Acclimation, shock and hardening in the cold

The role of species thermal plasticity for alien species invasibility in a changing climate: A case study of Lophocladia trichoclados

The Mediterranean Sea provides fertile ground for understanding the complex interplay between invasive species and native habitats, particularly within the context of climate change. This thermal tolerance study reveals the remarkable ability of Lophocladia trichoclados, a red algae species that has proven highly invasive, to adapt to varying temperatures, particularly thriving in colder Mediterranean waters, where it can withstand temperatures as low as 14 °C, a trait not observed in its native habitat. This rapid acclimation, occurring in less than a century, might entail a trade-off with high temperature resistance. Additionally, all sampled populations in the Mediterranean share the same haplotype, suggesting a common origin and the possibility that we might be facing an exceptionally acclimatable and invasive strain. This high degree of acclimatability could determine the future spread capacity in a changing scenario, highlighting the importance of considering both acclimation and adaptation in understanding the expansion of invasive species' ranges.

Geographic variation in larval cold tolerance and exposure across the invasion front of a widely established forest insect

Under global climate change, high and low temperature extremes can drive shifts in species distributions. Across the range of a species, thermal tolerance is based on acclimatization, plasticity, and may undergo selection, shaping resilience to temperature stress. In this study, we measured variation in cold temperature tolerance of early instar larvae of an invasive forest insect, Lymantria dispar dispar L. (Lepidoptera: Erebidae), using populations sourced from a range of climates within the current introduced range in the Eastern United States. We tested for population differences in chill coma recovery (CCR) by measuring recovery time following a period of exposure to a nonlethal cold temperature in 2 cold exposure experiments. A 3rd experiment quantified growth responses after CCR to evaluate sublethal effects. Our results indicate that cold tolerance is linked to regional climate, with individuals from populations sourced from colder climates recovering faster from chill coma. While this geographic gradient is seen in many species, detecting this pattern is notable for an introduced species founded from a single point-source introduction. We demonstrate that the cold temperatures used in our experiments occur in nature during cold spells after spring egg hatch, but impacts to growth and survival appear low. We expect that population differences in cold temperature performance manifest more from differences in temperature-dependent growth than acute exposure. Evaluating intraspecific variation in cold tolerance increases our understanding of the role of climatic gradients on the physiology of an invasive species, and contributes to tools for predicting further expansion.

Effects of a high cholesterol diet on chill tolerance are highly context-dependent in Drosophila

Chill susceptible insects are thought to be injured through different mechanisms depending on the duration and severity of chilling. While chronic chilling causes "indirect" injury through disruption of metabolic and ion homeostasis, acute chilling is suspected to cause "direct" injury, in part through phase transitions of cell membrane lipids. Dietary supplementation of cholesterol can reduce acute chilling injury in Drosophila melanogaster (Shreve et al., 2007), but the generality of this effect and the mechanisms underlying it remain unclear. To better understand how and why cholesterol has this effect, we assessed how a high cholesterol diet and thermal acclimation independently and interactively impact several measures of chill tolerance. Cholesterol supplementation positively affected tolerance to acute chilling in warm-acclimated flies (as reported previously). Conversely, feeding on the high-cholesterol diet negatively affected tolerance to chronic chilling in both cold and warm acclimated flies, as well as tolerance to acute chilling in cold acclimated flies. Cholesterol had no effect on the ability of flies to remain active in the cold or recover movement after a cold stress. Our findings support the idea that dietary cholesterol reduces mechanical injury to membranes caused by direct chilling injury, and that acute and chronic chilling are associated with distinct mechanisms of injury. Feeding on a high-cholesterol diet may interfere with mechanisms involved in cold acclimation, leaving cholesterol augmented flies more susceptible to chilling injury under some conditions.

Sex dependent transcriptome responses of the diamondback moth, Plutella xylostella L. to cold stress

Temperature has fundamental influences on the performance and distribution of insects. While considerable attention has been devoted to extreme conditions, particularly extreme cold conditions, few studies have investigated effects of mild cold conditions on insects. We examined the transcriptomic changes in mid-fourth instar larvae of both sexes reared at 10 °C and 25 °C to investigate sex-dependent responses of Plutella xylostella to mild cold stress. There were 624 differentially expressed genes (DEGs) in females, the majority of which (n = 386) were down-regulated. In males 3239 genes were differentially expressed and the majority (n = 2341) were up-regulated. Only 280 DEGs were common to both sexes. In females, there were no DEGs encoding heat shock or cold shock proteins, but six of these DEGs were found in males. These differences suggest that females and males might adopt some different strategies to cope with cold stress and/or that they were affected by rearing under cold conditions to different degrees and in different ways. In addition, DEGs encoding antimicrobial peptides, cytochrome P450 monooxygenases, fatty acid-related enzymes, cuticle proteins, myofilament, and hormone-related proteins were found in both sexes under cold stress. The transcriptome study reveals unexpected sex-dependent thermal responses and provides new information of how an insect that does not diapause copes with low temperatures.

A viewpoint on ecological and evolutionary study of plant thermal performance curves in a warming world

We can understand the ecology and evolution of plant thermal niches through thermal performance curves (TPCs), which are unimodal, continuous reaction norms of performance across a temperature gradient. Though there are numerous plant TPC studies, plants remain under-represented in syntheses of TPCs. Further, few studies quantify plant TPCs from fitness-based measurements (i.e. growth, survival and reproduction at the individual level and above), limiting our ability to draw conclusions from the existing literature about plant thermal adaptation. We describe recent plant studies that use a fitness-based TPC approach to test fundamental ecological and evolutionary hypotheses, some of which have uncovered key drivers of climate change responses. Then, we outline three conceptual questions in ecology and evolutionary biology for future plant TPC studies: (i) Do populations and species harbour genetic variation for TPCs? (ii) Do plant TPCs exhibit plastic responses to abiotic and biotic factors? (iii) Do fitness-based TPCs scale up to population-level thermal niches? Moving forward, plant ecologists and evolutionary biologists can capitalize on TPCs to understand how plasticity and adaptation will influence plant responses to climate change.

Stress memory responses and seed priming correlate with drought tolerance in plants: an overview

Environmental-friendly techniques based on plant stress memory, cross-stress tolerance, and seed priming help sustainable agriculture by mitigating negative effects of dehydration stress. The frequently uneven rainfall distribution caused by global warming will lead to more irregular and multiple abiotic stresses, such as heat stress, dehydration stress, cold stress or the combination of these stresses. Dehydration stress is one of the major environmental factors affecting the survival rate and productivity of plants. Hence, there is an urgent need to develop improved resilient varieties. Presently, technologies based on plant stress memory, cross-stress tolerance and priming of seeds represent fruitful and promising areas of future research and applied agricultural science. In this review, we will provide an overview of plant drought stress memory from physiological, biochemical, molecular and epigenetic perspectives. Drought priming-induced cross-stress tolerance to cold and heat stress will be discussed and the application of seed priming will be illustrated for different species.

Acclimation, duration and intensity of cold exposure determine the rate of cold stress accumulation and mortality in Drosophila suzukii

The spotted wing drosophila (SWD), Drosophila suzukii, is a major invasive fruit pest. There is strong consensus that low temperature is among the main drivers of SWD population distribution, and the invasion success of SWD is also linked to its thermal plasticity. Most studies on ectotherm cold tolerance focus on exposure to a single stressful temperature but here we investigated how cold stress intensity affected survival duration across a broad range of low temperatures (-7 to +3 °C). The analysis of Lt₅₀ at different stressful temperatures (Thermal Death Time curve - TDT) is based on the suggestion that cold injury accumulation rate increases exponentially with the intensity of thermal stress. In accordance with the hypothesis, Lt₅₀ of SWD decreased exponentially with temperature. Further, comparison of TDT curves from flies acclimated to 15, 19 and 23 °C, respectively, showed an almost full compensation with acclimation such that the temperature required to induce mortality over a fixed time decreased almost 1 °C per °C lowering of acclimation temperature. Importantly, this change in cold tolerance with acclimation was uniform across the range of moderate to intense cold stress exposures examined. To understand if cold stress at moderate and intense exposures affects the same physiological systems we examined how physiological markers/symptoms of chill injury developed at different intensities of the cold stress. Specifically, hsp23 expression and extracellular [K⁺] were measured in flies exposed to different intensities of cold stress (-6, -2 and +2 °C) and at various time points corresponding to the same progression of injury (equivalent to 1/3, 2/3 or 3/3 of Lt₅₀). The different cold stress intensities all triggered hsp23 expression following 2 h of recovery, but patterns of expression differed. At the most intense cold stress (-6 and -2 °C) a gradual increase with time was found. In contrast, at +2 °C an initial increase was followed by a dissipating expression. A gradual perturbation of ion balance (hyperkalemia) was also found at all three cold stress intensities examined, with only slight dissimilarities between treatment temperatures. Despite some differences between the three cold intensities examined, the results generally support the hypothesis that intense and moderate cold stress induces the same physiological perturbation. This suggests that cold stress experienced during natural fluctuating conditions is additive and the results also illustrate that the rate of injury accumulation increases dramatically (exponentially) with decreasing temperature (increasing stress).

N-acetyltyrosine-induced redox signaling in hormesis

A suite of adaptations allows insects to survive in hostile terrestrial environments for long periods of time. Temperature represents a key environmental factor for most ectothermic insects, and they rapidly acclimate to high and low temperatures. Vast amounts of data in this research field support the idea that an insect's ability to tolerate fluctuating temperatures can be regarded as a biphasic hormetic dose response. Observation indicates that their thermal hormetic response represents a conservative estimate of their intrinsic capacity for rapid adaptation to environmental changes in nature because they naturally experience diel or seasonal temperature fluctuations. It is therefore reasonable to suppose that the hormetic response in insects reflects a surplus physiological capacity to deal with temperature changes that they would experience naturally. Although it has been unknown how thermal acclimation is induced, a stress-dependent increase in N-acetyltyrosine (NAT) was recently found to occur in insect larvae who had endured high temperatures. NAT treatment was demonstrated to induce thermotolerance in several tested insect species. NAT was also identified in the serum of humans as well as mice, and its concentration in mice was shown to be increased by heat and restraint stress, with NAT pretreatment lowering the concentrations of corticosterone and peroxidized lipids in stressed mice. These recent findings may give us some hints about how long a hormetic response lasts. Here, I will discuss recent findings underlying hormetic responses induced by an intrinsic factor, NAT, and how the hormetic response may begin and end.

Small heat shock protein Hsp67Bc plays a significant role in Drosophila melanogaster cold stress tolerance

Hsp67Bc in Drosophila melanogaster is a member of the small heat shock protein family, the main function of which is to prevent the aggregation of misfolded or damaged proteins. Hsp67Bc interacts with Starvin and Hsp23, which are known to be a part of the cold stress response in the fly during the recovery phase. In this study, we investigated the role of the Hsp67Bc gene in the cold stress response. We showed that in adult Drosophila, Hsp67Bc expression increases after cold stress and decreases after 1.5 h of recovery, indicating the involvement of Hsp67Bc in short-term stress recovery. We also implemented a deletion in the D. melanogaster Hsp67Bc gene using imprecise excision of a P-element, and analysed the cold tolerance of Hsp67Bc-null mutants at different developmental stages. We found that Hsp67Bc-null homozygous flies are viable and fertile but display varying cold stress tolerance throughout the stages of ontogenesis: the survival after cold stress is slightly impaired in late third instar larvae, unaffected in pupae, and notably affected in adult females. Moreover, the recovery from chill coma is delayed in Hsp67Bc-null adults of both sexes. In addition, the deletion in the Hsp67Bc gene caused more prominent up-regulation of Hsp70 following cold stress, suggesting the involvement of Hsp70 in compensation of the lack of the Hsp67Bc protein. Taken together, our results suggest that Hsp67Bc is involved in the recovery of flies from a comatose state and contributes to the protection of the fruit fly from cold stress.

Beyond thermal melanism: association of wing melanization with fitness and flight behaviour in a butterfly

Cold developmental conditions can greatly affect adult life history of ectotherms in seasonal habitats. Such effects are mostly negative, but sometimes adaptive. Here, we tested how cold conditions experienced during pupal development affect adult wing melanization of an insect ectotherm, the Glanville fritillary butterfly, Melitaea cinxia. We also assessed how in turn previous cold exposure and increased melanization can shape adult behaviour and fitness, by monitoring individuals in a seminatural set-up. We found that, despite pupal cold exposure inducing more melanization, wing melanization was not linked to adult thermoregulation preceding flight, under the conditions tested. Conversely, wing-vibrating behaviour had a major role in producing heat preceding flight. Moreover, more melanized individuals were more mobile across the experimental set-up. This may be caused by a direct impact of melanization on flight ability or a more indirect impact of coloration on behaviours such as mate search strategies and/or eagerness to disperse to more suitable mating habitats. We also found that more melanized individuals of both sexes had reduced mating success and produced fewer offspring, which suggests a clear fitness cost of melanization. Whether the reduced mating success is dictated by impaired mate search behaviour, reduced physical condition leading to a lower dominance status or weakened visual signalling remains unknown. In conclusion, while there was no clear role of melanization in providing a thermal advantage under our seminatural conditions, we found a fitness cost of being more melanized, which potentially impacted adult space use behaviour.

Effect of Intermittent and Mild Cold Stimulation on the Immune Function of Bursa in Broilers

Cold stress causes growth performance to decrease and increases production costs. Cold adaptation can enhance immune function and alleviate the negative impact caused by the stress condition. The study investigated the effect of intermittent and mild cold stimulation on the immune function of the bursa of Fabricius in broilers. A total of 400 healthy one-day-old broilers were divided into the control group (CC) and cold stimulation (CS) groups. The CC group was raised at a conventional raising temperature of broilers, while the CS groups were raised at 3°C below the temperature of the CC for three-, four-, five-, or six-hour periods at one-day intervals from 15 to 35 days of age (D35), denoted CS3, CS4, CS5, and CS6, respectively. Subsequently, they were raised at 20°C from 36 to 49 days of age (D49). The expression levels of TLRs, cytokines, and AvBDs were determined to access the immune function of bursa in broilers. After 21-day IMCS (at D36), the expression levels of TLR1, TLR15 and TLR21, interleukin (IL)-8, and interferon (IFN)-γ, as well as AvBD8 in CS groups, were lower than those in CC (p < 0.05). The expression levels of TLR3, TLR4 and TLR7, were decreased in the CS3, CS5, and CS6 groups (p < 0.05), but there were no significant differences in both the CC and CS4 groups (p > 0.05). When the IMCS ended for 14 days (at D49), the expression levels of TLR2, TLR3, TLR5, TLR7, TLR15, and TLR21, and IL-8, as well as AvBD2, AvBD4 and AvBD7 in CS groups, were lower than those in CC (p < 0.05). In addition to CS4, the expression levels of TLR1, IFN-γ, and AvBD8 in CS3, CS5, and CS6 were still lower than those in CC (p < 0.05). We concluded that the intermittent and mild cold stimulation could regulate immunoreaction by modulating the production of TLRs, cytokines, and AvBDs in the bursa, which could help broilers adapt to low ambient temperature and maintain homeostasis.

Phenotypic Responses to and Genetic Architecture of Sterility Following Exposure to Sub-Lethal Temperature During Development

Thermal tolerance range, based on temperatures that result in incapacitating effects, influences species’ distributions and has been used to predict species’ response to increasing temperature. Reproductive performance may also be negatively affected at less extreme temperatures, but such sublethal heat-induced sterility has been relatively ignored in studies addressing the potential effects of, and ability of species’ to respond to, predicted climate warming. The few studies examining the link between increased temperature and reproductive performance typically focus on adults, although effects can vary between life history stages. Here we assessed how sublethal heat stress during development impacted subsequent adult fertility and its plasticity, both of which can provide the raw material for evolutionary responses to increased temperature. We quantified phenotypic and genetic variation in fertility of Drosophila melanogaster reared at standardized densities in three temperatures (25, 27, and 29°C) from a set of lines of the Drosophila Genetic Reference Panel (DGRP). We found little phenotypic variation at the two lower temperatures with more variation at the highest temperature and for plasticity. Males were more affected than females. Despite reasonably large broad-sense heritabilities, a genome-wide association study found little evidence for additive genetic variance and no genetic variants were robustly linked with reproductive performance at specific temperatures or for phenotypic plasticity. We compared results on heat-induced male sterility with other DGRP results on relevant fitness traits measured after abiotic stress and found an association between male susceptibility to sterility and male lifespan reduction following oxidative stress. Our results suggest that sublethal stress during development has profound negative consequences on male adult reproduction, but despite phenotypic variation in a population for this response, there is limited evolutionary potential, either through adaptation to a specific developmental temperature or plasticity in response to developmental heat-induced sterility.

Temperature Limits for the Brown Wheat Mite, in Colorado

Brown wheat mites, Petrobia latens (Müller 1776, Acari: Tetranychidae), are sporadic yet economically damaging pests of winter cereals. In Colorado, their life history is closely tied to the development of winter wheat, where they are present in the field from crop planting in late September through harvest in early June. In order to withstand winter months, these mites are able to survive cold temperatures. However, the mechanisms of cold hardening and their temperature limits are unknown. This research documents the seasonal supercooling points of the brown wheat mite. Their seasonal average supercooling point stayed consistent throughout the year, never varying more than a degree from the overall average supercooling point of -17°C. The greatest variation in supercooling point was seen in the spring, during which supercooling point temperatures ranged from -9.2 to -25.5°C. We also documented the upper and lower lethal temperatures for the brown wheat mite. When comparing small nymphs to large nymph and adult stages, small nymphs were slightly more cold tolerant (lethal temperature estimates required to kill 99% of the population [LT99] were -30.8 and -30.6°C, respectively), but less heat tolerant (LT99 was 50 and 56°C, respectively).

Performance in a variable world: using Jensen's inequality to scale up from individuals to populations

Body temperature affects plants' and animals' performance, but these effects are complicated by thermal variation through time within an individual and variation through space among individuals in a population. This review and synthesis describes how the effects of thermal variation-in both time and space-can be estimated by applying a simple, nonlinear averaging scheme. The method is first applied to the temporal variation experienced by an individual, providing an estimate of the individual's average performance. The method is then applied to the scale-dependent thermal variation among individuals, which is modelled as a 1/f-noise phenomenon. For an individual, thermal variation reduces average performance, lowers the temperature of maximum performance (Topt ) and contracts the range of viable temperatures. Thermal variation among individuals similarly reduces performance and lowers Topt , but increases the viable range of average temperatures. These results must be viewed with caution, however, because they do not take into account the time-dependent interaction between body temperature and physiological plasticity. Quantifying these interactions is perhaps the largest challenge for ecological and conservation physiologists as they attempt to predict the effects of climate change.

Transcriptional and metabolic changes in the desiccation tolerant plant Craterostigma plantagineum during recurrent exposures to dehydration

Multiple dehydration/rehydration treatments improve the adaptation of Craterostigma plantagineum to desiccation by accumulating stress-inducible transcripts, proteins and metabolites. These molecules serve as stress imprints or memory and can lead to increased stress tolerance. It has been reported that repeated exposure to dehydration may generate stronger reactions during a subsequent dehydration treatment in plants. This stimulated us to address the question whether the desiccation tolerant resurrection plant Craterostigma plantagineum has a stress memory. The expression of four representative stress-related genes gradually increased during four repeated dehydration/rehydration treatments in C. plantagineum. These genes reflect a transcriptional memory and are trainable genes. In contrast, abundance of chlorophyll synthesis/degradation-related transcripts did not change during dehydration and remained at a similar level as in the untreated tissues during the recovery phase. During the four dehydration/rehydration treatments the level of ROS pathway-related transcripts, superoxide dismutase (SOD) activity, proline, and sucrose increased, whereas H₂O₂ content and electrolyte leakage decreased. Malondialdehyde (MDA) content did not change during the dehydration, which indicates a gain of stress tolerance. At the protein level, increased expression of four representative stress-related proteins showed that the activated stress memory can persist over several days. The phenomenon described here could be a general feature of dehydration stress memory responses in resurrection plants.

Rapid adaptation to high temperatures in Chironomus riparius

Effects of seasonal or daily temperature variation on fitness and physiology of ectothermic organisms and their ways to cope with such variations have been widely studied. However, the way multivoltines organisms cope with temperature variations from one generation to the next is still not well understood. The aim of this study was to investigate whether the multivoltine midge Chironomus riparius Meigen (1803) responds mainly via acclimation as predicted by current theories or whether rapid genetic adaptation is involved. To investigate this issue, a common garden approach has been applied. A mix of larvae from five European populations was raised in the laboratory at three different pre-exposure temperatures (PET): 14, 20, and 26°C. After three and five generations, respectively, larvae were exposed to three treatment temperatures (TT): 14, 20, and 26°C. Mortality was monitored for the first 48 hr and after emergence. After three generations, significant mortality rate differences depended on an interaction of PET and TT. This finding supports the hypothesis that chironomids respond rapidly to climatic variation via adaptive mechanisms and to a lesser extent via phenotypic plasticity. The result of the experiment indicates that three generations were sufficient to adapt to warm temperature, decreasing the mortality rate, highlighting the potential for chironomids to rapidly respond to seasonally changing conditions.

Cold Acclimation Favors Metabolic Stability in Drosophila suzukii

The invasive fruit fly pest, Drosophila suzukii, is a chill susceptible species, yet it is capable of overwintering in rather cold climates, such as North America and North Europe, probably thanks to a high cold tolerance plasticity. Little is known about the mechanisms underlying cold tolerance acquisition in D. suzukii. In this study, we compared the effect of different forms of cold acclimation (at juvenile or at adult stage) on subsequent cold tolerance. Combining developmental and adult cold acclimation resulted in a particularly high expression of cold tolerance. As found in other species, we expected that cold-acclimated flies would accumulate cryoprotectants and would be able to maintain metabolic homeostasis following cold stress. We used quantitative target GC-MS profiling to explore metabolic changes in four different phenotypes: control, cold acclimated during development or at adult stage or during both phases. We also performed a time-series GC-MS analysis to monitor metabolic homeostasis status during stress and recovery. The different thermal treatments resulted in highly distinct metabolic phenotypes. Flies submitted to both developmental and adult acclimation were characterized by accumulation of cryoprotectants (carbohydrates and amino acids), although concentrations changes remained of low magnitude. After cold shock, non-acclimated chill-susceptible phenotype displayed a symptomatic loss of metabolic homeostasis, correlated with erratic changes in the amino acids pool. On the other hand, the most cold-tolerant phenotype was able to maintain metabolic homeostasis after cold stress. These results indicate that cold tolerance acquisition of D. suzukii depends on physiological strategies similar to other drosophilids: moderate changes in cryoprotective substances and metabolic robustness. In addition, the results add to the body of evidence supporting that mechanisms underlying the different forms of acclimation are distinct.

Prey-specific impact of cold pre-exposure on kill rate and reproduction

Temperature influences biological processes of ectotherms including ecological interactions, but interaction strengths may depend on species-specific traits. Furthermore, ectotherms acclimate to prevailing thermal conditions by adjusting physiological parameters, which often implies costs to other fitness-related parameters. Both predators and prey may therefore pay thermal acclimation costs following exposure to suboptimal temperatures. However, these costs may be asymmetrical between predator and prey, and between the predator and different species of concurrent prey. We investigated whether thermal pre-exposure affected subsequent kill rate and predator fitness when foraging on prey that differ in ease of capture, and whether changes were primarily caused by predator or by prey pre-exposure effects. Specifically, we were interested in whether there were interactions between predator pre-exposed temperature and specific prey. Using the mesostigmatid mite Gaeolaelaps aculeifer as a generalist predator and the collembolans Folsomia candida and Protaphorura fimata as prey, we measured the impact of present temperature, predator pre-exposure temperature, prey pre-exposure temperature (all 10 or 20°C), prey species, and all interactions on prey numbers killed, predator eggs produced, and exploitation of killed prey in a full factorial design. Mites killed P. fimata in equal numbers independent of the presence of F. candida, but killed F. candida when P. fimata was absent. Mite kill rate and reproduction were significantly affected by mite pre-exposure temperature and test temperature, but not by prey pre-exposure temperature. Significantly more of the slower prey was killed than of the quicker prey. Importantly, we found significant synergistic negative interaction effects between predator cold pre-exposure and hunting prey of higher agility on predator kill rate and reproduction. Our findings show that the negative effects of cold and cold pre-exposure on kill rate and reproduction may be more severe when predators forage on quick prey. The study implies that predator cold exposure has consequences for specific prey survival following cold due to altered predation pressures, which in nature should influence the specific prey population dynamics and apparent competition outcomes. The findings exemplify how not only current but also preceding conditions affect ecological interactions, and that effect strength depends on the species involved.

Ovary Development and Cold Tolerance of the Invasive Pest Drosophila suzukii (Matsumura) in the Central Plains of Kansas, United States

Environmental challenges presented by temperature variation can be overcome through phenotypic plasticity in small invasive ectotherms. We tested the effect of thermal exposure to 21, 18, and 11°C throughout the whole life cycle of individuals, thermal exposure of adults reared at 25°C to 15 and 11°C for a 21-d period, and long (14:10 hr) and short (10:14 hr) photoperiod on ovary size and development in Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) cultured from a recently established population in Topeka, Kansas (United States). Examination of the response to temperature and photoperiod variation in this central plains population provides insight into the role of phenotypic plasticity in a climate that is warmer than regions in North America where D. suzukii was initially established. We found both low temperature and short photoperiod resulted in reduced ovary size and level of development. In particular, reduced ovary development was observed following exposure to 15°C, indicating that ovary development in females from the central plains population is more sensitive to lower temperature compared with populations examined from the northern United States and southern Canada. We also provide evidence that D. suzukii reared at 25°C are capable of short-term hardening when exposed to -6°C following 4°C acclimation, contrary to previous reports indicating flies reared at warm temperatures do not rapidly-cold harden. Our study highlights the central role of phenotypic plasticity in response to winter-like laboratory conditions and provides an important geographic comparison to previously published assessments of ovary development and short-term hardening survival response for D. suzukii collected in cooler climates.

Costs of cold acclimation on survival and reproductive behavior in Drosophila melanogaster

Fitness is determined by the ability of an organism to both survive and reproduce; however, the mechanisms that lead to increased survival may not have the same effect on reproductive success. We used nineteen natural Drosophila melanogaster genotypes from the Drosophila Genetic Reference Panel to determine if adaptive plasticity following short-term acclimation through rapid cold-hardening (RCH) affects mating behavior and mating success. We confirmed that exposure to the acclimation temperature is beneficial to survival following cold stress; however, we found that this same acclimation temperature exposure led to less efficient male courtship and a significant decrease in the likelihood of mating. Cold tolerance and the capacity to respond plastically to cold stress were not correlated with mating behavior following acclimation, suggesting that the genetic control of the physiological effects of the cold temperature exposure likely differ between survival and behavioral responses. We also tested whether the exposure of males to the acclimation temperature influenced courtship song. This exposure again significantly increased courtship duration; however, courtship song was unchanged. These results illustrate costs of short-term acclimation on survival and reproductive components of fitness and demonstrate the pronounced effect that short-term thermal environment shifts can have on reproductive success.

Environmental and genetic control of cold tolerance in the Glanville fritillary butterfly

Thermal tolerance has a major effect on individual fitness and species distributions and can be determined by genetic variation and phenotypic plasticity. We investigate the effects of developmental and adult thermal conditions on cold tolerance, measured as chill coma recovery (CCR) time, during the early and late adult stage in the Glanville fritillary butterfly. We also investigate the genetic basis of cold tolerance by associating CCR variation with polymorphisms in candidate genes that have a known role in insect physiology. Our results demonstrate that a cooler developmental temperature leads to reduced cold tolerance in the early adult stage, whereas cooler conditions during the adult stage lead to increased cold tolerance. This suggests that adult acclimation, but not developmental plasticity, of adult cold tolerance is adaptive. This could be explained by the ecological conditions the Glanville fritillary experiences in the field, where temperature during early summer, but not spring, is predictive of thermal conditions during the butterfly's flight season. In addition, an amino acid polymorphism (Ala-Glu) in the gene flightin, which has a known function in insect flight and locomotion, was associated with CCR. These amino acids have distinct biochemical properties and may thus affect protein function and/or structure. To our knowledge, our study is the first to link genetic variation in flightin to cold tolerance, or thermal adaptation in general.

Critical thermal limits of bumblebees (Bombus impatiens) are marked by stereotypical behaviors and are unchanged by acclimation, age or feeding status

Critical thermal limits often determine species distributions for diverse ectotherms and have become a useful tool for understanding past and predicting future range shifts in response to changing climates. Despite recently documented population declines and range shifts of bumblebees (genus Bombus), the few measurements of thermal tolerance available for the group have relied on disparate measurement approaches. We describe a novel stereotypical behavior expressed by bumblebee individuals during entry into chill coma. This behavioral indicator of minimum critical temperature (CT_min) occurred at ambient temperatures of 3-5°C (approximately 7-9°C core temperatures) and was accompanied by a pronounced CO₂ pulse, indicative of loss of spiracle function. Maximum critical temperature (CT_max) was indicated by the onset of muscular spasms prior to entering an unresponsive state and occurred at ambient temperatures of approximately 52-55°C (42-44°C core temperatures). Measurements of CT_min and CT_max were largely unaffected by acclimation, age or feeding status, but faster ramping rates significantly increased CT_max and decreased CT_min This high-throughput approach allows rapid measurement of critical thermal limits for large numbers of individuals, facilitating large-scale comparisons among bumblebee populations and species - a key step in determining current and future effects of climate on these critical pollinators.

Thermal acclimation is not induced by habitat-of-origin, maintenance temperature, or acute exposure to low or high temperatures in a pit-building wormlion (Vermileo sp.)

Wormlions are sit-and-wait insect predators that construct pit-traps to capture arthropod prey. They require loose soil and shelter from direct sun, both common in Mediterranean cities, and explaining their high abundance in urban habitats. We studied different aspects of thermal acclimation in wormlions. We compared chill-coma recovery time (CCRT) and heat-shock recovery time (HSRT) of wormlions from urban, semi-urban and natural habitats, expecting those originating from the urban habitat to be more heat tolerant and less cold tolerant. However, no differences were detected among the three habitats. We then examined whether maintenance temperature affects CCRT and HSRT, and expected beneficial acclimation. However, CCRT was unaffected by maintenance temperature, while temperature affected HSRT in an opposite direction to our prediction: wormlions maintained under the higher temperatures took longer to recover. When testing with two successive thermal shocks, wormlions took longer to recover from both cold and heat shock after applying an initial cold shock. We therefore conclude that cold shock inflicts some damage rather than induces acclimation. Finally, both cold- and heat-shocked wormlions constructed smaller pits than wormlions of a control group. Smaller pits probably translate to a lower likelihood of capturing prey and also limit the size of the prey, indicating a concrete cost of thermal shock. In summary, we found no evidence for thermal acclimation related either to the habitat-of-origin or to maintenance temperatures, but, rather, negative effects of unfavorable temperatures.

Cold tolerance of third-instar Drosophila suzukii larvae

Drosophila suzukii is an emerging global pest of soft fruit; although it likely overwinters as an adult, larval cold tolerance is important both for determining performance during spring and autumn, and for the development of temperature-based control methods aimed at larvae. We examined the low temperature biology of third instar feeding and wandering larvae in and out of food. We induced phenotypic plasticity of thermal biology by rearing under short days and fluctuating temperatures (5.5-19°C). Rearing under fluctuating temperatures led to much slower development (42.1days egg-adult) compared to control conditions (constant 21.5°C; 15.7days), and yielded larger adults of both sexes. D. suzukii larvae were chill-susceptible, being killed by low temperatures not associated with freezing, and freezing survival was not improved when ice formation was inoculated externally via food or silver iodide. Feeding larvae were more cold tolerant than wandering larvae, especially after rearing under fluctuating temperatures, and rearing under fluctuating temperatures improved survival of prolonged cold (0°C) to beyond 72h in both larval stages. There was no evidence that acute cold tolerance could be improved by rapid cold-hardening. We conclude that D. suzukii has the capacity to develop at low temperatures under fluctuating temperatures, but that they have limited cold tolerance. However, phenotypic plasticity of prolonged cold tolerance must be taken into account when developing low temperature treatments for sanitation of this species.

CRISPR-induced null alleles show that Frost protects Drosophila melanogaster reproduction after cold exposure

The ability to survive and reproduce after cold exposure is important in all kingdoms of life. However, even in a sophisticated genetic model system like Drosophila melanogaster, few genes have been identified as functioning in cold tolerance. The accumulation of the Frost (Fst) gene transcript increases after cold exposure, making it a good candidate for a gene that has a role in cold tolerance. However, despite extensive RNAi knockdown analysis, no role in cold tolerance has been assigned to Fst. CRISPR is an effective technique for completely knocking down genes, and less likely to produce off-target effects than GAL4-UAS RNAi systems. We have used CRISPR-mediated homologous recombination to generate Fst null alleles, and these Fst alleles uncovered a requirement for FST protein in maintaining female fecundity following cold exposure. However, FST does not have a direct role in survival following cold exposure. FST mRNA accumulates in the Malpighian tubules, and the FST protein is a highly disordered protein with a putative signal peptide for export from the cell. Future work is needed to determine whether FST is exported from the Malpighian tubules and directly interacts with female reproductive tissues post-cold exposure, or if it is required for other repair/recovery functions that indirectly alter energy allocation to reproduction.

Concurrent effects of cold and hyperkalaemia cause insect chilling injury

Chilling injury and death are the ultimate consequence of low temperature exposure for chill susceptible insects, and low temperature tolerance is considered one of the most important factors determining insect distribution patterns. The physiological mechanisms that cause chilling injury are unknown, but chronic cold exposure that causes injury is consistently associated with elevated extracellular [K(+)], and cold tolerant insects possess a greater capacity to maintain ion balance at low temperatures. Here, we use the muscle tissue of the migratory locust (Locusta migratoria) to examine whether chill injury occurs during cold exposure or following return to benign temperature and we specifically examine if elevated extracellular [K(+)], low temperature, or a combination thereof causes cell death. We find that in vivo chill injury occurs during the cold exposure (when extracellular [K(+)] is high) and that there is limited capacity for repair immediately following the cold stress. Further, we demonstrate that that high extracellular [K(+)] causes cell death in situ, but only when experienced at low temperatures. These findings strongly suggest that that the ability to maintain ion (particularly K(+)) balance is critical to insect low temperature survival, and highlight novel routes of study in the mechanisms regulating cell death in insects in the cold.

All or nothing: Survival, reproduction and oxidative balance in Spotted Wing Drosophila (Drosophila suzukii) in response to cold

Winter severity and overwintering capacity are key ecological factors in successful invasions, especially in ectotherms. The integration of physiological approaches into the study of invasion processes is emerging and promising. Physiological information describes the mechanisms underlying observed survival and reproductive capacities, and it can be used to predict an organism's response to environmental perturbations such as cold temperatures. We investigated the effects of various cold treatments on life history and physiological traits of an invasive pest species, Drosophila suzukii, such as survival, fertility and oxidative balance. This species, a native of temperate Asian areas, is known to survive where cold temperatures are particularly harsh and has been recently introduced into Europe and North America. We found that cold treatments had a strong impact on adult survival but no effect on female's fertility. Although only minor changes were observed after cold treatment on studied physiological traits, a strong sex-based difference was observed in both survival and physiological markers (antioxidant defences and oxidative markers). Females exhibited higher survival, reduced oxidative defences, less damage to nucleic acids, and more damage to lipids. These results suggest that D. suzukii relies on a pathway other than oxidative balance to resist cold injury. Altogether, our results provide information concerning the mechanisms of successful invasion by D. suzukii. These findings may assist in the development of population models that predict the current and future geographic ranges of this species.

Thermal Tolerances of the Spotted-Wing Drosophila Drosophila suzukii (Diptera: Drosophilidae)

The spotted-wing drosophila (Drosophila suzukii Matsumura) is an invasive species of Asian origin that is now widely distributed in North America and Europe. Because of the female’s serrated ovipositor, eggs are laid in preharvest fruit, causing large economic losses in cultivated berries and stone fruit. Modeling D. suzukii population dynamics and potential distribution will require information on its thermal tolerance. Large summer populations have been found in regions with severe winter conditions, though little is known about responses to prolonged low-temperature exposure. We used controlled chambers to examine D. suzukii fecundity, development rate, and mortality across a range of temperatures encompassing the upper and lower thresholds (5–35°C). Optimal temperatures (Topt) were found to be 28.2°C for the development of the egg-to-adult stage, and 22.9°C for reproductive output. No adult eclosion occurred below 8.1°C (Tlower) or above 30.9°C (Tupper). We also investigated survival outcomes following prolonged (42-d) low-temperature exposure to a simulated cold winter (−5, −3, −1, 1, 3, and 5°C). Adult survival was dependent on temperature, with a mean LT50 of 4.9°C. There were no effects of sex, mating status, geographic strain, and photoperiod preexposure on overwintering survival. Thirty-eight percent of females that were mated prior, but not after, prolonged low-temperature exposure produced viable offspring, suggesting that this species may undergo sperm storage. This study provides data on the thermal tolerances of D. suzukii, which can be used for models of D. suzukii population dynamics, degree-day, and distribution models.

Parallel molecular routes to cold adaptation in eight genera of New Zealand stick insects

The acquisition of physiological strategies to tolerate novel thermal conditions allows organisms to exploit new environments. As a result, thermal tolerance is a key determinant of the global distribution of biodiversity, yet the constraints on its evolution are not well understood. Here we investigate parallel evolution of cold tolerance in New Zealand stick insects, an endemic radiation containing three montane-occurring species. Using a phylogeny constructed from 274 orthologous genes, we show that stick insects have independently colonized montane environments at least twice. We compare supercooling point and survival of internal ice formation among ten species from eight genera, and identify both freeze tolerance and freeze avoidance in separate montane lineages. Freeze tolerance is also verified in both lowland and montane populations of a single, geographically widespread, species. Transcriptome sequencing following cold shock identifies a set of structural cuticular genes that are both differentially regulated and under positive sequence selection in each species. However, while cuticular proteins in general are associated with cold shock across the phylogeny, the specific genes at play differ among species. Thus, while processes related to cuticular structure are consistently associated with adaptation for cold, this may not be the consequence of shared ancestral genetic constraints.

Adult plasticity of cold tolerance in a continental-temperate population of Drosophila suzukii

Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) is a worldwide emerging pest of soft fruits, but its cold tolerance has not been thoroughly explored. We determined the cold tolerance strategy, low temperature thermal limits, and plasticity of cold tolerance in both male and female adult D. suzukii. We reared flies under common conditions (long days, 21°C; control) and induced plasticity by rapid cold-hardening (RCH, 1h at 0°C followed by 1h recovery), cold acclimation (CA, 5 days at 6°C) or acclimation under fluctuating temperatures (FA). D. suzukii had supercooling points (SCPs) between -16 and -23°C, and were chill-susceptible. 80% of control flies were killed after 1h at -7.2°C (males) or -7.5°C (females); CA and FA improved survival of this temperature in both sexes, but RCH did not. 80% of control flies were killed after 70 h (male) or 92 h (female) at 0°C, and FA shifted this to 112 h (males) and 165 h (females). FA flies entered chill coma (CTmin) at approximately -1.7°C, which was ca. 0.5°C colder than control flies; RCH and CA increased the CTmin compared to controls. Control and RCH flies exposed to 0°C for 8h took 30-40 min to recover movement, but this was reduced to <10 min in CA and FA. Flies placed outside in a field cage in London, Ontario, were all killed by a transient cold snap in December. We conclude that adult phenotypic plasticity is not sufficient to allow D. suzukii to overwinter in temperate habitats, and suggest that flies could overwinter in association with built structures, or that there may be additional cold tolerance imparted by developmental plasticity.

Thermal ecology of the fiddler crab Uca panacea: Thermal constraints and organismal responses

Temperature is one of the primary environmental variables limiting organismal performance, fitness, and species distributions. Yet, understanding temperature effects requires thorough exploration of thermal constraints and organismal responses that can translate to fitness and non-lethal long-term consequences under both constant and changing thermal regimes. We examined the thermal ecology of the fiddler crab Uca panacea, including critical thermal limits, thermal sensitivity of locomotion, operative environmental temperatures, preferred body temperatures, and acclimation ability. Operative environmental temperatures frequently reached the critical thermal maximum (41.8±0.8°C, mean ± s.e.m.), especially in unvegetated microhabitats, indicating the need for behavioral thermoregulation to maintain diurnal activity patterns. Preferred body temperatures (21.1-28.6°C) were substantially below the thermal optimum (30-40°C), although further research is needed to determine the driver of this mismatch. Critical thermal limits shifted 2-4°C in response to exposure to low (20°C) or high (35°C) temperatures, with full acclimation occurring in approximately 9d. This capacity for rapid acclimation, combined with the capacity for behavioral thermoregulation, is a strong candidate mechanism that explains the broad habitat use and could help explain the successful pantropical distribution of fiddler crabs.

Chill-tolerant Gryllus crickets maintain ion balance at low temperatures

Insect cold tolerance is both phenotypically-plastic and evolutionarily labile, but the mechanisms underlying this variation are uncertain. Chill-susceptible insects lose ion and water homeostasis in the cold, which contributes to the development of injuries and eventually death. We thus hypothesized that more cold-tolerant insects will better maintain ion and water balance at low temperatures. We used rapid cold-hardening (RCH) and cold acclimation to improve cold tolerance of male Gryllus pennsylvanicus, and also compared this species to its cold-tolerant relative (Gryllus veletis). Cold acclimation and RCH decreased the critical thermal minimum (CTmin) and chill coma recovery time (CCR) in G. pennsylvanicus, but while cold acclimation improved survival of 0 °C, RCH did not; G. veletis was consistently more cold-tolerant (and had lower CCR and CTmin) than G. pennsylvanicus. During cold exposure, hemolymph water and Na(+) migrated to the gut of warm-acclimated G. pennsylvanicus, which increased hemolymph [K(+)] and decreased muscle K(+) equilibrium potentials. By contrast, cold-acclimated G. pennsylvanicus suffered a smaller loss of ion and water homeostasis during cold exposure, and this redistribution did not occur at all in cold-exposed G. veletis. The loss of ion and water balance was similar between RCH and warm-acclimated G. pennsylvanicus, suggesting that different mechanisms underlie decreased CCR and CTmin compared to increased survival at 0 °C. We conclude that increased tolerance of chilling is associated with improved maintenance of ion and water homeostasis in the cold, and that this is consistent for both phenotypic plasticity and evolved cold tolerance.

The roles of thermal transient receptor potential channels in thermotactic behavior and in thermal acclimation in the red flour beetle, Tribolium castaneum

To survive in variable or fluctuating temperature, organisms should show appropriate behavioral and physiological responses which must be mediated through properly attuned thermal sensory mechanisms. Transient receptor potential channels (TRPs) are a family of cation channels a number of which, called thermo-TRPs, are known to function as thermosensors. We investigated the potential role of thermo-TPRs that have been previously identified in the fruit fly, Drosophila melanogaster, in thermotaxis and thermal acclimation in the red flour beetle, Tribolium castaneum. Phylogenetic analysis of the trp genes showed generally one-to-one orthology between those in D. melanogaster and in T. castaneum, although there are putative gene-losses in two TRP subfamilies of D. melanogaster. With RNA interference (RNAi) of T. castaneum thermo-TRP candidates painless, pyrexia and trpA1, we measured thermal avoidance behavior. RNAi of trpA1 resulted in reduced avoidance of high temperatures, 39 and 42 °C. We also measured the effects of RNAi on heat-induced knockout and death under a short exposure to high temperature (1min at 52 °C) either with or without a 10-min acclimation period at 42 °C. Relatively short exposure to high temperature was enough to induce high temperature thermal acclimation. RNAi of trpA1 led to faster knockout at 52 °C. RNAi of painless showed lower recovery rates from heat-induced knockout after thermal acclimation, and RNAi of pyrexia showed lower long-term survivorship without thermal acclimation. Therefore, we concluded that trpA1 is important in high temperature sensing and also in enhanced tolerance to high-temperature induced knockout; painless plays a role in rapid acclimation to high temperature; and pyrexia functions in protecting beetles from acute heat stress without acclimation.

Constraints, independence, and evolution of thermal plasticity: probing genetic architecture of long- and short-term thermal acclimation

Seasonal and daily thermal variation can limit species distributions because of physiological tolerances. Low temperatures are particularly challenging for ectotherms, which use both basal thermotolerance and acclimation, an adaptive plastic response, to mitigate thermal stress. Both basal thermotolerance and acclimation are thought to be important for local adaptation and persistence in the face of climate change. However, the evolutionary independence of basal and plastic tolerances remains unclear. Acclimation can occur over longer (seasonal) or shorter (hours to days) time scales, and the degree of mechanistic overlap is unresolved. Using a midlatitude population of Drosophila melanogaster, we show substantial heritable variation in both short- and long-term acclimation. Rapid cold hardening (short-term plasticity) and developmental acclimation (long-term plasticity) are positively correlated, suggesting shared mechanisms. However, there are independent components of these traits, because developmentally acclimated flies respond positively to short-term acclimation. A strong negative correlation between basal cold tolerance and developmental acclimation suggests that basal cold tolerance may constrain developmental acclimation, whereas a weaker negative correlation between basal cold tolerance and short-term acclimation suggests less constraint. Using genome-wide association mapping, we show the genetic architecture of rapid cold hardening and developmental acclimation responses are nonoverlapping at the SNP and corresponding gene level. However, genes associated with each trait share functional similarities, including genes involved in apoptosis and autophagy, cytoskeletal and membrane structural components, and ion binding and transport. These results indicate substantial opportunity for short-term and long-term acclimation responses to evolve separately from each other and for short-term acclimation to evolve separately from basal thermotolerance.

Sensitivity to thermal extremes in Australian Drosophila implies similar impacts of climate change on the distribution of widespread and tropical species

Climatic factors influence the distribution of ectotherms, raising the possibility that distributions of many species will shift rapidly under climate change and/or that species will become locally extinct. Recent studies have compared performance curves of species from different climate zones and suggested that tropical species may be more susceptible to climate change than those from temperate environments. However, in other comparisons involving responses to thermal extremes it has been suggested that mid-latitude populations are more susceptible. Using a group of 10 closely related Drosophila species with known tropical or widespread distribution, we undertake a detailed investigation of their growth performance curves and their tolerance to thermal extremes. Thermal sensitivity of life history traits (fecundity, developmental success, and developmental time) and adult heat resistance were similar in tropical and widespread species groups, while widespread species had higher adult cold tolerance under all acclimation regimes. Laboratory measurements of either population growth capacity or acute tolerance to heat and cold extremes were compared to daily air temperature under current (2002-2007) and future (2100) conditions to investigate if these traits could explain current distributions and, therefore, also forecast future effects of climate change. Life history traits examining the thermal sensitivity of population growth proved to be a poor predictor of current species distributions. In contrast, we validate that adult tolerance to thermal extremes provides a good correlate of current distributions. Thus, in their current distribution range, most of the examined species experience heat exposure close to, but rarely above, the functional heat resistance limit. Similarly, adult functional cold resistance proved a good predictor of species distribution in cooler climates. When using the species' functional tolerance limits under a global warming scenario, we find that both tropical and widespread Drosophila species will face a similar proportional reduction in distribution range under future warming.

Heritability and inter-population differences in lipid profiles of Drosophila melanogaster

Characterizing and understanding the complex spectrum of lipids in higher organisms lags far behind our analysis of genome and transcriptome sequences. Here we generate and evaluate comprehensive lipid profiles (>200 lipids) of 92 inbred lines from five different Drosophila melanogaster populations. We find that the majority of lipid species are highly heritable, and even lipids with odd-chain fatty acids, which cannot be generated by the fly itself, also have high heritabilities. Abundance of the endosymbiont Wolbachia, a potential provider of odd-chained lipids, was positively correlated with this group of lipids. Additionally, we show that despite years of laboratory rearing on the same medium, the lipid profiles of the five geographic populations are sufficiently distinct for population discrimination. Our data predicts a strikingly different membrane fluidity for flies from the Netherlands, which is supported by their increased ethanol tolerance. We find that 18% of lipids show strong concentration differences between males and females. Through an analysis of the correlation structure of the lipid classes, we find modules of co-regulated lipids and begin to associate these with metabolic constraints. Our data provide a foundation for developing associations between variation in lipid composition with variation in other metabolic attributes, with genome-wide variation, and with metrics of health and overall reproductive fitness.

Rapid cold hardening improves recovery of ion homeostasis and chill coma recovery in the migratory locust Locusta migratoria

Chill tolerance of insects is defined as the ability of insects to tolerate low temperature under circumstances not involving freezing of intra- or extracellular fluids. For many insects chill tolerance is crucial for their ability to persist in cold environments and mounting evidence indicate that chill tolerance is associated with the ability to maintain ion- and water-homeostasis, thereby ensuring muscular function and preventing chill injury at low temperature. The present study describes the relationship between muscle and hemolymph ion-homeostasis and time to regain posture following cold shock (CS, 2h at -4°C) in the chill susceptible locust, Locusta migratoria. This relationship is examined in animals with and without a prior rapid cold hardening treatment (RCH, 2h at 0°C) to investigate the physiological underpinnings of RCH. Cold shock elicited a doubling of hemolymph [K+] and this disturbance was greater in locusts pre-exposed to RCH. Recovery of ion homeostasis was, however, markedly faster in RCH treated animals which correlated well with whole organism performance as hardened individuals regained posture more than 2 minutes faster than non-hardened individuals following CS. The present study indicates that loss and recovery of muscular function is associated with resting membrane potential of excitable membranes as attested from the changes in the equilibrium potential for K+ (EK) following CS. Both hardened and non-hardened animals recovered movement once K+ homeostasis was recovered to a fixed level (EK≈ -41 mV). RCH is therefore not associated with altered sensitivity to ion disturbance but instead a faster recovery of hemolymph [K+].

Mortality from desiccation contributes to a genotype-temperature interaction for cold survival in Drosophila melanogaster

Survival at cold temperatures is a complex trait, primarily because of the fact that the physiological cause of injury may differ across degrees of cold exposure experienced within the lifetime of an ectothermic individual. In order to better understand how chill-sensitive insects experience and adapt to low temperatures, we investigated the physiological basis for cold survival across a range of temperature exposures from -4 to 6°C in five genetic lines of the fruit fly Drosophila melanogaster. Genetic effects on cold survival were temperature dependent and resulted in a significant genotype-temperature interaction for survival across cold temperature exposures that differ by as little as 2°C. We investigated desiccation as a potential mechanism of injury across these temperature exposures. Flies were dehydrated following exposures near 6°C, whereas flies were not dehydrated following exposures near -4°C. Furthermore, decreasing humidity during cold exposure decreased survival, and increasing humidity during cold exposure increased survival at 6°C, but not at -4°C. These results support the conclusion that in D. melanogaster there are multiple physiological mechanisms of cold-induced mortality across relatively small differences in temperature, and that desiccation contributes to mortality for exposures near 6°C but not for subzero temperatures. Because D. melanogaster has recently expanded its range from tropical to temperate latitudes, the complex physiologies underlying cold tolerance are likely to be important traits in the recent evolutionary history of this fruit fly.

Myo-inositol as a main metabolite in overwintering flies: seasonal metabolomic profiles and cold stress tolerance in a northern drosophilid fly

Coping with seasonal changes in temperature is an important factor underlying the ability of insects to survive over the harsh winter conditions in the northern temperate zone, and only a few drosophilids have been able to colonize sub-polar habitats. Information on their winter physiology is needed as it may shed light on the adaptive mechanisms of overwintering when compared with abundant data on the thermal physiology of more southern species, such as Drosophila melanogaster. Here we report the first seasonal metabolite analysis in a Drosophila species. We traced changes in the cold tolerance and metabolomic profiles in adult Drosophila montana flies that were exposed to thermoperiods and photoperiods similar to changes in environmental conditions of their natural habitat in northern Finland. The cold tolerance of diapausing flies increased noticeably towards the onset of winter; their chill coma recovery times showed a seasonal minimum between late autumn and early spring, whereas their survival after cold exposure remained high until late spring. The flies had already moderately accumulated glucose, trehalose and proline in autumn, but the single largest change occurred in myo-inositol concentrations. This increased up to 400-fold during the winter and peaked at 147 nmol mg(-1) fresh mass, which is among the largest reported accumulations of this compound in insects.

Cold resistance depends on acclimation and behavioral caste in a temperate ant

Adjusting to low temperatures is important for animals living in cold environments. We studied the chill-coma recovery time in temperate ant workers (Temnothorax nylanderi) from colonies collected in autumn and spring in Germany. We experimentally acclimated these ant colonies to cold temperatures followed by warm temperatures. As expected, cold-acclimated workers recovered faster from freezing temperatures, but subsequent heat acclimation did not change the short recovery times observed after cold acclimation. Hence, either heat acclimation improves cold tolerance, possibly as a general response to stress, or at least it does not negate enhanced cold tolerance following cold acclimation. Colonies collected in spring showed similar cold tolerance levels to cold-acclimated colonies in the laboratory. Next, we compared the chill-coma recovery time of different worker castes and found that exterior workers recovered faster than interior workers. This difference may be related to their more frequent exposure to cold, higher activity level, or distinct physiology. Interior workers were also heavier and showed a higher gaster-to-head ratio and thorax ratio compared to exterior workers. An obvious difference between exterior and interior workers is activity level, but we found no link between activity and cold tolerance. This suggests that physiology rather than behavioral differences could cause the increased cold tolerance of exterior workers. Our study reveals the importance of acclimation for cold tolerance under natural and standardized conditions and demonstrates differences in cold tolerance and body dimensions in monomorphic behavioral castes of an ant.