Transmembrane ion distribution during recovery from freezing in the woolly bear caterpillar Pyrrharctia isabella (Lepidoptera: Arctiidae)

Management of inorganic elements by overwintering physiology of cold hardy larvae of European corn borer (Ostrinia nubilalis, Hbn.)

The European corn borer (Ostrinia nubilalis, Hbn.), enters diapause, a strategy characterized by arrest of development and reproduction, reduction of metabolic rate and the emergence of increased resistance to challenging seasonal conditions as low sub-zero winter temperatures. The aim of this study was to investigate the potential role of inorganic elements in the ecophysiology of O. nubilalis, analysing their content in the whole body, hemolymph and fat body, both metabolically active, non-diapausing and overwintering diapausing larvae by ICP-OES spectrometer following the US EPA method 200.7:2001. O nubilalis as many phytophagous lepidopteran species maintain a very low extracellular sodium concentration and has potassium as dominant cation in hemolymph of their larvae. Changes in hemolymph and the whole body sodium content occur already at the onset of diapause (when the mean environmental temperatures are still high above 0 ºC) and remain stable during the time course of diapause when larvae of this species cope with sub-zero temperatures, it seems that sodium content regulation is rather a part of diapausing program than the direct effect of exposure to low temperatures. Compared to non-diapausing O. nubilalis larvae, potassium levels are much higher in the whole body and fat body of diapausing larvae and substantially increase approaching the end of diapause. The concentration of Ca, Mg, P and S differed in the whole body, hemolymph and fat body between non-diapausing and diapausing larvae without a unique trend during diapause, except an increase in their contents at the end of diapause.

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.

L-proline feeding for augmented freeze tolerance of Camponotus japonicus Mayr

The successful cryopreservation of organs is a strong and widespread demand around the world but faces great challenges. The mechanisms of cold tolerance of organisms in nature inspirit researchers to find new solutions for these challenges. Especially, the thermal, mechanical, biological and biophysical changes during the regulation of freezing tolerance process should be studied and coordinated to improve the cryopreservation technique and quality of complex organs. Here the cold tolerance of the Japanese carpenter ants, Camponotus japonicus Mayr, was greatly improved by using optimal protocols and feeding on L-proline-augmented diets for 5 days. When cooling to -27.66 °C, the survival rate of frozen ants increased from 37.50% ± 1.73% to 83.88% ± 3.67%. Profiling of metabolites identified the concentration of whole-body L-proline of ants increased from 1.78 to 4.64 ng g^-1 after 5-day feeding. High L-proline level, together with a low rate of osmotically active water and osmotically inactive water facilitated the prevention of cryoinjury. More importantly, gene analysis showed that the expression of ribosome genes was significantly up-regulated and played an important role in manipulating freezing tolerance. To the best of our knowledge, this is the first study to link genetic variation to the enhancement of ants' cold tolerance by feeding exogenous cryoprotective compound. It is worth noting that the findings provide the theoretical and technical foundation for the cryopreservation of more complex tissues, organs, and living organisms.

Loss of ion homeostasis is not the cause of chill coma or impaired dispersal in false codling moth Thaumatotibia leucotreta (Lepidoptera: Tortricidae)

Dispersal is a central requirement of a successful sterile insect release programme, but field-released false codling moth (FCM) typically suffer from poor dispersal ability, especially at low ambient temperatures. Here we test the hypothesis that poor activity and dispersal in FCM is caused by delayed or perturbed recovery of ion and/or water homeostasis after chilling for handling and transport prior to field release. Hemolymph and flight muscle were collected from two treatment groups at three time points that targeted thermal conditions above and below the chill coma induction threshold of ~ 6 °C: 1) control moths kept at 25 °C, 2) moths exposed to 3 °C or 9 °C for 4 h, and 3) moths allowed to recover at 25 °C for 24 h after exposure to either 3 °C or 9 °C. We measured concentrations of Na⁺, K⁺ and Mg²⁺ in the hemolymph and muscle collected at each time point. Exposure to a chill-coma inducing temperature had little effect overall on ion balance in the hemolymph and flight muscle of false codling moth, but hemolymph [Na⁺] decreased from 10.4 ± 0.4 mM to 6.9 ± 0.7 mM as moths were chilled to 3 °C and then increased to 10.4 ± 0.9 mM after the 24 h recovery period. In the 9 °C cooling treatment, [K⁺] increased from 8.2 ± 0.5 mM during chilling to 14.1 ± 1.9 mM after the 24 h recovery period. No changes were seen in equilibrium potentials in either of the ions measured. Thus, we did not find evidence that water and ion homeostasis are lost by the moths in chill coma and conclude that reduced dispersal in field-released moths is not direct a consequence of the costs of re-establishment of homeostasis.

Physiological responses to fluctuating temperatures are characterized by distinct transcriptional profiles in a solitary bee

Exposure to stressful low temperatures during development can result in the accumulation of deleterious physiological effects called chill injury. Metabolic imbalances, disruptions in ion homeostasis and oxidative stress contribute to the increased mortality of chill-injured insects. Interestingly, survival can be significantly increased when chill-susceptible insects are exposed to a daily warm-temperature pulse during chilling. We hypothesize that warm pulses allow for the repair of damage associated with chill injury. Here, we describe transcriptional responses during exposure to a fluctuating thermal regime, relative to constant chilled temperatures, during pupal development in the alfalfa leafcutting bee, Megachile rotundata, using a combination of RNA-seq and qPCR. Pupae were exposed to either a constant, chilled temperature of 6°C, or 6°C with a daily pulse of 20°C for 7 days. RNA-seq after experimental treatment revealed differential expression of transcripts involved in construction of cell membranes, oxidation-reduction and various metabolic processes. These mechanisms provide support for shared physiological responses to chill injury across taxa. The large number of differentially expressed transcripts observed after 7 days of treatment suggests that the initial divergence in expression profiles between the two treatments occurred upstream of the time point sampled. Additionally, the differential expression profiles observed in this study show little overlap with those differentially expressed during temperature stress in the diapause state of M. rotundata While the mechanisms governing the physiological response to low-temperature stress are shared, the specific transcripts associated with the response differ between life stages.

Physiological correlates of chill susceptibility in Lepidoptera

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

Cold-hardening during long-term acclimation in a freeze-tolerant woolly bear caterpillar, Pyrrharctia isabella

The banded woolly bear caterpillar, Pyrrharctia isabella (Lepidoptera: Erebidae), overwinters in leaf litter and survives freezing under natural conditions. Following 18 weeks of cold acclimation at 5°C, all caterpillars could survive 1 week of continuous freezing at -20°C or seven cycles of freezing-thawing at -20°C, but none survived freezing at -80°C. Field-collected caterpillars had a temperature of crystallization of -7.7±0.5°C that decreased significantly to -9.5±0.6°C after 12 weeks of acclimation at 5°C. Hemolymph levels of free proline, total amino acids and proteins reached a peak during the first 4 weeks of acclimation; concomitantly, hemolymph osmolality increased markedly during this interval (from 364 to 1282 mosmol kg(-1)). In contrast, hemolymph pH decreased during the first 4 weeks of acclimation before this trend reversed and pH values gradually returned to initial values. However, pH reached its peak value following 1 week at -20°C, but decreased after longer periods of freezing. During cold acclimation, cholesterol levels decreased in the hemolymph and the membrane fraction of fat body but not in other tissues. Lethal freezing at -80°C reduced cell survival in foregut tissue and caused leakage of free proline, total amino acids and proteins from tissues into the hemolymph. The addition of glycerol to the bathing medium reduced freezing injury in fat body cells, as evidenced by reduced leakage of amino acids and proteins.

Sodium distribution predicts the chill tolerance of Drosophila melanogaster raised in different thermal conditions

Many insects, including the model holometabolous insect Drosophila melanogaster, display remarkable plasticity in chill tolerance in response to the thermal environment experienced during development or as adults. At low temperatures, many insects lose the ability to regulate Na+ balance, which is suggested to cause a secondary loss of hemolymph water to the tissues and gut lumen that concentrates the K+ remaining in the hemolymph. The resultant increase in extracellular [K+] inhibits neuromuscular excitability and is proposed to cause cellular apoptosis and injury. The present study investigates whether and how variation in chill tolerance induced through developmental and adult cold acclimation is associated with changes in Na+, water, and K+ balance. Developmental and adult cold acclimation improved the chilling tolerance of D. melanogaster in an additive manner. In agreement with the proposed model, these effects were intimately related to differences in Na+ distribution prior to cold exposure, such that chill-tolerant flies had low hemolymph [Na+], while intracellular [Na+] was similar among treatment groups. The low hemolymph Na+ of cold-acclimated flies allowed them to maintain hemolymph volume, prevent hyperkalemia, and avoid injury following chronic cold exposure. These findings extend earlier observations of hemolymph volume disruption during cold exposure to the most ubiquitous model insect ( D. melanogaster), highlight shared mechanisms of developmental and adult thermal plasticity and provide strong support for ionoregulatory failure as a central mechanism of insect chill susceptibility.

A hierarchy of factors influence discontinuous gas exchange in the grasshopper Paracinema tricolor (Orthoptera: Acrididae)

The evolutionary origin and maintenance of discontinuous gas exchange (DGE) in tracheate arthropods are poorly understood and highly controversial. We investigated prioritization of abiotic factors in the gas exchange control cascade by examining oxygen, water and haemolymph pH regulation in the grasshopper Paracinema tricolor. Using a full-factorial design, grasshoppers were acclimated to hypoxic or hyperoxic (5% O2, 40% O2) gas conditions, or dehydrated or hydrated, whereafter their CO2 release was measured under a range of O2 and relative humidity (RH) conditions (5%, 21%, 40% O2 and 5%, 60%, 90% RH). DGE was significantly less common in grasshoppers acclimated to dehydrating conditions compared with the other acclimations (hypoxia, 98%; hyperoxia, 100%; hydrated, 100%; dehydrated, 67%). Acclimation to dehydrating conditions resulted in a significant decrease in haemolymph pH from 7.0±0.3 to 6.6±0.1 (mean ± s.d., P=0.018) and also significantly increased the open (O)-phase duration under 5% O2 treatment conditions (5% O2, 44.1±29.3 min; 40% O2, 15.8±8.0 min; 5% RH, 17.8±1.3 min; 60% RH, 24.0±9.7 min; 90% RH, 20.6±8.9 min). The observed acidosis could potentially explain the extension of the O-phase under low RH conditions, when it would perhaps seem more useful to reduce the O-phase to lower respiratory water loss. The results confirm that DGE occurrence and modulation are affected by multiple abiotic factors. A hierarchical framework for abiotic factors influencing DGE is proposed in which the following stressors are prioritized in decreasing order of importance: oxygen supply, CO2 excretion and pH modulation, oxidative damage protection and water savings.