Genomic basis of adaptation to climate and parasite prevalence and the importance of odorant perception in the ant Temnothorax longispinosus

A co-evolutionary arms race ensues when parasites exhibit exploitative behaviour, which prompts adaptations in their hosts, in turn triggering counter-adaptations by the parasites. To unravel the genomic basis of this coevolution from the host's perspective, we collected ants of the host species Temnothorax longispinosus, parasitized by the social parasite Temnothorax americanus, from 10 populations in the northeastern United States exhibiting varying levels of parasite prevalence and living under different climatic conditions. We conducted a genome-wide association study (GWAS) to identify single nucleotide polymorphisms (SNPs) associated with both prevalence and climate. Our investigation highlighted a multitude of candidate SNPs associated with parasite prevalence, particularly in genes responsible for sensory perception of smell including odorant receptor genes. We further focused on population-specific compositions of cuticular hydrocarbons, a complex trait important for signalling, communication and protection against desiccation. The relative abundances of n-alkanes were correlated with climate, while there was only a trend between parasite prevalence and the relative abundances of known recognition cues. Furthermore, we identified candidate genes likely involved in the synthesis and recognition of specific hydrocarbons. In addition, we analysed the population-level gene expression in the antennae, the primary organ for odorant reception, and established a strong correlation with parasite prevalence. Our comprehensive study highlights the intricate genomic patterns forged by the interplay of diverse selection factors and how these are manifested in the expression of various phenotypes.

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.

Insecticidal efficacy and risk assessment of different neuropeptide analog combinations against the peach-potato aphid following topical exposure

BACKGROUND

Insect neuropeptides control essential physiological metabolic activities. In our previous studies, Capability/CAP2b (PK/CAPA) analog 1895 applied alone or as a combination of CAPA analogs (1895 + 2315) was reported to decrease aphid fitness. While this was obtained with the combination of two peptide analogs of the same neuropeptide class, the effect of combining peptide analogs of different neuropeptide classes has not been explored so far.

RESULTS

In this study, we assessed the effect of combinations of the PK/CAPA analog 1895 with neuropeptide analogs of four different classes [adipokinetic hormone (AKH) analog: 2271; myosuppressin analog: 2434; kinin analog: 2460; tachykinin-related peptide analog: 2463] on the fitness of aphids. We found that the combination of 1895 and AKH analog 2271 was the most effective one to control Myzus persicae. The triple combination 1895 + 2271 + 2315 provided a synergistic effect by further increasing aphid mortality and reducing reproduction relative to 1895 + 2315. Additionally, a biosafety assessment of the combination 1895 + 2271 + 2315 showed no significant lethal nor sub-lethal effects on survival rates and food intake for the pollinator (Bombus terrestris) and the two representative natural enemies (Harmonia axyridis and Nasonia vitripennis).

CONCLUSION

These results could facilitate establishment of the triple combination 1895 + 2271 + 2315, and/or inclusion of second generation analogs, as alternatives to broad spectrum and less friendly insecticides. © 2022 Society of Chemical Industry.

Physiological responses of freshwater insects to salinity: molecular-, cellular- and organ-level studies

Salinization of freshwater is occurring throughout the world, affecting freshwater biota that inhabit rivers, streams, ponds, marshes and lakes. There are many freshwater insects, and these animals are important for ecosystem health. These insects have evolved physiological mechanisms to maintain their internal salt and water balance based on a freshwater environment that has comparatively little salt. In these habitats, insects must counter the loss of salts and dilution of their internal body fluids by sequestering salts and excreting water. Most of these insects can tolerate salinization of their habitats to a certain level; however, when exposed to salinization they often exhibit markers of stress and impaired development. An understanding of the physiological mechanisms for controlling salt and water balance in freshwater insects, and how these are affected by salinization, is needed to predict the consequences of salinization for freshwater ecosystems. Recent research in this area has addressed the whole-organism response, but the purpose of this Review is to summarize the effects of salinization on the osmoregulatory physiology of freshwater insects at the molecular to organ level. Research of this type is limited, and pursuing such lines of inquiry will improve our understanding of the effects of salinization on freshwater insects and the ecosystems they inhabit.

Updates on ion and water transport by the Malpighian tubule

The Malpighian (renal) tubule is capable of transporting fluid at remarkable rates. This review will focus on recent insights into the mechanisms by which these high rates are achieved and controlled, with particular reference to the tubules of Drosophila melanogaster, in which the combination of physiology and genetics has led to particularly rapid progress. Like many vertebrate epithelia, the Drosophila tubule has specialized cell types, with active cation transport confined to a large, metabolically active principal cell; whereas the smaller intercalated stellate cell controls chloride and water shunts to achieve net fluid secretion. Recently, the genes underlying many of these processes have been identified, functionally validated and localized within the tubule. The imminent arrival of new types of post-genomic data (notably single cell sequencing) will herald an exciting era of new discovery.

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.

Genome-wide identification of neuropeptides and their receptor genes in Bemisia tabaci and their transcript accumulation change in response to temperature stresses

Insect neuropeptides play an important role in regulating physiological functions such as growth, development, behavior and reproduction. We identified temperature-sensitive neuropeptides and receptor genes of the cotton whitefly, Bemisia tabaci. We identified 38 neuropeptide precursor genes and 35 neuropeptide receptors and constructed a phylogenetic tree using additional data from other insects. As temperature adaptability enables B. tabaci to colonize a diversity of habitats, we performed quantitative polymerase chain reaction with two temperature stresses (low = 4 °C and high = 40 °C) to screen for temperature-sensitive neuropeptides. We found many neuropeptides and receptors that may be involved in the temperature adaptability of B. tabaci. This study is the first to identify B. tabaci neuropeptides and their receptors, and it will help to reveal the roles of neuropeptides in temperature adaptation of B. tabaci.

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 preserves hindgut reabsorption capacity at low temperature in a chill-susceptible insect, Locusta migratoria

Cold acclimation increases cold tolerance of chill-susceptible insects and the acclimation response often involves improved organismal ion balance and osmoregulatory function at low temperature. However, the physiological mechanisms underlying plasticity of ion regulatory capacity are largely unresolved. Here we used Ussing chambers to explore the effects of cold exposure on hindgut KCl reabsorption in cold- (11 °C) and warm-acclimated (30 °C) Locusta migratoria. Cooling (from 30 to 10 °C) reduced active reabsorption across recta from warm-acclimated locusts, while recta from cold-acclimated locusts maintained reabsorption at 10 °C. The differences in transport capacity were not linked to major rearrangements of membrane phospholipid profiles. Yet, the stimulatory effect of two signal transduction pathways were altered by temperature and/or acclimation. cAMP-stimulation increased reabsorption in both acclimation groups, with a strong stimulatory effect at 30 °C and a moderate stimulatory effect at 10 °C. cGMP-stimulation also increased reabsorption in both acclimation groups at 30 °C, but their response to cGMP differed at 10 °C. Recta from warm-acclimated locusts, characterised by reduced reabsorption at 10 °C, recovered reabsorption capacity following cGMP-stimulation at 10 °C. In contrast, recta from cold-acclimated locusts, characterised by sustained reabsorption at 10 °C, were unaffected by cGMP-stimulation. Furthermore, cold-exposed recta from warm-acclimated locusts were insensitive to bafilomycin-α1, a V-type H⁺-ATPase inhibitor, whereas this blocker reduced reabsorption across cold-exposed recta from cold-acclimated animals. In conclusion, bafilomycin-sensitive and cGMP-dependent transport mechanism(s) are likely blocked during cold exposure in warm-acclimated animals while preserved in cold-acclimated animals. These may in part explain the large differences in rectal ion transport capacity between acclimation groups at low temperature.

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.

Probing the Honey Bee Diet-Microbiota-Host Axis Using Pollen Restriction and Organic Acid Feeding

Microbial metabolites are considered important drivers of diet-based microbiota influence on the host, however, mechanistic models are confounded by interactions between diet, microbiota function, and host physiology. The honey bee harbors a simple microbiota that produces organic acids as fermentation products of dietary nectar and pollen, making it a model for gut microbiota research. Herein, we demonstrate that bacterial abundance in the honey bee gut is partially associated with the anterior rectum epithelium. We used dietary pollen restriction and organic acid feeding treatments to obtain information about the role of undigested pollen as a microbiota growth substrate and the impact of bacterial fermentation products on honey bee enteroendocrine signaling. Pollen restriction markedly reduced total and specific bacterial 16S rRNA abundance in the anterior rectum but not in the ileum. Anterior rectum expression levels of bacterial fermentative enzyme gene transcripts (acetate kinase, lactate dehydrogenase, and hydroxybutyryl-CoA dehydrogenase) were reduced in association with diet-induced microbiota shifts. To evaluate the effects of fermentative metabolites on host enteroendocrine function, pollen-restricted bees were fed an equimolar mixture of organic acid sodium salts (acetate, lactate, butyrate, formate, and succinate). Organic acid feeding significantly impacted hindgut enteroendocrine signaling gene expression, rescuing some effects of pollen restriction. This was specifically manifested by tissue-dependent expression patterns of neuropeptide F and allatostatin pathways, which are implicated in energy metabolism and feeding behaviors. Our findings provide new insights into the diet-microbiota-host axis in honey bees and may inform future efforts to improve bee health through diet-based microbiota manipulations.

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.

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).

Delayed mortality and sublethal effects of cold stress in Drosophila melanogaster

Analysis of sublethal responses in cold-stressed insects can provide important information about fitness costs and a better understanding of the physiological mechanisms used to prevent and/or to cope with cold injury. Yet, such responses are understudied and often neglected in the literature. Here, we analyzed the effects of cold stress applied to larvae on the mortality/survival and fitness parameters of survivor adults of the vinegar fly, Drosophila melanogaster. Third instar larvae (either cold-sensitive or cold-acclimated) were exposed to either supercooling or freezing stress, both at -5 °C. A whole array of sublethal effects were observed, from mortality that occurs with some delay after cold stress, through delayed development to the pupal stage, to shortened life-span of the adult, and decreased female fecundity. Taking the sublethal effects into account improves the ecological meaningfulness of cold hardiness assay outcomes. For instance, we observed that although more than 80% of cold-acclimated larvae survive freezing to -5 °C, less than 10% survive until adulthood, and survivor females exhibit more than 50% reduction in their fecundity relative to controls. Female fecundity was positively correlated with dry mass and negatively correlated with total protein and glycogen stores. Hence, these parameters may serve as good predictors of survivor adult female fecundity. Further, we provide the concept of a two-component defense system, which (based on analysis of sublethal effects on fitness parameters) distinguishes between physiological mechanisms that help insects to resist (reduce or avoid) or tolerate (survive or repair) injuries linked to cold stress.

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.

Anatomy and Physiology of the Digestive Tract of Drosophila melanogaster

The gastrointestinal tract has recently come to the forefront of multiple research fields. It is now recognized as a major source of signals modulating food intake, insulin secretion and energy balance. It is also a key player in immunity and, through its interaction with microbiota, can shape our physiology and behavior in complex and sometimes unexpected ways. The insect intestine had remained, by comparison, relatively unexplored until the identification of adult somatic stem cells in the Drosophila intestine over a decade ago. Since then, a growing scientific community has exploited the genetic amenability of this insect organ in powerful and creative ways. By doing so, we have shed light on a broad range of biological questions revolving around stem cells and their niches, interorgan signaling and immunity. Despite their relatively recent discovery, some of the mechanisms active in the intestine of flies have already been shown to be more widely applicable to other gastrointestinal systems, and may therefore become relevant in the context of human pathologies such as gastrointestinal cancers, aging, or obesity. This review summarizes our current knowledge of both the formation and function of the Drosophila melanogaster digestive tract, with a major focus on its main digestive/absorptive portion: the strikingly adaptable adult midgut.

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.