposed individuals. Phenotypic plasticity within and across generations might be a solution to cope with the detrimental effects of heat waves, especially for fast-developing, small arthropods with limited dispersal abilities. Therefore, we studied whether severe heat may affect the reproduction of a pest species, the spider mite Tetranychus urticae, and its counterpart, the predatory mite Phytoseiulus persimilis. Single offspring females with different parental thermal origins (reared under mild or extreme heat waves) of both species were exposed to mild or extreme heat waves on bean leaves over 10 days, and the oviposition, egg sizes, survival, and escape behavior of the females were evaluated daily. The total losses of predators mainly via escapers were very high compared to prey, which makes a separation between selective and plastic effects on shifted reproductive traits impossible. Predator females laid smaller eggs, while their consumption and oviposition rates were unaffected during extreme heat waves. In comparison, larger prey females fed more and produced more, but smaller, eggs due to within- and trans-generational effects. These advantages for the prey in comparison to its predator when exposed to extreme heat waves during the reproductive phase support the trophic sensitivity hypothesis: higher trophic levels (i.e., the predator) are more sensitive to thermal stress than lower trophic levels (i.e., the prey). Furthermore, the species-specific responses may reflect their lifestyles. The proactive and mobile predator should be selected for behavioral thermoregulation under heat waves via spatiotemporal avoidance of heat-exposed locations rather than relying on physiological adaptations in contrast to the more sessile prey. Whether these findings also influence predator-prey interactions and their population dynamics under heat waves remains an open question.

Evolutionary constraints mediate extinction risk under climate change

Mounting evidence suggests that rapid evolutionary adaptation may rescue some organisms from the impacts of climate change. However, evolutionary constraints might hinder this process, especially when different aspects of environmental change generate antagonistic selection on genetically correlated traits. Here, we use individual-based simulations to explore how genetic correlations underlying the thermal physiology of ectotherms might influence their responses to the two major components of climate change-increases in mean temperature and thermal variability. We found that genetic correlations can influence population dynamics under climate change, with declines in population size varying three-fold depending on the type of correlation present. Surprisingly, populations whose thermal performance curves were constrained by genetic correlations often declined less rapidly than unconstrained populations. Our results suggest that accurate forecasts of the impact of climate change on ectotherms will require an understanding of the genetic architecture of the traits under selection.

Variation in fine-scale recombination rate in temperature-evolved Drosophila melanogaster populations in response to selection

Meiotic recombination plays a critical evolutionary role in maintaining fitness in response to selective pressures due to changing environments. Variation in recombination rate has been observed amongst and between species and populations and within genomes across numerous taxa. Studies have demonstrated a link between changes in recombination rate and selection, but the extent to which fine-scale recombination rate varies between evolved populations during the evolutionary period in response to selection is under active research. Here, we utilize a set of 3 temperature-evolved Drosophila melanogaster populations that were shown to have diverged in several phenotypes, including recombination rate, based on the temperature regime in which they evolved. Using whole-genome sequencing data from these populations, we generated linkage disequilibrium-based fine-scale recombination maps for each population. With these maps, we compare recombination rates and patterns among the 3 populations and show that they have diverged at fine scales but are conserved at broader scales. We further demonstrate a correlation between recombination rates and genomic variation in the 3 populations. Lastly, we show variation in localized regions of enhanced recombination rates, termed warm spots, between the populations with these warm spots and associated genes overlapping areas previously shown to have diverged in the 3 populations due to selection. These data support the existence of recombination modifiers in these populations which are subject to selection during evolutionary change.

No evidence for short-term evolutionary response to a warming environment in Drosophila

Adaptive evolution is key in mediating responses to global warming and may sometimes be the only solution for species to survive. Such evolution will expectedly lead to changes in the populations' thermal reaction norm and improve their ability to cope with stressful conditions. Conversely, evolutionary constraints might limit the adaptive response. Here, we test these expectations by performing a real-time evolution experiment in historically differentiated Drosophila subobscura populations. We address the phenotypic change after nine generations of evolution in a daily fluctuating environment with average constant temperature, or in a warming environment with increasing average and amplitude temperature across generations. Our results showed that (1) evolution under a global warming scenario does not lead to a noticeable change in the thermal response; (2) historical background appears to be affecting responses under the warming environment, particularly at higher temperatures; and (3) thermal reaction norms are trait dependent: although lifelong exposure to low temperature decreases fecundity and productivity but not viability, high temperature causes negative transgenerational effects on productivity and viability, even with high fecundity. These findings in such an emblematic organism for thermal adaptation studies raise concerns about the short-term efficiency of adaptive responses to the current rising temperatures.

Negative relationship between thermal tolerance and plasticity in tolerance emerges during experimental evolution in a widespread marine invertebrate

Whether populations can adapt to predicted climate change conditions, and how rapidly, are critical questions for the management of natural systems. Experimental evolution has become an important tool to answer these questions. In order to provide useful, realistic insights into the adaptive response of populations to climate change, there needs to be careful consideration of how genetic differentiation and phenotypic plasticity interact to generate observed phenotypic changes. We exposed three populations of the widespread copepod Acartia tonsa (Crustacea) to chronic, sublethal temperature selection for 15 generations. We generated thermal survivorship curves at regular intervals both during and after this period of selection to track the evolution of thermal tolerance. Using reciprocal transplants between ambient and warming conditions, we also tracked changes in the strength of phenotypic plasticity in thermal tolerance. We observed significant increases in thermal tolerance in the Warming lineages, while plasticity in thermal tolerance was strongly reduced. We suggest these changes are driven by a negative relationship between thermal tolerance and plasticity in thermal tolerance. Our results indicate that adaptation to warming through an increase in thermal tolerance might not reduce vulnerability to climate change if the increase comes at the expense of tolerance plasticity. These results illustrate the importance of considering changes in both a trait of interest and the trait plasticity during experimental evolution.

Insects and their pathogens in a changing climate

The complex nature of climate change-mediated multitrophic interaction is an underexplored area, but has the potential to dramatically shift transmission and distribution of many insects and their pathogens, placing some populations closer to the brink of extinction. However, for individual insect-pathogen interactions climate change will have complicated hard-to-anticipate impacts. Thus, both pathogen virulence and insect host immunity are intrinsically linked with generalized stress responses, and in both pathogen and host have extensive trade-offs with nutrition (e.g., host plant quality), growth and reproduction. Potentially alleviating or exasperating these impacts, some pathogens and hosts respond genetically and rapidly to environmental shifts. This review identifies many areas for future research including a particular need to identify how altered global warming interacts with other environmental changes and stressors, and how consistent these impacts are across pathogens and hosts. With that achieved we would be closer to producing an overarching framework to integrate knowledge on all environmental interplay and infectious disease events.

Multiple paths to cold tolerance: the role of environmental cues, morphological traits and the circadian clock gene vrille

Background

Tracing the association between insect cold tolerance and latitudinally and locally varying environmental conditions, as well as key morphological traits and molecular mechanisms, is essential for understanding the processes involved in adaptation. We explored these issues in two closely-related species, Drosophila montana and Drosophila flavomontana, originating from diverse climatic locations across several latitudes on the coastal and mountainous regions of North America. We also investigated the association between sequence variation in one of the key circadian clock genes, vrille, and cold tolerance in both species. Finally, we studied the impact of vrille on fly cold tolerance and cold acclimation ability by silencing it with RNA interference in D. montana.

Results

We performed a principal component analysis (PCA) on variables representing bioclimatic conditions on the study sites and used latitude as a proxy of photoperiod. PC1 separated the mountainous continental sites from the coastal ones based on temperature variability and precipitation, while PC2 arranged the sites based on summer and annual mean temperatures. Cold tolerance tests showed D. montana to be more cold-tolerant than D. flavomontana and chill coma resistance (CT_min) of this species showed an association with PC2. Chill coma recovery time (CCRT) of both species improved towards northern latitudes, and in D. flavomontana this trait was also associated with PC1. D. flavomontana flies were darkest in the coast and in the northern mountainous populations, but coloration showed no linkage with cold tolerance. Body size decreased towards cold environments in both species, but only within D. montana populations largest flies showed fastest recovery from cold. Finally, both the sequence analysis and RNAi study on vrille suggested this gene to play an essential role in D. montana cold resistance and acclimation, but not in recovery time.

Conclusions

Our study demonstrates the complexity of insect cold tolerance and emphasizes the need to trace its association with multiple environmental variables and morphological traits to identify potential agents of natural selection. It also shows that a circadian clock gene vrille is essential both for short- and long-term cold acclimation, potentially elucidating the connection between circadian clock system and cold tolerance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01849-y.

Genomics of Recombination Rate Variation in Temperature-Evolved Drosophila melanogaster Populations

Meiotic recombination is a critical process that ensures proper segregation of chromosome homologs through DNA double-strand break repair mechanisms. Rates of recombination are highly variable among various taxa, within species, and within genomes with far-reaching evolutionary and genomic consequences. The genetic basis of recombination rate variation is therefore crucial in the study of evolutionary biology but remains poorly understood. In this study, we took advantage of a set of experimental temperature-evolved populations of Drosophila melanogaster with heritable differences in recombination rates depending on the temperature regime in which they evolved. We performed whole-genome sequencing and identified several chromosomal regions that appear to be divergent depending on temperature regime. In addition, we identify a set of single-nucleotide polymorphisms and associated genes with significant differences in allele frequency when the different temperature populations are compared. Further refinement of these gene candidates emphasizing those expressed in the ovary and associated with DNA binding reveals numerous potential candidate genes such as Hr38, EcR, and mamo responsible for observed differences in recombination rates in these experimental evolution lines thus providing insight into the genetic basis of recombination rate variation.

Generalist grasshoppers from thermally variable sites do not have higher thermal tolerance than grasshoppers from thermally stable sites - A study of five populations

Thermal tolerance allows many organisms, including insects, to withstand stressful temperatures. Thermal generalists are expected to have higher thermal tolerance than specialists, but the environmental conditions leading to the evolution of a thermal generalist life history are not fully understood. Thermal variability has been put forth as an evolutionary driver of high thermal tolerance, but rarely has this been empirically tested. We used a generalist agricultural pest grasshopper, Melanoplus differentialis, to test upper and lower thermal limits of populations that experienced different levels of thermal variability. We quantified thermal heterogeneity at five sites in a longitudinal transect in the Midwestern U.S. by examining, over a 101-year period, 1) variance in daily thermal maxima and minima; and 2) daily range. Also, as a measure of a biologically relevant thermal extreme, we depicted days per month at each site that reached a stressfully high temperature for M. differentialis. We collected individuals from these sites and tested their upper and lower thermal limits. We found that most of our metrics of thermal heterogeneity differed among sites, while all sites experienced an average of at least two stressfully high temperature events per month. We found that heavier males from these sites were able to withstand both warmer and colder temperatures than smaller males, while heavier females had no thermal advantage over lighter females. However, site of origin had no effect on thermal tolerance. Our findings indicate three things: 1) there is no clear correlation between thermal variability and thermal tolerance in the populations we studied; 2) weight affects thermal tolerance range among sites for M. differentialis males, and 3) thermal extremes may be more important than thermal variability in determining CTMax in this species.

Heat knockdown resistance and chill-coma recovery as correlated responses to selection on mating success at high temperature in Drosophila buzzatii

Reproduction and related traits such as mating success are strongly affected by thermal stress. We tested direct and correlated responses to artificial selection in replicated lines of Drosophila buzzatii that were selected for mating success at high temperature. Knockdown resistance at high temperature (KRHT) and chill-coma recovery (CCR) were tested as correlated selection responses. Virgin flies were allowed to mate for four hours at 33°C in three replicated lines (S lines) to obtain the selected flies and then returned at 25°C to lay eggs. Other three replicated lines were maintained at 25°C without any selection as control (C lines). After 15 selection generations, KRHT and CCR were measured. Both traits were assessed in flies that did not receive any hardening pretreatments as well as in flies that were either heat or cold hardened. Thermotolerance traits showed significant correlated responses with higher KRHT in S than in C lines, both with a heat-hardening pretreatment and without a heat-hardening pretreatment. CCR time was longer in S than in C lines both with a cold-hardening pretreatment and without a cold-hardening pretreatment. Hardening treatments improved both KRHT and CCR in all cases excepting KRHT in C lines. Overall, KRHT and CCR showed an antagonistic pattern of correlated responses to our selection regime, suggesting either pleiotropy or tightly linked trait-specific genes partially affecting KRHT and CCR.

Broad thermal tolerance is negatively correlated with virulence in an opportunistic bacterial pathogen

Predicting the effects of global increase in temperatures on disease virulence is challenging, especially for environmental opportunistic bacteria, because pathogen fitness may be differentially affected by temperature within and outside host environment. So far, there is very little empirical evidence on the connections between optimal temperature range and virulence in environmentally growing pathogens. Here, we explored whether the virulence of an environmentally growing opportunistic fish pathogen, Flavobacterium columnare, is malleable to evolutionary changes via correlated selection on thermal tolerance. To this end, we experimentally quantified the thermal performance curves (TPCs) for maximum biomass yield of 49 F. columnare isolates from eight different geographic locations in Finland over ten years (2003-2012). We also characterized virulence profiles of these strains in a zebra fish (Danio rerio) infection model. We show that virulence among the strains increased over the years, but thermal generalism, and in particular tolerance to higher temperatures, was negatively associated with virulence. Our data suggest that temperature has a strong effect on the pathogen genetic diversity and therefore presumably also on disease dynamics. However, the observed increase in frequency and severity of F. columnare epidemics over the last decade cannot be directly linked to bacterial evolution due to increased mean temperature, but is most likely associated with factors related to increased length of growing season, or other time-dependent change in environment. Our study demonstrates that complex interactions between the host, the pathogen and the environment influence disease virulence of an environmentally growing opportunistic pathogen.

Renal neuroendocrine control of desiccation and cold tolerance by Drosophila suzukii

BACKGROUND

Neuropeptides are central to the regulation of physiological and behavioural processes in insects, directly impacting cold and desiccation survival. However, little is known about the control mechanisms governing these responses in Drosophila suzukii. The close phylogenetic relationship of D. suzukii with Drosophila melanogaster allows, through genomic and functional studies, an insight into the mechanisms directing stress tolerance in D. suzukii.

RESULTS

Capability (Capa), leucokinin (LK), diuretic hormone 44 (DH44 ) and DH31 neuropeptides demonstrated a high level of conservation between D. suzukii and D. melanogaster with respect to peptide sequences, neuronal expression, receptor localisation, and diuretic function in the Malpighian tubules. Despite D. suzukii's ability to populate cold environments, it proved sensitive to both cold and desiccation. Furthermore, in D. suzukii, Capa acts as a desiccation- and cold stress-responsive gene, while DH44 gene expression is increased only after desiccation exposure, and the LK gene after nonlethal cold stress recovery.

CONCLUSION

This study provides a comparative investigation into stress tolerance mediation by neuroendocrine signalling in two Drosophila species, providing evidence that similar signalling pathways control fluid secretion in the Malpighian tubules. Identifying processes governing specific environmental stresses affecting D. suzukii could lead to the development of targeted integrated management strategies to control insect pest populations. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Long-term laboratory culture causes contrasting shifts in tolerance to two marine pollutants in copepods of the genus Tigriopus

Organismal chemical tolerance is often used to assess ecological risk and monitor water quality, yet tolerance can differ between field- and lab-raised organisms. In this study, we examined how tolerance to copper (Cu) and tributyltin oxide (TBTO) in two species of marine copepods, Tigriopus japonicus and T. californicus, changed across generations under benign laboratory culture (in the absence of pre-exposure to chemicals). Both copepod species exhibited similar chemical-specific changes in tolerance, with laboratory maintenance resulting in increased Cu tolerance and decreased TBTO tolerance. To assess potential factors underlying these patterns, chemical tolerance was measured in conjunction with candidate environmental variables (temperature, UV radiation, diet type, and starvation). The largest chemical-specific effect was found for starvation, which decreased TBTO tolerance but had no effect on Cu tolerance. Understanding how chemical-specific tolerance can change in the laboratory will be critical in strengthening bioassays and their applications for environmental protection and chemical management.

More oxygen during development enhanced flight performance but not thermal tolerance of Drosophila melanogaster

High temperatures can stress animals by raising the oxygen demand above the oxygen supply. Consequently, animals under hypoxia could be more sensitive to heating than those exposed to normoxia. Although support for this model has been limited to aquatic animals, oxygen supply might limit the heat tolerance of terrestrial animals during energetically demanding activities. We evaluated this model by studying the flight performance and heat tolerance of flies (Drosophila melanogaster) acclimated and tested at different concentrations of oxygen (12%, 21%, and 31%). We expected that flies raised at hypoxia would develop into adults that were more likely to fly under hypoxia than would flies raised at normoxia or hyperoxia. We also expected flies to benefit from greater oxygen supply during testing. These effects should have been most pronounced at high temperatures, which impair locomotor performance. Contrary to our expectations, we found little evidence that flies raised at hypoxia flew better when tested at hypoxia or tolerated extreme heat better than did flies raised at normoxia or hyperoxia. Instead, flies raised at higher oxygen levels performed better at all body temperatures and oxygen concentrations. Moreover, oxygen supply during testing had the greatest effect on flight performance at low temperature, rather than high temperature. Our results poorly support the hypothesis that oxygen supply limits performance at high temperatures, but do support the idea that hyperoxia during development improves performance of flies later in life.

Basal tolerance to heat and cold exposure of the spotted wing drosophila, Drosophila suzukii

The spotted wing Drosophila, Drosophila suzukii, is a new pest in Europe and America which causes severe damages, mostly to stone fruit crops. Temperature and humidity are among the most important abiotic factors governing insect development and fitness. In many situations, temperature can become stressful thus compromising survival. The ability to cope with thermal stress depends on basal level of thermal tolerance. Basic knowledge on temperature-dependent mortality of D. suzukii is essential to facilitate management of this pest. The objective of the present study was to investigate D. suzukii basal cold and heat tolerance. Adults and pupae were subjected to six low temperatures (-5-7.5 °C) and seven high temperatures (30-37 °C) for various durations, and survival-time-temperature relationships were investigated. Data showed that males were globally more cold tolerant than females. At temperature above 5 °C, adult cold mortality became minor even after prolonged exposures (e.g., only 20% mortality after one month at 7.5 °C). Heat tolerance of males was lower than that of females at the highest tested temperatures (34, 35 and 37 °C). Pupae appeared much less cold tolerant than adults at all temperatures (e.g., Lt50 at 5° C: 4-5 d for adults vs. 21 h for pupae). Pupae were more heat tolerant than adults at the most extreme high temperatures (e.g., Lt50 at 37 °C: 30 min for adults vs. 4 h for pupae). The pupal thermal tolerance was further investigated under low vs. high humidity. Low relative humidity did not affect pupal cold survival, but it reduced survival under heat stress. Overall, this study shows that survival of D. suzukii under heat and cold conditions can vary with stress intensity, duration, humidity, sex and stage, and the methodological approach used here, which was based on thermal tolerance landscapes, provides a comprehensive description of D. suzukiithermal tolerance and limits.

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.

Heat resistance throughout ontogeny: body size constrains thermal tolerance

Heat tolerance is a trait of paramount ecological importance and may determine a species' ability to cope with ongoing climate change. Although critical thermal limits have consequently received substantial attention in recent years, their potential variation throughout ontogeny remained largely neglected. We investigate whether such neglect may bias conclusions regarding a species' sensitivity to climate change. Using a tropical butterfly, we found that developmental stages clearly differed in heat tolerance. It was highest in pupae followed by larvae, adults and finally eggs and hatchlings. Strikingly, most of the variation found in thermal tolerance was explained by differences in body mass, which may thus impose a severe constraint on adaptive variation in stress tolerance. Furthermore, temperature acclimation was beneficial by increasing heat knock-down time and therefore immediate survival under heat stress, but it affected reproduction negatively. Extreme temperatures strongly reduced survival and subsequent reproductive success even in our highly plastic model organism, exemplifying the potentially dramatic impact of extreme weather events on biodiversity. We argue that predictions regarding a species' fate under changing environmental conditions should consider variation in thermal tolerance throughout ontogeny, variation in body mass and acclimation responses as important predictors of stress tolerance.

Flies evolved small bodies and cells at high or fluctuating temperatures

Recent theory predicts that the sizes of cells will evolve according to fluctuations in body temperature. Smaller cells speed metabolism during periods of warming but require more energy to maintain and repair. To evaluate this theory, we studied the evolution of cell size in populations of Drosophila melanogaster held at either a constant temperature (16°C or 25°C) or fluctuating temperatures (16 and 25°C). Populations that evolved at fluctuating temperatures or a constant 25°C developed smaller thoraxes, wings, and cells than did flies exposed to a constant 16°C. The cells of flies from fluctuating environments were intermediate in size to those of flies from constant environments. Most genetic variation in cell size was independent of variation in wing size, suggesting that cell size was a target of selection. These evolutionary patterns accord with patterns of developmental plasticity documented previously. Future studies should focus on the mechanisms that underlie the selective advantage of small cells at high or fluctuating temperatures.

How Extreme Temperatures Impact Organisms and the Evolution of their Thermal Tolerance

SynopsisUnderstanding the biological impacts of extreme temperatures requires translating meteorological estimates into organismal responses, but that translation is complex. In general, the physiological stress induced by a given thermal extreme should increase with the extreme's magnitude and duration, though acclimation may buffer that stress. However, organisms can differ strikingly in their exposure to and tolerance of a given extreme temperatures. Moreover, their sensitivity to extremes can vary during ontogeny, across seasons, and among species; and that sensitivity and its variation should be subject to selection. We use a simple quantitative genetic model and demonstrate that thermal extremes-even when at low frequency-can substantially influence the evolution of thermal sensitivity, particularly when the extremes cause mortality or persistent physiological injury, or when organisms are unable to use behavior to buffer exposure to extremes. Thermal extremes can drive organisms in temperate and tropical sites to have similar thermal tolerances despite major differences in mean temperatures. Indeed, the model correctly predicts that Australian Drosophila should have shallower latitudinal gradients in thermal tolerance than would be expected based only on gradients in mean conditions. Predicting responses to climate change requires understanding not only how past selection to tolerate thermal extremes has helped establish existing geographic gradients in thermal tolerances, but also how increasing the incidence of thermal extremes will alter geographic gradients in the future.

Plastic and evolutionary responses to heat stress in a temperate dung fly: negative correlation between basal and induced heat tolerance?

Extreme weather events such as heat waves are becoming more frequent and intense. Populations can cope with elevated heat stress by evolving higher basal heat tolerance (evolutionary response) and/or stronger induced heat tolerance (plastic response). However, there is ongoing debate about whether basal and induced heat tolerance are negatively correlated and whether adaptive potential in heat tolerance is sufficient under ongoing climate warming. To evaluate the evolutionary potential of basal and induced heat tolerance, we performed experimental evolution on a temperate source population of the dung fly Sepsis punctum. Offspring of flies adapted to three thermal selection regimes (Hot, Cold and Reference) were subjected to acute heat stress after having been exposed to either a hot-acclimation or non-acclimation pretreatment. As different traits may respond differently to temperature stress, several physiological and life history traits were assessed. Condition dependence of the response was evaluated by exposing juveniles to different levels of developmental (food restriction/rearing density) stress. Heat knockdown times were highest, whereas acclimation effects were lowest in the Hot selection regime, indicating a negative association between basal and induced heat tolerance. However, survival, adult longevity, fecundity and fertility did not show such a pattern. Acclimation had positive effects in heat-shocked flies, but in the absence of heat stress hot-acclimated flies had reduced life spans relative to non-acclimated ones, thereby revealing a potential cost of acclimation. Moreover, body size positively affected heat tolerance and unstressed individuals were less prone to heat stress than stressed flies, offering support for energetic costs associated with heat tolerance. Overall, our results indicate that heat tolerance of temperate insects can evolve under rising temperatures, but this response could be limited by a negative relationship between basal and induced thermotolerance, and may involve some but not other fitness-related traits.

Keeping your options open: Maintenance of thermal plasticity during adaptation to a stable environment

Phenotypic plasticity may allow species to cope with environmental variation. The study of thermal plasticity and its evolution helps understanding how populations respond to variation in temperature. In the context of climate change, it is essential to realize the impact of historical differences in the ability of populations to exhibit a plastic response to thermal variation and how it evolves during colonization of new environments. We have analyzed the real-time evolution of thermal reaction norms of adult and juvenile traits in Drosophila subobscura populations from three locations of Europe in the laboratory. These populations were kept at a constant temperature of 18ºC, and were periodically assayed at three experimental temperatures (13ºC, 18ºC, and 23ºC). We found initial differentiation between populations in thermal plasticity as well as evolutionary convergence in the shape of reaction norms for some adult traits, but not for any of the juvenile traits. Contrary to theoretical expectations, an overall better performance of high latitude populations across temperatures in early generations was observed. Our study shows that the evolution of thermal plasticity is trait specific, and that a new stable environment did not limit the ability of populations to cope with environmental challenges.

Variability in thermal and phototactic preferences in Drosophila may reflect an adaptive bet-hedging strategy

Organisms use various strategies to cope with fluctuating environmental conditions. In diversified bet‐hedging, a single genotype exhibits phenotypic heterogeneity with the expectation that some individuals will survive transient selective pressures. To date, empirical evidence for bet‐hedging is scarce. Here, we observe that individual Drosophila melanogaster flies exhibit striking variation in light‐ and temperature‐preference behaviors. With a modeling approach that combines real world weather and climate data to simulate temperature preference‐dependent survival and reproduction, we find that a bet‐hedging strategy may underlie the observed interindividual behavioral diversity. Specifically, bet‐hedging outcompetes strategies in which individual thermal preferences are heritable. Animals employing bet‐hedging refrain from adapting to the coolness of spring with increased warm‐seeking that inevitably becomes counterproductive in the hot summer. This strategy is particularly valuable when mean seasonal temperatures are typical, or when there is considerable fluctuation in temperature within the season. The model predicts, and we experimentally verify, that the behaviors of individual flies are not heritable. Finally, we model the effects of historical weather data, climate change, and geographic seasonal variation on the optimal strategies underlying behavioral variation between individuals, characterizing the regimes in which bet‐hedging is advantageous.

Exceptional thermal tolerance and water resistance in the mite Paratarsotomus macropalpis (Erythracaridae) challenge prevailing explanations of physiological limits

Physiological performance and tolerance limits in metazoans have been widely studied and have informed our understanding of processes such as extreme heat and cold tolerance, and resistance to water loss. Because of scaling considerations, very small arthropods with extreme microclimatic niches provide promising extremophiles for testing predictive physiological models. Corollaries of small size include rapid heating and cooling (small thermal time constants) and high mass-specific metabolic and water exchange rates. This study examined thermal tolerance and water loss in the erythracarid mite Paratarsotomus macropalpis (Banks, 1916), a species that forages on the ground surface of the coastal sage scrub habitat of Southern California, USA. Unlike most surface-active diurnal arthropods, P. macropalpis remains active during the hottest parts of the day in midsummer. We measured water-loss gravimetrically and estimated the critical thermal maximum (CTmax) by exposing animals to a given temperature for 1h and then increasing temperature sequentially. The standardized water flux of 4.4ngh(-1)cm(-2)Pa(-1), averaged for temperatures between 22 and 40°C, is among the lowest values reported in the literature. The CTmax of 59.4°C is, to our knowledge, the highest metazoan value reported for chronic (1-h) exposure, and closely matches maximum field substrate temperatures during animal activity. The extraordinary physiological performance seen in P. macropalpis likely reflects extreme selection resulting from its small size and resultant high mass-specific water loss rate and low thermal time-constant. Nevertheless, the high water resistance attained with a very thin lipid barrier, and the mite's exceptional thermal tolerance, challenge existing theories seeking to explain physiological limits.