Cold-induced depolarization of insect muscle: differing roles of extracellular K+ during acute and chronic chilling

Dietary potassium and cold acclimation additively increase cold tolerance in Drosophila melanogaster

In the cold, chill susceptible insects lose the ability to regulate ionic and osmotic gradients. This leads to hemolymph hyperkalemia that drives a debilitating loss of cell membrane polarization, triggering cell death pathways and causing organismal injury. Biotic and abiotic factors can modulate insect cold tolerance by impacting the ability to mitigate or prevent this cascade of events. In the present study, we test the combined and isolated effects of dietary manipulations and thermal acclimation on cold tolerance in fruit flies. Specifically, we acclimated adult Drosophila melanogaster to 15 or 25 °C and fed them either a K+-loaded diet or a control diet. We then tested the ability of these flies to recover from and survive a cold exposure, as well as their capacity to protect transmembrane K+ gradients, and intracellular Na+ concentration. As predicted, cold-exposed flies experienced hemolymph hyperkalemia and cold-acclimated flies had improved cold tolerance due to an improved maintenance of the hemolymph K+ concentration at low temperature. Feeding on a high-K+ diet improved cold tolerance additively, but paradoxically reduced the ability to maintain extracellular K+ concentrations. Cold-acclimation and K+-feeding additively increased the intracellular K+ concentration, aiding in maintenance of the transmembrane K+ gradient during cold exposure despite cold-induced hemolymph hyperkalemia. There was no effect of acclimation of diet on intracellular Na+ concentration. These findings suggest intracellular K+ loading and reduced muscle membrane K+ sensitivity as mechanisms through which cold-acclimated and K+-fed flies are able to tolerate hemolymph hyperkalemia.

A rapid return to normal: temporal gene expression patterns following cold exposure in the bumble bee Bombus impatiens

Bumble bees are common in cooler climates and many species likely experience periodic exposure to very cold temperatures, but little is known about the temporal dynamics of cold response mechanisms following chill exposure, especially how persistent effects of cold exposure may facilitate tolerance of future events. To investigate molecular processes involved in the temporal response by bumble bees to acute cold exposure, we compared mRNA transcript abundance in Bombus impatiens workers exposed to 0°C for 75 min (inducing chill coma) and control bees maintained at a constant ambient temperature (28°C). We sequenced the 3' end of mRNA transcripts (TagSeq) to quantify gene expression in thoracic tissue of bees at several time points (0, 10, 30, 120 and 720 min) following cold exposure. Significant differences from control bees were only detectable within 30 min after the treatment, with most occurring at the 10 min recovery time point. Genes associated with gluconeogenesis and glycolysis were most notably upregulated, while genes related to lipid and purine metabolism were downregulated. The observed patterns of expression indicate a rapid recovery after chill coma, suggesting an acute differential transcriptional response during recovery from chill coma and return to baseline expression levels within an hour, with no long-term gene expression markers of this cold exposure. Our work highlights the functions and pathways important for acute cold recovery, provides an estimated time frame for recovery from cold exposure in bumble bees, and suggests that cold hardening may be less important for these heterothermic 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.

Sub-optimal host plants have developmental and thermal fitness costs to the invasive fall armyworm

The fall armyworm (FAW) Spodoptera frugiperda (J.E. Smith) is a global invasive pest of cereals. Although this pest uses maize and sorghum as its main hosts, it is associated with a wide range of host plants due to its polyphagous nature. Despite the FAW's polyphagy being widely reported in literature, few studies have investigated the effects of the non-preferred conditions or forms (e.g., drought-stressed forms) of this pest's hosts on its physiological and ecological fitness. Thus, the interactive effects of biotic and abiotic stresses on FAW fitness costs or benefits have not been specifically investigated. We therefore assessed the effects of host plant quality on the developmental rates and thermal tolerance of the FAW. Specifically, we reared FAW neonates on three hosts (maize, cowpeas, and pearl millet) under two treatments per host plant [unstressed (well watered) and stressed (water deprived)] until the adult stage. Larval growth rates and pupal weights were determined. Thermal tolerance traits viz critical thermal maxima (CTmax), critical thermal minima (CTmin), heat knockdown time (HKDT), chill-coma recovery time (CCRT), and supercooling points (SCPs) were measured for the emerging adults from each treatment. The results showed that suboptimal diets significantly prolonged the developmental time of FAW larvae and reduced their growth rates and ultimate body weights, but did not impair their full development. Suboptimal diets (comprising non-cereal plants and drought-stressed cereal plants) increased the number of larval instars to eight compared to six for optimal natural diets (unstressed maize and pearl millet). Apart from direct effects, in all cases, suboptimal diets significantly reduced the heat tolerance of FAWs, but their effect on cold tolerance was recorded only in select cases (e.g., SCP). These results suggest host plant effects on the physical and thermal fitness of FAW, indicating a considerable degree of resilience against multiple stressors. This pest's resilience can present major drawbacks to its cultural management using suboptimal hosts (in crop rotations or intercrops) through its ability to survive on most host plants despite their water stress condition and gains in thermal fitness. The fate of FAW population persistence under multivariate environmental stresses is therefore not entirely subject to prior environmental host plant history or quality.

Locust gut epithelia do not become more permeable to fluorescent dextran and bacteria in the cold

The insect gut, which plays a role in ion and water balance, has been shown to leak solutes in the cold. Cold stress can also activate insect immune systems, but it is unknown whether the leak of the gut microbiome is a possible immune trigger in the cold. We developed a novel feeding protocol to load the gut of locusts (Locusta migratoria) with fluorescent bacteria before exposing them to -2°C for up to 48 h. No bacteria were recovered from the hemolymph of cold-exposed locusts, regardless of exposure duration. To examine this further, we used an ex vivo gut sac preparation to re-test cold-induced fluorescent FITC-dextran leak across the gut and found no increased rate of leak. These results question not only the validity of FITC-dextran as a marker of paracellular barrier permeability in the gut, but also to what extent the insect gut becomes leaky in the cold.

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.

Rapid stress hardening in the Antarctic midge improves male fertility by increasing courtship success and preventing decline of accessory gland proteins following cold exposure

Rapid hardening is a process that quickly improves an animal's performance following exposure to potentially damaging stress. In this study of the Antarctic midge, Belgica antarctica (Diptera, Chironomidae), we examined how rapid hardening in response to dehydration (RDH) or cold (RCH) improves male pre- and post-copulatory function when the insects are subsequently subjected to a damaging cold exposure. Neither RDH nor RCH improved survival in response to lethal cold stress, but male activity and mating success following sublethal cold exposure were enhanced. Egg viability decreased following direct exposure of the mating males to sublethal cold but improved following RCH and RDH. Sublethal cold exposure reduced the expression of four accessory gland proteins, while expression remained high in males exposed to RCH. Though rapid hardening may be cryptic in males, this study shows that it can be revealed by pre- and post-copulatory interactions with females.

Identification of a neural basis for cold acclimation in Drosophila larvae

Low temperatures can be fatal to insects, but many species have evolved the ability to cold acclimate, thereby increasing their cold tolerance. It has been previously shown that Drosophila melanogaster larvae perform cold-evoked behaviors under the control of noxious cold-sensing neurons (nociceptors), but it is unknown how the nervous system might participate in cold tolerance. Herein, we describe cold-nociceptive behavior among 11 drosophilid species; we find that the predominant cold-evoked larval response is a head-to-tail contraction behavior, which is likely inherited from a common ancestor, but is unlikely to be protective. We therefore tested the hypothesis that cold nociception functions to protect larvae by triggering cold acclimation. We found that Drosophila melanogaster Class III nociceptors are sensitized by and critical to cold acclimation and that cold acclimation can be optogenetically evoked, sans cold. Collectively, these findings demonstrate that cold nociception constitutes a peripheral neural basis for Drosophila larval cold acclimation.

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.

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

Background

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

Results

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

Conclusions

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

Supplementary Information

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

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.

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.

Rapid subcellular calcium responses and dynamics by calcium sensor G-CatchER. iScience 2021;24:102129. [PMID: 33665552 PMCID: PMC7900224 DOI: 10.1016/j.isci.2021.102129]

The precise spatiotemporal characteristics of subcellular calcium (Ca²⁺) transients are critical for the physiological processes. Here we report a green Ca²⁺ sensor called "G-CatchER⁺" using a protein design to report rapid local ER Ca²⁺ dynamics with significantly improved folding properties. G-CatchER⁺ exhibits a superior Ca²⁺ on rate to G-CEPIA1er and has a Ca²⁺-induced fluorescence lifetimes increase. G-CatchER⁺ also reports agonist/antagonist triggered Ca²⁺ dynamics in several cell types including primary neurons that are orchestrated by IP₃Rs, RyRs, and SERCAs with an ability to differentiate expression. Upon localization to the lumen of the RyR channel (G-CatchER⁺-JP45), we report a rapid local Ca²⁺ release that is likely due to calsequestrin. Transgenic expression of G-CatchER⁺ in Drosophila muscle demonstrates its utility as an in vivo reporter of stimulus-evoked SR local Ca²⁺ dynamics. G-CatchER⁺ will be an invaluable tool to examine local ER/SR Ca²⁺ dynamics and facilitate drug development associated with ER dysfunction.

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.

Hyperkalaemia, not apoptosis, accurately predicts insect chilling injury

There is a growing appreciation that insect distribution and abundance are associated with the limits of thermal tolerance, but the physiology underlying thermal tolerance remains poorly understood. Many insects, like the migratory locust (Locusta migratoria), suffer a loss of ion and water balance leading to hyperkalaemia (high extracellular [K⁺]) in the cold that indirectly causes cell death. Cells can die in several ways under stress, and how they die is of critical importance to identifying and understanding the nature of thermal adaptation. Whether apoptotic or necrotic cell death pathways are responsible for low-temperature injury is unclear. Here, we use a caspase-3 specific assay to indirectly quantify apoptotic cell death in three locust tissues (muscle, nerves and midgut) following prolonged chilling and recovery from an injury-inducing cold exposure. Furthermore, we obtain matching measurements of injury, extracellular [K⁺] and muscle caspase-3 activity in individual locusts to gain further insight into the mechanistic nature of chilling injury. We found a significant increase in muscle caspase-3 activity, but no such increase was observed in either nervous or gut tissue from the same animals, suggesting that chill injury primarily relates to muscle cell death. Levels of chilling injury measured at the whole animal level, however, were strongly correlated with the degree of haemolymph hyperkalaemia, and not apoptosis. These results support the notion that cold-induced ion balance disruption triggers cell death but also that apoptosis is not the main form of cell damage driving low-temperature injury.

Cold acclimation increases depolarization resistance and tolerance in muscle fibers from a chill-susceptible insect, Locusta migratoria

Cold exposure depolarizes cells in insects due to a reduced electrogenic ion transport and a gradual increase in extracellular K⁺ concentration ([K⁺]). Cold-induced depolarization is linked to cold injury in chill-susceptible insects, and the locust, Locusta migratoria, has been shown to improve cold tolerance following cold acclimation through depolarization resistance. Here we investigate how cold acclimation influences depolarization resistance and how this resistance relates to improved cold tolerance. To address this question, we investigated if cold acclimation affects the electrogenic transport capacity and/or the relative K⁺ permeability during cold exposure by measuring membrane potentials of warm- and cold-acclimated locusts in the presence and absence of ouabain (Na⁺-K⁺ pump blocker) or 4-aminopyridine (4-AP; voltage-gated K⁺ channel blocker). In addition, we compared the membrane lipid composition of muscle tissue from warm- and cold-acclimated locust and the abundance of a range transcripts related to ion transport and cell injury accumulation. We found that cold-acclimated locusts are depolarization resistant due to an elevated K⁺ permeability, facilitated by opening of 4-AP-sensitive K⁺ channels. In accordance, cold acclimation was associated with an increased abundance of Shaker transcripts (gene encoding 4-AP-sensitive voltage-gated K⁺ channels). Furthermore, we found that cold acclimation improved muscle cell viability following exposure to cold and hyperkalemia even when muscles were depolarized substantially. Thus cold acclimation confers resistance to depolarization by altering the relative ion permeability, but cold-acclimated locusts are also more tolerant to depolarization.

Chilling induces unidirectional solute leak through the locust gut epithelia

Chill-susceptible insects, like the migratory locust, often die when exposed to low temperatures from an accumulation of tissue damage that is unrelated to freezing (chilling injury). Chilling injury is often associated with a loss of ion balance across the gut epithelia. It has recently been suggested that this imbalance is at least partly caused by a cold-induced disruption of epithelial barrier function. Here, we aimed to test this hypothesis in the migratory locust (Locustamigratoria). First, chill tolerance was quantified by exposing locusts to -2°C and recording chill coma recovery time and survival 24 h post-cold exposure. Longer exposure times significantly increased recovery time and caused injury and death. Ion-selective microelectrodes were also used to test for a loss of ion balance in the cold. We found a significant increase of haemolymph K+ and decrease of haemolymph Na+ concentration over time. Next, barrier failure along the gut was tested by monitoring the movement of an epithelial barrier marker (FITC-dextran) across the gut epithelia during exposure to -2°C. We found a significant increase in haemolymph FITC-dextran concentration over time in the cold when assayed in the mucosal to serosal direction. However, when tested in the serosal to mucosal direction, we saw minimal marker movement across the gut epithelia. This suggests that while cold-induced barrier disruption is present, it is apparently unidirectional. It is important to note that these data reveal only the phenomenon itself. The location of this leak as well as the underlying mechanisms remain unclear and require further investigation.

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.

Is rapid cold-hardening an aerobic process? Characterization of changes in metabolic activity during its induction and effects of anoxia in flesh fly

Rapid cold-hardening (RCH) is a type of phenotypic plasticity that promotes a swift improvement of cold tolerance in insects. A brief exposure to mild cold dramatically increases insect survival to a subsequent cold exposure that would be lethal otherwise. In adult male flesh fly, Sarcophaga bullata, as little as 15 min at 5 °C significantly improved organismal survival at -7°C from 0 to 66.7 ± 11.1%. In this study, we investigated whether this RCH response is an aerobic process in S. bullata by characterizing changes in metabolic activity during its induction. At the level of whole organism, CO₂ production continued at a level above our detection limit, and a relatively greater rate was observed during the early phase before it stabilized after ~1 h of the RCH induction. Similarly, in isolated flight muscle tissues, those maintained at 5 °C for 10 min exhibited significantly greater rates of oxygen consumption, compared to those maintained at 5 °C for 1 h (2.82 ± 0.29 vs. 1.36 ± 0.22 μl O₂ mg^-1 DM h^-1). When these tissues were exposed to LaCl₃, a treatment that should inhibit RCH ex vivo, oxygen consumption rates of the muscles were reduced significantly to a level similar to those that had been maintained at 5 °C for 1 h. Interestingly, however, the RCH response was still evident among individuals exposed to chilling under anoxia. Compared to those exposed to anoxia for 30 min only at 25 °C, flies exposed to 5 °C for 2 h under anoxia following the initial exposure exhibited a significantly greater level of cold tolerance at -7.5 °C (41.7 ± 7.1 vs. 91.8 ± 3.9%). Our results suggest that while relatively greater rates of metabolic activity are associated with the early phase of the RCH induction, it can proceed under the anoxic condition, thereby suggesting its independence to aerobic respiration.

An impressive capacity for cold tolerance plasticity protects against ionoregulatory collapse in the disease vector Aedes aegypti

The mosquito Aedes aegypti is largely confined to tropical and subtropical regions, but its range has recently been spreading to colder climates. As insect biogeography is tied to environmental temperature, understanding the limits of A. aegypti thermal tolerance and their capacity for phenotypic plasticity is important in predicting the spread of this species. In this study, we report on the chill coma onset (CCO) and recovery time (CCRT), as well as low-temperature survival phenotypes of larvae and adults of A. aegypti that developed or were acclimated to 15°C (cold) or 25°C (warm). Cold acclimation did not affect CCO temperatures of larvae but substantially reduced CCO in adults. Temperature and the duration of exposure both affected CCRT, and cold acclimation strongly mitigated these effects and increased rates of survival following prolonged chilling. Female adults were far less likely to take a blood meal when cold acclimated, and exposing females to blood (without feeding) attenuated some of the beneficial effects of cold acclimation on CCRT. Lastly, larvae suffered from haemolymph hyperkalaemia when chilled, but cold acclimation attenuated the imbalance. Our results demonstrate that A. aegypti larvae and adults have the capacity to acclimate to low temperatures, and do so at least in part by better maintaining ion balance in the cold. This ability for cold acclimation may facilitate the spread of this species to higher latitudes, particularly in an era of climate change.

Cold acclimation triggers major transcriptional changes in Drosophila suzukii

BACKGROUND

Insects have the capacity to adjust their physiological mechanisms during their lifetime to promote cold tolerance and cope with sublethal thermal conditions, a phenomenon referred to as thermal acclimation. The spotted wing drosophila, Drosophila suzukii, is an invasive fruit pest that, like many other species, enhances its thermotolerance in response to thermal acclimation. However, little is known about the underlying mechanisms of this plastic response. Here, we promoted flies' cold tolerance by gradually increasing acclimation duration (i.e. pre-exposure from 2 h to 9 days at 10 °C), and then compared transcriptomic responses of cold hardy versus cold susceptible phenotypes using RNA sequencing.

RESULTS

Cold tolerance of D. suzukii increased with acclimation duration; the longer the acclimation, the higher the cold tolerance. Cold-tolerant flies that were acclimated for 9 days were selected for transcriptomic analyses. RNA sequencing revealed a total of 2908 differentially expressed genes: 1583 were up- and 1325 were downregulated in cold acclimated flies. Functional annotation revealed many enriched GO-terms among which ionic transport across membranes and signaling were highly represented in acclimated flies. Neuronal activity and carbohydrate metabolism were also enriched GO-terms in acclimated flies. Results also revealed many GO-terms related to oogenesis which were underrepresented in acclimated flies.

CONCLUSIONS

Involvement of a large cluster of genes related to ion transport in cold acclimated flies suggests adjustments in the capacity to maintain ion and water homeostasis. These processes are key mechanisms underlying cold tolerance in insects. Down regulation of genes related to oogenesis in cold acclimated females likely reflects that females were conditioned at 10 °C, a temperature that prevents oogenesis. Overall, these results help to understand the molecular underpinnings of cold tolerance acquisition in D. suzukii. These data are of importance considering that the invasive success of D. suzukii in diverse climatic regions relates to its high thermal plasticity.

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.

Cold acclimation modulates voltage gated Ca2+ channel currents and fiber excitability in skeletal muscles of Locusta migratoria

Cold exposure is known to induce stressful imbalances in chill susceptible insects, including loss of hemolymph water, hyperkalemia and cell depolarization. Cold induced depolarization induces uncontrolled Ca²⁺ influx and accumulation of injury through necrosis/apoptosis. Conversely cold induced Ca²⁺ influx has been shown to induce rapid cold hardening and therefore also play a role to reduce cold injury. Cold acclimation is known to reduce cold injury in insects and due to the involvement of depolarization and Ca²⁺ in the pathophysiology of hypothermia, we hypothesized that cold acclimation modulates voltage gated Ca²⁺ channels and fiber excitability. Using intracellular electrodes or force transducers, we measured the Ca²⁺ currents, fiber excitability and muscle contractility in warm (31 °C) and cold (11 °C) acclimated locusts. Experiments were performed under conditions ranging from mild conditions where the membrane potential is well regulated to stressful conditions, where the membrane potential is very depolarized and the tissue is at risk of accumulating injury. These experiments found that cold acclimation modulates Ca²⁺ currents and fiber excitability in a manner that depends on the cold exposure. Thus, under mild conditions, Ca²⁺ currents and fiber excitability was increased whilst muscle contractility was unaffected by cold acclimation. Conversely, fiber excitability and muscle contractility was decreased under stressful conditions. Further work is required to fully understand the adaptive effects of these modulations. However, we propose a model which reconciles the dualistic role of the Ca²⁺ ion in cold exposure and cold acclimation. Thus, increased Ca²⁺ currents at mild temperatures could help to enhance cold sensing capacity whereas reduced fiber excitability under stressful conditions could help to reduce catastrophic Ca²⁺ influx during periods of severe cold exposure.

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

When insects are cooled, they initially lose their ability to perform coordinated movements at their critical thermal minima (CT_min). At a slightly lower temperature, they enter a state of complete paralysis (chill coma onset temperature - CCO) and if they are returned to permissive temperatures they regain function after a recovery period which is termed chill coma recovery time (CCRT). These three phenotypes (CT_min, CCO, and CCRT) are all popular measures of insect cold tolerance and it is therefore important to characterize the physiological processes that are responsible for these phenotypes. In the present study we measured extracellular field potentials in the central nervous system (CNS) and muscle membrane potential (V_m) during cooling and recovery in three Drosophila species that have different cold tolerances. With these measurements we assess the role of the CNS and muscle V_m in setting the lower thermal limits (CT_min and CCO) and in delaying chill coma recovery (CCRT). The experiments suggest that entry into chill coma is primarily caused by the onset of a spreading depolarization in the CNS for all three species. In the two most cold-sensitive species we observed that the loss of CNS function was followed closely by a depolarization of muscle V_m which is known to compromise muscle function. When flies are returned to benign temperature after a cold exposure we observe a rapid recovery of CNS function, but functional recovery was delayed by a slower recovery of muscle polarization. Thus, we demonstrate the primacy of different physiological systems (CNS vs. muscle) as determinants of the most commonly used cold tolerance measures for insects (CT_min vs. CCRT).

Dissecting cause from consequence: a systematic approach to thermal limits

Thermal limits mark the boundaries of ectotherm performance, and are increasingly appreciated as strong correlates and possible determinants of animal distribution patterns. The mechanisms setting the thermal limits of ectothermic animals are under active study and rigorous debate as we try to reconcile new observations in the lab and field with the knowledge gained from a long history of research on thermal adaptation. Here, I provide a perspective on our divided understanding of the mechanisms setting thermal limits of ectothermic animals. I focus primarily on the fundamental differences between high and low temperatures, and how animal form and environment can place different constraints on different taxa. Together, complexity and variation in animal form drive complexity in the interactions within and among levels of biological organization, creating a formidable barrier to determining mechanistic cause and effect at thermal limits. Progress in our understanding of thermal limits will require extensive collaboration and systematic approaches that embrace this complexity and allow us to separate the causes of failure from the physiological consequences that can quickly follow. I argue that by building integrative models that explain causal links among multiple organ systems, we can more quickly arrive at a holistic understanding of the varied challenges facing animals at extreme temperatures.

Cold stress results in sustained locomotor and behavioral deficits in Drosophila melanogaster

Tolerance of climatic stressors is an important predictor of the current distribution of insect species, their potential to invade new environments, and their responses to rapid climate change. Cold stress causes acute injury to nerves and muscles, and here we tested the hypothesis that low temperature causes sublethal deficits in locomotor behaviors that are dependent on neuromuscular function. To do so, we applied a previously developed assay, the rapid iterative negative geotaxis (RING) assay, to investigate behavioral consequences of cold stress in Drosophila melanogaster. The RING assay allows for rapid assessment of negative geotaxis behavior by quantifying climbing height and willingness to climb after cold stress. We exposed flies to cold stress at 0°C and assessed the extent to which duration of cold stress, recovery time, and cold acclimation influenced climbing performance. There was a clear dose-response relationship between cold exposure and performance deficits, with climbing height and willingness decreasing as cold exposure increased from 2 to 24 hr. Following cold exposure of an intermediate duration (12 hr), climbing height and willingness gradually improved as recovery time increased from 4 to 72 hr but flies never fully recovered. Finally, cold acclimation improved overall climbing height and willingness in both untreated and cold-stressed flies but did not prevent a reduction in climbing performance. Thus, cold stress causes deficits in locomotor and behavior that are dependent on the dose of cold exposure and persist long after the stress subsides. These results likely have implications for the ecological and evolutionary responses of insect populations to thermally variable environments.

Cold Acclimation of Trogoderma granarium Everts Is Tightly Linked to Regulation of Enzyme Activity, Energy Content, and Ion Concentration

In this study, cold hardiness and some physiological characteristics of the Khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae) larvae, were investigated under different thermal regimes, i.e., control, cold-acclimated (CA), fluctuating-acclimated (FA), and rapid cold-hardened (RCH). In all the regimes, the larval survival rate decreased with a decrease in temperature. CA larvae showed the highest cold hardiness following 24 h exposure at -15 and -20°C. Control larvae had the highest glycogen content (34.4 ± 2.3 μg/dry weight). In contrast, CA larvae had the lowest glycogen content (23.0 ± 1.6 μg/dry weight). Change in trehalose content was reversely proportional to changes in glycogen content. The highest myo-inositol and glucose contents were detected in CA larvae (10.7 ± 0.4 μg/dry weight) and control (0.49 ± 0.03 μg/dry weight), respectively. In control and treated larvae, [Na⁺] decreased, though [K⁺] increased, with increasing exposure time. The shape of the thermal reaction curve of AMP-depended protein kinase and protein phosphatase 2C followed the same norm, which was different from protein phosphatase 1 and protein phosphatase 2A. Protein phosphatase 2A and 2C showed a complete difference in thermal reaction norms. Indeed, thermal fluctuation caused the highest changes in the activity of the enzymes, whereas the RCH showed the lowest changes in the activity of the enzymes. Our results showed a significant enhancement of larval cold tolerance under CA regime, which is related to the high levels of low molecular weight carbohydrates under this regime. Our results showed that among the different thermal regimes tested, the CA larvae had the lowest supercooling point (about -22°C) and the highest cold hardiness following 24 h exposure at -15 and -20°C.

Anti-diuretic activity of a CAPA neuropeptide can compromise Drosophila chill tolerance

For insects, chilling injuries that occur in the absence of freezing are often related to a systemic loss of ion and water balance that leads to extracellular hyperkalemia, cell depolarization and the triggering of apoptotic signalling cascades. The ability of insect ionoregulatory organs (e.g. the Malpighian tubules) to maintain ion balance in the cold has been linked to improved chill tolerance, and many neuroendocrine factors are known to influence ion transport rates of these organs. Injection of micromolar doses of CAPA (an insect neuropeptide) have been previously demonstrated to improve Drosophila cold tolerance, but the mechanisms through which it impacts chill tolerance are unclear, and low doses of CAPA have been previously demonstrated to cause anti-diuresis in insects, including dipterans. Here, we provide evidence that low (femtomolar) and high (micromolar) doses of CAPA impair and improve chill tolerance, respectively, via two different effects on Malpighian tubule ion and water transport. While low doses of CAPA are anti-diuretic, reduce tubule K⁺ clearance rates and reduce chill tolerance, high doses facilitate K⁺ clearance from the haemolymph and increase chill tolerance. By quantifying CAPA peptide levels in the central nervous system, we estimated the maximum achievable hormonal titres of CAPA and found further evidence that CAPA may function as an anti-diuretic hormone in Drosophila melanogaster We provide the first evidence of a neuropeptide that can negatively affect cold tolerance in an insect and further evidence of CAPA functioning as an anti-diuretic peptide in this ubiquitous insect model.

Cold exposure causes cell death by depolarization-mediated Ca2+ overload in a chill-susceptible insect

Cold tolerance of insects is arguably among the most important traits defining their geographical distribution. Even so, very little is known regarding the causes of cold injury in this species-rich group. In many insects it has been observed that cold injury coincides with a cellular depolarization caused by hypothermia and hyperkalemia that develop during chronic cold exposure. However, prior studies have been unable to determine if cold injury is caused by direct effects of hypothermia, by toxic effects of hyperkalemia, or by the depolarization that is associated with these perturbations. Here we use a fluorescent DNA-staining method to estimate cell viability of muscle and hindgut tissue from Locusta migratoria and show that the cellular injury is independent of the direct effects of hypothermia or toxic effects of hyperkalemia. Instead, we show that chill injury develops due to the associated cellular depolarization. We further hypothesized that the depolarization-induced injury was caused by opening of voltage-sensitive Ca²⁺ channels, causing a Ca²⁺ overload that triggers apoptotic/necrotic pathways. In accordance with this hypothesis, we show that hyperkalemic depolarization causes a marked increase in intracellular Ca²⁺ levels. Furthermore, using pharmacological manipulation of intra- and extracellular Ca²⁺ concentrations as well as Ca²⁺ channel conductance, we demonstrate that injury is prevented if transmembrane Ca²⁺ flux is prevented by removing extracellular Ca²⁺ or blocking Ca²⁺ influx. Together these findings demonstrate a causal relationship between cold-induced hyperkalemia, depolarization, and the development of chill injury through Ca²⁺-mediated necrosis/apoptosis.

The effect of cold acclimation on active ion transport in cricket ionoregulatory tissues

Cold-acclimated insects defend ion and water transport function during cold exposure. We hypothesized that this is achieved via enhanced active transport. The Malpighian tubules and rectum are likely targets for such transport modifications, and recent transcriptomic studies indicate shifts in Na+-K+ ATPase (NKA) and V-ATPase expression in these tissues following cold acclimation. Here we quantify the effect of cold acclimation (one week at 12°C) on active transport in the ionoregulatory organs of adult Gryllus pennsylvanicus field crickets. We compared primary urine production of warm- and cold-acclimated crickets in excised Malpighian tubules via Ramsay assay at a range of temperatures between 4 and 25°C. We then compared NKA and V-ATPase activities in Malpighian tubule and rectal homogenates from warm- and cold-acclimated crickets via NADH-linked photometric assays. Malpighian tubules of cold-acclimated crickets excreted fluid at lower rates at all temperatures compared to warm-acclimated crickets. This reduction in Malpighian tubule excretion rates may be attributed to increased NKA activity that we observed for cold-acclimated crickets, but V-ATPase activity was unchanged. Cold acclimation had no effect on rectal NKA activity at either 21°C or 6°C, and did not modify rectal V-ATPase activity. Our results suggest that an overall reduction, rather than enhancement of active transport in the Malpighian tubules allows crickets to maintain hemolymph water balance during cold exposure, and increased Malpighian tubule NKA activity may help to defend and/or re-establish ion homeostasis.

Central nervous shutdown underlies acute cold tolerance in tropical and temperate Drosophila species

When cooled, insects first lose their ability to perform coordinated movements (CTmin) after which they enter chill coma (chill coma onset, CCO). Both these behaviours are popular measures of cold tolerance that correlate remarkably well with species distribution. To identify and understand the neuromuscular impairment that causes CTmin and CCO we used inter- and intraspecific model systems of Drosophila species that have varying cold tolerance as a consequence of adaptation or cold acclimation. Our results demonstrate that CTmin and CCO correlate strongly with a spreading depolarization (SD) within the central nervous system (CNS). We show that this SD is associated with a rapid increase in extracellular [K+] within the CNS causing neuronal depolarization that silences the CNS. The CNS shutdown is likely caused by a mismatch between passive and active ion transport within the CNS and in a different set of experiments we examine inter- and intraspecific differences in sensitivity to SD events during anoxic exposure. These experiments show that cold adapted or acclimated flies are better able to maintain ionoregulatory balance when active transport is compromised within the CNS. Combined, we demonstrate that a key mechanism underlying chill coma entry of Drosophila is CNS shutdown, and the ability to prevent this CNS shutdown is therefore an important component of acute cold tolerance, thermal adaptation and cold acclimation in insects.

Functional plasticity of the gut and the Malpighian tubules underlies cold acclimation and mitigates cold-induced hyperkalemia in Drosophila melanogaster

At low temperatures, Drosophila, like most insects, lose the ability to regulate ion and water balance across the gut epithelia, which can lead to a lethal increase of [K+] in the hemolymph (hyperkalemia). Cold-acclimation, the physiological response to a prior low temperature exposure, can mitigate or entirely prevent these ion imbalances, but the physiological mechanisms that facilitate this process are not well understood. Here, we test whether plasticity in the ionoregulatory physiology of the gut and Malpighian tubules of Drosophila may aid in preserving ion homeostasis in the cold. Upon adult emergence, D. melanogaster females were subjected to seven days at warm (25°C) or cold (10°C) acclimation conditions. The cold acclimated flies had a lower critical thermal minimum (CTmin), recovered from chill coma more quickly, and better maintained hemolymph K+ balance in the cold. The improvements in chill tolerance coincided with increased Malpighian tubule fluid secretion and better maintenance of K+ secretion rates in the cold, as well as reduced rectal K+ reabsorption in cold-acclimated flies. To test whether modulation of ion-motive ATPases, the main drivers of epithelial transport in the alimentary canal, mediate these changes, we measured the activities of Na+-K+-ATPase and V-type H+-ATPase at the Malpighian tubules, midgut, and hindgut. Na+/K+-ATPase and V-type H+-ATPase activities were lower in the midgut and the Malpighian tubules of cold-acclimated flies, but unchanged in the hindgut of cold acclimated flies, and were not predictive of the observed alterations in K+ transport. Our results suggest that modification of Malpighian tubule and gut ion and water transport likely prevents cold-induced hyperkalemia in cold-acclimated flies and that this process is not directly related to the activities of the main drivers of ion transport in these organs, Na+/K+- and V-type H+-ATPases.

Cold tolerance is linked to osmoregulatory function of the hindgut in Locusta migratoria

There is growing evidence that maintenance of ion and water balance determine cold tolerance in many insects. The hindgut of terrestrial insects is critical for maintaining organismal homeostasis as it regulates solute- and water-balance of the hemolymph. Here we used ex vivo everted gut sacs of L. migratoria to examine the effects of temperature (0 - 30°C), thermal-acclimation, hypoxia, and ionic and osmotic forces on bulk water and ion (Na+, K+ and Cl−) movement across the rectal epithelium. These findings were related to simultaneous in vivo measurements of water and ion balance in locusts exposed to similar temperatures. As predicted, we observed a critical inhibition of net water and ion reabsorption at low temperature that is proportional to the in vivo loss of water and ion homeostasis. Further, cold-acclimated locust, known to defend ion and water balance at low temperature, were characterised by improved reabsorptive capacity at low temperature. These findings strongly support the hypothesis that transport mechanisms in the hindgut at low temperature are essential for cold tolerance. The loss of osmoregulatory capacity at low temperature was primarily caused by reduced active transport while rectal paracellular permeability to fluorescein isothiocyanate dextran was unchanged at 0 and 30°C. During cold exposure, water reabsorption was independent of major cation gradients across the epithelia while reduction in mucosal Cl− availability and increase in mucosal osmolality markedly depressed water reabsorption. These findings are discussed in perspective of existing knowledge and with suggestions for future physiological studies on cold acclimation and adaptation in insects.

Chill coma in the locust, Locusta migratoria, is initiated by spreading depolarization in the central nervous system

The ability of chill-sensitive insects to function at low temperatures limits their geographic ranges. They have species-specific temperatures below which movements become uncoordinated prior to entering a reversible state of neuromuscular paralysis. In spite of decades of research, which in recent years has focused on muscle function, the role of neural mechanisms in determining chill coma is unknown. Spreading depolarization (SD) is a phenomenon that causes a shutdown of neural function in the integrating centres of the central nervous system. We investigated the role of SD in the process of entering chill coma in the locust, Locusta migratoria. We used thermolimit respirometry and electromyography in whole animals and extracellular and intracellular recording techniques in semi-intact preparations to characterize neural events during chilling. We show that chill-induced SD in the central nervous system is the mechanism underlying the critical thermal minimum for coordinated movement in locusts. This finding will be important for understanding how insects adapt and acclimate to changing environmental temperatures.

Effects of cold-acclimation on gene expression in Fall field cricket (Gryllus pennsylvanicus) ionoregulatory tissues

Background

Cold tolerance is a key determinant of temperate insect distribution and performance. Chill-susceptible insects lose ion and water homeostasis during cold exposure, but prior cold acclimation improves both cold tolerance and defense of homeostasis. The mechanisms underlying these processes are mostly unknown; cold acclimation is thought to enhance ion transport in the cold and/or prevent leak of water and ions. To identify candidate mechanisms of cold tolerance plasticity we generated transcriptomes of ionoregulatory tissues (hindgut and Malpighian tubules) from Gryllus pennsylvanicus crickets and compared gene expression in warm- and cold-acclimated individuals.

Results

We assembled a G. pennsylvanicus transcriptome de novo from 286 million 50-bp reads, yielding 70,037 contigs (~44% of which had putative BLAST identities). We compared the transcriptomes of warm- and cold-acclimated hindguts and Malpighian tubules. Cold acclimation led to a ≥ 2-fold change in the expression of 1493 hindgut genes (733 downregulated, 760 upregulated) and 2008 Malpighian tubule genes (1009 downregulated, 999 upregulated). Cold-acclimated crickets had altered expression of genes putatively associated with ion and water balance, including: a downregulation of V-ATPase and carbonic anhydrase in the Malpighian tubules and an upregulation of Na⁺-K⁺ ATPase in the hindgut. We also observed acclimation-related shifts in the expression of cytoskeletal genes in the hindgut, including actin and actin-anchoring/stabilizing proteins, tubulin, α-actinin, and genes involved in adherens junctions organization. In both tissues, cold acclimation led to differential expression of genes encoding cytochrome P450s, glutathione-S-transferases, apoptosis factors, DNA repair, and heat shock proteins.

Conclusions

This is the first G. pennsylvanicus transcriptome, and our tissue-specific approach yielded new candidate mechanisms of cold tolerance plasticity. Cold acclimation may reduce loss of hemolymph volume in the cold by 1) decreasing primary urine production via reduced expression of carbonic anhydrase and V-ATPase in the Malpighian tubules and 2) by increasing Na⁺ (and therefore water) reabsorption across the hindgut via increase in Na⁺-K⁺ ATPase expression. Cold acclimation may reduce chilling injury by remodeling and stabilizing the hindgut epithelial cytoskeleton and cell-to-cell junctions, and by increasing the expression of genes involved in DNA repair, detoxification, and protein chaperones.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3711-9) contains supplementary material, which is available to authorized users.

Physiological correlates of chill susceptibility in Lepidoptera

The majority of insects enter a state of reversible coma if temperature is lowered sufficiently. If the cold treatment is not too severe these insects recover gradually when returned to benign temperatures in a time-dependent manner that often depends on the duration and intensity of the cold exposure. Previous studies have associated these phenotypes to changes in membrane potential (V_m) and ion balance, and especially hemolymph [K⁺] is known to be of importance for the recovery time. In the present study we examined this link in three species of Lepidoptera as insects from this order are known to possess resting hemolymph [K⁺] that would severely compromise V_m in other insects. Specifically, we exposed larval and adult Manduca sexta, larval Bombyx mori, and adult Heliconius cydno to stressful cold (0°C) for extended periods of time. Subsequently we measured chill coma recovery time (CCRT), ion- and water balance, and muscle V_m. As expected we find that resting hemolymph [K⁺] is high and that resting hemolymph [Na⁺] is low compared to most other insect species. Muscle V_m depolarised considerably during acute cold exposure, but did so in a manner that was not associated with changes in ion balance. However, prolonged cold exposure coincided with an increase of hemolymph [K⁺] and further depolarisation of V_m which correlated well with prolongation of CCRT. Combined this demonstrates how insects with different ionic compositions generally suffer from similar consequences of cold stress as other species, such that cold tolerance of chill-susceptible insects within Lepidoptera is also intimately linked to maintenance of ion balance and membrane polarisation.

Cold acclimation allows Drosophila flies to maintain mitochondrial functioning under cold stress

Environmental stress generally disturbs cellular homeostasis. Researchers have hypothesized that chilling injury is linked to a shortage of ATP. However, previous studies conducted on insects exposed to nonfreezing low temperatures presented conflicting results. In this study, we investigated the mitochondrial bioenergetics of Drosophila melanogaster flies exposed to chronic cold stress (4 °C). We assessed mitochondrial oxygen consumption while monitoring the rate of ATP synthesis at various times (0, 1, 2, and 3 days) during prolonged cold stress and at two assay temperatures (25 and 4 °C). We compared organelle responses between cold-susceptible and cold-acclimated phenotypes. Continuous exposure to low temperature provoked temporal declines in the rates of mitochondrial respiration and ATP synthesis. Respiratory control ratios (RCRs) suggested that mitochondria were not critically uncoupled. Nevertheless, after 3 days of continuous cold stress, a sharp decline in the mitochondrial ATP synthesis rate was observed in control flies when they were assayed at low temperature. This change was associated with reduced survival capacity in control flies. In contrast, cold-acclimated flies exhibited high survival and maintained higher rates of mitochondrial ATP synthesis and coupling (i.e., higher RCRs). Adaptive changes due to cold acclimation observed in the whole organism were thus manifested in isolated mitochondria. Our observations suggest that cold tolerance is linked to the ability to maintain bioenergetics capacity under cold stress.

Chronic dietary salt stress mitigates hyperkalemia and facilitates chill coma recovery in Drosophila melanogaster

Chill susceptible insects like Drosophila lose the ability to regulate water and ion homeostasis at low temperatures. This loss of hemolymph ion and water balance drives a hyperkalemic state that depolarizes cells, causing cellular injury and death. The ability to maintain ion homeostasis at low temperatures and/or recover ion homeostasis upon rewarming is closely related to insect cold tolerance. We thus hypothesized that changes to organismal ion balance, which can be achieved in Drosophila through dietary salt loading, could alter whole animal cold tolerance phenotypes. We put Drosophila melanogaster in the presence of diets highly enriched in NaCl, KCl, xylitol (an osmotic control) or sucrose (a dietary supplement known to impact cold tolerance) for 24h and confirmed that they consumed the novel food. Independently of their osmotic effects, NaCl, KCl, and sucrose supplementation all improved the ability of flies to maintain K⁺ balance in the cold, which allowed for faster recovery from chill coma after 6h at 0°C. These supplements, however, also slightly increased the CT_min and had little impact on survival rates following chronic cold stress (24h at 0°C), suggesting that the effect of diet on cold tolerance depends on the measure of cold tolerance assessed. In contrast to prolonged salt stress, brief feeding (1.5h) on diets high in salt slowed coma recovery, suggesting that the long-term effects of NaCl and KCl on chilling tolerance result from phenotypic plasticity, induced in response to a salty diet, rather than simply the presence of the diet in the gut lumen.

Concurrent effects of cold and hyperkalaemia cause insect chilling injury

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

Ion and water balance in Gryllus crickets during the first twelve hours of cold exposure

Insects lose ion and water balance during chilling, but the mechanisms underlying this phenomenon are based on patterns of ion and water balance observed in the later stages of cold exposure (12 or more hours). Here we quantified the distribution of ions and water in the hemolymph, muscle, and gut in adult Gryllus field crickets during the first 12h of cold exposure to test mechanistic hypotheses about why homeostasis is lost in the cold, and how chill-tolerant insects might maintain homeostasis to lower temperatures. Unlike in later chill coma, hemolymph [Na(+)] and Na(+) content in the first few hours of chilling actually increased. Patterns of Na(+) balance suggest that Na(+) migrates from the tissues to the gut lumen via the hemolymph. Imbalance of [K(+)] progressed gradually over 12h and could not explain chill coma onset (a finding consistent with recent studies), nor did it predict survival or injury following 48h of chilling. Gryllus veletis avoided shifts in muscle and hemolymph ion content better than Gryllus pennsylvanicus (which is less chill-tolerant), however neither species defended water, [Na(+)], or [K(+)] balance during the first 12h of chilling. Gryllus veletis better maintained balance of Na(+) content and may therefore have greater tissue resistance to ion leak during cold exposure, which could partially explain faster chill coma recovery for that species.

Physiological Limits along an Elevational Gradient in a Radiation of Montane Ground Beetles

A central challenge in ecology and biogeography is to determine the extent to which physiological constraints govern the geographic ranges of species along environmental gradients. This study tests the hypothesis that temperature and desiccation tolerance are associated with the elevational ranges of 12 ground beetle species (genus Nebria) occurring on Mt. Rainier, Washington, U.S.A. Species from higher elevations did not have greater cold tolerance limits than lower-elevation species (all species ranged from -3.5 to -4.1°C), despite a steep decline in minimum temperature with elevation. Although heat tolerance limits varied among species (from 32.0 to 37.0°C), this variation was not generally associated with the relative elevational range of a species. Temperature gradients and acute thermal tolerance do not support the hypothesis that physiological constraints drive species turnover with elevation. Measurements of intraspecific variation in thermal tolerance limits were not significant for individuals taken at different elevations on Mt. Rainier, or from other mountains in Washington and Oregon. Desiccation resistance was also not associated with a species' elevational distribution. Our combined results contrast with previously-detected latitudinal gradients in acute physiological limits among insects and suggest that other processes such as chronic thermal stress or biotic interactions might be more important in constraining elevational distributions in this system.

Reduced L-type Ca2+ current and compromised excitability induce loss of skeletal muscle function during acute cooling in locust

Low temperature causes most insects to enter a state of neuromuscular paralysis, termed chill coma. Susceptibility of insect species to enter chill coma is tightly correlated to the species distribution limits and for this reason it is important to understand the cellular processes that underlie chill coma. It is known that muscle function is markedly depressed at low temperature and this suggests that chill coma is partly caused by impairment in the muscle per se. To find the cellular mechanism(s) underlying muscle dysfunction at low temperature, we examined the effect of low temperature (5°C) on several events in the excitation-contraction-coupling in the migratory locust (Locusta migratoria). Intracellular membrane potential recordings during single nerve stimulations showed that 70% of fibers at 20°C produced an action potential (AP), while only 55% of the fibers were able to fire AP at 5°C. Reduced excitability at low temperature was caused by ∼80% drop in L-type Ca2+ current and a depolarizing shift in its activation of around 20 mV, which means that a larger endplate potential would be needed to activate the muscle AP at low temperature. In accordance we showed that intracellular Ca2+ transients were largely absent at low temperature following nerve stimulation. In contrast, maximum contractile force was unaffected by low temperature in chemically skinned muscle bundles which demonstrates that the function of the contractile filaments are preserved at low temperature. These findings demonstrate that reduced L-type Ca2+ current is likely the most important factor contributing to loss of muscle function at low temperature in locust.

Cold-acclimation improves chill tolerance in the migratory locust through preservation of ion balance and membrane potential

Most insects have the ability to alter their cold tolerance in response to temporal temperature fluctuations, and recent studies have shown that insect cold tolerance is closely tied to the ability to maintain transmembrane ion-gradients that are important for the maintenance of cell membrane potential (Vm). Accordingly, several studies have suggested a link between preservation of Vm and cellular survival after cold stress, but none have measured Vm in this context. We tested this hypothesis by acclimating locusts (Locusta migratoria) to high (31°C) and low temperature (11°C) for four days before exposing them to cold stress (0°C) for up to 48 hours and subsequently measuring ion balance, cell survival, muscle Vm, and whole animal performance. Cold stress caused gradual muscle cell death which coincided with a loss of ion balance and depolarisation of muscle Vm. The loss of ion-balance and cell polarisation were, however, dampened markedly in cold-acclimated locusts such that the development of chill injury was reduced. To further examine the association between cellular injury and Vm we exposed in vitro muscle preparations to cold buffers with low, intermediate, or high [K+]. These experiments revealed that cellular injury during cold exposure occurs when Vm becomes severely depolarised. Interestingly we found that cellular sensitivity to hypothermic hyperkalaemia was lower in cold-acclimated locusts that were better able to defend Vm whilst exposed to high extracellular [K+]. Together these results demonstrate a mechanism of cold-acclimation in locusts that improves survival after cold stress: Increased cold tolerance is accomplished by preservation of Vm through maintenance of ion homeostasis and decreased K+-sensitivity.

Hemolymph metabolites and osmolality are tightly linked to cold tolerance of Drosophila species: a comparative study

Drosophila, like most insects, are susceptible to low temperatures, and will succumb to temperatures above the freezing point of their hemolymph. For these insects, cold exposure causes a loss of extracellular ion and water homeostasis, leading to chill injury and eventually death. Chill tolerant species are characterized by lower hemolymph [Na+] than chill susceptible species and this lowered hemolymph [Na+] is suggested to improve ion and water homeostasis during cold exposure. It has therefore also been hypothesized that hemolymph Na+ is replaced by other “cryoprotective” osmolytes in cold tolerant species. Here, we compare the hemolymph metabolite profiles of five drosophilid species with marked difference in chill tolerance. All species were examined under “normal” thermal conditions (i.e. 20°C) and following cold exposure (4 hours at 0°C). Under benign conditions total hemolymph osmolality was similar among all species despite chill tolerant species having lower hemolymph [Na+]. Using NMR spectroscopy we found that chill tolerant species instead have higher levels of sugars and free amino acids in their hemolymph, including classical “cryoprotectants” such as trehalose and proline. In addition, we found that chill tolerant species maintain a relatively stable hemolymph osmolality and metabolite profile when exposed to cold stress while sensitive species suffer from large increases in osmolality and massive changes in their metabolic profiles during a cold stress. We suggest that the larger contribution of classical “cryoprotectants” in chill tolerant Drosophila play a non-colligative role for cold tolerance that contributes to osmotic and ion homeostasis during cold exposures and in addition we discuss how these comparative differences may represent an evolutionary pathway toward more extreme cold tolerance of insects.

The capacity to maintain ion and water homeostasis underlies interspecific variation in Drosophila cold tolerance

Many insects, including Drosophila, succumb to the physiological effects of chilling at temperatures well above those causing freezing. Low temperature causes a loss of extracellular ion and water homeostasis in such insects, and chill injuries accumulate. Using an integrative and comparative approach, we examined the role of ion and water balance in insect chilling susceptibility/ tolerance. The Malpighian tubules (MT), of chill susceptible Drosophila species lost [Na(+)] and [K(+)] selectivity at low temperatures, which contributed to a loss of Na(+) and water balance and a deleterious increase in extracellular [K(+)]. By contrast, the tubules of chill tolerant Drosophila species maintained their MT ion selectivity, maintained stable extracellular ion concentrations, and thereby avoided injury. The most tolerant species were able to modulate ion balance while in a cold-induced coma and this ongoing physiological acclimation process allowed some individuals of the tolerant species to recover from chill coma during low temperature exposure. Accordingly, differences in the ability to maintain homeostatic control of water and ion balance at low temperature may explain large parts of the wide intra- and interspecific variation in insect chilling tolerance.