How insects survive the cold: molecular mechanisms—a review

Dermal white adipose tissue: a new component of the thermogenic response

Recent literature suggests that the layer of adipocytes embedded in the skin below the dermis is far from being an inert spacer material. Instead, this layer of dermal white adipose tissue (dWAT) is a regulated lipid layer that comprises a crucial environmental defense. Among all the classes of biological molecules, lipids have the lowest thermal conductance and highest insulation potential. This property can be exploited by mammals to reduce heat loss, suppress brown adipose tissue activation, reduce the activation of thermogenic programs, and increase metabolic efficiency. Furthermore, this layer responds to bacterial challenge to provide a physical barrier and antimicrobial disinfection, and its expansion supports the growth of hair follicles and regenerating skin. In sum, this dWAT layer is a key defensive player with remarkable potential for modifying systemic metabolism, immune function, and physiology. In this review, we discuss the key literature illustrating the properties of this recently recognized adipose depot.

Protein expression parallels thermal tolerance and ecologic changes in the diversification of a diving beetle species complex

Physiological changes associated with evolutionary and ecological processes such as diversification, range expansion or speciation are still incompletely understood, especially for non-model species. Here we study differences in protein expression in response to temperature in a western Mediterranean diving beetle species complex, using two-dimensional differential gel electrophoresis with one Moroccan and one Iberian population each of Agabus ramblae and Agabus brunneus. We identified proteins with significant expression differences after thermal treatments comparing them with a reference EST library generated from one of the species of the complex (A. ramblae). The colonisation during the Middle Pleistocene of the Iberian peninsula by A. ramblae, where maximum temperatures and seasonality are lower than in the ancestral north African range, was associated with changes in the response to 27 °C in proteins related to energy metabolism. The subsequent speciation of A. brunneus from within populations of Iberian A. ramblae was associated with changes in the expression of several stress-related proteins (mostly chaperons) when exposed to 4 °C. These changes are in agreement with the known tolerance to lower temperatures of A. brunneus, which occupies a larger geographical area with a wider range of climatic conditions. In both cases, protein expression changes paralleled the evolution of thermal tolerance and the climatic conditions experienced by the species. However, although the colonisation of the Iberian peninsula did not result in morphological change, the speciation process of A. brunneus within Iberia involved genetic isolation and substantial differences in male genitalia and body size and shape.

Cloning and differential expression of five heat shock protein genes associated with thermal stress and development in the polyphagous predatory mite Neoseiulus cucumeris (Acari: Phytoseiidae)

In order to explore the role of heat shock proteins (Hsps) during thermal stress and development in the predatory mite Neoseiulus cucumeris (Oudemans), we cloned and characterized five full-length Hsp genes. We investigated the expression levels of these genes by quantitative real-time PCR. The five genes characterized here were NcHsp90, NcHsp75, NcHsp70, NcHsp60, and NcHsp40. These Hsps showed high sequence conservation and had greatest identity with heat shock proteins of Metaseiulus occidentalis and other mite and insect species. All five NcHsp genes showed changes in their levels of expression during development. Higher levels of expression were observed in adult females than in adult males, but there were no significant changes between pre-oviposition and post-oviposition stages in the females. NcHsp90, NcHsp75, and NcHsp70 expression levels were up-regulated after a heat shock, and the increases in NcHsp75 and NcHsp70 expression levels were maintained for at least 3 h. Up-regulation of NcHsp60 and NcHsp40 was not detected after 1 h at a high temperature (35-45 °C); however, a significant down-regulation was observed after 3 h heat exposure at 35 °C and 3 h recovery at 25 °C. Cold shock treatment (-5 to 15 °C) for 1 h did not acute elicit changes in the expression levels of any of the genes. At 5 °C, the expression levels of NcHsp90 significantly increased after 6 or 24 h exposure compared to the levels after 1 h exposure. Thus, expression of Hsp genes in N. cucumeris reflected developmental changes, sexual difference, and variable induced response to thermal stress. Increased expression of Hsps might protect N. cucumeris individuals under extreme temperature conditions. Therefore, it may be possible to enhance the thermal tolerance of commercially available N. cucumeris using temperature acclimation. Treatment at 35 °C should be suitable for such acclimation.

Insect glycerol transporters evolved by functional co-option and gene replacement

Transmembrane glycerol transport is typically facilitated by aquaglyceroporins in Prokaryota and Eukaryota. In holometabolan insects however, aquaglyceroporins are absent, yet several species possess polyol permeable aquaporins. It thus remains unknown how glycerol transport evolved in the Holometabola. By combining phylogenetic and functional studies, here we show that a more efficient form of glycerol transporter related to the water-selective channel AQP4 specifically evolved and multiplied in the insect lineage, resulting in the replacement of the ancestral branch of aquaglyceroporins in holometabolan insects. To recapitulate this evolutionary process, we generate specific mutants in distantly related insect aquaporins and human AQP4 and show that a single mutation in the selectivity filter converted a water-selective channel into a glycerol transporter at the root of the crown clade of hexapod insects. Integration of phanerozoic climate models suggests that these events were associated with the emergence of complete metamorphosis and the unparalleled radiation of insects.

Studying translational control in non-model stressed organisms by polysomal profiling

In stressed organisms, strategic proteins are selectively translated even if the global process of protein synthesis is compromised. The determination of protein concentrations in tissues of non-model organisms (thus with limited genomic information) is challenging due to the absence of specific antibodies. Moreover, estimating protein levels quantifying transcriptional responses may be misleading, because translational control mechanisms uncouple protein and mRNAs abundances. Translational control is increasingly recognized as a hub where regulation of gene expression converges to shape proteomes, but it is almost completely overlooked in molecular ecology studies. An interesting approach to study translation and its control mechanisms is the analysis of variations of gene-specific translational efficiencies by quantifying mRNAs associated to ribosomes. In this paper, we propose a robust and streamlined pipeline for purifying ribosome-associated mRNAs and calculating global and gene-specific translation efficiencies from non-model insect's species. This method might found applications in molecular ecology to study responses to environmental stressors in non-model organisms.

PARASITOID VENOM INDUCES METABOLIC CASCADES IN FLY HOSTS

Parasitoid wasps inject insect hosts with a cocktail of venoms to manipulate the physiology, development, and immunity of the hosts and to promote development of the parasitoid offspring. The jewel wasp Nasonia vitripennis is a model parasitoid with at least 79 venom proteins. We conducted a high-throughput analysis of Nasonia venom effects on temporal changes of 249 metabolites in pupae of the flesh fly host (Sarcophaga bullata), over a five-day time course. Our results show that venom does not simply arrest the metabolism of the fly host. Rather, it targets specific metabolic processes while keeping hosts alive for at least five days post venom injection by the wasp. We found that venom: (a) Activates the sorbitol biosynthetic pathway while maintaining stable glucose levels, (b) Causes a shift in intermediary metabolism by switching to anaerobic metabolism and blocking the tricarboxylic acid cycle, (c) Arrests chitin biosynthesis that likely reflects developmental arrest of adult fly structures, (d) Elevates the majority of free amino acids, and (e) May be increasing phospholipid degradation. Despite sharing some metabolic effects with cold treatment, diapause, and hypoxia, the venom response is distinct from these conditions. Because Nasonia venom dramatically increases sorbitol levels without changing glucose levels, it could be a useful model for studying the regulation of the sorbitol pathway, which is relevant to diabetes research. Our findings generally support the view that parasitoid venoms are a rich source of bioactive molecules with potential biomedical applications.

Frostbite protection in mice expressing an antifreeze glycoprotein

Ectotherms in northern latitudes are seasonally exposed to cold temperatures. To improve survival under cold stress, they use diverse mechanisms to increase temperature resistance and prevent tissue damage. The accumulation of anti-freeze proteins that improve cold hardiness occurs in diverse species including plants, arthropods, fish, and amphibians. We previously identified an Ixodes scapularis anti-freeze glycoprotein, named IAFGP, and demonstrated its cold protective function in the natural tick host and in a transgenic Drosophila model. Here we show, in a transgenic mouse model expressing an anti-freeze glycoprotein, that IAFGP protects mammalian cells and mice from cold shock and frostbite respectively. Transgenic skin samples showed reduced cell death upon cold storage ex vivo and transgenic mice demonstrated increased resistance to frostbite injury in vivo. IAFGP actively protects mammalian tissue from freezing, suggesting its application for the prevention of frostbite, and other diseases associated with cold exposure.

Invariance and plasticity in the Drosophila melanogaster metabolomic network in response to temperature

Background

Metabolomic responses to extreme thermal stress have recently been investigated in Drosophila melanogaster. However, a network level understanding of metabolomic responses to longer and less drastic temperature changes, which more closely reflect variation in natural ambient temperatures experienced during development and adulthood, is currently lacking. Here we use high-resolution, non-targeted metabolomics to dissect metabolomic changes in D. melanogaster elicited by moderately cool (18°C) or warm (27°C) developmental and adult temperature exposures.

Results

We find that temperature at which larvae are reared has a dramatic effect on metabolomic network structure measured in adults. Using network analysis, we are able to identify modules that are highly differentially expressed in response to changing developmental temperature, as well as modules whose correlation structure is strongly preserved across temperature.

Conclusions

Our results suggest that the effect of temperature on the metabolome provides an easily studied and powerful model for understanding the forces that influence invariance and plasticity in biological networks.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-014-0139-6) contains supplementary material, which is available to authorized users.

Novel synthetic Medea selfish genetic elements drive population replacement in Drosophila; a theoretical exploration of Medea-dependent population suppression

Insects act as vectors for diseases of plants, animals, and humans. Replacement of wild insect populations with genetically modified individuals unable to transmit disease provides a potentially self-perpetuating method of disease prevention. Population replacement requires a gene drive mechanism in order to spread linked genes mediating disease refractoriness through wild populations. We previously reported the creation of synthetic Medea selfish genetic elements able to drive population replacement in Drosophila. These elements use microRNA-mediated silencing of myd88, a maternally expressed gene required for embryonic dorso-ventral pattern formation, coupled with early zygotic expression of a rescuing transgene, to bring about gene drive. Medea elements that work through additional mechanisms are needed in order to be able to carry out cycles of population replacement and/or remove existing transgenes from the population, using second-generation elements that spread while driving first-generation elements out of the population. Here we report the synthesis and population genetic behavior of two new synthetic Medea elements that drive population replacement through manipulation of signaling pathways involved in cellular blastoderm formation or Notch signaling, demonstrating that in Drosophila Medea elements can be generated through manipulation of diverse signaling pathways. We also describe the mRNA and small RNA changes in ovaries and early embryos associated from Medea-bearing females. Finally, we use modeling to illustrate how Medea elements carrying genes that result in diapause-dependent female lethality could be used to bring about population suppression.

Analysis of survival, gene expression and behavior following chill-coma in the medfly Ceratitis capitata: effects of population heterogeneity and age

The medfly Ceratitis capitata is an agricultural pest distributed worldwide thanks, in part, to its phenotypic plasticity of thermal tolerance. Cold exposure has been shown to reduce C. capitata survival, which may affect its distribution in areas with subfreezing temperatures. When insects are increasingly cooled, they attain a critical thermal threshold and enter a chill-coma state characterized by cessation of movement. It is not clear how a rapid cold exposure affects the physiological state of medflies, and how this is influenced by age and population heterogeneity. In order to approach these questions, C. capitata single-sex laboratory populations of 15 and 30 days old were subjected to a chill-coma recovery assay, and separated according to their recovery time in three subgroups: Fast-Subgroups, Intermediate-Subgroups, and Slow-Subgroups. Thereafter, we analyzed their survival, behavioral, and gene expression outputs. In female and old male populations, we found that flies with the slowest recovery time had a reduced life expectancy, a higher initial mortality rate, and a worse climbing performance compared with flies that recovered faster. Therefore, we were able to separate subgroups that developed chilling-injury from subgroups that had a reversible full recovery after cold exposure. The gene expression analysis of the heat shock protein genes hsp70 and hsp83 showed no clear association with the parameters studied. Interestingly, thorax expression levels of the Cu/Zn superoxide dismutase gene were elevated during the recovery phase in the Fast-Subgroups, but remained constant in the Slow-Subgroups that developed chilling-injury. On the other hand, none of the young male subgroups seemed to have suffered irreversible damage. Thus, we concluded that depending on age and population heterogeneity, chill-coma recovery time points out significant differences on individual cold tolerance. Moreover, the inability to properly induce the antioxidant defense system to counteract the oxidative damage caused by cold seems to contribute to the development of chilling-injury.

Molecular basis of chill resistance adaptations in poikilothermic animals

Chill and freeze represent very different components of low temperature stress. Whilst the principal mechanisms of tissue damage and of acquired protection from freeze-induced effects are reasonably well established, those for chill damage and protection are not. Non-freeze cold exposure (i.e. chill) can lead to serious disruption to normal life processes, including disruption to energy metabolism, loss of membrane perm-selectivity and collapse of ion gradients, as well as loss of neuromuscular coordination. If the primary lesions are not relieved then the progressive functional debilitation can lead to death. Thus, identifying the underpinning molecular lesions can point to the means of building resistance to subsequent chill exposures. Researchers have focused on four specific lesions: (i) failure of neuromuscular coordination, (ii) perturbation of bio-membrane structure and adaptations due to altered lipid composition, (iii) protein unfolding, which might be mitigated by the induced expression of compatible osmolytes acting as 'chemical chaperones', (iv) or the induced expression of protein chaperones along with the suppression of general protein synthesis. Progress in all these potential mechanisms has been ongoing but not substantial, due in part to an over-reliance on straightforward correlative approaches. Also, few studies have intervened by adoption of single gene ablation, which provides much more direct and compelling evidence for the role of specific genes, and thus processes, in adaptive phenotypes. Another difficulty is the existence of multiple mechanisms, which often act together, thus resulting in compensatory responses to gene manipulations, which may potentially mask disruptive effects on the chill tolerance phenotype. Consequently, there is little direct evidence of the underpinning regulatory mechanisms leading to induced resistance to chill injury. Here, we review recent advances mainly in lower vertebrates and in arthropods, but increasingly in genetic model species from a broader range of taxa.

De novo transcriptome of the desert beetle Microdera punctipennis (Coleoptera: Tenebrionidae) using Illumina RNA-seq technology

Insects in Tenebrionidae have unique stress adaptations that allow them to survive temperature extremes. We report here a gene expression profiling of Microdera punctipennis, a beetle in desert region, to gain a global view of its environmental adaptations. A total of 48,158,004 reads were obtained by transcriptome sequencing, and the de novo assembly yielded 56,348 unigenes with an average length of 666 bp. Based on similarity searches with a cut-off E-value of 10⁻⁵ against two protein sequence databases, 41,109 of the unigenes (about 72.96 %) were matched to known proteins. An in-depth analysis of the data revealed a large number of genes were associated with environmental stress, including genes that encode heat shock proteins, antifreeze proteins, and enzymes such as chitinase, trehalose, and trehalose-6-phosphate synthase. This study generated a substantial number of M. punctipennis transcript sequences that can be used to discover novel genes associated with stress adaptation. These sequences are a valuable resource for future studies of the desert beetle and other insects in Tenebrionidae. Transcriptome analysis based on Illumina paired-end sequencing is a powerful approach for gene discovery and molecular marker development for non-model species.

Are the Antarctic dipteran, Eretmoptera murphyi, and Arctic collembolan, Megaphorura arctica, vulnerable to rising temperatures?

Polar terrestrial invertebrates are suggested as being vulnerable to temperature change relative to lower latitude species, and hence possibly also to climate warming. Previous studies have shown Antarctic and Arctic Collembola and Acari to possess good heat tolerance and survive temperature exposures above 30 °C. To test this feature further, the heat tolerance and physiological plasticity of heat stress were explored in the Arctic collembolan, Megaphorura arctica, from Svalbard and the Antarctic midge, Eretmoptera murphyi, from Signy Island. The data obtained demonstrate considerable heat tolerance in both species, with upper lethal temperatures ≥35 °C (1 h exposures), and tolerance of exposure to 10 and 15 °C exceeding 56 days. This tolerance is far beyond that required in their current environment. Average microhabitat temperatures in August 2011 ranged between 5.1 and 8.1 °C, and rarely rose above 10 °C, in Ny-Ålesund, Svalbard. Summer soil microhabitat temperatures on Signy Island have previously been shown to range between 0 and 10 °C. There was also evidence to suggest that E. murphyi can recover from high-temperature exposure and that M. arctica is capable of rapid heat hardening. M. arctica and E. murphyi therefore have the physiological capacity to tolerate current environmental conditions, as well as future warming. If the features they express are characteristically more general, such polar terrestrial invertebrates will likely fare well under climate warming scenarios.

Characterization of the small heat shock protein Hsp27 gene in Chironomus riparius (Diptera) and its expression profile in response to temperature changes and xenobiotic exposures

Small heat shock proteins constitute the most diverse and least conserved group within the large family of heat shock proteins, which play a crucial role in cell response to environmental insults. Chironomus riparius larvae are widely used in environmental research for testing pollutant toxicity in sediments and freshwater environments. Different genes, such as Hsp70, Hsc70, Hsp90, and Hsp40, have been identified in this species as sensitive biomarkers for xenobiotics, but small Hsps genes remain largely unknown. In this study, the Hsp27 has been characterized in C. riparius and its transcriptional response evaluated under several environmental stimuli. The Hsp27 gene was mapped by FISH on polytene chromosomes at region I-C4 and was found to encode a 195 aa protein, which contains an α-crystallin domain bounded by three conserved regions. This protein shows homology with Drosophila melanogaster HSP27, Ceratitis capitata HSP27, and Sarcophaga crassipalpis HSP25. Real-time reverse transcriptase-polymerase chain reaction analysis showed that heat shock (35 °C) and cadmium dramatically upregulate this gene. Moreover, exposures to triclosan and bisphenol A were able to significantly increase mRNA levels. However, neither nonylphenol nor tributyltin altered Hsp27 gene expression. The transcriptional activity of Hsp27 gene was modulated during cold stress. Interestingly, cold shock (4 °C) significantly reduced Hsp27 transcripts, but this gene was significantly overexpressed during the recovery time at the normal growing temperature. These results show that the Hsp27 gene is sensitive to different environmental stimuli, including endocrine-disrupting pollutants, suggesting its potential as a suitable biomarker for ecotoxicological studies in aquatic systems.

Responses of invertebrates to temperature and water stress: A polar perspective

As small bodied poikilothermic ectotherms, invertebrates, more so than any other animal group, are susceptible to extremes of temperature and low water availability. In few places is this more apparent than in the Arctic and Antarctic, where low temperatures predominate and water is unusable during winter and unavailable for parts of summer. Polar terrestrial invertebrates express a suite of physiological, biochemical and genomic features in response to these stressors. However, the situation is not as simple as responding to each stressor in isolation, as they are often faced in combination. We consider how polar terrestrial invertebrates manage this scenario in light of their physiology and ecology. Climate change is also leading to warmer summers in parts of the polar regions, concomitantly increasing the potential for drought. The interaction between high temperature and low water availability, and the invertebrates' response to them, are therefore also explored.

Organ protective mechanisms common to extremes of physiology: a window through hibernation biology

Supply and demand relationships govern survival of animals in the wild and are also key determinants of clinical outcomes in critically ill patients. Most animals' survival strategies focus on the supply side of the equation by pursuing territory and resources, but hibernators are able to anticipate declining availability of nutrients by reducing their energetic needs through the seasonal use of torpor, a reversible state of suppressed metabolic demand and decreased body temperature. Similarly, in clinical medicine the majority of therapeutic interventions to care for critically ill or trauma patients remain focused on elevating physiologic supply above critical thresholds by increasing the main determinants of delivery of oxygen to the tissues (cardiac output, perfusion pressure, hemoglobin concentrations, and oxygen saturation), as well as increasing nutritional support, maintaining euthermia, and other general supportive measures. Techniques, such as induced hypothermia and preconditioning, aimed at diminishing a patient's physiologic requirements as a short-term strategy to match reduced supply and to stabilize their condition, are few and underutilized in clinical settings. Consequently, comparative approaches to understand the mechanistic adaptations that suppress metabolic demand and alter metabolic use of fuel as well as the application of concepts gleaned from studies of hibernation, to the care of critically ill and injured patients could create novel opportunities to improve outcomes in intensive care and perioperative medicine.

Supercooling capacity and cold hardiness of band-winged grasshopper eggs (Orthoptera: Acrididae)

The band-winged grasshopper, Oedaleus asiaticus Bei-Bienko, is one of the most dominant and economically important grasshopper species in the steppe grasslands and farming-pastoral ecotone in northern China. It is a univoltine species and overwinters as eggs in soil. The cold hardiness of its eggs was examined in the laboratory. Water content in soil significantly affected the supercooling points (SCPs), water content and fat content of prediapause eggs. With the increase of water content in soil, the SCP, and water content of prediapause eggs rose whereas the fat content declined. There was a significant relationship between the SCP and water content or fat content of prediapause eggs. The SCPs of prediapause and diapause eggs varied from -7.6 to -28.4°C and the SCPs of eggs 30 d after oviposition could be divided into two groups. The means of high SCP group (-11.0 to -11.9°C) were much higher than those of low SCP group (-21.8 to -21.9°C), and the majority belonged to the latter (90.48-93.33%). The SCPs of prediapause eggs and early-diapause eggs 30 d after oviposition were significantly higher than those of deep-diapause eggs 60 d after oviposition. The survival rates of diapause eggs were significantly different among different temperature treatments. The survival rate was higher than 88% at greater than -20°C and declined significantly to 57% at -25°C, and suddenly dropped to zero at -30°C. The lower lethal temperature (Ltemp50) for 12 h exposure was -25.3°C and the lower lethal time (Ltime50) at -20°C was 32.8 d. As the mean SCPs of diapause eggs were similar to their Ltemp50, the SCP of eggs can be considered as a good indicator of cold hardiness for O. asiaticus and that this grasshopper is a freeze-intolerant insect.

Genome wide gene-expression analysis of facultative reproductive diapause in the two-spotted spider mite Tetranychus urticae

BACKGROUND

Diapause or developmental arrest, is one of the major adaptations that allows mites and insects to survive unfavorable conditions. Diapause evokes a number of physiological, morphological and molecular modifications. In general, diapause is characterized by a suppression of the metabolism, change in behavior, increased stress tolerance and often by the synthesis of cryoprotectants. At the molecular level, diapause is less studied but characterized by a complex and regulated change in gene-expression. The spider mite Tetranychus urticae is a serious polyphagous pest that exhibits a reproductive facultative diapause, which allows it to survive winter conditions. Diapausing mites turn deeply orange in color, stop feeding and do not lay eggs.

RESULTS

We investigated essential physiological processes in diapausing mites by studying genome-wide expression changes, using a custom built microarray. Analysis of this dataset showed that a remarkable number, 11% of the total number of predicted T. urticae genes, were differentially expressed. Gene Ontology analysis revealed that many metabolic pathways were affected in diapausing females. Genes related to digestion and detoxification, cryoprotection, carotenoid synthesis and the organization of the cytoskeleton were profoundly influenced by the state of diapause. Furthermore, we identified and analyzed an unique class of putative antifreeze proteins that were highly upregulated in diapausing females. We also further confirmed the involvement of horizontally transferred carotenoid synthesis genes in diapause and different color morphs of T. urticae.

CONCLUSIONS

This study offers the first in-depth analysis of genome-wide gene-expression patterns related to diapause in a member of the Chelicerata, and further adds to our understanding of the overall strategies of diapause in arthropods.

Transcriptomic characterization of cold acclimation in larval zebrafish

Background

Temperature is one of key environmental parameters that affect the whole life of fishes and an increasing number of studies have been directed towards understanding the mechanisms of cold acclimation in fish. However, the adaptation of larvae to cold stress and the cold-specific transcriptional alterations in fish larvae remain largely unknown. In this study, we characterized the development of cold-tolerance in zebrafish larvae and investigated the transcriptional profiles under cold stress using RNA-seq.

Results

Pre-exposure of 96 hpf zebrafish larvae to cold stress (16°C) for 24 h significantly increased their survival rates under severe cold stress (12°C). RNA-seq generated 272 million raw reads from six sequencing libraries and about 92% of the processed reads were mapped to the reference genome of zebrafish. Differential expression analysis identified 1,431 up- and 399 down-regulated genes. Gene ontology enrichment analysis of cold-induced genes revealed that RNA splicing, ribosome biogenesis and protein catabolic process were the most highly overrepresented biological processes. Spliceosome, proteasome, eukaryotic ribosome biogenesis and RNA transport were the most highly enriched pathways for genes up-regulated by cold stress. Moreover, alternative splicing of 197 genes and promoter switching of 64 genes were found to be regulated by cold stress. A shorter isoform of stk16 that lacks 67 amino acids at the N-terminus was specifically generated by skipping the second exon in cold-treated larvae. Alternative promoter usage was detected for per3 gene under cold stress, which leading to a highly up-regulated transcript encoding a truncated protein lacking the C-terminal domains.

Conclusions

These findings indicate that zebrafish larvae possess the ability to build cold-tolerance under mild low temperature and transcriptional and post-transcriptional regulations are extensively involved in this acclimation process.

Physiological responses to fluctuating thermal and hydration regimes in the chill susceptible insect, Thaumatotibia leucotreta

Fluctuating thermal regimes (FTR), consisting of cycles between stressful low and benign temperatures, are known to improve survival and fecundity in a variety of insects. By contrast, fluctuating hydration regimes (FHR) consisting of cycles between dehydrating and benign conditions have been less comprehensively researched. Hypothetically, either repeated stress accumulates damage and reduces survival, or the recovery periods may act as a protective mechanism by allowing low temperature- or dehydration-induced damage to be repaired. Using false codling moth (Thaumatotibia leucotreta) larvae, we investigated whether FTR and FHR resulted in protection, or accumulated damage, at the cellular and whole-organism levels. Time- and age-matched controls were used to verify that the effects were due to the fluctuating stressors and not age- or time-dependent responses. Results showed that larval body water-(BWC) and lipid content (BLC) remained unchanged in response to FTR. Importantly though, FTR are protective when compared to constant low temperature exposures, potentially due to an increase in heat shock protein 70 (HSP70). However, larvae may suffer long-term fitness consequences compared to constant benign exposures. Results for FHR appear equivocal when compared to constant controls, due to high survival rates for all experiments, although the physiological responses to FHR included a decrease in larval BWC and BLC, a decrease in cuticular water loss rates, and a depletion of HSP70 during the final dehydration cycle. In conclusion, it appears that fluctuating stressors are protective in T. leucotreta when compared to constant stress conditions, likely through regulation of whole-animal metabolic rate and HSP70, although other mechanisms (e.g. ion homeostasis) are also implicated.

Fatty acid modifications during autumnal cold-hardening in an obligatory ectoparasite, the deer ked (Lipoptena cervi)

Poikilothermic organisms often modify their tissue fatty acids (FA) in response to cold exposure by increased unsaturation. In insects, this has been found to be accompanied by increases in the activities or mRNA expression of desaturase enzymes. In the present study, the FA composition of an obligatory ectoparasite, the deer ked (Lipoptena cervi), was analyzed in August-November. In addition to studying the general FA profile of the species, the possible contribution of FA to autumnal cold-hardening was examined. The FA composition of the deer ked imago was relatively similar to previously studied dipteran species, with high percentages of monounsaturated FA (especially 18:1n-9 and 16:1n-7) and 16:0. The individuals caught later in autumn had significantly higher values for the ratio of unsaturated to saturated FA and, regarding individual FA, the percentages of 18:1n-9, 18:2n-6, 18:3n-3, 20:4n-6 and 22:6n-3 were higher but those of 16:0 and 16:1n-7 lower than in August. Potential selective use of particular FA for energy could not account for the large increase in the levels of polyunsaturated FA (PUFA). The observed increased degree of FA unsaturation may have resulted from cold-induced desaturation, as observed previously in other species, or increased survival of the keds with relatively large PUFA contents. The PUFA with low melting points probably allow lipid membranes to maintain sufficient fluidity required to maintain protein functions at low ambient temperatures.

Dietary sterols/steroids and the generalist caterpillar Helicoverpa zea: physiology, biochemistry and midgut gene expression

Sterols are essential nutrients for insects because, in contrast to mammals, no insect (or arthropod for that matter) can synthesize sterols de novo. Plant-feeding insects typically generate their sterols, commonly cholesterol, by metabolizing phytosterols. However, not all phytosterols are readily converted to cholesterol. In this study we examined, using artificial diets containing single sterols/steroids, how typical (cholesterol and stigmasterol) and atypical (cholestanol and cholestanone) sterols/steroids affect the performance of a generalist caterpillar (Helicoverpa zea). We also performed sterols/steroids analyses, using GC/MS techniques, to explore the metabolic fate of these different dietary sterols/steroids. Finally, we used a microarray approach to measure, and compare, midgut gene expression patterns that arise as a function of dietary sterols/steroids. In general, H. zea performed best on the cholesterol and stigmasterol diets, with cholesterol as the dominant tissue sterol on these two treatments. Compared to the cholesterol and stigmasterol diets, performance was reduced on the cholestanol and cholestanone diets; on these latter treatments stanols were the dominant tissue sterol. Finally, midgut gene expression patterns differed as a function of dietary sterol/steroid; using the cholesterol treatment as a reference, gene expression differences were smallest on stigmasterol, intermediate on cholestanol, and greatest on cholestanone. Inspection of our data revealed two broad insights. First, they identify a number of genes potentially involved in sterol/steroid metabolism and absorption. Second, they provide unique mechanistic insights into how variation in dietary sterol/steroid structure can affect insect herbivores.

Does cold tolerance plasticity correlate with the thermal environment and metabolic profiles of a parasitoid wasp?

Tolerance of ectotherm species to cold stress is highly plastic according to thermal conditions experienced prior to cold stress. In this study, we investigated how cold tolerance varies with developmental temperature (at 17, 25 and 30°C) and whether developmental temperature induces different metabolic profiles. Experiments were conducted on the two populations of the parasitoid wasp, Venturia canescens, undergoing contrasting thermal regimes in their respective preferential habitat (thermally variable vs. buffered). We predicted the following: i) development at low temperatures improves the cold tolerance of parasitoid wasps, ii) the shape of the cold tolerance reaction norm differs between the two populations, and iii) these phenotypic variations are correlated with their metabolic profiles. Our results showed that habitat origin and developmental acclimation interact to determine cold tolerance and metabolic profiles of the parasitoid wasps. Cold tolerance was promoted when developmental temperatures declined and population originating from variable habitat presented a higher cold tolerance. Cold tolerance increases through the accumulation of metabolites with an assumed cryoprotective function and the depression of metabolites involved in energy metabolism. Our data provide an original example of how intraspecific cold acclimation variations correlate with metabolic response to developmental temperature.

Changes in extreme cold tolerance, membrane composition and cardiac transcriptome during the first day of thermal acclimation in the porcelain crab Petrolisthes cinctipes

Intertidal zone organisms can experience transient freezing temperatures during winter low tides, but their extreme cold tolerance mechanisms are not known. Petrolisthes cinctipes is a temperate mid-high intertidal zone crab species that can experience wintertime habitat temperatures below the freezing point of seawater. We examined how cold tolerance changed during the initial phase of thermal acclimation to cold and warm temperatures, as well as the persistence of cold tolerance during long-term thermal acclimation. Thermal acclimation for as little as 6 h at 8°C enhanced cold tolerance during a 1 h exposure to -2°C relative to crabs acclimated to 18°C. Potential mechanisms for this enhanced tolerance were elucidated using cDNA microarrays to probe for differences in gene expression in cardiac tissue of warm- and cold-acclimated crabs during the first day of thermal acclimation. No changes in gene expression were detected until 12 h of thermal acclimation. Genes strongly upregulated in warm-acclimated crabs represented immune response and extracellular/intercellular processes, suggesting that warm-acclimated crabs had a generalized stress response and may have been remodelling tissues or altering intercellular processes. Genes strongly upregulated in cold-acclimated crabs included many that are involved in glucose production, suggesting that cold acclimation involves increasing intracellular glucose as a cryoprotectant. Structural cytoskeletal proteins were also strongly represented among the genes upregulated in only cold-acclimated crabs. There were no consistent changes in composition or the level of unsaturation of membrane phospholipid fatty acids with cold acclimation, which suggests that neither short- nor long-term changes in cold tolerance are mediated by changes in membrane fatty acid composition. Overall, our study demonstrates that initial changes in cold tolerance are likely not regulated by transcriptomic responses, but that gene-expression-related changes in homeostasis begin within 12 h, the length of a tidal cycle.

Trehalose-6-phosphate synthase from the cat flea Ctenocephalides felis and Drosophila melanogaster: gene identification, cloning, heterologous functional expression and identification of inhibitors by high throughput screening

Trehalose phosphate synthase (EC 2.4.1.15; TPS) is the crucial enzyme for the biosynthesis of trehalose, the main haemolymph sugar of insects, and therefore a potential insecticidal molecular target. In this study, we report the functional heterologous expression of Drosophila melanogaster TPS, the gene identification, full length cDNA cloning and functional expression of cat flea (Ctenocephalides felis) TPS, and the Michaelis-Menten constants for their specific substrates glucose-6-phosphate and uridinediphosphate-glucose. A novel high throughput screening-compatible TPS assay and its use for the identification of the first potent insect TPS inhibitors from a large synthetic compound collection (>115 000 compounds) is described. One compound class that emerged in this screening, the 4-substituted 2,6-diamino-3,5-dicyano-4H-thiopyrans, was further investigated by analysing preliminary structure-activity relationships. Here, compounds were identified that show low µM to high nM half maximal inhibitory concentrations on insect TPS and that may serve as lead compounds for the development of insecticides with a novel mode of action.

Role of high-fat diet in stress response of Drosophila

Obesity is associated with many diseases, one of the most common being obstructive sleep apnea (OSA), which in turn leads to blood gas disturbances, including intermittent hypoxia (IH). Obesity, OSA and IH are associated with metabolic changes, and while much mammalian work has been done, mechanisms underlying the response to IH, the role of obesity and the interaction of obesity and hypoxia remain unknown. As a model organism, Drosophila offers tremendous power to study a specific phenotype and, at a subsequent stage, to uncover and study fundamental mechanisms, given the conservation of molecular pathways. Herein, we characterize the phenotype of Drosophila on a high-fat diet in normoxia, IH and constant hypoxia (CH) using triglyceride and glucose levels, response to stress and lifespan. We found that female flies on a high-fat diet show increased triglyceride levels (p<0.001) and a shortened lifespan in normoxia, IH and CH. Furthermore, flies on a high-fat diet in normoxia and CH show diminished tolerance to stress, with decreased survival after exposure to extreme cold or anoxia (p<0.001). Of interest, IH seems to rescue this decreased cold tolerance, as flies on a high-fat diet almost completely recovered from cold stress following IH. We conclude that the cross talk between hypoxia and a high-fat diet can be either deleterious or compensatory, depending on the nature of the hypoxic treatment.

Suspended animation, diapause and quiescence: arresting the cell cycle in C. elegans

Developing organisms require nutrients to support cell division vital for growth and development. An adaptation to stress, used by many organisms, is to reversibly enter an arrested state by reducing energy-requiring processes, such as development and cell division. This "wait it out" approach to survive stress until the environment is conductive for growth and development is used by many metazoans. Much is known about the molecular regulation of cell division, metazoan development and responses to environmental stress. However, how these biological processes intersect is less understood. Here, we review studies conducted in Caenorhabditis elegans that investigate how stresses such as oxygen deprivation (hypoxia and anoxia), exogenous chemicals or starvation affect cellular processes in the embryo, larvae or adult germline. Using C. elegans to identify how stress signals biological arrest can help in our understanding of evolutionary pressures as well as human health-related issues.

Immune and stress response 'cross-talk' in the Drosophila Malpighian tubule

The success of insects is in large part due to their ability to survive environmental stress, including heat, cold, and dehydration. Insects are also exposed to infection, osmotic or oxidative stress, and to xenobiotics or toxins. The molecular mechanisms of stress sensing and response have been widely investigated in mammalian cell lines, and the area of stress research is now so vast to be beyond the scope of a single review article. However, the mechanisms by which stress inputs to the organism are sensed and integrated at the tissue and cellular level are less well understood. Increasingly, common molecular events between immune and other stress responses are observed in vivo; and much of this work stems of efforts in insect molecular science and physiology. We describe here the current knowledge in the area of immune and stress signalling and response at the level of the organism, tissue and cell, focussing on a key epithelial tissue in insects, the Malpighian tubule, and drawing together the known pathways that modulate responses to different stress insults. The tubules are critical for insect survival and are increasingly implicated in responses to multiple and distinct stress inputs. Importantly, as tubule function is central to survival, they are potentially key targets for insect control, which will be facilitated by increased understanding of the complexities of stress signalling in the organism.

Insect cold hardiness: metabolic, gene, and protein adaptation1This review is part of a virtual symposium on recent advances in understanding a variety of complex regulatory processes in insect physiology and endocrinology, including development, metabolism, cold hardiness, food intake and digestion, and diuresis, through the use of omics technologies in the postgenomic era

Winter survival for thousands of species of insects relies on adaptive strategies for cold hardiness. Two basic mechanisms are widely used (freeze avoidance by deep supercooling and freeze tolerance where insects endure ice formation in extracellular fluid spaces), whereas additional strategies (cryoprotective dehydration, vitrification) are also used by some polar species in extreme environments. This review assesses recent research on the biochemical adaptations that support insect cold hardiness. We examine new information about the regulation of cryoprotectant biosynthesis, mechanisms of metabolic rate depression, role of aquaporins in water and glycerol movement, and cell preservation strategies (chaperones, antioxidant defenses and metal binding proteins, mitochondrial suppression) for survival over the winter. We also review the new information coming from the use of genomic and proteomic screening methods that are greatly widening the scope for discovery of genes and proteins that support winter survival.

Rapid decline of cold tolerance at young age is associated with expression of stress genes in Drosophila melanogaster

Many endogenous factors influence thermal tolerance of insects. Among these, age contributes an important source of variation. Heat tolerance is typically high in newly-enclosed insects, before declining dramatically. It is not known whether this phenomenon relates to cold tolerance also. In addition, the underlying mechanisms of this variation are unresolved. In this study we tested whether cold tolerance declines in Drosophila melanogaster females aged from 0 to 5 days. We also assessed whether expression (basal and induced) of eight stress genes (hsp22, hsp23, hsp40, hsp68, hsp70Aa, hsp83, Starvin and Frost) varied post-eclosion in correspondence with changes found cold tolerance. We report that cold tolerance was very high at eclosion and then it rapidly declined in young flies. hsp23 and hsp68 showed a dramatic age-related variation of basal expression that was associated with cold tolerance proxies. Significant age-related plasticity of cold-induced expression was also found for hsp22, hsp23, hsp68, hsp70Aa, Frost and Starvin. hsp22 and hsp70Aa induced expression was high in newly-enclosed phenotypes before declining dramatically, whilst opposite age-related patterns were found for hsp23, hsp68, Starvin and Frost. This study shows a marked within-stage variation in cold tolerance. The involvement of the stress genes in setting basal thermal tolerance is discussed.

Interactions between Controlled Atmospheres and Low Temperature Tolerance: A Review of Biochemical Mechanisms

Controlled atmosphere treatments using carbon dioxide, oxygen, and/or nitrogen, together with controlled temperature and humidity, form an important method for post-harvest sterilization against insect-infested fruit. However, in insects, the cross tolerance and biochemical interactions between the various stresses of modified gas conditions and low temperature may either elicit or block standard stress responses which can potentiate (or limit) lethal low temperature exposure. Thus, the success of such treatments is sometimes erratic and does not always result in the desired pest mortality. This review focuses on the biochemical modes of action whereby controlled atmospheres affect insects low temperature tolerance, making them more (or occasionally, less) susceptible to cold sterilization. Insights into the integrated biochemical modes of action may be used together with the pests' low temperature tolerance physiology to determine which treatments may be of value in post-harvest sterilization.

Divergent transcriptomic responses to repeated and single cold exposures in Drosophila melanogaster

Insects in the field are exposed to multiple bouts of cold, and there is increasing evidence that the fitness consequences of repeated cold exposure differ from the impacts of a single cold exposure. We tested the hypothesis that different kinds of cold exposure (in this case, single short, prolonged and repeated cold exposure) would result in differential gene expression. We exposed 3 day old adult female wild-type Drosophila melanogaster (Diptera: Drosophilidae) to –0.5°C for a single 2 h exposure, a single 10 h exposure, or five 2 h exposures on consecutive days, and extracted RNA after 6 h of recovery. Global gene expression was quantified using an oligonucleotide microarray and validated with real-time PCR using different biological replicates. We identified 76 genes upregulated in response to multiple cold exposure, 69 in response to prolonged cold exposure and 20 genes upregulated in response to a single short cold exposure, with a small amount of overlap between treatments. Three genes – Turandot A, Hephaestus and CG11374 – were upregulated in response to all three cold exposure treatments. Key functional groups upregulated include genes associated with muscle structure and function, the immune response, stress response, carbohydrate metabolism and egg production. We conclude that cold exposure has wide-ranging effects on gene expression in D. melanogaster and that increased duration or frequency of cold exposure has impacts different to those of a single short cold exposure. This has important implications for extrapolating laboratory studies of insect overwintering that are based on only a single cold exposure.

Freezing tolerance and low molecular weight cryoprotectants in an invasive parasitic fly, the deer ked (Lipoptena cervi)

Insect cold hardiness is often mediated by low molecular weight cryoprotectants, such as sugars, polyols, and amino acids (AA). While many free-living northern insects must cope with extended periods of freezing ambient temperatures (Ta), the ectoparasitic deer ked Lipoptena cervi imago can encounter subfreezing Ta only during a short autumnal period between hatching and host location. Subsequently, it benefits from the body temperature of the cervid host for survival in winter. This study investigated the cold tolerance of the species by determining its lower lethal temperature (100% mortality, LLT100) during faster and slower cold acclimation, by determining the supercooling point (SCP) and by measuring the concentrations of potential low molecular weight cryoprotectants. The LLT100 of the deer ked was approximately -16 ° C, which would enable it to survive freezing nighttime Ta not only in its current area of distribution but also further north. The SCP was -7.8 ° C, clearly higher than the LLT100 , indicating that the deer ked displays freezing tolerance. The concentrations of free AA, especially nonessential AA, were higher in the cold-acclimated deer keds similar to several other insects. The concentrations of proline increased together with γ-aminobutyrate, arginine, asparagine, cystine, glutamate, glutamine, hydroxylysine, sarcosine, serine, and taurine. AA could be hypothesized to act as cryoprotectants by, e.g., protecting enzymes and lipid membranes from damage caused by cold.

A comparative study of the short term cold resistance response in distantly related Drosophila species: the role of regucalcin and frost

The molecular basis of short term cold resistance (indexed as chill-coma recovery time) has been mostly addressed in D. melanogaster, where candidate genes (Dca (also known as smp-30) and Frost (Fst)) have been identified. Nevertheless, in Drosophila, the ability to tolerate short term exposure to low temperatures evolved several times independently. Therefore, it is unclear whether variation in the same candidate genes is also responsible for short term cold resistance in distantly related Drosophila species. It should be noted that Dca is a candidate gene for cold resistance in the Sophophora subgenus only, since there is no orthologous gene copy in the Drosophila subgenus. Here we show that, in D. americana (Drosophila subgenus), there is a north-south gradient for a variant at the 5′ non-coding region of regucalcin (a Dca-like gene; in D. melanogaster the proteins encoded by the two genes share 71.9% amino acid identities) but in our D. americana F2 association experiment there is no association between this polymorphism and chill-coma recovery times. Moreover, we found no convincing evidence that this gene is up-regulated after cold shock in both D. americana and D. melanogaster. Size variation in the Fst PEST domain (putatively involved in rapid protein degradation) is observed when comparing distantly related Drosophila species, and is associated with short term cold resistance differences in D. americana. Nevertheless, this effect is likely through body size variation. Moreover, we show that, even at two hours after cold shock, when up-regulation of this gene is maximal in D. melanogaster (about 48 fold expression change), in D. americana this gene is only moderately up-regulated (about 3 fold expression change). Our work thus shows that there are important differences regarding the molecular basis of cold resistance in distantly related Drosophila species.

Chaperone proteins and winter survival by a freeze tolerant insect

The role of chaperone proteins in the winter survival of insects was evaluated in freeze tolerant gall fly larvae, Eurosta solidaginis. Levels of four heat shock proteins (Hsp110, Hsp70, Hsp60, Hsp40), two glucose-regulated proteins (Grp75, Grp78) and three others (tailless complex polypeptide 1 [TCP-1], αA-crystallin, αB-crystallin) were tracked in outdoor larvae from September to April and, in addition, laboratory experiments assessed chilling, freezing, and anoxia effects on these proteins. Gall fly larvae showed consistent elevation of Hsp110, Hsp70, Hsp40, Grp78 and αB-crystallin over the late autumn and winter months, generally 1.5-2.0-fold higher than September values. This suggests that these proteins contribute to cell preservation over the winter months via protection and stabilization of macromolecules. By contrast, levels of the mitochondrial Hsp60 fell to just 40% of September values by midwinter, paralleling the responses by numerous mitochondrial enzymes and consistent with a reduction in total mitochondria numbers over the winter. None of the proteins were altered when 15°C acclimated larvae were chilled to 3°C for 24h but Hsp70, Hsp40 and Grp75 increased during freezing at -16°C for 24h whereas others (Hsp110, TCP-1 and both crystallins) increased significantly after larvae thawed at 3°C. Anoxia exposure (24h under N2 gas at 15°C) elevated levels of Hsp70, Grp78 and the two crystallins. Levels of active hyperphosphorylated heat shock transcription factor (HSF1) were also analyzed, giving an indication of the state of hsp gene transcription in the larvae. HSF1 was high in September and October but fell to less than 40% of September values in midwinter consistent with suppression of gene transcription in diapause larvae. HSF1 levels responded positively to freezing and increased robustly by 4.9-fold under anoxia. Overall, the data provide strong evidence for the importance of protein chaperones as a mechanism of cell preservation in freeze tolerant insects.