Muscle membrane potential and insect chill coma

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.

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.

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.

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.

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.

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.

Local adaptation across a complex bioclimatic landscape in two montane bumble bee species

Understanding evolutionary responses to variation in temperature and precipitation across species ranges is of fundamental interest given ongoing climate change. The importance of temperature and precipitation for multiple aspects of bumble bee (Bombus) biology, combined with large geographic ranges that expose populations to diverse environmental pressures, make these insects well-suited for studying local adaptation. Here, we analyzed genome-wide sequence data from two widespread bumble bees, Bombus vosnesenskii and Bombus vancouverensis, using multiple environmental association analysis methods to investigate climate adaptation across latitude and altitude. The strongest signatures of selection were observed in B. vancouverensis, but despite unique responses between species for most loci, we detected several shared responses. Genes relating to neural and neuromuscular function and ion transport were especially evident with respect to temperature variables, while genes relating to cuticle formation, tracheal and respiratory system development, and homeostasis were associated with precipitation variables. Our data thus suggest that adaptive responses for tolerating abiotic variation are likely to be complex, but that several parallels among species can emerge even for these complex traits and landscapes. Results provide the framework for future work into mechanisms of thermal and desiccation tolerance in bumble bees and a set of genomic targets that might be monitored for future conservation efforts.

Heat shock sensitivity of adult male fertility in the parasitoid wasp Anisopteromalus calandrae (Hymenoptera, Pteromalidae)

In insects, decreased reproduction is a sublethal consequence of high temperatures, with males being more sensitive to this in many species. In hymenoptera, arrhenotokous parthenogenesis means that female offspring are produced using sperm and are thus diploid, while males are haploid. Consequently, sperm stocks in males and females (after copulation) are a key regulator of the sex ratio. Anisopteromalus calandrae is a parasitoid wasp in which males can suffer from subfertility due to a drastic decrease in sperm count after exposure to high temperatures during a critical early pupal stage. However, in this species spermatogenesis continues during adulthood, therefore the heat sensitivity of adult males remains to be studied. Laboratory studies were conducted on virgin and previously mated young adult males under control (30 °C) and heat shock (10 min at 48 °C) conditions to exhaust their initial sperm stock. After heat shock, in both virgin and already mated males, the individual sperm potential was half that of controls. Both groups continuously produced sperm, but sperm stock of heat shocked males' never reached that of the controls. Heat shock reduced survival at 10 days only in previously experienced males but had no impact on the mating ability in competition for a female compared to controls. Despite a reduced sperm count, heat shocked males had fully fertile spermatozoa. Such a physiological response to heat shock in a species with continuous sperm production could be of major interest for both wild populations in a context of temperature variations and parasitoid wasps introduced for agronomical purposes.

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.

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).

Fluctuating thermal regime preserves physiological homeostasis and reproductive capacity in Drosophila suzukii

Drosophila suzukii, an invasive species recently introduced in Europe, lays eggs in thin-skinned fruits and causes huge financial losses to fruit growers. One potential way to control this pest is the sterile insect technique (SIT) which demands a large stock of reproductive females to produce millions of sterile males to be released on demand. Unfortunately, Drosophila stocks age quickly, show declining fecundity when maintained at warm temperatures and conversely, they die from chill injury if they are maintained at constant low temperature. Here we investigate the potential of fluctuating thermal regime (FTR) as a storage method that harness the benefits of both warm and cold storage. Using a FTR with a daily warm period (1 h 20 at 25 °C) and cold period (20 h at 3 °C), interspaced by gradual heating and cooling, we compared longevity, fecundity and physiological condition between FTR females and females exposed to constant 25 °C and 3 °C. As hypothesised, FTR flies experienced much slower senescence (>3-fold increase in lifespan) and they preserved fecundity to a much higher age than flies from constant 25 °C. Flies maintained at constant 3 °C quickly died from chill injuries caused by a gradual loss of ion and water balance. In contrast, FTR flies were able to maintain ion and water balance (similar to 25 °C flies) as they were allowed to recover homeostasis during the short warm periods. Together these results demonstrate that FTR represents a useful protocol for storage of Drosophila stocks, and more broadly, this shows that the benefits of FTR are tightly linked with the insect ability to recover physiological homeostasis during the short warm periods.

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

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

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.

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.

Cold tolerance of Drosophila species is tightly linked to epithelial K+ transport capacity of the Malpighian tubules and rectal pads

Insect chill tolerance is strongly associated with the ability to maintain ion and water homeostasis during cold exposure. Maintenance of K+ balance is particularly important due to its role in setting the cell membrane potential that is involved in many aspects of cellular function and viability. In most insects, K+ balance is maintained through secretion at the Malpighian tubules balancing reabsorption from the hindgut and passive leak arising from the gut lumen. Here, we used a scanning ion-selective electrode technique (SIET) system at benign (23°C) and low (6°C) temperature to examine K+ flux across the Malpighian tubules and the rectal pads in the hindgut in five Drosophila species that differ in cold tolerance. We found that chill tolerant species were better at maintaining K+ secretion and supressing reabsorption during cold exposure. In contrast, chill susceptible species exhibited large reductions in secretion with no change, or a paradoxical increase, in K+ reabsorption. Using an assay to measure paracellular leak we found that chill susceptible species experience a large increase in leak during cold exposure, which could explain the increased K+ reabsorption found in these species. Our data therefore strongly support the hypothesis that cold tolerant Drosophila species are better at maintaining K+ homeostasis through an increased ability to maintain K+ secretion rates and through reduced leakage of K+ towards the hemolymph. These adaptations are manifested both at the Malpighian tubule and at the rectal pads in the hindgut and ensure that cold tolerant species experience less perturbation of K+ homeostasis during 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.

Cold acclimation wholly reorganizes the Drosophila melanogaster transcriptome and metabolome

Cold tolerance is a key determinant of insect distribution and abundance, and thermal acclimation can strongly influence organismal stress tolerance phenotypes, particularly in small ectotherms like Drosophila. However, there is limited understanding of the molecular and biochemical mechanisms that confer such impressive plasticity. Here, we use high-throughput mRNA sequencing (RNA-seq) and liquid chromatography – mass spectrometry (LC-MS) to compare the transcriptomes and metabolomes of D. melanogaster acclimated as adults to warm (rearing) (21.5 °C) or cold conditions (6 °C). Cold acclimation improved cold tolerance and led to extensive biological reorganization: almost one third of the transcriptome and nearly half of the metabolome were differentially regulated. There was overlap in the metabolic pathways identified via transcriptomics and metabolomics, with proline and glutathione metabolism being the most strongly-supported metabolic pathways associated with increased cold tolerance. We discuss several new targets in the study of insect cold tolerance (e.g. dopamine signaling and Na⁺-driven transport), but many previously identified candidate genes and pathways (e.g. heat shock proteins, Ca²⁺ signaling, and ROS detoxification) were also identified in the present study, and our results are thus consistent with and extend the current understanding of the mechanisms of insect chilling tolerance.

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.

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.

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.