The sub-lethal effects of repeated freezing in the woolly bear caterpillar Pyrrharctia isabella

Exploring cross-protective effects between cold and immune stress in Drosophila melanogaster

It is well established that environmental and biotic stressors like temperature and pathogens/parasites are essential for the life of small ectotherms. There are complex interactions between cold stress and pathogen infection in insects. Possible cross-protective mechanisms occur between both stressors, suggesting broad connectivity in insect stress responses. In this study, the functional significance of these interactions was tested, as well as the potential role of newly uncovered candidate genes, turandot. This was done using an array of factorial experiments exposing Drosophila melanogaster flies to a combination of different cold stress regimes (acute or chronic) and infections with the parasitic fungus Beauveria bassiana. Following these crossed treatments, phenotypic and molecular responses were assessed by measuring 1) induced cold tolerance, 2) immune resistance to parasitic fungus, and 3) activation of turandot genes. We found various responses in the phenotypic outcomes according to the various treatment combinations with higher susceptibility to infection following cold stress, but also significantly higher acute cold survival in flies that were infected. Regarding molecular responses, we found overexpression of turandot genes in response to most treatments, suggesting reactivity to both cold and infection. Moreover, maximum peak expressions were distinctly observed in the combined treatments (infection plus cold), indicating a marked synergistic effect of the stressors on turandot gene expression patterns. These results reflect the great complexity of cross-tolerance reactions between infection and abiotic stress, but could also shed light on the mechanisms underlying the activation of these responses.

Cold-induced immune activation in chill-susceptible insects

Chilling injuries in chill-susceptible insects, like the model dipteran Drosophila melanogaster, have been well-documented as a consequence of stressful low temperature exposures. Cold stress also causes upregulation of genes in the insect immune pathways, some of which are also upregulated following other forms of sterile stress. The adaptive significance and underlying mechanisms surrounding cold-induced immune activation, however, are still unclear. Here, we review recent work on the roles of ROS, DAMPs, and AMPs in insect immune signalling or function. Using this emerging knowledge, we propose a conceptual model linking biochemical and molecular causes of immune activation to its consequences during and following cold stress.

Proteostasis in ice: the role of heat shock proteins and ubiquitin in the freeze tolerance of the intertidal mussel, Mytilus trossulus

The bay mussel, Mytilus trossulus, is an animal that can survive extracellular ice formation. Depending on air and ocean temperatures, freeze tolerant intertidal organisms, like M. trossulus, may freeze and thaw many times during the winter. Freezing can cause protein denaturation, leading to an induction of the heat shock response with expression of chaperone proteins like the 70 kDa heat shock protein (HSP70), and an increase in ubiquitin-conjugated proteins. There has been little work on the mechanisms of freeze tolerance in intertidal species, limiting our understanding of this survival strategy. Additionally, this limited research has focused solely on the effects of single freezing events, but the act of repeatedly crossing the freezing threshold may present novel physiological or biochemical stressors that have yet to be discovered. Mytilus are important ecosystem engineers and provide habitat for other intertidal species, thus understanding their physiology under thermal extremes is important for preserving shoreline health. We predicted that repeated freeze exposures would increase mortality, upregulate HSP70 expression, and increase ubiquitin conjugates in mussels, relative to single, prolonged freeze exposures. Mytilus trossulus from Vancouver, Canada were repeatedly frozen for a combination of 1 × 8 h, 2 × 4 h, or 4 × 2 h. We then compared mortality, HSP70 expression, and the quantity of ubiquitin-conjugated proteins across experimental groups. We found a single 8-h freeze caused significantly more mortality than repeated freeze-thaw cycles. We also found that HSP70 and ubiquitinated protein was upregulated exclusively after freeze-thaw cycles, suggesting that freeze-thaw cycles offer a period of damage repair between freezes. This indicates that freeze-thaw cycles, which happen naturally in the intertidal, are crucial for M. trossulus survival in sub-zero temperatures.

Repeated freeze-thaw cycles in freeze-tolerant treefrogs: novel interindividual variation of integrative biochemical, cellular, and organismal responses

The freeze-tolerant anuran Dryophytes chrysoscelis, Cope's gray treefrog, mobilizes a complex cryoprotectant system that includes glycerol, glucose, and urea to minimize damage induced by freezing and thawing of up to 65% of body water. In this species' eastern Northern American temperate habitat, oscillations of temperature above and below freezing are common; however, the effects of repeated freezing and thawing in this species are unstudied. The biochemical and physiological effects of repeated freeze-thaw cycles were therefore evaluated and compared with cold acclimation and single freeze-thaw episodes. Glycerol was elevated in plasma, liver, and skeletal muscle of both singly and repeatedly frozen and thawed animals compared with cold-acclimated frogs. In contrast, urea was unchanged by freezing and thawing, whereas glucose was elevated in singly frozen and thawed animals but was reduced toward cold acclimation levels after repeated bouts of freezing. Overall, the cryoprotectant system was maintained, but not further elevated, in all tissues assayed in repeatedly frozen and thawed animals. For repeated freeze-thaw only, hepatic glycogen was depleted and plasma hemoglobin, indicative of erythrocyte hemolysis, increased. Postfreeze recovery of locomotor function, including limb and whole body movement, was delayed with repeated freeze-thaw and was associated with glycerol accumulation and glycogen depletion. Individuals that resumed locomotor function more quickly also accumulated greater cryoinjury. Integrated analyses of cryoprotectant and cryoinjury accumulation suggest that winter survival of D. chrysoscelis may be vulnerable to climate change, limited by carbohydrate stores, cellular repair mechanisms, and plasticity of the cryoprotectant system.

Evolution of cross-tolerance in Drosophila melanogaster as a result of increased resistance to cold stress

Cold stress is a critical environmental challenge that affects an organism's fitness-related traits. In Drosophila, increased resistance to specific environmental stress may lead to increased resistance to other kinds of stress. In the present study, we aimed to understand whether increased cold stress resistance in Drosophila melanogaster can facilitate their ability to tolerate other environmental stresses. For the current study, we used successfully selected replicate populations of D. melanogaster against cold shock and their control population. These selected populations have evolved several reproductive traits, including increased egg viability, mating frequency, male mating ability, ability to sire progenies, and faster recovery for mating latency under cold shock conditions. In the present work, we investigated egg viability and mating frequency with and without heat and cold shock conditions in the selected and their control populations. We also examined resistance to cold shock, heat shock, desiccation, starvation, and survival post-challenge with Staphylococcus succinus subsp. succinus PK-1 in the selected and their control populations. After cold-shock treatment, we found a 1.25 times increase in egg viability and a 1.57 times increase in mating frequency in the selected populations compared to control populations. Moreover, more males (0.87 times) and females (1.66 times) of the selected populations survived under cold shock conditions relative to their controls. After being subjected to heat shock, the selected population's egg viability and mating frequency increased by 0.30 times and 0.57 times, respectively, compared to control populations. Additionally, more selected males (0.31 times) and females (0.98 times) survived under heat shock conditions compared to the control populations. Desiccation resistance slightly increased in the females of the selected populations relative to their control, but we observed no change in the case of males. Starvation resistance decreased in males and females of the selected populations compared to their controls. Our findings suggest that the increased resistance to cold shock correlates with increased tolerance to heat stress, but this evolved resistance comes at a cost, with decreased tolerance to starvation.

Cold hardening improves larval tick questing under low temperatures at the expense of longevity

Questing in ticks is essential for locating a host, and this behavioral response can occur at regionally specific low temperatures for most tick species. Little is known about the dynamics between tick questing behavior and temperature in ticks, specifically how this may impact other aspects of tick biology. Here, we examine whether cold hardening increases questing in three larval tick species (Ixodes uriae, Dermacentor variabilis, and Amblyomma americanum) at low temperatures and whether cold hardening impacts longevity. Rapid cold hardening and prolonged cold acclimation benefitted ticks by decreasing the temperature of chill coma onset, and increased survival, activity, and questing in ticks at low temperatures. Oxygen consumption increased at low temperatures following acclimation in larvae, suggesting this process has a distinct metabolic expense. This increased metabolism associated with hardening led to a substantial reduction in larval longevity as nutrient reserves are limited and cannot be replenished until a host is located. These studies suggest that tick larvae, and likely other developmental stages, require a delicate balance between the need for questing at low temperatures and survival until the first blood meal.

Harnessing the potential of cross-protection stressor interactions for conservation: a review

Conservation becomes increasingly complex as climate change exacerbates the multitude of stressors that organisms face. To meet this challenge, multiple stressor research is rapidly expanding, and the majority of this work has highlighted the deleterious effects of stressor interactions. However, there is a growing body of research documenting cross-protection between stressors, whereby exposure to a priming stressor heightens resilience to a second stressor of a different nature. Understanding cross-protection interactions is key to avoiding unrealistic 'blanket' conservation approaches, which aim to eliminate all forms of stress. But, a lack of synthesis of cross-protection interactions presents a barrier to integrating these protective benefits into conservation actions. To remedy this, we performed a review of cross-protection interactions among biotic and abiotic stressors within a conservation framework. A total of 66 publications were identified, spanning a diverse array of stressor combinations and taxonomic groups. We found that cross-protection occurs in response to naturally co-occurring stressors, as well as novel, anthropogenic stressors, suggesting that cross-protection may act as a 'pre-adaptation' to a changing world. Cross-protection interactions occurred in response to both biotic and abiotic stressors, but abiotic stressors have received far more investigation. Similarly, cross-protection interactions were present in a diverse array of taxa, but several taxonomic groups (e.g. mammals, birds and amphibians) were underrepresented. We conclude by providing an overview of how cross-protection interactions can be integrated into conservation and management actions and discuss how future research in this field may be directed to improve our understanding of how cross-protection may shield animals from global change.

Changes in haemolymph parameters and insect ability to respond to immune challenge during overwintering

Overwintering is a challenging period in the life of temperate insects. A limited energy budget characteristic of this period can result in reduced investment in immune system. Here, we investigated selected physiological and immunological parameters in laboratory-reared and field-collected harlequin ladybirds (Harmonia axyridis). For laboratory-reared beetles, we focused on the effects of winter temperature regime (cold, average, or warm winter) on total haemocyte concentration aiming to investigate potential effects of ongoing climate change on immune system in overwintering insects. We recorded strong reduction in haemocyte concentration during winter; however, there were only limited effects of winter temperature regime on changes in haemocyte concentration in the course of overwintering. For field-collected beetles, we measured additional parameters, specifically: total protein concentration, antimicrobial activity against Escherichia coli, and haemocyte concentration before and after overwintering. The field experiment did not investigate effects of winter temperature, but focused on changes in inducibility of insect immune system during overwintering, that is, measured parameters were compared between naïve beetles and those challenged by Escherichia coli. Haemocyte concentration decreased during overwintering, but only in individuals challenged by Escherichia coli. Prior to overwintering, the challenged beetles had a significantly higher haemocyte concentration compared to naïve beetles, whereas no difference was observed after overwintering. A similar pattern was observed also for antimicrobial activity against Escherichia coli as challenged beetles outperformed naïve beetles before overwintering, but not after winter. In both sexes, total protein concentration increased in the course of overwintering, but females had a significantly higher total protein concentration in their hemolymph compared to males. In general, our results revealed that insect's ability to respond to an immune challenge is significantly reduced in the course of overwintering.

Optical cryomicroscopy and differential scanning calorimetry of buffer solutions containing cryoprotectants

In the pharmaceutical industry, cryoprotectants are added to buffer formulations to protect the active pharmaceutical ingredient from freeze- and thaw damage. We investigated the freezing and thawing of aqueous sodium citrate buffer with various cryoprotectants, specifically amino acids (cysteine, histidine, arginine, proline and lysine), disaccharides (trehalose and sucrose), polyhydric alcohols (glycerol and mannitol) and surfactants (polysorbate 20 and polysorbate 80). Hereby, we employed optical cryomicroscopy in combination with differential scanning calorimetry in the temperature range to -80 °C. The effect of cryoprotectants on the morphology of the ice crystals, the glass transition temperature and the initial melting temperature is presented. Some of the cryoprotectants have a significant impact on ice crystal size. Disaccharides restrict ice crystal growth, whereas surfactants and glycerol allow ice crystals to increase in size. Cysteine and mannitol cause dehydration after thawing. Either one or two glass transition temperatures were detected, where arginine, surfactants, glycerol, proline and lysine suppress the second, implying a uniform freeze-concentrated solution. The initial melting temperature of pure buffer solution can be shifted up by adding mannitol, both disaccharides and both surfactants; but down by glycerol, proline and lysine.

Sex-specific responses to cold in a very cold-tolerant, northern Drosophila species

Organisms can plastically alter resource allocation in response to changing environmental factors. For example, in harsh conditions, organisms are expected to shift investment from reproduction toward survival; however, the factors and mechanisms that govern the magnitude of such shifts are relatively poorly studied. Here we compared the impact of cold on males and females of the highly cold-tolerant species Drosophila montana at the phenotypic and transcriptomic levels. Although both sexes showed similar changes in cold tolerance and gene expression in response to cold treatment, indicating that the majority of changes are concordant between the sexes, we identified a clear reduction in sexually dimorphic gene expression, suggesting that preparing for the colder season involves reducing investment in sex-specific traits. This reduction was larger in males than females, as expected if male sexual traits are more condition-dependent than female traits, as predicted by theory. Gene expression changes were primarily associated with shifts in metabolic profile, which likely play a role in increasing cold tolerance. Finally, we found that the expression of immune genes was reduced following cold treatment, suggesting that reduced investment in costly immune function may be important in helping flies survive colder periods.

Costs of averting or prematurely terminating diapause associated with slow decline of metabolic rates at low temperature

Diapause, a form of insect dormancy, generally facilitates overwintering by increasing cold tolerance and decreasing energy drain at high temperatures via metabolic rate suppression. Averting or terminating diapause prior to winter is generally assumed to be a lethal phenotype. However, low temperature acclimation can also increase cold tolerance and decrease metabolic rates. Here, we tested the hypothesis that non- and post-diapause individuals in a cold-induced quiescence can achieve a diapause-like phenotype, compensating for the potential costs of averting diapause. We tested this in the apple maggot fly Rhagoletis pomonella, which typically overwinters in the soil as a diapause pupa, but can avert diapause (non-diapause) or terminate diapause early ('weak diapause') when reared at warm temperatures. Metabolic rates were initially higher in non- and post-diapause than diapause pupae at high (25 °C) and low (4 °C) temperatures, but quiescent non- and post-diapause pupae achieved diapause-like metabolic rates slowly over time when incubated at 4 °C for several weeks. We found that diapause and quiescent pupae were freeze-avoidant and had similar tolerance of extreme low temperatures (cooling to c. -18 °C) following 8 weeks acclimation at 4 °C. Despite high tolerance of subzero temperatures, quiescent pupae did not survive well when chilled for prolonged periods (8 weeks or more) at 4 °C. We conclude that cold acclimation can only partially compensate for costs associated with aversion or premature termination of diapause, and that energy drain at low (not just high) temperatures likely contributes to chilling mortality in quiescent insects.

Metabolic cost of freeze-thaw and source of CO2 production in the freeze-tolerant cricket Gryllus veletis

Freeze-tolerant insects can survive the conversion of a substantial portion of their body water to ice. While the process of freezing induces active responses from some organisms, these responses appear absent from freeze-tolerant insects. Recovery from freezing likely requires energy expenditure to repair tissues and re-establish homeostasis, which should be evident as elevations in metabolic rate after thaw. We measured carbon dioxide (CO2) production in the spring field cricket (Gryllus veletis) as a proxy for metabolic rate during cooling, freezing and thawing and compared the metabolic costs associated with recovery from freezing and chilling. We hypothesized that freezing does not induce active responses, but that recovery from freeze-thaw is metabolically costly. We observed a burst of CO2 release at the onset of freezing in all crickets that froze, including those killed by either cyanide or an insecticide (thiacloprid), implying that the source of this CO2 was neither aerobic metabolism nor a coordinated nervous system response. These results suggest that freezing does not induce active responses from G. veletis, but may liberate buffered CO2 from hemolymph. There was a transient 'overshoot' in CO2 release during the first hour of recovery, and elevated metabolic rate at 24, 48 and 72 h, in crickets that had been frozen compared with crickets that had been chilled (but not frozen). Thus, recovery from freeze-thaw and the repair of freeze-induced damage appears metabolically costly in G. veletis, and this cost persists for several days after thawing.

Tolerance of aquifer stoneflies to repeated hypoxia exposure and oxygen dynamics in an alluvial aquifer

Aquatic insects cope with hypoxia and anoxia using a variety of behavioral and physiological responses. Most stoneflies (Plecoptera) occur in highly oxygenated surface waters, but some species live underground in alluvial aquifers containing heterogeneous oxygen concentrations. Aquifer stoneflies appear to be supported by methane-derived food resources, which they may exploit using anoxia-resistant behaviors. We documented dissolved oxygen dynamics and collected stoneflies over 5 years in floodplain wells of the Flathead River, Montana. Hypoxia regularly occurred in two wells, and nymphs of Paraperla frontalis were collected during hypoxic periods. We measured mass-specific metabolic rates (MSMRs) at different oxygen concentrations (12, 8, 6, 4, 2, 0.5 mg l-1, and during recovery) for 111 stonefly nymphs to determine whether aquifer and benthic taxa differed in hypoxia tolerance. Metabolic rates of aquifer taxa were similar across oxygen concentrations spanning 2 to 12 mg l-1 (P>0.437), but the MSMRs of benthic taxa dropped significantly with declining oxygen (P<0.0001; 2.9-times lower at 2 vs. 12 mg l-1). Aquifer taxa tolerated short-term repeated exposure to extreme hypoxia surprisingly well (100% survival), but repeated longer-term (>12 h) exposures resulted in lower survival (38-91%) and lower MSMRs during recovery. Our work suggests that aquifer stoneflies have evolved a remarkable set of behavioral and physiological adaptations that allow them to exploit the unique food resources available in hypoxic zones. These adaptations help to explain how large-bodied consumers might thrive in the underground aquifers of diverse and productive river floodplains.

The influence of immune activation on thermal tolerance along a latitudinal cline

Global change is shifting both temperature patterns and the geographic distribution of pathogens, and infection has already been shown to substantially reduce host thermal performance, potentially placing populations at greater risk that previously thought. But what about individuals that are able to successfully clear an infection? Whilst the direct damage a pathogen causes will likely lead to reductions in host's thermal tolerance, the response to infection often shares many underlying pathways with the general stress response, potentially acting as a buffer against subsequent thermal stress. Here, by exposing Drosophila melanogaster to heat-killed bacterial pathogens, we investigate how activation of a host's immune system can modify any response to both heat and cold temperature stress. In a single focal population, we find that immune activation can improve a host's knockdown times during heat shock, potentially offsetting some of the damage that would subsequently arise as an infection progresses. Conversely, immune activation had a detrimental effect on CT_max and did not influence lower thermal tolerance as measured by chill-coma recovery time. However, we also find that the influence of immune activation on heat knockdown times is not generalizable across an entire cline of locally adapted populations. Instead, immune activation led to signals of local adaptation to temperature being lost, erasing the previous advantage that populations in warmer regions had when challenged with heat stress. Our results suggest that activation of the immune system may help buffer individuals against the detrimental impact of infection on thermal tolerance; however, any response will be population specific and potentially not easily predicted across larger geographic scales, and dependent on the form of thermal stress faced by a host.

Laboratory acclimation to autumn-like conditions induces freeze tolerance in the spring field cricket Gryllus veletis (Orthoptera: Gryllidae)

Many temperate insects encounter temperatures low enough to freeze their body fluids. Remarkably, some insects are freeze-tolerant, surviving this internal ice formation. However, the mechanisms underlying freeze tolerance are not well-understood, in part due to a lack of tractable model organisms. We describe a novel laboratory model to study insect freeze tolerance, the spring field cricket Gryllus veletis (Orthopera: Gryllidae). Following acclimation to six weeks of decreasing temperature and photoperiod, G. veletis become freeze-tolerant, similar to those exposed to natural autumn conditions in London, Ontario, Canada. Acclimated crickets suppress their metabolic rate by c. 33%, and survive freezing for up to one week at -8 °C, and to temperatures as low as -12 °C. Freeze-tolerant G. veletis protect fat body cells from freeze injury in vivo, and fat body tissue from freeze-tolerant cricket survives brief freeze treatments when frozen ex vivo. Freeze-tolerant crickets freeze at c. -6 °C, which may be initiated by accumulation of ice-nucleating agents in hemolymph or gut tissue. Although we hypothesize that control of ice formation facilitates freeze tolerance, initiating ice formation at high subzero temperatures does not confer freeze tolerance on freeze-intolerant nymphs. Acclimation increases hemolymph osmolality from c. 400 to c. 650 mOsm, which may facilitate freeze tolerance by reducing ice content. Hemolymph ion concentrations do not change with acclimation, and we therefore predict that freeze-tolerant G. veletis elevate hemolymph osmolality by accumulating other molecules. Gryllus veletis is easily reared and manipulated in a controlled laboratory environment, and is therefore a suitable candidate for further investigating the mechanisms underlying freeze tolerance.

Insect fat body cell morphology and response to cold stress is modulated by acclimation

Mechanistic understanding about the nature of cellular cryoinjury and mechanisms by which some animals survive freezing while others do not is currently lacking. Here, we exploited the broadly manipulable freeze tolerance of larval malt flies (Chymomyza costata) to uncover cell and tissue morphological changes associated with freeze mortality. Diapause induction, cold acclimation and dietary proline supplementation generate malt fly variants ranging from weakly to extremely freeze tolerant. Using confocal microscopy and immunostaining of the fat body, Malpighian tubules and anterior midgut, we described tissue and cytoskeletal (F-actin and α-tubulin) morphologies among these variants after exposure to various cold stresses (from chilling at -5°C to extreme freezing at -196°C), and upon recovery from cold exposure. Fat body tissue appeared to be the most susceptible to cryoinjury: freezing caused coalescence of lipid droplets, loss of α-tubulin structure and apparent aggregation of F-actin. A combination of diapause and cold acclimation substantially lowered the temperature at which these morphological disruptions occurred. Larvae that recovered from a freezing challenge repaired F-actin aggregation but not lipid droplet coalescence or α-tubulin structure. Our observations indicate that lipid coalescence and damage to α-tubulin are non-lethal forms of freeze injury, and suggest that repair or removal (rather than protection) of actin proteins is a potential mechanism of acquired freeze tolerance.

Eco-immunology in the cold: the role of immunity in shaping the overwintering survival of ectotherms

The effect of temperature on physiology mediates many of the challenges that ectotherms face under climate change. Ectotherm immunity is thermally sensitive and, as such, environmental change is likely to have complex effects on survival, disease resistance and transmission. The effects of temperature on immunity will be particularly profound in winter because cold and overwintering are important triggers and regulators of ectotherm immune activity. Low temperatures can both suppress and activate immune responses independent of parasites, which suggests that temperature not only affects the rate of immune responses but also provides information that allows overwintering ectotherms to balance investment in immunity and other physiological processes that underlie winter survival. Changing winter temperatures are now shifting ectotherm immunity, as well as the demand for energy conservation and protection against parasites. Whether an ectotherm can survive the winter will thus depend on whether new immune phenotypes will shift to match the conditions of the new environment, or leave ectotherms vulnerable to infection or energy depletion. Here, we synthesise patterns of overwintering immunity in ectotherms and examine how new winter conditions might affect ectotherm immunity. We then explore whether it is possible to predict the effects of changing winter conditions on ectotherm vulnerability to the direct and indirect effects of parasites.

Repeated freezing induces a trade-off between cryoprotection and egg production in the goldenrod gall fly, Eurosta solidaginis

Internal ice formation leads to wholesale changes in ionic, osmotic and pH homeostasis, energy metabolism, and mechanical damage, across a small range of temperatures, and is thus an abiotic stressor that acts at a distinct, physiologically-relevant, threshold. Insects that experience repeated freeze-thaw cycles over winter will cross this stressor threshold many times over their lifespan. Here we examine the effect of repeatedly crossing the freezing threshold on short-term physiological parameters (metabolic reserves and cryoprotectant concentration) as well as long-term fitness-related performance (survival and egg production) in the freeze-tolerant goldenrod gall fly Eurosta solidaginis. We exposed overwintering prepupae to a series of low temperatures (-10, -15, or -20 °C) with increasing numbers of freezing events (3, 6, or 10) with differing recovery periods between events (1, 5, or 10 days). Repeated freezing increased sorbitol concentration by about 50% relative to a single freezing episode, and prompted prepupae to modify long chain triacylglycerols to acetylated triacylglycerols. Long-term, repeated freezing did not significantly reduce survival, but did reduce egg production by 9.8% relative to a single freezing event. Exposure temperature did not affect any of these measures, suggesting that threshold crossing events may be more important to fitness than the intensity of stress in E. solidaginis overwintering.

Learning to starve: impacts of food limitation beyond the stress period

Starvation is common among wild animal populations, and many individuals experience repeated bouts of starvation over the course of their lives. Although much information has been gained through laboratory studies of acute starvation, little is known about how starvation affects an animal once food is again available (i.e. during the refeeding and recovery phases). Many animals exhibit a curious phenomenon – some seem to ‘get better’ at starving following exposure to one or more starvation events – by this we mean that they exhibit potentially adaptive responses, including reduced rates of mass loss, reduced metabolic rates, and lower costs of digestion. During subsequent refeedings they may also exhibit improved digestive efficiency and more rapid mass gain. Importantly, these responses can last until the next starvation bout or even be inherited and expressed in the subsequent generation. Currently, however, little is known about the molecular regulation and physiological mechanisms underlying these changes. Here, we identify areas of research that can fill in the most pressing knowledge gaps. In particular, we highlight how recently refined techniques (e.g. stable isotope tracers, quantitative magnetic resonance and thermal measurement) as well as next-generation sequencing approaches (e.g. RNA-seq, proteomics and holobiome sequencing) can address specific starvation-focused questions. We also describe outstanding unknowns ripe for future research regarding the timing and severity of starvation, and concerning the persistence of these responses and their interactions with other ecological stressors.

Insect Immunity Varies Idiosyncratically During Overwintering

Overwintering insects face multiple stressors, including pathogen and parasite pressures that shift with seasons. However, we know little of how the insect immune system fluctuates with season, particularly in the overwintering period. To understand how immune activity changes across autumn, winter, and spring, we tracked immune activity of three temperate insects that overwinter as larvae: a weevil (Curculio sp., Coleoptera), gallfly (Eurosta solidaginis, Diptera), and larvae of the lepidopteran Pyrrharctia isabella. We measured baseline circulating hemocyte numbers, phenoloxidase activity, and humoral antimicrobial activity, as well as survival of fungal infection and melanization response at 12°C and 25°C to capture any potential plasticity in thermal performance. In Curculio sp. and E. solidaginis, hemocyte concentrations remained unchanged across seasons and antimicrobial activity against Gram-positive bacteria was lowest in autumn; however, Curculio sp. were less likely to survive fungal infection in autumn, whereas E. solidaginis were less likely to survive infection during the winter. Furthermore, hemocyte concentrations and antimicrobial activity decreased in P. isabella overwintering beneath snow cover. Overall, seasonal changes in activity were largely species dependent, thus it may be difficult to create generalizable predictions about the effects of a changing climate on seasonal immune activity in insects. However, we suggest that the relationship between the response to multiple stressors (e.g., cold and pathogens) drives changes in immune activity, and that understanding the physiology underlying these relationships will inform our predictions of the effects of environmental change on insect overwintering success.

Impact of cold on the immune system of burying beetle, Nicrophorus vespilloides (Coleoptera: Silphidae)

Insect overwintering is one of the most astonishing phases of the insect life cycle. Despite vast amounts of knowledge available about the physiological mechanisms of this phenomenon, the impact of stress factors on insect immune system functioning during the winter is still unknown. The aim of this study is to analyze how low temperatures influence the immune system of the beetle Nicrophorus vespilloides. The results show that the beetle's immune system is differently modulated by cold induced in laboratory settings than that which occurs in natural conditions. Among beetles cultured in conditions similar to summer, low temperatures, did not influence the number of circulating haemocytes, phenoloxidase activity, haemocytes morphology, and percentage ratio of haemocyte types. In these beetles, differences were noted only in the ability of haemocytes to perform phagocytosis. Individuals acclimated in natural conditions in autumn had a higher level of humoral response and a different percentage ratio of haemocyte types. During the winter period, the number of haemocytes in the beetles decreased, but the percentage ratio of phagocytic haemocytes increased. Furthermore, we noted an increase of phenoloxidase activity. Our study also showed mitotic divisions of haemocytes in haemolymph collected from burying beetles after cold exposure and from burying beetles collected from natural conditions during autumn and winter. Differences in response to low temperatures in laboratory conditions and the natural environment suggest that the simultaneous presence of other stress factors during winter such as desiccation and starvation have a significant influence on the activity of burying beetle's immune system.

Does cold activate the Drosophila melanogaster immune system?

Cold exposure appears to activate aspects of the insect immune system; however, the functional significance of the relationship between cold and immunity is unclear. Insect success at low temperatures is shaped in part by interactions with biotic stressors, such as pathogens, thus it is important to understand how and why immunity might be activated by cold. Here we explore which components of the immune system are activated, and whether those components differ among different kinds of cold exposure. We exposed Drosophila melanogaster to both acute (2h, -2°C) and sustained (10h, -0.5°C) cold, and measured potential (antimicrobial peptide expression, phenoloxidase activity, haemocyte counts) and realised (survival of fungal infection, wound-induced melanisation, bacterial clearance) immunity following recovery. Acute cold increased circulating haemocyte concentration and the expression of Turandot-A and diptericin, but elicited a short-term decrease in the clearance of gram-positive bacteria. Sustained cold increased the expression of Turandot-A, with no effect on other measures of potential or realised immunity. We show that measures of potential immunity were up-regulated by cold, whereas realised immunity was either unaffected or down-regulated. Thus, we hypothesize that cold-activation of potential immunity in Drosophila may be a compensatory mechanism to maintain stable immune function during or after low temperature exposure.

The physiological consequences of varied heat exposure events in adult Myzus persicae: a single prolonged exposure compared to repeated shorter exposures

The study of environmental stress tolerance in aphids has primarily been at low temperatures. In these cases, and in the rare cases of high temperature tolerance assessments, all exposures had been during a single stress event. In the present study, we examined the physiological consequences of repeated high temperature exposure with recovery periods between these stress events in Myzus persicae. We subjected individuals to either a single prolonged three hour heating event, or three one hour heating events with a recovery time of 24 h between bouts. Aphids exposed to repeated bouts of high temperatures had more glucose and higher expression of proteins and osmolyte compounds, such as glycerol, compared to the prolonged exposure group. However, aphids exposed to the repeated high temperature treatment had reduced sources of energy such as trehalose and triglyceride compounds than the prolonged exposure group. Recovery time had more physiological costs (based on production of more protein and consumption of more trehalose and triglyceride) and benefits (based on production of more osmolytes) in repeated high temperature treatments. As aphids are known to respond differently to constant versus ‘natural’ fluctuating temperature regimes, conclusions drawn from constant temperature data sets may be problematic. We suggest future experiments assessing insect responses to thermal stress incorporate a repeated stress and recovery pattern into their methodologies.

Life in the Frequency Domain: the Biological Impacts of Changes in Climate Variability at Multiple Time Scales

Over the last few decades, biologists have made substantial progress in understanding relationships between changing climates and organism performance. Much of this work has focused on temperature because it is the best kept of climatic records, in many locations it is predicted to keep rising into the future, and it has profound effects on the physiology, performance, and ecology of organisms, especially ectothermic organisms which make up the vast majority of life on Earth. Nevertheless, much of the existing literature on temperature-organism interactions relies on mean temperatures. In reality, most organisms do not directly experience mean temperatures; rather, they experience variation in temperature over many time scales, from seconds to years. We propose to shift the focus more directly on patterns of temperature variation, rather than on means per se, and present a framework both for analyzing temporal patterns of temperature variation and for incorporating those patterns into predictions about organismal biology. In particular, we advocate using the Fourier transform to decompose temperature time series into their component sinusoids, thus allowing transformations between the time and frequency domains. This approach provides (1) standardized ways of visualizing the contributions that different frequencies make to total temporal variation; (2) the ability to assess how patterns of temperature variation have changed over the past half century and may change into the future; and (3) clear approaches to manipulating temporal time series to ask "what if" questions about the potential effects of future climates. We first summarize global patterns of change in temperature variation over the past 40 years; we find meaningful changes in variation at the half day to yearly times scales. We then demonstrate the utility of the Fourier framework by exploring how power added to different frequencies alters the overall incidence of long-term waves of high and low temperatures, and find that power added to the lowest frequencies greatly increases the probability of long-term heat and cold waves. Finally, we review what is known about the time scales over which organismal thermal performance curves change in response to variation in the thermal environment. We conclude that integrating information characterizing both the frequency spectra of temperature time series and the time scales of resulting physiological change offers a powerful new avenue for relating climate, and climate change, to the future performance of ectothermic organisms.

Effect of freeze-thaw cycles and 4-nonylphenol on cellular energy allocation in the freeze-tolerant enchytraeid Enchytraeus albidus

Due to climate change and intense anthropogenic activity, organisms from cold regions are often exposed to combined effects of temperature fluctuations and contaminants. In this investigation, we assessed the lipid, protein, and carbohydrate energy budgets; the energy available (Ea); consumed (Ec); and cellular energy allocation (CEA) of the freeze-tolerant Enchytraeus albidus, when exposed to sublethal concentrations of 4-nonylphenol (a lipophilic contaminant) for 7 days, followed by exposure to different temperature regimes (continuous 2 °C, continuous -4 °C, and daily freeze-thaw cycles (FTC) (2 to -4 °C) for additional 10 days. Results showed that a pre-exposure to 4-nonylphenol (4-NP) induced important changes in the worms' energy budgets and CEA and increased mortality with most severe effects observed for the FTC events. For FTC, lipids were the most accumulated energy source, whereas during freezing (-4 °C), proteins were the most used. FTC caused the highest Ec, indicating the higher energy requirements for organisms when shifting between freezing and thawing events. This is also in line with the higher mortality observed in FTC compared to continuous -4 °C or 2 °C. Worms exposed to continuous freezing presented relatively stable and positive levels of Ea and low levels of Ec, possibly related with the decrease in metabolism.

Critical thermal limits of Poecilia caucana (Steindachner, 1880) (Cyprinodontiformes: Poeciliidae)

Abstract Although temperature has far-reaching effects on fish biology, the thermal tolerance ranges of most freshwater fish species are unknown. This lack of information precludes forecasting responses to climatic change and does not allow for comparative analyses that may inform evolutionary and biogeographic studies. We used the critical thermal methodology to quantify acclimation capacity and thermal tolerance in the Neotropical freshwater species Poecilia caucana . For fish acclimated to 20˚C, 25ºC, and 28ºC, critical thermal minima (CTmin) were 12.52 ± 0.62ºC, 13.41 ± 0.56ºC and 14.24 ± 0.43ºC respectively, and critical thermal maxima (CTmax) were 38.43 ± 0.64ºC, 40.28 ± 0.92ºC and 41.57 ± 0.27ºC, respectively. Both CTmin and CTmax changed with acclimation temperatures, indicating that P. caucana was effectively acclimatable. Relative to values reported for other freshwater fish species, the acclimation capacity of P. caucana for CTmin was low, but it was average for CTmax. The data, together with similar work in other species, can be used in analyses focusing on broad ecological and evolutionary questions.

Cold-hardening during long-term acclimation in a freeze-tolerant woolly bear caterpillar, Pyrrharctia isabella

The banded woolly bear caterpillar, Pyrrharctia isabella (Lepidoptera: Erebidae), overwinters in leaf litter and survives freezing under natural conditions. Following 18 weeks of cold acclimation at 5°C, all caterpillars could survive 1 week of continuous freezing at -20°C or seven cycles of freezing-thawing at -20°C, but none survived freezing at -80°C. Field-collected caterpillars had a temperature of crystallization of -7.7±0.5°C that decreased significantly to -9.5±0.6°C after 12 weeks of acclimation at 5°C. Hemolymph levels of free proline, total amino acids and proteins reached a peak during the first 4 weeks of acclimation; concomitantly, hemolymph osmolality increased markedly during this interval (from 364 to 1282 mosmol kg(-1)). In contrast, hemolymph pH decreased during the first 4 weeks of acclimation before this trend reversed and pH values gradually returned to initial values. However, pH reached its peak value following 1 week at -20°C, but decreased after longer periods of freezing. During cold acclimation, cholesterol levels decreased in the hemolymph and the membrane fraction of fat body but not in other tissues. Lethal freezing at -80°C reduced cell survival in foregut tissue and caused leakage of free proline, total amino acids and proteins from tissues into the hemolymph. The addition of glycerol to the bathing medium reduced freezing injury in fat body cells, as evidenced by reduced leakage of amino acids and proteins.

Cryoprotectants and extreme freeze tolerance in a subarctic population of the wood frog

Wood frogs (Rana sylvatica) exhibit marked geographic variation in freeze tolerance, with subarctic populations tolerating experimental freezing to temperatures at least 10-13 degrees Celsius below the lethal limits for conspecifics from more temperate locales. We determined how seasonal responses enhance the cryoprotectant system in these northern frogs, and also investigated their physiological responses to somatic freezing at extreme temperatures. Alaskan frogs collected in late summer had plasma urea levels near 10 μmol ml-1, but this level rose during preparation for winter to 85.5 ± 2.9 μmol ml-1 (mean ± SEM) in frogs that remained fully hydrated, and to 186.9 ± 12.4 μmol ml-1 in frogs held under a restricted moisture regime. An osmolality gap indicated that the plasma of winter-conditioned frogs contained an as yet unidentified osmolyte(s) that contributed about 75 mOsmol kg-1 to total osmotic pressure. Experimental freezing to -8°C, either directly or following three cycles of freezing/thawing between -4 and 0°C, or -16°C increased the liver's synthesis of glucose and, to a lesser extent, urea. Concomitantly, organs shed up to one-half (skeletal muscle) or two-thirds (liver) of their water, with cryoprotectant in the remaining fluid reaching concentrations as high as 0.2 and 2.1 M, respectively. Freeze/thaw cycling, which was readily survived by winter-conditioned frogs, greatly increased hepatic glycogenolysis and delivery of glucose (but not urea) to skeletal muscle. We conclude that cryoprotectant accrual in anticipation of and in response to freezing have been greatly enhanced and contribute to extreme freeze tolerance in northern R. sylvatica.

Effect of repeated freeze-thaw cycles on geographically different populations of the freeze-tolerant worm Enchytraeus albidus (Oligochaeta)

Freeze-tolerant organisms survive internal ice formation; however, the adaptations to repeated freeze-thaw cycles are often not well investigated. Here we report how three geographically different populations of Enchytraeus albidus (Germany, Iceland and Svalbard) respond to three temperature treatments - constant thawed (0°C), constant freezing (-5°C) and fluctuating temperature (0 to -5°C) - over a period of 42 days. Survival varied between treatments and populations such that enchytraeids from arctic locations had a higher survival following prolonged freeze periods compared with temperate populations. However, enchytraeids from temperate locations had the same survival rate as arctic populations when exposed to repeated freeze-thaw events. Across all populations, metabolic rate decreased markedly in frozen animals (-5°C) compared with thawed controls (0°C). This decrease is likely due to the lower temperature of frozen animals, but also to the transition to the frozen state per se. Animals exposed to repeated freeze-thaw events had an intermediate metabolic rate and freeze-thaw events were not associated with pronounced excess energetic costs. Overwintering under either condition was not associated with a decrease in lipid content; however, during exposure to constant freezing and repeated freeze-thaw events there was a noticeable decrease in carbohydrate stores over time. Thus, animals exposed to constant freezing showed a decrease in glycogen stores, while both glucose and glycogen content decreased over time when the organisms were exposed to repeated freezing. The results therefore suggest that carbohydrate resources are important as a fuel for E. albidus during freezing whereas lipid resources are of marginal importance.

A conceptual framework for the emerging discipline of conservation physiology

Current rates of biodiversity decline are unprecedented and largely attributed to anthropogenic influences. Given the scope and magnitude of conservation issues, policy and management interventions must maximize efficiency and efficacy. The relatively new field of conservation physiology reveals the physiological mechanisms associated with population declines, animal-environment relationships and population or species tolerance thresholds, particularly where these relate to anthropogenic factors that necessitate conservation action. We propose a framework that demonstrates an integrative approach between physiology, conservation and policy, where each can inform the design, conduct and implementation of the other. Each junction of the conservation physiology process has the capacity to foster dialogue that contributes to effective implementation, monitoring, assessment and evaluation. This approach enables effective evaluation and implementation of evidence-based conservation policy and management decisions through a process of ongoing refinement, but may require that scientists (from the disciplines of both physiology and conservation) and policy-makers bridge interdisciplinary knowledge gaps. Here, we outline a conceptual framework that can guide and lead developments in conservation physiology, as well as promote innovative research that fosters conservation-motivated policy.

Importance of freeze-thaw events in low temperature ecotoxicology of cold tolerant enchytraeids

Due to global warming it is predicted that freeze-thaw cycles will increase in Arctic and cold temperate regions. The effects of this variation becomes of particular ecological importance to freeze-tolerant species when it is combined with chemical pollutants. We compared the effect of control temperature (2 °C), daily freeze-thaw cycles (2 to -4 °C) and constant freezing (-2 °C) temperatures on the cold-tolerance of oligochaete worms (Enchytraeus albidus) and tested how survival was influenced by pre-exposure to 4-nonylphenol (4-NP), a common nonionic detergent found in sewage sludge amended soils. Results showed that combined effect of 4-NP and daily freeze-thaw cycles can cause higher mortality to worms as compared with sustained freezing or control temperature. Exposure to 4-NP caused a substantial depletion of glycogen reserves which is catabolized during freezing to produce cryoprotective concentrations of free glucose. Further, exposure to freeze-thaw cycles resulted in higher concentrations of 4-NP in worm tissues as compared to constant freezing or control temperature (2 °C). Thus, worms exposed to combined effect of freeze-thaw cycles and 4-NP suffer higher consequences, with the toxic effect of the chemical potentiating the deleterious effects of freezing and thawing.

Repeated freezing induces oxidative stress and reduces survival in the freeze-tolerant goldenrod gall fly, Eurosta solidaginis

Freeze tolerant insects must not only survive extracellular ice formation but also the generation of reactive oxygen species (ROS) during oxygen reperfusion upon thawing. Furthermore, diurnal fluctuations in temperature place temperate insects at risk of being exposed to multiple freeze-thaw cycles, yet few studies have examined metrics of survival and oxidative stress in freeze-tolerant insects subjected to successive freezing events. To address this, we assessed survival in larvae of the goldenrod gall fly Eurosta solidaginis, after being subjected to 0, 5, 10, 20, or 30 diurnally repeated cold exposures (RCE) to -18°C or a single freeze to -18°C for 20days. In addition, we measured indicators of oxidative stress, levels of cryoprotectants, and total aqueous antioxidant capacity in animals exposed to the above treatments at 8, 32, or 80h after their final thaw. Repeated freezing and thawing, rather than time spent frozen, reduced survival as only 30% of larvae subjected to 20 or 30 RCE successfully pupated, compared to those subjected to fewer RCE or a single 20d freeze, of which 82% pupated. RCE had little effect on the concentration of the cryoprotectant glycerol (4.26±0.66μgglycerol·ngprotein(-1) for all treatments and time points) or sorbitol (18.8±2.9μgsorbitol·mgprotein(-1) for all treatments and time points); however, sorbitol concentrations were more than twofold higher than controls (16.3±2.2μgsorbitol·mgprotein(-1)) initially after a thaw in larvae subjected to a single extended freeze, but levels returned to values similar to controls at 80h after thaw. Thawing likely produced ROS as total aqueous antioxidant capacities peaked at 1.8-fold higher than controls (14.7±1.6mmoltrolox·ngprotein(-1)) in animals exposed to 5, 10, or 20 RCE. By contrast, aqueous antioxidant capacities were similar to controls in larvae subjected to 30 RCE or the single 20d freeze regardless of time post final thaw, indicating these animals may have had an impaired ability to produce primary antioxidants. Larvae lacking an antioxidant response also had elevated levels of oxidized proteins, nearly twice that of controls (21.8±3.2mmolchloramine-T·mgprotein(-1)). Repeated freezing also lead to substantial oxidative damage to lipids that was independent of aqueous antioxidant capacity; peroxides were, on average, 5.6-fold higher in larvae subjected to 10, 20 or 30 RCE compared to controls (29.1±7.3mmolTMOP·μgprotein(-1)). These data suggest that oxidative stress due to repeated freeze-thaw cycles reduces the capacity of E. solidaginis larvae to survive freezing.

Response to cold acclimation in diapause pupae of Hyles euphorbiae (Lepidoptera: Sphingidae): candidate biomarker identification using proteomics

The distribution range of Hyles euphorbiae covers distinct climates across the Palaearctic. Previous investigations showed a correlation between mitochondrial DNA identity of populations and climatic conditions related to winter; however, the lack of biomarkers hampers investigations to test whether geographically distinct populations do show specific molecular responses to low temperatures or whether they possess specific genetic identity at loci functionally related to cold response. The present study was designed to identify candidate protein biomarkers and biological processes that are associated with cold acclimation of overwintering H. euphorbiae diapause pupae. Specimens taken from a single central European population were gradually cooled from 20 °C to -2 °C over 36 days and 12 differentially abundant proteins were identified. In addition, DeepSuperSAGE sequencing technology was applied to study differentially regulated genes. There was incongruence between differentially abundant proteins and differentially expressed genes, but functional characteristics of regulated proteins and analyses of gene ontology term enrichment among differentially regulated genes pointed to activation of the same biological processes, e.g. oxidative stress response. As proteins represent biologically active molecules, candidate biomarkers derived from proteomics are considered well suited to explore intraspecific patterns of local adaptation to different climates.

Physiological responses of Corythucha ciliata adults to high temperatures under laboratory and field conditions

Under high temperature conditions, insects can tolerate to survive through various physiological mechanisms, which have been well documented in laboratory studies. However, it is still unclear as to whether these laboratory data can scale up to those in the field. Here we studied dynamics of heat-induced metabolites in Corythucha ciliata adults under both laboratory and field conditions to examine their significance in thermal tolerance of the species. We compared the effects of controlled thermal treatments (2h at 33-43 °C at 2 °C intervals in the laboratory) and naturally increasing thermal conditions (10:00-14:00 at 2-h intervals (33.5-37.2 °C) on a hot summer day in a field in Shanghai, China) on water content and levels of water-soluble protein, triglycerides, mannitol, and sorbitol in the adult bodies. The results showed that water content significantly decreased and all other metabolic parameters significantly increased in response to temperature stresses with similar patterns in both the laboratory and field, although the respective threshold temperatures were different under the two conditions. The close linkage observed in the two conditions suggests that a short period of heat stress induces water loss and accumulation of thermal metabolites in C. ciliata adults. This heat-resistance provides a defense mechanism counteracting thermal damage in C. ciliata.

Cold truths: how winter drives responses of terrestrial organisms to climate change

Winter is a key driver of individual performance, community composition, and ecological interactions in terrestrial habitats. Although climate change research tends to focus on performance in the growing season, climate change is also modifying winter conditions rapidly. Changes to winter temperatures, the variability of winter conditions, and winter snow cover can interact to induce cold injury, alter energy and water balance, advance or retard phenology, and modify community interactions. Species vary in their susceptibility to these winter drivers, hampering efforts to predict biological responses to climate change. Existing frameworks for predicting the impacts of climate change do not incorporate the complexity of organismal responses to winter. Here, we synthesise organismal responses to winter climate change, and use this synthesis to build a framework to predict exposure and sensitivity to negative impacts. This framework can be used to estimate the vulnerability of species to winter climate change. We describe the importance of relationships between winter conditions and performance during the growing season in determining fitness, and demonstrate how summer and winter processes are linked. Incorporating winter into current models will require concerted effort from theoreticians and empiricists, and the expansion of current growing-season studies to incorporate winter.

The impacts of repeated cold exposure on insects

Insects experience repeated cold exposure (RCE) on multiple time scales in natural environments, yet the majority of studies of the effects of cold on insects involve only a single exposure. Three broad groups of experimental designs have been employed to examine the effects of RCE on insect physiology and fitness, defined by the control treatments: 'RCE vs cold', which compares RCE with constant cold conditions; 'RCE vs warm', which compares RCE with constant warm conditions; and 'RCE vs matched cold' which compares RCE with a prolonged period of cold matched by time to the RCE condition. RCE are generally beneficial to immediate survival, and increase cold hardiness relative to insects receiving a single prolonged cold exposure. However, the effects of RCE depend on the study design, and RCE vs warm studies cannot differentiate between the effects of cold exposure in general vs RCE in particular. Recent studies of gene transcription, immune function, feeding and reproductive output show that the responses of insects to RCE are distinct from the responses to single cold exposures. We suggest that future research should attempt to elucidate the mechanistic link between physiological responses and fitness parameters. We also recommend that future RCE experiments match the time spent at the stressful low temperature in all experimental groups, include age controls where appropriate, incorporate a pilot study to determine time and intensity of exposure, and measure sub-lethal impacts on fitness.

Short-term anoxic conditioning hormesis boosts antioxidant defenses, lowers oxidative damage following irradiation and enhances male sexual performance in the Caribbean fruit fly, Anastrepha suspensa

Most organisms are repeatedly exposed to oxidative stress from multiple sources throughout their lifetimes, potentially affecting all aspects of organismal performance. Here we test whether exposure to a conditioning bout of anoxia early in adulthood induces a hormetic response that confers resistance to oxidative stress and enhances male sexual performance later in life in the Caribbean fruit fly, Anastrepha suspensa. Anoxic conditioning of adults prior to emergence led to an increase in antioxidant capacity driven by mitochondrial superoxide dismutase and glutathione peroxidase. When exposed to gamma irradiation, a strong oxidative stressor, males that received anoxic conditioning had lower lipid and protein oxidative damage at sexual maturity. Anoxia conditioning led to greater male sexual competitiveness compared with unconditioned males when both were irradiated, although there was no effect of anoxia conditioning on mating competitiveness in unirradiated males. Anoxia also led to higher adult emergence rates and greater flight ability in irradiation-stressed flies while preserving sterility. Thus, hormetic treatments that increased antioxidant enzyme activity also improved male performance after irradiation, suggesting that antioxidant enzymes play an important role in mediating the relationship between oxidative stress and sexual selection. Furthermore, our work has important applied implications for the sterile insect technique (SIT), an environmentally friendly method of insect pest control where males are sterilized by irradiation and deployed in the field to disrupt pest populations via mating. We suggest that hormetic treatments specifically designed to enhance antioxidant activity may produce more sexually competitive sterile males, thus improving the efficacy and economy of SIT programs.

Energetic consequences of repeated and prolonged dehydration in the Antarctic midge, Belgica antarctica

Larvae of the Antarctic midge, Belgica antarctica, routinely face periods of limited water availability in their natural environments on the Antarctic Peninsula. As a result, B. antarctica is one of the most dehydration-tolerant insects studied, surviving up to 70% loss of its body water. While previous studies have characterized the physiological effects of a single bout of dehydration, in nature larvae are likely to experience multiple bouts of dehydration throughout their lifetime. Thus, we examined the physiological consequences of repeated dehydration and compared results to larvae exposed to a single, prolonged period of dehydration. For the repeated dehydration experiment, larvae were exposed to 1-5 cycles of 24 h dehydration at 75% RH followed by 24 h rehydration. Each bout of dehydration resulted in 30-40% loss of body water, with a concomitant 2- to 3-fold increase in body fluid osmolality. While nearly 100% of larvae survived a single bout of dehydration, <65% of larvae survived five such cycles. Larvae subjected to multiple bouts of dehydration also experienced severe depletion of carbohydrate energy reserves; glycogen and trehalose content decreased with each successive cycle, with larvae losing 89% and 48% of their glycogen and trehalose, respectively, after five cycles of dehydration/rehydration. Larvae exposed to prolonged dehydration (99% RH for 10d) had 26% less water, 43% less glycogen, and 27% less lipid content than controls, but did not experience any mortality. Thus, both repeated and prolonged dehydration results in substantial energetic costs that are likely to negatively impact fitness.

Threshold temperatures mediate the impact of reduced snow cover on overwintering freeze-tolerant caterpillars

Decreases in snow cover due to climate change could alter the energetics and physiology of ectothermic animals that overwinter beneath snow, yet how snow cover interacts with physiological thresholds is unknown. We applied numerical simulation of overwintering metabolic rates coupled with field validation to determine the importance of snow cover and freezing to the overwintering lipid consumption of the freeze-tolerant Arctiid caterpillar Pyrrharctia isabella. Caterpillars that overwintered above the snow experienced mean temperatures 1.3°C lower than those below snow and consumed 18.36 mg less lipid of a total 68.97-mg reserve. Simulations showed that linear temperature effects on metabolic rate accounted for only 30% of the difference in lipid consumption. When metabolic suppression by freezing was included, 93% of the difference between animals that overwintered above and below snow was explained. Our results were robust to differences in temperature sensitivity of metabolic rate, changes in freezing point, and the magnitude of metabolic suppression by freezing. The majority of the energy savings was caused by the non-continuous reduction in metabolic rate due to freezing, the first example of the importance of temperature thresholds in the lipid use of overwintering insects.

Divergent transcriptomic responses to repeated and single cold exposures in Drosophila melanogaster

Insects in the field are exposed to multiple bouts of cold, and there is increasing evidence that the fitness consequences of repeated cold exposure differ from the impacts of a single cold exposure. We tested the hypothesis that different kinds of cold exposure (in this case, single short, prolonged and repeated cold exposure) would result in differential gene expression. We exposed 3 day old adult female wild-type Drosophila melanogaster (Diptera: Drosophilidae) to –0.5°C for a single 2 h exposure, a single 10 h exposure, or five 2 h exposures on consecutive days, and extracted RNA after 6 h of recovery. Global gene expression was quantified using an oligonucleotide microarray and validated with real-time PCR using different biological replicates. We identified 76 genes upregulated in response to multiple cold exposure, 69 in response to prolonged cold exposure and 20 genes upregulated in response to a single short cold exposure, with a small amount of overlap between treatments. Three genes – Turandot A, Hephaestus and CG11374 – were upregulated in response to all three cold exposure treatments. Key functional groups upregulated include genes associated with muscle structure and function, the immune response, stress response, carbohydrate metabolism and egg production. We conclude that cold exposure has wide-ranging effects on gene expression in D. melanogaster and that increased duration or frequency of cold exposure has impacts different to those of a single short cold exposure. This has important implications for extrapolating laboratory studies of insect overwintering that are based on only a single cold exposure.