Role of HSF activation for resistance to heat, cold and high-temperature knock-down

The heat shock response in Polistes spp. brood from differing climates following heat stress

Temperature is a crucial factor in many physiological processes, especially in small ectotherms whose body temperature is highly influenced by ambient temperature. Polistes (paper wasps) is a genus of primitively eusocial wasps found in widely varying thermal environments throughout the world. Paper wasps construct open-faced combs in which the brood is exposed to varying ambient temperatures. The Heat Shock Response is a physiological mechanism that has been shown to help cope with thermal stress. We investigated the expression of heat shock proteins in different life stages of three species of Polistes from different climates with the aim of deducing adaptive patterns. This was done by assaying heat shock protein (hsp70, hsp83, hsc70) expression during control conditions (25 °C) or a heat insult (35 or 45 °C) in individuals collected from natural populations in Alpine, Temperate, or Mediterranean climates. Basal expression of hsc70 and hsp83 was found to be high, while hsp70 and hsp83 expression was found to be highly responsive to severe heat stress. As expression levels varied based on species, geographical origin, and life stage as well as between heat shock proteins, the Heat Shock Response of Polistes was found to be complex. The results suggest that adaptive utilization of the heat shock response contributes to the ability of Polistes spp. to inhabit widely different thermal environments.

Integrative biology of injury in animals

Mechanical injury is a prevalent challenge in the lives of animals with myriad potential consequences for organisms, including reduced fitness and death. Research on animal injury has focused on many aspects, including the frequency and severity of wounding in wild populations, the short- and long-term consequences of injury at different biological scales, and the variation in the response to injury within or among individuals, species, ontogenies, and environmental contexts. However, relevant research is scattered across diverse biological subdisciplines, and the study of the effects of injury has lacked synthesis and coherence. Furthermore, the depth of knowledge across injury biology is highly uneven in terms of scope and taxonomic coverage: much injury research is biomedical in focus, using mammalian model systems and investigating cellular and molecular processes, while research at organismal and higher scales, research that is explicitly comparative, and research on invertebrate and non-mammalian vertebrate species is less common and often less well integrated into the core body of knowledge about injury. The current state of injury research presents an opportunity to unify conceptually work focusing on a range of relevant questions, to synthesize progress to date, and to identify fruitful avenues for future research. The central aim of this review is to synthesize research concerning the broad range of effects of mechanical injury in animals. We organize reviewed work by four broad and loosely defined levels of biological organization: molecular and cellular effects, physiological and organismal effects, behavioural effects, and ecological and evolutionary effects of injury. Throughout, we highlight the diversity of injury consequences within and among taxonomic groups while emphasizing the gaps in taxonomic coverage, causal understanding, and biological endpoints considered. We additionally discuss the importance of integrating knowledge within and across biological levels, including how initial, localized responses to injury can lead to long-term consequences at the scale of the individual animal and beyond. We also suggest important avenues for future injury biology research, including distinguishing better between related yet distinct injury phenomena, expanding the subjects of injury research to include a greater variety of species, and testing how intrinsic and extrinsic conditions affect the scope and sensitivity of injury responses. It is our hope that this review will not only strengthen understanding of animal injury but will contribute to building a foundation for a more cohesive field of 'injury biology'.

A functional study of the role of Turandot genes in Drosophila melanogaster: An emerging candidate mechanism for inducible heat tolerance

Plastic responses to heat stress have been shown to temporarily increase heat stress tolerance in many small ectotherms. Heat shock proteins (Hsps) have previously been shown to play a role in this induced heat stress tolerance. The heat shock response is fast but short lived, with the cellular Hsp concentration peaking within a few hours after induction. Induced heat stress tolerance, on the other hand, peaks 16-32 h after induction. Therefore, the inducible heat stress response must depend on additional mechanisms. The Turandot gene family has been suggested as a candidate. It contains eight genes that are all upregulated to some degree following heat stress in Drosophila melanogaster. Previously, Turandot A (totA) and Turandot X (totX) have been linked to induced heat stress tolerance. The study presented here aimed to investigate the temporal dynamics of Turandot expression and the functional role of totA and totC for heat stress tolerance. This was done by assaying the temporal heat tolerance and Turandot gene expression after a heat insult, and by exposing Turandot gene knock down flies to a range of heat hardening treatments, and evaluating the effects on heat tolerance. Successful gene knock down was verified by gene expression assays. In addition, expression of hsp70A was included. Both totA, totC, and hsp70A expression increased following a heat hardening treatment, while the results for totX were less clear. The expression of totC temporally co-occurred with and was functionally linked to increased heat tolerance. Expression of totA did not have a significant effect on heat stress tolerance. The complexity of inducible heat tolerance was underlined by the result that knock down of Turandot genes led to increased expression of hsp70. The results suggest that heat tolerance is determined by the interaction between several mechanisms, of which Turandot genes constitute one such mechanism.

Transcriptional regulation of small heat shock protein genes by heat shock factor 1 (HSF1) in Liriomyza trifolii under heat stress

Small heat shock proteins (sHSPs) function as molecular chaperones in multiple physiological processes and are active during thermal stress. sHSP expression is controlled by heat shock transcription factor (HSF); however, few studies have been conducted on HSF in agricultural pests. Liriomyza trifolii is an introduced insect pest of horticultural and vegetable crops in China. In this study, the master regulator, HSF1, was cloned and characterized from L. trifolii, and the expression levels of HSF1 and five sHSPs were studied during heat stress. HSF1 expression in L. trifolii generally decreased with rising temperatures, whereas expression of the five sHSPs showed an increasing trend that correlated with elevated temperatures. All five sHSPs and HSF1 showed an upward trend in expression with exposure to 40 ℃ without a recovery period. When a recovery period was incorporated after thermal stress, the expression patterns of HSF1 and sHSPs in L. trifolii exposed to 40 °C was significantly lower than expression with no recovery period. To elucidate potential interactions between HSF1 and sHSPs, double-stranded RNA was synthesized to knock down HSF1 in L. trifolii by RNA interference. The knockdown of HSF1 by RNAi decreased the survival rate and expression of HSP19.5, HSP20.8, and HSP21.3 during high-temperature stress. This study expands our understanding of HSF1-regulated gene expression in L. trifolii exposed to heat stress.

Molecular Characterization of Heat-Induced HSP11.0 and Master-Regulator HSF from Cotesia chilonis and Their Consistent Response to Heat Stress

Simple Summary

Small heat shock proteins (sHSPs) are members of the heat shock protein (HSP) family that play an important role in heat stress, and heat shock factors (HSFs) are transcriptional activators that mainly regulate the expression of HSPs. Cotesia chilonis, the major endoparasitoid of Chilo suppressalis, widely distributes in China and other Asian regions. Previous studies have shown that C. chilonis has a certain thermal tolerance. Here, heat-induced HSP11.0 and master-regulator HSF were cloned and characterized from C. chilonis. The transcription patterns of them in response to different temperatures and time course after temperature treatment were analyzed. This study is the first report on the analysis on hsf gene of C. chilonis. The results of expression patterns will provide new insights into thermoregulation of C. chilonis in response to climate change.

Abstract

Small heat shock proteins (sHSPs) are members of the heat shock protein (HSP) family that play an important role in temperature stress, and heat shock factors (HSFs) are transcriptional activators that regulate HSP expression. Cotesia chilonis, the major endoparasitoid of Chilo suppressalis, modulates the C. suppressalis population in the field. In this study, we cloned and characterized two genes from C.chilonis: the heat-induced HSP11.0 gene (Cchsp11.0) that consisted of a 306-bp ORF, and the master regulator HSF (Cchsf) containing an 1875-bp ORF. CcHSP11.0 contained a chaperonin cpn10 signature motif that is conserved in other hymenopteran insects. CcHSF is a typical HSF and contains a DNA-binding domain, two hydrophobic heptad repeat domains, and a C-terminal trans-activation domain. Neither Cchsp11.0 or Cchsf contain introns. Real-time quantitative PCR revealed that Cchsp11.0 and Cchsf were highly induced at 36 °C and 6 °C after a 2-h exposure. Overall, the induction of Cchsf was lower than Cchsp11.0 at low temperatures, whereas the opposite was true at high temperatures. In conclusion, both Cchsp11.0 and Cchsf are sensitive to high and low temperature stress, and the expression pattern of the two genes were positively correlated during temperature stress.

Heat Shock Factor Is Involved in Regulating the Transcriptional Expression of Two Potential Hsps (AhHsp70 and AhsHsp21) and Its Role in Heat Shock Response of Agasicles hygrophila

Heat shock proteins are molecular chaperones that are involved in numerous normal cellular processes and stress responses, and heat shock factors are transcriptional activators of heat shock proteins. Heat shock factors and heat shock proteins are coordinated in various biological processes. The regulatory function of heat shock factors in the expression of genes encoding heat shock proteins (Hsps) has been documented in some model insects, however, the role of transcription factors in modulating Hsps in other insects is still limited. In this study, one heat shock factor gene (AhHsf) was isolated and its two potential target genes (AhHsp70 and AhsHsp21) were confirmed from Agasicles hygrophila. AhHsf sequence analysis indicated that it belongs to the Hsfs gene family. RT-qPCR showed that expression levels of heat shock factors and of two heat shock proteins significantly increased under heat stress. Injection with double-stranded Hsf RNA in freshly emerged adult beetles significantly inhibited expression of AhHsp70 and AhsHsp21, shortened the adult survival, drastically reduced egg production, and ultimately led to a decrease in fecundity. RNA interference (RNAi)-mediated suppression of AhHsp70 or AhsHsp21 expression also significantly affected expression of AhHsf. Our findings revealed a potential transcriptional function of AhHsf to regulate expression of AhHsp70 and AhsHsp21, which may play a key role in A. hygrophila thermotolerance. Our results improve our understanding of the molecular mechanisms of the AhHsf - AhHsps signaling pathway in A. hygrophila.

Age-dependent expression profiles of two adaptogenic systems and thermotolerance in Drosophila melanogaster

Here, we monitored the expression of three genes (hsp70, hsp22, and hsf1) involved in heat shock response in Drosophila melanogaster in males and females of different age. Also, we investigated age- and sex-dependent expression of three major genes participating in the production of hydrogen sulfide (H2S) (cse, cbs, and mst), implicated in stress resistance and aging. In addition to the control strain, we monitored the expression of all of these genes in a cbs knockout strain (cbs-/-) generated using the CRISPR technique. The tested strains differ in the induction capacities of the studied genes. Relative to the control strain, under normal conditions, the cbs-/- strain expresses all of the studied genes more abundantly, especially hsp22. In the control strain, aging leads to a dramatic increase in hsp22 synthesis, whereas in the cbs-/- strain, hsp22 induction is not pronounced. Furthermore, in 30-day-old cbs-/- flies, the constitutive expression of hsp70 and mst is decreased. Surprisingly, in the cbs-/- strain, we detected an upregulation of hsf1 transcription in the 30-day-old females. After heat shock in the control strain, hsp70 and hsp22 induction decreased with age in males and hsp22 decreased in females, while in the cbs-/- strain, a pronounced drop in the induction capacity of both hsp genes was seen in 30-day-old males and females. However, in most cases, the expression levels of hsf1 and H2S-producing genes do not exhibit pronounced changes depending on sex, age, or heat shock. Flies of control and cbs-/- strain exhibited strong reduction in basal thermotolerance with age. Our data suggest a cross-talk between the two studied ancient and universal adaptive systems.

Effects of abiotic stress on the expression of Hsp70 genes in Sogatella furcifera (Horváth)

Sogatella furcifera (Horváth), a prominent rice pest in Asia, is a typical R-strategic and highly adaptable insect. Heat shock proteins (Hsps) are highly conserved molecular chaperones regulating responses to various abiotic stresses; however, limited information is available regarding their role in responding to abiotic stress in S. furcifera. This study aimed to investigate the effect of abiotic stresses on the expression of Hsp70 genes in the S. furcifera. Five Hsp70 genes were isolated from S. furcifera, and the expression patterns at different developmental stages and temperatures, upon treatment with different insecticides and ultraviolet A (UV-A) stress, were analyzed. Hsp70 genes were expressed at different developmental stages. Hsp70-2, Hsp70-5, and Hsp70-6 were significantly upregulated upon heat shock at 40 °C for 30 min. Hsp70-3 and Hsp70-4 were significantly upregulated upon heat shock at 30 °C for 30 min. Under UV-A stress, Hsp70-3, Hsp70-4, Hsp70-5, and Hsp70-6 were significantly upregulated. Conversely, Hsp70-2 was significantly downregulated under UV-A stress. The five Hsp70 genes were significantly downregulated in 3rd-instar nymphs on exposure to thiamethoxam, buprofezin, and avermectin at LC10 and LC25 concentrations. Hence, Hsp70 genes significantly contribute to the tolerance of S. furcifera to temperature and UV-A stress; however, they are not involved in the response to insecticides.

Protein Kinase D Is Dispensable for Development and Survival of Drosophila melanogaster

Members of the Protein Kinase D (PKD) family are involved in numerous cellular processes in mammals, including cell survival after oxidative stress, polarized transport of Golgi vesicles, as well as cell migration and invasion. PKD proteins belong to the PKC/CAMK class of serine/threonine kinases, and transmit diacylglycerol-regulated signals. Whereas three PKD isoforms are known in mammals, Drosophila melanogaster contains a single PKD homolog. Previous analyses using overexpression and RNAi studies indicated likewise multi-facetted roles for Drosophila PKD, including the regulation of secretory transport and actin-cytoskeletal dynamics. Recently, involvement in growth regulation has been proposed based on the hypomorphic dPKDH allele. We have generated PKD null alleles that are homozygous viable without apparent phenotype. They largely match control flies regarding fertility, developmental timing and weight. Males, but not females, are slightly shorter lived and starvation sensitive. Furthermore, migration of pole cells in embryos and border cells in oocytes appears normal. PKD mutants tolerate heat, cold and osmotic stress like the control but are sensitive to oxidative stress, conforming to the described role for mammalian PKDs. A candidate screen to identify functionally redundant kinases uncovered genetic interactions of PKD with Pkcδ, sqa and Drak mutants, further supporting the role of PKD in oxidative stress response, and suggesting its involvement in starvation induced autophagy and regulation of cytoskeletal dynamics. Overall, PKD appears dispensable for fly development and survival presumably due to redundancy, but influences environmental responses.

Identification, Expression Patterns and RNA Interference of Aquaporins in Dendroctonus armandi (Coleoptera: Scolytinae) Larvae During Overwintering

The ability to survive annual temperature minima could be a key determinant of distribution limits for insects under global climate change. Recent studies have suggested that insect aquaporins are indispensable for cellular water management under conditions that lead to dehydration and cold stress. Aquaporins are integral membrane water channel proteins in the major intrinsic protein superfamily and promote selected solutes and the movement of water across biological membranes. We cloned and characterized nine full-length aquaporins from Dendroctonus armandi (DaAqps), the most destructive forest pest in the Qinling Mountains of Shaanxi Province, China. Eight of the DaAqps belong to three classical aquaporin grades, including the Drosophila integral protein, the Pyrocoelia rufa integral protein, the entomoglyceroporins and one that belongs to the unorthodox grade of aquaporin 12-like channels. The DaAqps were increasingly expressed during different developmental stages and in different larval tissues, and expression peaked in mid-winter. They were tested under cold conditions for different lengths of time, and the expression of almost all DaAqps was down regulated with decreasing temperatures and long-term exposure to cold conditions. However, when the lowest temperatures were reached, the levels were immediately upregulated. These genes indicate that cold tolerance can improve through mortality responses at low temperatures after RNA interference of DaAqps. In our study, we analyzed the molecular response, expression patterns, and RNA interference of DaAqps and clarified the crucial role of protective compounds (aquaporins) underlying D. armandi cold tolerance and provide a new pest control method.

Expression status of heat shock proteins in response to cold, heat, or insecticide exposure in the Colorado potato beetle Leptinotarsa decemlineata

The Colorado potato beetle (Leptinotarsa decemlineata (Say)) is an agricultural pest that threatens the potato industry worldwide. This insect is widely regarded as one of the most difficult-to-control pests, as it can thrive in a wide range of temperature conditions and routinely develops resistance towards various insecticides. The molecular changes associated with response to these challenges have not been fully investigated in L. decemlineata. While differential expression and characterization of heat shock proteins (HSPs) in response to stress have been conducted in several insects, data regarding HSPs in L. decemlineata are limited. The overarching objective of this study consisted of evaluating the expression of various HSPs in L. decemlineata exposed to different temperatures or treated with the insecticides imidacloprid and chlorantraniliprole. Expression levels of HSP60, HSP70, HSP90, and HSP Beta-1 were evaluated by qRT-PCR and insect mortality was assessed using dsRNAs aimed at select HSP targets. Elevated HSP70 and HSP90 transcript levels were observed in heat-exposed L. decemlineata while downregulation of HSP70 transcript levels was measured in insects submitted to cold conditions. Chlorantraniliprole exposure was associated with reduced HSP Beta-1 transcript levels while no change in expression was monitored in insects exposed to imidacloprid. RNAi-based knockdown of HSP60 levels correlated with significant insect mortality 14 days after dsRNA injection. These results highlight the modulation of HSPs that occur in L. decemlineata exposed to fluctuating temperatures and position HSPs as interesting candidates in the identification of novel molecular leads that could be targeted to control this insect.

Larvae of Drosophila melanogaster exhibit transcriptional activation of immune response pathways and antimicrobial peptides during recovery from supercooling stress

The biochemical and molecular mechanisms underlying insect cold acclimation prior to cold stress are relatively well explored, but the mechanisms linked to recovery and repair after cold stress have received much less attention. Here we focus on recovery from cold stress in the larvae of the vinegar fly (Drosophila melanogaster) that were exposed to two physiologically distinct cold stress situations: supercooling (S, survival > 95%) and freezing (F, survival < 10%), both at -5 °C. We analysed the metabolic and transcriptomic responses to cold stress via GC-MS/LC-MS and whole-genome microarrays, respectively. Both stresses (S and F) caused metabolic perturbations which were transient in supercooled larvae but deeper and irreversible in frozen larvae. Differential gene expression analysis revealed a clear disparity in responses to supercooling and freezing (less than 10% of DE genes overlapped between S and F larvae). Using GO term enrichment analysis and KEGG pathway mapping, we identified the stimulation of immune response pathways as a strong candidate mechanism for coping with supercooling. Supercooling caused complex transcriptional activation of innate immunity potential: from Lysozyme-mediated degradation of bacterial cell walls, recognition of pathogen signals, through phagocytosis and lysosomal degradation, Toll and Imd signaling, to upregulation of genes coding for different antimicrobial peptides. The transcriptomic response to freezing was instead dominated by degradation of macromolecules and death-related processes such as autophagy and apoptosis. Of the 45 upregulated DE genes overlapping in responses to supercooling and freezing, 26 were broadly ascribable to defense and repair functions.

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.

Transcriptome Analysis Reveals Candidate Genes for Cold Tolerance in Drosophila ananassae

Coping with daily and seasonal temperature fluctuations is a key adaptive process for species to colonize temperate regions all over the globe. Over the past 18,000 years, the tropical species Drosophila ananassae expanded its home range from tropical regions in Southeast Asia to more temperate regions. Phenotypic assays of chill coma recovery time (CCRT) together with previously published population genetic data suggest that only a small number of genes underlie improved cold hardiness in the cold-adapted populations. We used high-throughput RNA sequencing to analyze differential gene expression before and after exposure to a cold shock in cold-tolerant lines (those with fast chill coma recovery, CCR) and cold-sensitive lines (slow CCR) from a population originating from Bangkok, Thailand (the ancestral species range). We identified two candidate genes with a significant interaction between cold tolerance and cold shock treatment: GF14647 and GF15058. Further, our data suggest that selection for increased cold tolerance did not operate through the increased activity of heat shock proteins, but more likely through the stabilization of the actin cytoskeleton and a delayed onset of apoptosis.

Variation in adult stress resistance does not explain vulnerability to climate change in copper butterflies

Ongoing climate change is a major threat to biodiversity. However, although many species clearly suffer from ongoing climate change, others benefit from it, for example, by showing range expansions. However, which specific features determine a species' vulnerability to climate change? Phenotypic plasticity, which has been described as the first line of defence against environmental change, may be of utmost importance here. Against this background, we here compare plasticity in stress tolerance in 3 copper butterfly species, which differ arguably in their vulnerability to climate change. Specifically, we investigated heat, cold and desiccation resistance after acclimatization to different temperatures in the adult stage. We demonstrate that acclimation at a higher temperature increased heat but decreased cold tolerance and desiccation resistance. Contrary to our predictions, species did not show pronounced variation in stress resistance, though plastic capacities in temperature stress resistance did vary across species. Overall, our results seemed to reflect population-rather than species-specific patterns. We conclude that the geographical origin of the populations used should be considered even in comparative studies. However, our results suggest that, in the 3 species studied here, vulnerability to climate change is not in the first place determined by stress resistance in the adult stage. As entomological studies focus all too often on adults only, we argue that more research effort should be dedicated to other developmental stages when trying to understand insect responses to environmental change.

Recovery from supercooling, freezing, and cryopreservation stress in larvae of the drosophilid fly, Chymomyza costata

Physiological adjustments accompanying insect cold acclimation prior to cold stress have been relatively well explored. In contrast, recovery from cold stress received much less attention. Here we report on recovery of drosophilid fly larvae (Chymomyza costata) from three different levels of cold stress: supercooling to −10 °C, freezing at −30 °C, and cryopreservation at −196 °C. Analysis of larval CO₂ production suggested that recovery from all three cold stresses requires access to additional energy reserves to support cold-injury repair processes. Metabolomic profiling (targeting 41 metabolites using mass spectrometry) and custom microarray analysis (targeting 1,124 candidate mRNA sequences) indicated that additional energy was needed to: clear by-products of anaerobic metabolism, deal with oxidative stress, re-fold partially denatured proteins, and remove damaged proteins, complexes and/or organelles. Metabolomic and transcriptomic recovery profiles were closely similar in supercooled and frozen larvae, most of which successfully repaired the cold injury and metamorphosed into adults. In contrast, the majority of cryopreseved larvae failed to proceed in ontogenesis, showed specific metabolic perturbations suggesting impaired mitochondrial function, and failed to up-regulate a set of 116 specific genes potentially linked to repair of cold injury.

Rapid Cold Hardening Capacity and Its Impact on Performance of Russian Wheat Aphid (Hemiptera: Aphididae)

The Russian wheat aphid, Diuraphis noxia (Kurdjumov), is one of the most important pests of wheat and barley in most wheat-producing countries. Rapid cold hardiness (RCH) is a capacity of insects to develop, within hours, protection against subfreezing temperatures that plays an important role in aphid survival in response to sudden decreases in air temperature. In this research, we investigated the duration and rate of cooling on the induction of RCH of D. noxia and the costs of RCH on aphid development and fecundity. By transferring aphids directly from 20 °C to a range of subzero temperatures for 2 h, the lower lethal temperature for 80% mortality (LT80) was determined to be - 11.9 °C. Preconditioning the aphids at 0 °C for 1-3 h prior to exposure at (LT80) (-11.9 °C) resulted in a sharp increase in survival, with little change with longer durations of preconditioning. The slowest cooling rate (0.05 °C/min) increased survival fourfold, whereas rates from 0.1 to 1 °C/min increased survival twofold compared with a direct transfer to 0 °C, regardless of aphid stage used. Deleterious effects of RCH were not observed on aphid development, longevity, or fecundity. The present study indicates that RCH is induced in D. noxia in just a few hours in response to sudden lowering of temperatures to freezing, with little or no cost in reproductive capacity.

Evolutionary adaptation to environmental stressors: a common response at the proteomic level

Mechanistic trade-offs between traits under selection can shape and constrain evolutionary adaptation to environmental stressors. However, our knowledge of the quantitative and qualitative overlap in the molecular machinery among stress tolerance traits is highly restricted by the challenges of comparing and interpreting data between separate studies and laboratories, as well as to extrapolating between different levels of biological organization. We investigated the expression of the constitutive proteome (833 proteins) of 35 Drosophila melanogaster replicate populations artificially selected for increased resistance to six different environmental stressors. The evolved proteomes were significantly differentiated from replicated control lines. A targeted analysis of the constitutive proteomes revealed a regime-specific selection response among heat-shock proteins, which provides evidence that selection also adjusts the constitutive expression of these molecular chaperones. Although the selection response in some proteins was regime specific, the results were dominated by evidence for a "common stress response." With the exception of high temperature survival, we found no evidence for negative correlations between environmental stress resistance traits, meaning that evolutionary adaptation is not constrained by mechanistic trade-offs in regulation of functional important proteins. Instead, standing genetic variation and genetic trade-offs outside regulatory domains likely constrain the evolutionary responses in natural populations.

Concurrent effects of cold and hyperkalaemia cause insect chilling injury

Chilling injury and death are the ultimate consequence of low temperature exposure for chill susceptible insects, and low temperature tolerance is considered one of the most important factors determining insect distribution patterns. The physiological mechanisms that cause chilling injury are unknown, but chronic cold exposure that causes injury is consistently associated with elevated extracellular [K(+)], and cold tolerant insects possess a greater capacity to maintain ion balance at low temperatures. Here, we use the muscle tissue of the migratory locust (Locusta migratoria) to examine whether chill injury occurs during cold exposure or following return to benign temperature and we specifically examine if elevated extracellular [K(+)], low temperature, or a combination thereof causes cell death. We find that in vivo chill injury occurs during the cold exposure (when extracellular [K(+)] is high) and that there is limited capacity for repair immediately following the cold stress. Further, we demonstrate that that high extracellular [K(+)] causes cell death in situ, but only when experienced at low temperatures. These findings strongly suggest that that the ability to maintain ion (particularly K(+)) balance is critical to insect low temperature survival, and highlight novel routes of study in the mechanisms regulating cell death in insects in the cold.

Freezing of body fluids induces metallothionein gene expression in earthworms (Dendrobaena octaedra)

The molecular mechanisms activated by environmental contaminants and natural stressors such as freezing need to be investigated in order to better understand the mechanisms of interaction and potential effects that combined stressors may have on organisms. Using the freeze-tolerant earthworm Dendrobaena octaedra as model species, we exposed worms to freezing and exposure to sublethal copper in a factorial design and investigated the transcription of candidate genes for metal and cold stress. We hypothesised that both freezing and copper would induce transcription of genes coding for heat shock proteins (hsp10 and hsp70), metallothioneins (mt1 and mt2), and glutathione-S-transferase (gst), and that the combined effects of these two stressors would be additive. The gene transcripts hsp10, hsp70, and gst were significantly upregulated by freezing, but only hsp10 was upregulated by copper. We found that copper at the time of sampling had no effect on transcription of two metallothionein genes whereas transcription was strongly upregulated by freezing. Moreover, there was a significant interaction causing more than additive transcription rates of mt1 in the copper/freezing treatment suggesting that freeze-induced cellular dehydration increases the concentration of free copper ions in the cytosol. This metallothionein response to freezing is likely adaptive and possibly provides protection against freeze-induced elevated metal concentrations in the cytosol and excess ROS levels due to hypoxia during freezing.

The Role of Inducible Hsp70, and Other Heat Shock Proteins, in Adaptive Complex of Cold Tolerance of the Fruit Fly (Drosophila melanogaster)

Background

The ubiquitous occurrence of inducible Heat Shock Proteins (Hsps) up-regulation in response to cold-acclimation and/or to cold shock, including massive increase of Hsp70 mRNA levels, often led to hasty interpretations of its role in the repair of cold injury expressed as protein denaturation or misfolding. So far, direct functional analyses in Drosophila melanogaster and other insects brought either limited or no support for such interpretations. In this paper, we analyze the cold tolerance and the expression levels of 24 different mRNA transcripts of the Hsps complex and related genes in response to cold in two strains of D. melanogaster: the wild-type and the Hsp70^- null mutant lacking all six copies of Hsp70 gene.

Principal Findings

We found that larvae of both strains show similar patterns of Hsps complex gene expression in response to long-term cold-acclimation and during recovery from chronic cold exposures or acute cold shocks. No transcriptional compensation for missing Hsp70 gene was seen in Hsp70^- strain. The cold-induced Hsps gene expression is most probably regulated by alternative splice variants C and D of the Heat Shock Factor. The cold tolerance in Hsp70^- null mutants was clearly impaired only when the larvae were exposed to severe acute cold shock. No differences in mortality were found between two strains when the larvae were exposed to relatively mild doses of cold, either chronic exposures to 0°C or acute cold shocks at temperatures down to -4°C.

Conclusions

The up-regulated expression of a complex of inducible Hsps genes, and Hsp70 mRNA in particular, is tightly associated with cold-acclimation and cold exposure in D. melanogaster. Genetic elimination of Hsp70 up-regulation response has no effect on survival of chronic exposures to 0°C or mild acute cold shocks, while it negatively affects survival after severe acute cold shocks at temperaures below -8°C.

Transcriptome analysis of neonatal larvae after hyperthermia-induced seizures in the contractile silkworm, Bombyx mori

The ability to respond quickly and efficiently to transient extreme environmental conditions is an important property of all biota. However, the physiological basis of thermotolerance in different species is still unclear. Here, we found that the cot mutant showed a seizure phenotype including contraction of the body, rolling, vomiting gut juice and a momentary cessation of movement, and the heartbeat rhythm of the dorsal vessel significantly increases after hyperthermia. To comprehensively understand this process at the molecular level, the transcriptomic profile of cot mutant, which is a behavior mutant that exhibits a seizure phenotype, was investigated after hyperthermia (42°C) that was induced for 5 min. By digital gene expression profiling, we determined the gene expression profile of three strains (cot/cot ok/ok, +/+ ok/ok and +/+ +/+) under hyperthermia (42°C) and normal (25°C) conditions. A Venn diagram showed that the most common differentially expressed genes (DEGs, FDR<0.01 and log2 Ratio≥1) were up-regulated and annotated with the heat shock proteins (HSPs) in 3 strains after treatment with hyperthermia, suggesting that HSPs rapidly increased in response to high temperature; 110 unique DEGs, could be identified in the cot mutant after inducing hyperthermia when compared to the control strains. Of these 110 unique DEGs, 98.18% (108 genes) were up-regulated and 1.82% (two genes) were down-regulated in the cot mutant. KEGG pathways analysis of these unique DEGs suggested that the top three KEGG pathways were “Biotin metabolism,” “Fatty acid biosynthesis” and “Purine metabolism,” implying that diverse metabolic processes are active in cot mutant induced-hyperthermia. Unique DEGs of interest were mainly involved in the ubiquitin system, nicotinic acetylcholine receptor genes, cardiac excitation–contraction coupling or the Notch signaling pathway. Insights into hyperthermia-induced alterations in gene expression and related pathways could yield hints for understanding the relationship between behaviors and environmental stimuli (hyperthermia) in insects.

Anoxic stress and rapid cold hardening enhance cold tolerance of the migratory locust

Anoxia and rapid cold hardening (RCH) can increase the cold tolerance of many animals. However, mechanisms underlying these two kinds of stresses remain unclear. In this study, we aimed to explore the relationship of acclimation to cold stress with acclimation to anoxic stress in the migratory locust, Locusta migratoria. RCH at 0°C for 3h promoted the survival of cold stress-exposed locusts. Anoxic hypercapnia (CO2 anoxic treatment) for 40 min exerted an effect similar to that of RCH. Anoxic hypercapnia within 1h can all promote the cold hardiness of locusts. We investigated the transcript levels of six heat shock protein (Hsp) genes, namely, Hsp20.5, Hsp20.6, Hsp20.7, Hsp40, Hsp70, and Hsp90. Four genes, namely, Hsp90, Hsp40, Hsp20.5, and Hsp20.7, showed differential responses to RCH and anoxic hypercapnia treatments. Under cold stress, locusts exposed to the two regimens showed different responses for Hsp90, Hsp20.5, and Hsp20.7. However, the varied responses disappeared after recovery from cold stress. Compared with the control group, the transcript levels of six Hsp genes were generally downregulated in locusts subjected to anoxic hypercapnia or/and RCH. These results indicate that anoxic stress and RCH have different mechanisms of regulating the transcription of Hsp family members even if the two treatments exerted similar effects on cold tolerance of the migratory locust. However, Hsps may not play a major role in the promotion of cold hardiness by the two treatments.

Genome sequence and transcriptome analyses of the thermophilic zygomycete fungus Rhizomucor miehei

BACKGROUND

The zygomycete fungi like Rhizomucor miehei have been extensively exploited for the production of various enzymes. As a thermophilic fungus, R. miehei is capable of growing at temperatures that approach the upper limits for all eukaryotes. To date, over hundreds of fungal genomes are publicly available. However, Zygomycetes have been rarely investigated both genetically and genomically.

RESULTS

Here, we report the genome of R. miehei CAU432 to explore the thermostable enzymatic repertoire of this fungus. The assembled genome size is 27.6-million-base (Mb) with 10,345 predicted protein-coding genes. Even being thermophilic, the G + C contents of fungal whole genome (43.8%) and coding genes (47.4%) are less than 50%. Phylogenetically, R. miehei is more closerly related to Phycomyces blakesleeanus than to Mucor circinelloides and Rhizopus oryzae. The genome of R. miehei harbors a large number of genes encoding secreted proteases, which is consistent with the characteristics of R. miehei being a rich producer of proteases. The transcriptome profile of R. miehei showed that the genes responsible for degrading starch, glucan, protein and lipid were highly expressed.

CONCLUSIONS

The genome information of R. miehei will facilitate future studies to better understand the mechanisms of fungal thermophilic adaptation and the exploring of the potential of R. miehei in industrial-scale production of thermostable enzymes. Based on the existence of a large repertoire of amylolytic, proteolytic and lipolytic genes in the genome, R. miehei has potential in the production of a variety of such enzymes.

The rapid cold hardening response of Drosophila melanogaster: complex regulation across different levels of biological organization

Rapid cold hardening (RCH) is a form of thermal acclimation that allows ectotherms to fine-tune their physiological state to match rapid changes in thermal environment. Despite progress in recent years, there is still a considerable uncertainty regarding the physiological basis of RCH in insects. Here we investigated the physiological response of adult Drosophila melanogaster to a gradual reduction of temperature from 25 to 0°C followed by 1h at 0°C. As expected, this RCH treatment promoted cold tolerance, and so we hypothesized that this change could be detected at the proteomic level. Using 2D-DIGE, we found that only a few proteins significantly changed in abundance, and of these, we identified a set of four proteins of particular interest. These were identified as two different variants of glycogen phosphorylase (GlyP) of which three spots were up-regulated and another was down regulated. In subsequent experiments, we quantified upstream events by measuring the GlyP mRNA amount, but we found no marked effect of RCH. We also examined downstream events by measuring GlyP activity and the level of free sugars. We found no effect of RCH on GlyP activity. On the other hand, screening of whole animal sugar contents revealed a small increase in glucose levels following RCH while trehalose content was unaltered. This study highlights a complex regulation of GlyP in relation to RCH where we found associations between the cold tolerance, the protein abundance and the metabolite concentrations but no changes in mRNA expression and enzyme activity. These data stress the necessity of combining the hypothesis-generating power of an 'Omics' approach with subsequent targeted validations across several levels of the biological organization. We discuss reasons why different biological linked levels do not necessarily change stoichiometrically.

Heat shock factor 2 levels are associated with the severity of ulcerative colitis

BACKGROUND AND AIMS

The morbidity of ulcerative colitis (UC) is increasing in China every year. In addition, there is a lack of accurate diagnostic indices with which to evaluate the activity of the disease. The aim of this study was to identify UC-associated proteins as biomarkers for the diagnosis, and objective assessment of disease activity.

METHODS

Differential expression of serum proteins from UC patients compared to normal controls was analyzed by two-dimensional electrophoresis (2-DE) and matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF-MS). The expression of heat shock factor 2(HSF2)in colonic mucosa in Crohn's disease, Behcet's disease, ulcerative colitis, intestinal tuberculosis, infective enteritis, intestinal lymphoma, and normal controls was investigated by immunohistochemistry (IHC). The expression of the HSF2 in colonic mucosa of UC subjects with varying severity of disease was measured by real time-PCR and Western Blots. The expression of HSF2 was inhibited by HSF2 small interfering RNA (siRNA) transfection in Caco-2 cells. The concentrations of HSF2, IL-1β, and TNF-α in serum and IL-1β, and TNF-α in the supernatants of transfected Caco-2 cells were determined by ELISA.

RESULTS

HSF2 was differentially expressed in UC patients compared to normal controls. HSF2 expression was significantly higher in the intestinal mucosa of UC patients compared to other six groups. The results of immunohistochemistry, real time-PCR, Western Blots, and ELISA showed that the expression of HSF2 increased in parallel with the severity of UC. The serum concentration of HSF2 also positively correlated with levels of IL-1β and TNF-α. After down-regulation expression of HSF2 in Caco-2 cells by RNA interference, the productions of IL-1β and TNF-α stimulated by lipopolysaccharide (LPS) increased dramatically.

CONCLUSIONS

HSF2 appears to be a potential novel molecular marker for UC activity, and may provide a basis for studies on the pathogenesis and novel therapeutic targets for UC.

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.

Induced cold tolerance mechanisms depend on duration of acclimation in the chill sensitive Folsomia candida (Collembola)

During cold periods ectotherms may improve low temperature tolerance via rapid cold hardening (RCH) over a period of hours and/or long-term cold acclimation (LTCA) during days, weeks or months. However, the effect of duration and the major underlying mechanisms of these processes are still not fully understood. In the present study, the molecular and biochemical responses to RCH (1-3 hours) and LTCA (1-3 days) and the corresponding benefits to survival were investigated using the chill sensitive collembolan, Folsomia candida. We investigated osmolyte accumulation, membrane restructuring and transcription of candidate genes as well as survival benefits in response to RCH and LTCA. RCH induced significant up-regulation of targeted genes encoding enzymes related to carbohydrate metabolic pathways and genes encoding small and constitutively expressed Hsps, indicating that the animals rely on protein protection from a subset of Hsps during RCH and probably also LTCA. The up-regulation of genes involved in carbohydrate metabolic processes initiated during RCH was likely responsible for a transient accumulation of myoinositol during LTCA, which may support the protection of protein and membrane function and structure. Membrane restructuring, composed especially of a significantly increased ratio of unsaturated to saturated phospholipid fatty acids seems to be a supplementary mechanism to activation of Hsps and myoinositol accumulation in LTCA. Thus, the moderate increase in cold shock tolerance conferred by RCH seems to be dominated by effects of heat shock proteins, whereas the substantially better cold tolerance achieved after LTCA is dominated by post-transcriptional processes increasing membrane fluidity and cryoprotectant concentration.

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.

Cellular damage as induced by high temperature is dependent on rate of temperature change – investigating consequences of ramping rates on molecular and organismal phenotypes in Drosophila melanogaster Meigen 1830

Ecological relevance and repeatability of results obtained in different laboratories are key issues when assessing thermal tolerance of ectotherms. Traditionally assays have used acute exposures to extreme temperatures. The outcomes of ecologically more relevant ramping experiments, however, are dependent on the rate of temperature change leading to uncertainty of the causal factor for loss of function. Here, we test the physiological consequences of exposing female Drosophila melanogaster to gradually increasing temperatures in so called ramping assays. We exposed flies to ramping at rates of 0.06 and 0.1 °C per minute, respectively. Flies were sampled from the two treatments at 28, 30, 32, 34, 36 and 38 °C and tested for heat tolerance and expression levels of the heat shock genes hsp23 and hsp70 as well as Hsp70 protein. Heat shock genes were up-regulated more with a slow as compared to a faster ramping rate and heat knock down tolerance was higher in flies exposed to the faster rate. The fact that slow ramping induces a stronger stress response (Hsp expression) compared to faster ramping suggests that slow ramping induces more heat damage at the cellular level due to longer exposure time. This is supported by the observation that fast ramped flies have higher heat knock down tolerance. Thus, we observed both accumulation of thermal damage on the molecular level and heat hardening on the phenotypic level as a consequence of heat exposure. The balance between these processes is dependent on ramping rate leading to the observed variation in thermal tolerance when using different rates.

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.

Rapid cold hardening and expression of heat shock protein genes in the B-biotype Bemisia tabaci

This paper describes the rapid cold hardening processes of the sweetpotato whitefly, Bemisia tabaci (Gennadius). It was found that all developmental stages of B. tabaci have the capacity of rapid cold hardening and the length of time required to induce maximal cold hardiness at 0 °C varies with stage. There was only 18.3% survival when adult whiteflies were transferred directly from 26 °C to -8.5 °C for 2 h. However, exposure to 0 °C for 1 h before transfer to -8.5 °C increased the survival to 81.2%. The whiteflies show "prefreeze" mortality when they were exposed to temperatures above the supercooling point (SCP), although the range of SCP of whiteflies is -26 °C to -29 °C. The rapid cold hardening had no effect on SCP and reduced the lower lethal temperature of adults from -9 °C to -11 °C. Rapid cold-hardened adults had a similar lifespan as the control group but deposited fewer eggs than nonhardened individuals. The expression profiles during cold hardening and recovery from this process revealed that HSP90 did not respond to cold stress. However, HSP70 and HSP20 were significantly induced by cold with different temporal expression patterns. These results suggest that the rapid cold hardening response is possibly advantageous to whiteflies that are often exposed to drastic temperature fluctuations in spring or autumn in northern China, and the expression of HSP70 and HSP20 may be associated with the cold tolerance of B. tabaci.

The effect of soil pH and temperature on Folsomia candida transcriptional regulation

Differences in abiotic factors like temperature and soil pH can have a significant physiological impact on soil dwelling invertebrates and may confound results in ecotoxicological testing. In this study we exposed Folsomia candida to a range of two abiotic stress treatments (pH and temperature) for 3 days and measured gene expression of a panel of nine stress response genes with real-time Q-PCR. The exposure to different pH values had a minimal effect on the expression of the nine selected genes: only V-ATPase expression was significantly increased due to decreasing pH. ATPase expression was up-regulated, possibly due to increased proton trafficking across the cell membrane, at a lower pH. HSP70 was up-regulated in collembolans exposed to 30 degrees C, and along with HSP40 at 0 degrees C. We speculate that the minor pH effect on gene expression, compared to the temperature treatment, can be explained by the spatial restricted exposure to the external pH in the gut. Our data showed that only 1 or 2 stress response genes were transcriptionally affected by pH and temperature thus exerting minimal effects. The physiological effects of these treatments on F. candida might indicate interesting novel molecular mechanisms.

Temporal expression of heat shock genes during cold stress and recovery from chill coma in adult Drosophila melanogaster

A common physiological response of organisms to environmental stresses is the increase in expression of heat shock proteins (Hsps). In insects, this process has been widely examined for heat stress, but the response to cold stress has been far less studied. In the present study, we focused on 11 Drosophila melanogaster Hsp genes during the stress exposure and recovery phases. The temporal gene expression of adults was analyzed during 9 h of cold stress at 0 degrees C and during 8 h of recovery at 25 degrees C. Increased expression of some, but not all, Hsp genes was elicited in response to cold stress. The transcriptional activity of Hsp genes was not modulated during the cold stress, and peaks of expression occurred during the recovery phase. On the basis of their response, we consider that Hsp60, Hsp67Ba and Hsc70-1 are not cold-inducible, whereas Hsp22, Hsp23, Hsp26, Hsp27, Hsp40, Hsp68, Hsp70Aa and Hsp83 are induced by cold. This study suggests the importance of the recovery phase for repairing chilling injuries, and highlights the need to further investigate the contributions of specific Hsp genes to thermal stress responses. Parallels are drawn between the stress response networks resulting from heat and cold stress.

Stress specific correlated responses in fat content, Hsp70 and dopamine levels in Drosophila melanogaster selected for resistance to environmental stress

Studies of adaptation to stressful environments have frequently encountered cross resistance. This has prompted the hypothesis that certain adaptations confer resistance to multiple stressors. Some of the genes and mechanisms conferring stress resistance have been identified, however, the generality and basis of stress adaptation and cross resistance is still unclear. We investigated several physiological traits that have been previously linked to increased stress resistance: Hsp70 expression, fat content and dopamine levels. Additionally, we studied a behavioural trait, locomotor activity, as a proxy for the physiological state of the organisms. Physiology is the mechanistic link between resistance phenotype and underlying genetic background, and provides insights into the background and generality of cross resistance and correlated responses to selection for stress resistance. We assessed the relationship between the measured traits and stress resistance in a set of lines selected for increased resistance to several environmental stressors. We found that, although all physiological traits displayed significant differentiation among selection regimes, none were consistently associated with increased general stress resistance. This demonstrates that directional changes in Hsp70 expression level, dopamine level and fat content occur in response to the specific requirements of the different stress regimes, rather than as a general response to stress.

Exposure to mercury reduces heat tolerance and heat hardening ability of the springtail Folsomia candida

We investigated the combined effects of mercury (HgCl(2)) and acute heat on survival of the springtail Folsomia candida. The springtails were exposed to a range of aqueous concentrations (0-48 mg Hg(2+)/L) of HgCl(2) for 24 h. Subsequently, the same individuals were exposed to a range of high temperatures, from 20 to 35.5 degrees C. We found a highly significant synergistic interaction between effects of mercury and heat, with a reduced tolerance to heat after exposure to sublethal concentrations of mercury. Further, the heat hardening ability of F. candida was studied at sublethal concentrations of mercury. F. candida was able to heat harden (exposure to a mild heat treatment increasing survival of subsequent severe heat); however, when the springtails experienced a previous exposure to as little as 1 mg Hg(2+)/L, heat hardening failed to improve survival of heat shock at 34.5 degrees C, even though this was much lower than concentrations affecting survival without heat stress. Mild heat stress is known to induce the heat shock protein, HSP70, and real-time quantitative PCR confirmed that pre-acclimation to 32 degrees C did indeed cause >5-fold up-regulation of HSP70 expression. This up-regulation was not affected by previous exposure to 1 mg Hg(2+)/L.

Membrane remodeling and glucose in Drosophila melanogaster: a test of rapid cold-hardening and chilling tolerance hypotheses

Insect cold tolerance varies at both the population and species levels. Carbohydrate cryoprotectants and membrane remodeling are two main mechanisms hypothesised to increase chilling tolerance in Drosophila melanogaster, as part of both long-term (i.e., evolutionary) change and rapid cold-hardening (RCH). We used cold-selected lines of D. melanogaster with and without a pre-exposure that induces RCH to test three hypotheses: (1) that increased cold tolerance would be associated with increased free glucose; (2) that increased cold tolerance would be associated with desaturation of membrane phospholipid fatty acids; and (3) that increased cold tolerance would be associated with a change in phospholipid head group composition. We used colourimetric assays to measure free glucose and a combination of thin layer chromatography-flame ionization detection and gas chromatography to measure membrane composition. We observed a consistent decrease in free glucose with RCH, and no relationship between free glucose and basal cold tolerance. Also, phospholipid head group ratios and fatty acid composition showed no change following an RCH treatment. Thus, we conclude that changes in free glucose and membrane composition are unlikely to be significant determinants of variation in cold tolerance of D. melanogaster.

Short-term hardening effects on survival of acute and chronic cold exposure by Drosophila melanogaster larvae

We quantified the variation and plasticity in cold tolerance among four larval stages of four laboratory strains of Drosophila melanogaster in response to both acute (<2h of cold exposure) and chronic ( approximately 7h of cold exposure) cold exposure. We observed significant differences in basal cold tolerance between the strains and among larval stages. Early larval instars were generally more tolerant of acute cold exposures than third-instar larvae. However, wandering larvae were more tolerant of chronic cold exposures than the other stages. Early stages also displayed a more pronounced rapid cold-hardening response than the later stages. Heat pre-treatment did not confer a significant increase in cold tolerance to any of the strains at any stage, pointing to different mechanisms being involved in resolving heat- and cold-elicited damage. However, when heat pre-treatment was combined with rapid cold-hardening as sequential pre-treatments, both positive (heat first) and negative (heat second) effects on cold tolerance were observed. We discuss possible mechanisms underlying cold-hardening and the effects of acute and chronic cold exposures.

Effects of acclimation temperature on thermal tolerance and membrane phospholipid composition in the fruit fly Drosophila melanogaster

Adaptative responses of ectothermic organisms to thermal variation typically involve the reorganization of membrane glycerophospholipids (GPLs) to maintain membrane function. We investigated how acclimation at 15, 20 and 25 degrees C during preimaginal development influences the thermal tolerance and the composition of membrane GPLs in adult Drosophila melanogaster. Long-term cold survival was significantly improved by low acclimation temperature. After 60 h at 0 degrees C, more than 80% of the 15 degrees C-acclimated flies survived while none of the 25 degrees C-acclimated flies survived. Cold shock tolerance (1h at subzero temperatures) was also slightly better in the cold acclimated flies. LT50 shifted down by ca 1.5 degrees C in 15 degrees C-acclimated flies in comparison to those acclimated at 25 degrees C. In contrast, heat tolerance was not influenced by acclimation temperature. Low temperature acclimation was associated with the increase in proportion of ethanolamine (from 52.7% to 58.5% in 25 degrees C-acclimated versus 15 degrees C-acclimated flies, respectively) at the expense of choline in GPLs. Relatively small, but statistically significant changes in lipid molecular composition were observed with decreasing acclimation temperature. In particular, the proportions of glycerophosphoethanolamines with linoleic acid (18:2) at the sn-2 position increased. No overall change in the degree of fatty acid unsaturation was observed. Thus, cold tolerance but not heat tolerance was influenced by preimaginal acclimation temperature and correlated with the changes in GPL composition in membranes of adult D. melanogaster.