Physiological correlates of chill susceptibility in Lepidoptera

Measuring insect osmoregulation in vitro: A reference guide

Osmoregulation is influenced by a wide variety of biotic and abiotic variables, and maintenance of systemic osmoregulatory homeostasis is critical to insect fitness. Because insects are so small, accurately quantifying renal organ function is technically challenging, and often requires specialized equipment. On top of this, nearly a century of toiling in the laboratory has led to a wide and still growing variety of methods that can be difficult for novice researchers to disentangle. Here, we provide a reference guide for the most used in vitro approaches in the study of insect osmoregulation, including the Ramsay assay, Ussing chamber, epithelial potential measurement, scanning ion-selective electrode technique, and hindgut assays. Along the way, we highlight the history of each methodological innovation.

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 or 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.

Experimental evidence of a Neotropical pest insect moderately tolerant to complete freezing

Due to climate change, many regions are experiencing progressively milder winters. Consequently, pest insects from warm regions, particularly those with some tolerance to low temperatures, could expand their geographic range into these traditionally colder regions. The palm borer moth (Paysandisia archon) is a Neotropical insect that in recent decades has reached Europe and Asia as one of the worst pests of palm trees. Little is known about its ability to tolerate moderately cold winters and, therefore, to colonize new areas. In this work, we characterized the cold tolerance of Paysandisia archon by measuring its thermal limits: median lethal-temperature, LT50, chill-coma onset temperature, CTmin, supercooling point, SCP, freezing time and freezing survival. We found that this species was able to survive short periods of complete freezing, with survival rates of 87% after a 30-min freezing exposure, and 33% for a 1 h-exposure. It is then a moderately freeze-tolerant species, in contrast to all other lepidopterans native to warm areas, which are freeze-intolerant. Additionally, we investigated whether this insect improved its cold tolerance after either short or long pre-exposure to sub-lethal low temperatures. To that end, we studied potential changes in the main thermo-tolerance parameters and, using X-ray Computed Tomography, also in the morphological components of pretreated animals. We found that short pre-exposures did not imply significant changes in the SCP and CTmin values. In contrast, larvae with long pretreatments improved their survival to both freezing and low temperatures, and required longer times for complete freezing than the other groups. These long-term pre-exposed larvae also presented several morphological changes, including a reduction in water content that probably explained, at least in part, their longer freezing time and higher freezing survival. Our results represent the first cold tolerance characterization of this pest insect, which could be relevant to better design strategies to combat it.

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.

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.

Temporal regulation of temperature tolerances and gene expression in an arctic insect

Terrestrial arthropods in the Arctic are exposed to highly variable temperatures that frequently reach cold and warm extremes. Yet, ecophysiological studies on arctic insects typically focus on the ability of species to tolerate low temperatures, whereas studies investigating physiological adaptations of species to periodically warm and variable temperatures are few. In this study, we investigated temporal changes in thermal tolerances and the transcriptome in the Greenlandic seed bug Nysius groenlandicus, collected in the field across different times and temperatures in Southern Greenland. We found that plastic changes in heat and cold tolerances occurred rapidly (within hours) and at a daily scale in the field, and that these changes are correlated with diurnal temperature variation. Using RNA sequencing, we provide molecular underpinnings of the rapid adjustments in thermal tolerance across ambient field temperatures and in the laboratory. We show that transcriptional responses are sensitive to daily temperature changes, and days characterized by high temperature variation induced markedly different expression patterns than thermally stable days. Further, genes associated with laboratory-induced heat responses, including expression of heat shock proteins and vitellogenins, were shared across laboratory and field experiments, but induced at time points associated with lower temperatures in the field. Cold stress responses were not manifested at the transcriptomic level.

Osmoregulatory capacity at low temperature is critical for insect cold tolerance

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

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.

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.

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

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

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.

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.

Inter and Intraspecific Genomic Divergence in Drosophila montana Shows Evidence for Cold Adaptation

The genomes of species that are ecological specialists will likely contain signatures of genomic adaptation to their niche. However, distinguishing genes related to ecological specialism from other sources of selection and more random changes is a challenge. Here, we describe the genome of Drosophila montana, which is the most extremely cold-adapted Drosophila species known. We use branch tests to identify genes showing accelerated divergence in contrasts between cold- and warm-adapted species and identify about 250 genes that show differences, possibly driven by a lower synonymous substitution rate in cold-adapted species. We also look for evidence of accelerated divergence between D. montana and D. virilis, a previously sequenced relative, but do not find strong evidence for divergent selection on coding sequence variation. Divergent genes are involved in a variety of functions, including cuticular and olfactory processes. Finally, we also resequenced three populations of D. montana from across its ecological and geographic range. Outlier loci were more likely to be found on the X chromosome and there was a greater than expected overlap between population outliers and those genes implicated in cold adaptation between Drosophila species, implying some continuity of selective process at these different evolutionary scales.

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.

Thermal acclimation mitigates cold-induced paracellular leak from the Drosophila gut

Chill susceptible insects suffer tissue damage and die at low temperatures. The mechanisms that cause chilling injury are not well understood but a growing body of evidence suggests that a cold-induced loss of ion and water homeostasis leads to hemolymph hyperkalemia that depolarizes cells, leading to cell death. The apparent root of this cascade is the net leak of osmolytes down their concentration gradients in the cold. Many insects, however, are capable of adjusting their thermal physiology, and cold-acclimated Drosophila can maintain homeostasis and avoid injury better than warm-acclimated flies. Here, we test whether chilling causes a loss of epithelial barrier function in female adult Drosophila, and provide the first evidence of cold-induced epithelial barrier failure in an invertebrate. Flies had increased rates of paracellular leak through the gut epithelia at 0 °C, but cold acclimation reduced paracellular permeability and improved cold tolerance. Improved barrier function was associated with changes in the abundance of select septate junction proteins and the appearance of a tortuous ultrastructure in subapical intercellular regions of contact between adjacent midgut epithelial cells. Thus, cold causes paracellular leak in a chill susceptible insect and cold acclimation can mitigate this effect through changes in the composition and structure of transepithelial barriers.

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.