The central nervous system and muscular system play different roles for chill coma onset and recovery in insects

The freeze-avoiding mountain pine beetle (Dendroctonus ponderosae) survives prolonged exposure to stressful cold by mitigating ionoregulatory collapse

Insect performance is linked to environmental temperature, and surviving through winter represents a key challenge for temperate, alpine and polar species. To overwinter, insects have adapted a range of strategies to become truly cold hardy. However, although the mechanisms underlying the ability to avoid or tolerate freezing have been well studied, little attention has been given to the challenge of maintaining ion homeostasis at frigid temperatures in these species, despite this limiting cold tolerance for insects susceptible to mild chilling. Here, we investigated how prolonged exposure to temperatures just above the supercooling point affects ion balance in freeze-avoidant mountain pine beetle (Dendroctonus ponderosae) larvae in autumn, mid-winter and spring, and related it to organismal recovery times and survival. Hemolymph ion balance was gradually disrupted during the first day of exposure, characterized by hyperkalemia and hyponatremia, after which a plateau was reached and maintained for the rest of the 7-day experiment. The degree of ionoregulatory collapse correlated strongly with recovery times, which followed a similar asymptotical progression. Mortality increased slightly during extensive cold exposures, where hemolymph K+ concentration was highest, and a sigmoidal relationship was found between survival and hyperkalemia. Thus, the cold tolerance of the freeze-avoiding larvae of D. ponderosae appears limited by the ability to prevent ionoregulatory collapse in a manner similar to that of chill-susceptible insects, albeit at much lower temperatures. Based on these results, we propose that a prerequisite for the evolution of insect freeze avoidance may be a convergent or ancestral ability to maintain ion homeostasis during extreme cold stress.

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.

Rapid cold hardening modifies ion regulation to delay anoxia-induced spreading depolarization in the CNS of the locust

Insects experience different kinds of environmental stresses that can impair neural performance, leading to spreading depolarization (SD) of nerve cells and neural shutdown underlying coma. SD is associated with a sudden loss of ion, notably K+, homeostasis in the central nervous system. The sensitivity of an insect's nervous system to stress (e.g., anoxia) can be modulated by acute pre-treatment. Rapid cold hardening (RCH) is a form of preconditioning, in which a brief exposure to low temperature can enhance the stress tolerance of insects. We used a pharmacological approach to investigate whether RCH affects anoxia-induced SD in the locust, Locusta migratoria, via one or more of the following homeostatic mechanisms: (1) Na+/K+-ATPase (NKA), (2) Na+/K+/2Cl- co-transporter (NKCC), and (3) voltage-gated K+ (Kv) channels. We also assessed abundance and phosphorylation of NKCC using immunoblotting. We found that inhibition of NKA or Kv channels delayed the onset of anoxia-induced SD in both control and RCH preparations. However, NKCC inhibition preferentially abrogated the effect of RCH. Additionally, we observed a higher abundance of NKCC in RCH preps but no statistical difference in its phosphorylation level, indicating the involvement of NKCC expression or degradation as part of the RCH mechanism.

A neurophysiological limit and its biogeographic correlations: cold-induced spreading depolarization in tropical butterflies

The physiology of insects is directly influenced by environmental temperature, and thermal tolerance is therefore intrinsically linked to their thermal niche and distribution. Understanding the mechanisms that limit insect thermal tolerance is crucial to predicting biogeography and range shifts. Recent studies on locusts and flies suggest that the critical thermal minimum (CTmin) follows a loss of CNS function via a spreading depolarization. We hypothesized that other insect taxa share this phenomenon. Here, we investigate whether spreading depolarization events occur in butterflies exposed to cold. Supporting our hypothesis, we found that exposure to stressful cold induced spreading depolarization in all 12 species tested. This reinforces the idea that spreading depolarization is a common mechanism underlying the insect CTmin. Furthermore, our results highlight how CNS function is tuned to match the environment of a species. Further research into the physiology underlying spreading depolarization will likely elucidate key mechanisms determining insect thermal tolerance and ecology.

Naturally segregating genetic variants contribute to thermal tolerance in a D. melanogaster model system

Thermal tolerance is a fundamental physiological complex trait for survival in many species. For example, everyday tasks such as foraging, finding a mate, and avoiding predation, are highly dependent on how well an organism can tolerate extreme temperatures. Understanding the general architecture of the natural variants of the genes that control this trait is of high importance if we want to better comprehend how this trait evolves in natural populations. Here, we take a multipronged approach to further dissect the genetic architecture that controls thermal tolerance in natural populations using the Drosophila Synthetic Population Resource (DSPR) as a model system. First, we used quantitative genetics and Quantitative Trait Loci (QTL) mapping to identify major effect regions within the genome that influences thermal tolerance, then integrated RNA-sequencing to identify differences in gene expression, and lastly, we used the RNAi system to 1) alter tissue-specific gene expression and 2) functionally validate our findings. This powerful integration of approaches not only allows for the identification of the genetic basis of thermal tolerance but also the physiology of thermal tolerance in a natural population, which ultimately elucidates thermal tolerance through a fitness-associated lens.

A cold and quiet brain: mechanisms of insect CNS arrest at low temperatures

Exposure to cold causes insects to enter a chill coma at species-specific temperatures and such temperature sensitivity contributes to geographic distribution and phenology. Coma results from abrupt spreading depolarization (SD) of neural tissue in the integrative centers of the CNS. SD abolishes neuronal signaling and the operation of neural circuits, like an off switch for the CNS. Turning off the CNS by allowing ion gradients to collapse will conserve energy and may offset negative consequences of temporary immobility. SD is modified by prior experience via rapid cold hardening (RCH) or cold acclimation which alter properties of K_v channels, Na⁺/K⁺-ATPase and Na⁺/K⁺/2Cl^- cotransporter. The stress hormone octopamine mediates RCH. Future progress depends on developing a more complete understanding of ion homeostasis in and of the insect CNS.

Plasticity in Na+/K+-ATPase thermal kinetics drives variation in the temperature of cold-induced neural shutdown of adult Drosophila melanogaster

Most insects can acclimate to changes in their thermal environment and counteract temperature effects on neuromuscular function. At the critical thermal minimum, a spreading depolarization (SD) event silences central neurons, but the temperature at which this event occurs can be altered through acclimation. SD is triggered by an inability to maintain ion homeostasis in the extracellular space in the brain and is characterized by a rapid surge in extracellular K+ concentration, implicating ion pump and channel function. Here, we focused on the role of the Na+/K+-ATPase specifically in lowering the SD temperature in cold-acclimated Drosophila melanogaster. After first confirming cold acclimation altered SD onset, we investigated the dependency of the SD event on Na+/K+-ATPase activity by injecting the inhibitor ouabain into the head of the flies to induce SD over a range of temperatures. Latency to SD followed the pattern of a thermal performance curve, but cold acclimation resulted in a left-shift of the curve to an extent similar to its effect on the SD temperature. With Na+/K+-ATPase activity assays and immunoblots, we found that cold-acclimated flies have ion pumps that are less sensitive to temperature, but do not differ in their overall abundance in the brain. Combined, these findings suggest a key role for plasticity in Na+/K+-ATPase thermal sensitivity in maintaining central nervous system function in the cold, and more broadly highlight that a single ion pump can be an important determinant of whether insects can respond to their environment to remain active at low temperatures.

Chill coma recovery of Ceratitis capitata adults across the Northern Hemisphere

The Mediterranean fruit fly, Ceratitis capitata (Diptera: Tephritidae), is an invasive pest, that is currently expanding its geographic distribution from the Mediterranean coasts to more temperate areas of Europe. Given that low temperature is a primary determinant of insect species' range boundaries especially in the Northern Hemisphere with pronounced seasonality, we used chill coma recovery time for assessing latitudinal clines in basal chill tolerance of C. capitata adults. We selected six populations obtained from areas with broad climatic variability based on the main bioclimatic variables of temperature and precipitation, spanning a latitudinal range of about 19° from Middle East to Central Europe. Adults were exposed to 0 °C for 4 h, and time to regain the typical standing position of a fly at 25 °C were recorded. The post-stress survival after a period of 8 days was also recorded. Results revealed that adults from Israel and Austria were less chill tolerant than those from Greece, resulting in curvilinear trends with latitude. Analysis of macroclimatic conditions revealed combined effects of latitude (as a proxy of photoperiod) and macroclimatic conditions on chill coma recovery time. Nonetheless, there was not a deleterious effect on post-recovery survival, except for flies obtained from the northern most point (Vienna, Austria). Overall, it seems that evolutionary patterns of basal chill coma recovery time of C. capitata adults are driven mainly by local climatic variability.

Temperature and livestock grazing trigger transcriptome responses in bumblebees along an elevational gradient

Climate and land-use changes cause increasing stress to pollinators but the molecular pathways underlying stress responses are poorly understood. Here, we analyzed the transcriptomic response of Bombus lucorum workers to temperature and livestock grazing. Bumblebees sampled along an elevational gradient, and from differently managed grassland sites (livestock grazing vs unmanaged) in the German Alps did not differ in the expression of genes known for thermal stress responses. Instead, metabolic energy production pathways were upregulated in bumblebees sampled in mid- or high elevations or during cool temperatures. Extensive grazing pressure led to an upregulation of genetic pathways involved in immunoregulation and DNA-repair. We conclude that widespread bumblebees are tolerant toward temperature fluctuations in temperate mountain environments. Moderate temperature increases may even release bumblebees from metabolic stress. However, transcriptome responses to even moderate management regimes highlight the completely underestimated complexity of human influence on natural pollinators.

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Upregulation of energy metabolism pathways in Bombus lucorum with increasing elevation

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Genes known for thermal stress responses did not change with increased elevation

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Bombus lucorum are tolerant toward relatively broad temperature fluctuations

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Grazing lead to an upregulation in genetic information processes in B. lucorum

Rapid cold hardening increases axonal Na+/K+-ATPase activity and enhances performance of a visual motion detection circuit in Locusta migratoria

Rapid cold hardening (RCH) is a type of phenotypic plasticity that delays the occurrence of chill coma in insects. Chill coma is mediated by a spreading depolarization of neurons and glia in the CNS, triggered by a failure of ion homeostasis. We used biochemical and electrophysiological approaches in the locust, Locusta migratoria, to test the hypothesis that the protection afforded by RCH is mediated by activation of the Na+/K+-ATPase (NKA) in neural tissue. RCH did not affect NKA activity measured in a biochemical assay of homogenized thoracic ganglia. However, RCH hyperpolarized the axon of a visual interneuron (DCMD) and increased the amplitude of an activity-dependent hyperpolarization (ADH) shown previously to be blocked by ouabain. RCH also improved performance of the visual circuitry presynaptic to DCMD to minimize habituation and increase excitability. We conclude that RCH enhances in situ NKA activity in the nervous system but also affects other neuronal properties that promote visual processing in locusts.

Rapid cold hardening delays the onset of anoxia-induced coma via an octopaminergic pathway in Locusta migratoria

Rapid cold hardening (RCH) is a short-term hormesis that occurs in many invertebrate species, especially in insects. Although RCH is best known as enhancing cold tolerance, it can also enhance anoxic tolerance. When exposed to prolonged anoxia, insects enter a reversible coma, which is associated with spreading depolarization (SD) in the central nervous system (CNS). In this study, we investigated the effects of RCH and octopamine (OA) on anoxia-induced SD in L. migratoria. OA is an insect stress hormone that has roles in many physiological processes. Thus, we hypothesized that OA is involved in the mechanism of RCH. First, we found that RCH affects the K⁺ sensitivity of the locust blood brain barrier (BBB) in a way similar to the previously described effects of OA. Next, using SD as an indicator of anoxia-induced coma, we took a pharmacological approach to investigate the effects of OA and epinastine (EP), an octopaminergic receptor (OctR) antagonist. We found that OA mimics, whereas EP blocks, the effect of RCH on anoxia-induced SD. This study demonstrates that OA is involved in the mechanism of RCH in delaying the onset of anoxia-induced locust coma and contributes to determining the mechanism of RCH that modulates insect stress tolerances.

A lack of repeatability creates the illusion of a trade-off between basal and plastic cold tolerance

The thermotolerance-plasticity trade-off hypothesis predicts that ectotherms with greater basal thermal tolerance have a lower acclimation capacity. This hypothesis has been tested at both high and low temperatures but the results often conflict. If basal tolerance constrains plasticity (e.g. through shared mechanisms that create physiological constraints), it should be evident at the level of the individual, provided the trait measured is repeatable. Here, we used chill-coma onset temperature and chill-coma recovery time (CCO and CCRT; non-lethal thermal limits) to quantify cold tolerance of Drosophila melanogaster across two trials (pre- and post-acclimation). Cold acclimation improved cold tolerance, as expected, but individual measurements of CCO and CCRT in non-acclimated flies were not (or only slightly) repeatable. Surprisingly, however, there was still a strong correlation between basal tolerance and plasticity in cold-acclimated flies. We argue that this relationship is a statistical artefact (specifically, a manifestation of regression to the mean; RTM) and does not reflect a true trade-off or physiological constraint. Thermal tolerance trade-off patterns in previous studies that used similar methodology are thus likely to be impacted by RTM. Moving forward, controlling and/or correcting for RTM effects is critical to determining whether such a trade-off or physiological constraint exists.

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).

Osmoregulatory capacity at low temperature is critical for insect cold tolerance

At low temperature many insects lose extracellular ion homeostasis and the capacity to mitigate homeostatic imbalance determines their cold tolerance. Extracellular homeostasis is ensured by the osmoregulatory organs and recent research has emphasized key roles for the Malpighian tubules and hindgut in modulating insect cold tolerance. Here, we review the effects of low temperature on transport capacity of osmoregulatory organs and outline physiological processes leading from cold exposure to disruption of ion homeostasis and cold-injury in insects. We show how cold adaptation and cold acclimation are associated with physiological modifications to transport capacity in Malpighian tubules and hindgut. These responses mitigate loss of homeostasis and we highlight how further study of molecular and cellular mechanisms are critical to fully appreciate the adaptations that facilitate insect cold tolerance.

Effect of a single cold stress exposure on the reproductive behavior of male crickets

Cold stress is an important abiotic factor that can impact insect physiology, behavior, and overall fitness. Upon exposure to cold temperature, many insects enter a reversible state of immobility called chill coma. If the cold stress is brief and mild enough, insects can recover and regain full mobility upon return to warmer temperatures. However, the long-term impact of sublethal cold stress on insect behavior has been understudied. Here, sexually naïve adult male Acheta domesticus crickets were exposed to a single 0 °C cold stress for 6 h. One week later, the ability of these males to mate with a female was examined. For mating trials, a cold stressed male cricket was paired with a non-cold stressed, control female. Control pairs were comprised of a non-cold stressed control male and control female. Cold exposed males were less successful at mating than control males because most did not carry a spermatophore at the time of their mating trials. However, when these cold stressed males were allowed 1 h of chemosensory contact with a female, most produced a spermatophore. Males that produced spermatophores were given the opportunity to mate once with a female, and stressed males that successfully mated sired as many offspring as did control males. However, our results support that a single cold stress exposure can negatively impact the reproductive fitness of male crickets since it reduced their capacity to carry spermatophores and, as a consequence, to attract females.

Body mass and sex, not local climate, drive differences in chill coma recovery times in common garden reared bumble bees

The time required to recover from cold exposure (chill coma recovery time) may represent an important metric of performance and has been linked to geographic distributions of diverse species. Chill coma recovery time (CCRT) has rarely been measured in bumble bees (genus Bombus) but may provide insights regarding recent changes in their distributions. We measured CCRT of Bombus vosnesenskii workers reared in common garden laboratory conditions from queens collected across altitude and latitude in the Western United States. We also compared CCRTs of male and female bumble bees because males are often overlooked in studies of bumble bee ecology and physiology and may differ in their ability to respond to cold temperatures. We found no relationship between CCRT and local climate at the queen collection sites, but CCRT varied significantly with sex and body mass. Because differences in the ability to recover from cold temperatures have been shown in wild-caught Bombus, we predict that variability in CCRT may be strongly influenced by plasticity.

Sestrin regulates acute chill coma recovery in Drosophila melanogaster

When chill-susceptible insects are exposed to low temperatures they enter a temporary state of paralysis referred to as a chill coma. The most well-studied physiological mechanism of chill coma onset and recovery involves regulation of ion homeostasis. Previous studies show that changes in metabolism may also underlie the ability to recovery quickly, but the roles of genes that regulate metabolic homeostasis in chill coma recovery time (CCRT) are not well understood. Here, we investigate the roles of Sestrin and Spargel (Drosophila homolog of PGC-1α), which are involved in metabolic homeostasis and substrate oxidation, on CCRT in Drosophila melanogaster. We find that sestrin and spargel mutants have impaired CCRT. sestrin is required in the muscle and nervous system tissue for normal CCRT and spargel is required in muscle and adipose. On the basis that exercise induces sestrin and spargel, we also test the interaction of cold and exercise. We find that pre-treatment with one of these stressors does not consistently confer acute protection against the other. We conclude that Sestrin and Spargel are important in the chill coma response, independent of their role in exercise.

Divergent metabolomic profiles of cold-exposed mature and immature females of tropical versus temperate Drosophila species

Temperate species, contrary to their tropical counterparts, are exposed not only to thermally variable environments with low temperatures but also to long winters. Different selective pressures may have driven divergent physiological adaptations in closely related species with different biogeographic origins. To survive unfavourable winter conditions, Drosophila species in temperate areas generally undergo a period of reproductive dormancy, associated with a cold-induced cessation of oogenesis and metabolic reorganization. This work aims to compare cold tolerance and metabolic signatures of cold-exposed females exhibiting different reproductive maturity status (mature and immature females) of four Drosophila species from tropical vs. temperate origins. We expected that the capacity for delayed reproduction of immature females could result in the redirection of the energy-related metabolites to be utilized for surviving the cold season. To do so, we studied an array of 45 metabolites using quantitative target GC-MS profiling. Reproductively immature females of temperate species showed the lower CT_min and the faster chill coma recovery time (i.e. the most cold-tolerant group). Principal component analysis captured differences across species, but also between reproductive maturity states. Notably, temperate species exhibited significantly higher levels of glucose, alanine, and gluconolactone than tropical ones. As proline and glycerol showed higher abundances in immature females of temperate species compared to the levels exhibited by the rest of the groups, we reasoned that glucose and alanine could serve as intermediates in the synthesis of these compatible solutes. All in all, our findings suggest that cold-exposed females of temperate species accumulate energy-related and protective metabolites (e.g. glycerol and proline) while delaying reproduction, and that these metabolites are relevant to cold tolerance even at modest concentrations.

Chill coma onset and recovery fail to reveal true variation in thermal performance among populations of Drosophila melanogaster

Species from colder climates tend to be more chill tolerant regardless of the chill tolerance trait measured, but for Drosophila melanogaster, population-level differences in chill tolerance among populations are not always found when a single trait is measured in the laboratory. We measured chill coma onset temperature, chill coma recovery time, and survival after chronic cold exposure in replicate lines derived from multiple paired African and European D. melanogaster populations. The populations in our study were previously found to differ in chronic cold survival ability, which is believed to have evolved independently in each population pair; however, they did not differ in chill coma onset temperature and chill coma recovery time in a manner that reflected their geographic origins, even though these traits are known to vary with origin latitude among Drosophila species and are among the most common metrics of thermal tolerance in insects. While it is common practice to measure only one chill tolerance trait when comparing chill tolerance among insect populations, our results emphasise the importance of measuring more than one thermal tolerance trait to minimize the risk of missing real adaptive variation in insect thermal tolerance.

Quantitative model analysis of the resting membrane potential in insect skeletal muscle: Implications for low temperature tolerance

Abiotic stressors, such as cold exposure, can depolarize insect cells substantially causing cold coma and cell death. During cold exposure, insect skeletal muscle depolarization occurs through a 2-stage process. Firstly, short-term cold exposure reduces the activity of electrogenic ion pumps, which depolarize insect muscle markedly. Secondly, during long-term cold exposure, extracellular ion homeostasis is disrupted causing further depolarization. Consequently, many cold hardy insects improve membrane potential stability during cold exposure through adaptations that secure maintenance of ion homeostasis during cold exposure. Less is known about the adaptations permitting cold hardy insects to maintain membrane potential stability during the initial phase of cold exposure, before ion balance is disrupted. To address this problem it is critical to understand the membrane components (channels and transporters) that determine the membrane potential and to examine this question the present study constructed a mathematical "charge difference" model of the insect muscle membrane potential. This model was parameterized with known literature values for ion permeabilities, ion concentrations and membrane capacitance and the model was then further developed by comparing model predictions against empirical measurements following pharmacological inhibitors of the Na⁺/K⁺ ATPase, Cl^- channels and symporters. Subsequently, we compared simulated and recorded membrane potentials at 0 and 31 °C and at 10-50 mM extracellular [K⁺] to examine if the model could describe membrane potentials during the perturbations occurring during cold exposure. Our results confirm the importance of both Na⁺/K⁺ ATPase activity and ion-selective Na⁺, K⁺ and Cl^- channels, but the model also highlights that additional electroneutral flux of Na⁺ and K⁺ is needed to describe how membrane potentials respond to temperature and [K⁺] in insect muscle. While considerable further work is still needed, we argue that this "charge difference" model can be used to generate testable hypotheses of how insects can preserve membrane polarization in the face of stressful cold exposure.

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.

Thermal acclimation alters Na+/K+-ATPase activity in a tissue-specific manner in Drosophila melanogaster

Insects, like the model species Drosophila melanogaster, lose neuromuscular function and enter a state of paralysis (chill coma) at a population- and species-specific low temperature threshold that is decreased by cold acclimation. Entry into this coma is related to a spreading depolarization in the central nervous system, while recovery involves restoration of electrochemical gradients across muscle cell membranes. The Na⁺/K⁺-ATPase helps maintain ion balance and membrane potential in both the brain and hemolymph (surrounding muscles), and changes in thermal tolerance traits have therefore been hypothesized to be closely linked to variation in the expression and/or activity of this pump in multiple tissues. Here, we tested this hypothesis by measuring activity and thermal sensitivity of the Na⁺/K⁺-ATPase at the tagma-specific level (head, thorax and abdomen) in warm- (25 °C) and cold-acclimated (15 °C) flies by measuring Na⁺/K⁺-ATPase activity at 15, 20, and 25 °C. We relate differences in pump activity to differences in chill coma temperature, spreading depolarization temperature, and thermal dependence of muscle cell polarization. Differences in pump activity and thermal sensitivity induced by cold acclimation varied in a tissue-specific manner: While thermal sensitivity remained unchanged, cold-acclimated flies had decreased Na⁺/K⁺-ATPase activity in the thorax (mainly muscle) and head (mainly composed of brain). We argue that these changes may assist in maintenance of K⁺ homeostasis and membrane potential across muscle membranes, and discuss how reduced Na⁺/K⁺-ATPase activity in the brain may counterintuitively help insects delay coma onset in the cold.

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.

Biogeographic parallels in thermal tolerance and gene expression variation under temperature stress in a widespread bumble bee

Global temperature changes have emphasized the need to understand how species adapt to thermal stress across their ranges. Genetic mechanisms may contribute to variation in thermal tolerance, providing evidence for how organisms adapt to local environments. We determine physiological thermal limits and characterize genome-wide transcriptional changes at these limits in bumble bees using laboratory-reared Bombus vosnesenskii workers. We analyze bees reared from latitudinal (35.7-45.7°N) and altitudinal (7-2154 m) extremes of the species' range to correlate thermal tolerance and gene expression among populations from different climates. We find that critical thermal minima (CTMIN) exhibit strong associations with local minimums at the location of queen origin, while critical thermal maximum (CTMAX) was invariant among populations. Concordant patterns are apparent in gene expression data, with regional differentiation following cold exposure, and expression shifts invariant among populations under high temperatures. Furthermore, we identify several modules of co-expressed genes that tightly correlate with critical thermal limits and temperature at the region of origin. Our results reveal that local adaptation in thermal limits and gene expression may facilitate cold tolerance across a species range, whereas high temperature responses are likely constrained, both of which may have implications for climate change responses of bumble bees.

Effects of brief chilling and desiccation on ion homeostasis in the central nervous system of the migratory locust, Locusta migratoria

In insects, chilling, anoxia, and dehydration are cues to trigger rapid physiological responses enhancing stress tolerance within minutes. Recent evidence suggests that responses elicited by different cues are mechanistically distinct from each other, though these differences have received little attention. Further, the effects are not well studied in neural tissue. In this study, we examined how brief exposure to desiccation and chilling affect ion homeostatic mechanisms in metathoracic ganglion of the migratory locust, Locusta migratoria. Both desiccation and chilling enhanced resistance to anoxia, though only chilling hastened recovery from anoxic coma. Similarly, only chilling enhanced resistance to pharmacological perturbation of neuronal ion homeostasis. Our results indicate that chilling and desiccation trigger mechanistically distinct responses and, while both may be important for neuronal ion homeostasis, chilling has a larger effect on this tissue. SUMMARY STATEMENT: This is one of few studies to demonstrate the importance of the central nervous system in rapid acclimatory responses in insects.

Neural dysfunction correlates with heat coma and CTmax in Drosophila but does not set the boundaries for heat stress survival

When heated, insects lose coordinated movement followed by the onset of heat coma (critical thermal maximum, CTmax). These traits are popular measures to quantify interspecific and intraspecific differences in insect heat tolerance, and CTmax correlates well with current species distributions of insects, including Drosophila Here, we examined the function of the central nervous system (CNS) in five species of Drosophila with different heat tolerances, while they were exposed to either constant high temperature or a gradually increasing temperature (ramp). Tolerant species were able to preserve CNS function at higher temperatures and for longer durations than sensitive species, and similar differences were found for the behavioural indices (loss of coordination and onset of heat coma). Furthermore, the timing and temperature (constant and ramp exposure, respectively) for loss of coordination or complete coma coincided with the occurrence of spreading depolarisation (SD) events in the CNS. These SD events disrupt neurological function and silence the CNS, suggesting that CNS failure is the primary cause of impaired coordination and heat coma. Heat mortality occurs soon after heat coma in insects; to examine whether CNS failure could also be the proximal cause of heat death, we used selective heating of the head (CNS) and abdomen (visceral tissues). When comparing the temperature causing 50% mortality (LT50) of each body part versus that of the whole animal, we found that the head was not particularly heat sensitive compared with the abdomen. Accordingly, it is unlikely that nervous failure is the principal/proximate cause of heat mortality in Drosophila.

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

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

Intraspecific variation in thermal acclimation and tolerance between populations of the winter ant, Prenolepis imparis

Thermal phenotypic plasticity, otherwise known as acclimation, plays an essential role in how organisms respond to short-term temperature changes. Plasticity buffers the impact of harmful temperature changes; therefore, understanding variation in plasticity in natural populations is crucial for understanding how species will respond to the changing climate. However, very few studies have examined patterns of phenotypic plasticity among populations, especially among ant populations. Considering that this intraspecies variation can provide insight into adaptive variation in populations, the goal of this study was to quantify the short-term acclimation ability and thermal tolerance of several populations of the winter ant, Prenolepis imparis. We tested for correlations between thermal plasticity and thermal tolerance, elevation, and body size. We characterized the thermal environment both above and below ground for several populations distributed across different elevations within California, USA. In addition, we measured the short-term acclimation ability and thermal tolerance of those populations. To measure thermal tolerance, we used chill-coma recovery time (CCRT) and knockdown time as indicators of cold and heat tolerance, respectively. Short-term phenotypic plasticity was assessed by calculating acclimation capacity using CCRT and knockdown time after exposure to both high and low temperatures. We found that several populations displayed different chill-coma recovery times and a few displayed different heat knockdown times, and that the acclimation capacities of cold and heat tolerance differed among most populations. The high-elevation populations displayed increased tolerance to the cold (faster CCRT) and greater plasticity. For high-temperature tolerance, we found heat tolerance was not associated with altitude; instead, greater tolerance to the heat was correlated with increased plasticity at higher temperatures. These current findings provide insight into thermal adaptation and factors that contribute to phenotypic diversity by revealing physiological variance among populations.

Warm periods in repeated cold stresses protect Drosophila against ionoregulatory collapse, chilling injury, and reproductive deficits

In some insects, repeated cold stresses, characterized by warm periods that interrupt a sustained cold period, have been found to yield survival benefits over continuous cold stresses, but at the cost of reproduction. During a cold stress, chill susceptible insects like Drosophila melanogaster suffer from a loss of ion and water balance, and the current model of recovery from chilling posits that re-establishment of ion homeostasis begins upon return to a warm environment, but that it takes minutes to hours for an insect to fully restore homeostasis. Following this ionoregulatory model of chill coma recovery, we predicted that the longer the duration of the warm periods between cold stresses, the better a fly will endure a subsequent chill coma event and the more likely they will be to survive. We also predicted, however, that this recovery may lead to reduced fecundity, possibly due to allocation of energy reserves away from reproduction. Here, female D.melanogaster were treated to a long continuous cold stress (25 h at 0 °C), or experienced the same total time in the cold with repeated short (15 min), or long (120 min) breaks at 22 °C. We found that warm periods in general improved survival outcomes, and individuals that recovered for more time in between cold periods had significantly lower rates of injury, faster recovery from chill coma, and produced greater, rather than fewer, offspring. These improvements in chill tolerance were associated with mitigation of ionoregulatory collapse, as flies that experienced either short or long warm periods better maintained low hemolymph [K⁺]. Thus, warm periods that interrupt cold periods improve cold tolerance and fertility in D. melanogaster females relative to a single sustained cold stress, potentially because this time allows for recovery of ion and water homeostasis.

Maintenance of hindgut reabsorption during cold exposure is a key adaptation for Drosophila cold tolerance

Maintaining extracellular osmotic and ionic homeostasis is crucial for organismal function. In insects, hemolymph volume and ion content is regulated by the secretory Malpighian tubules and reabsorptive hindgut. When exposed to stressful cold, homeostasis is gradually disrupted, characterized by a debilitating increase in extracellular K+ concentration (hyperkalemia). Accordingly, studies have found a strong link between species-specific cold tolerance and the ability to maintain ion and water homeostasis at low temperature. This is also true for drosophilids where inter- and intra-specific differences in cold tolerance are linked to the secretory capacity of Malpighian tubules. There is, however, little information on the reabsorptive capacity of the hindgut in Drosophila To address this, we developed a novel method that permits continuous measurements of hindgut ion and fluid reabsorption in Drosophila We demonstrate that this assay is temporally stable (∼2 h) and responsive to cAMP stimulation and pharmacological intervention in accordance with the current insect hindgut reabsorption model. We then investigated how cold acclimation or cold adaptation affected hindgut reabsorption at benign (24°C) and low temperature (3°C). Cold-tolerant Drosophila species and cold-acclimated D. melanogaster maintain superior fluid and Na+ reabsorption at low temperature. Furthermore, cold adaptation and acclimation caused a relative reduction in K+ reabsorption at low temperature. These characteristic responses of cold adaptation/acclimation will promote maintenance of ion and water homeostasis at low temperature. Our study of hindgut function therefore provides evidence that adaptations in the osmoregulatory capacity of insects are critical for their ability to tolerate cold.

Rapid cold hardening and octopamine modulate chill tolerance in Locusta migratoria

Temperature has profound effects on the neural function and behaviour of insects. When exposed to low temperature, chill-susceptible insects enter chill coma, a reversible state of neuromuscular paralysis. Despite the popularity of studying the effects of low temperature on insects, we know little about the physiological mechanisms controlling the entry to, and recovery from, chill coma. Spreading depolarization (SD) is a phenomenon that causes a neural shutdown in the central nervous system (CNS) and it is associated with a loss of K⁺ homeostasis in the CNS. Here, we investigated the effects of rapid cold hardening (RCH) on chill tolerance of the migratory locust. With an implanted thermocouple in the thorax, we determined the temperature associated with a loss of responsiveness (i.e. the critical thermal minimum - CT_min) in intact male adult locusts. In parallel experiments, we recorded field potential (FP) in the metathoracic ganglion (MTG) of semi-intact preparations to determine the temperature that would induce neural shutdown. We found that SD in the CNS causes a loss of coordinated movement immediately prior to chill coma and RCH reduces the temperature that evokes neural shutdown. Additionally, we investigated a role for octopamine (OA) in the locust chill tolerance and found that OA reduces the CT_min and mimics the effects of prior stress (anoxia) in locust.