Energetic consequences of repeated and prolonged dehydration in the Antarctic midge, Belgica antarctica

Belgica antarctica (Diptera: Chironomidae): A natural model organism for extreme environments

Belgica antarctica (Diptera: Chironomidae), a brachypterous midge endemic to the maritime Antarctic, was first described in 1900. Over more than a century of study, a vast amount of information has been compiled on the species (3 750 000 Google search results as of January 10, 2021), encompassing its ecology and biology, life cycle and reproduction, polytene chromosomes, physiology, biochemistry and, increasingly, omics. In 2014, B. antarctica's genome was sequenced, further boosting research. Certain developmental stages can be cultured successfully in the laboratory. Taken together, this wealth of information allows the species to be viewed as a natural model organism for studies of adaptation and function in extreme environments.

Combined metabolome and transcriptome analysis reveals key components of complete desiccation tolerance in an anhydrobiotic insect

Anhydrobiosis is a reversible ametabolic state that occurs in response to severe desiccation. The largest anhydrobiotic animal known is the larva of the African chironomid Polypedilum vanderplanki. Here, we investigated how the metabolism of larvae changes during the desiccation–rehydration cycle and how simple biochemical processes determine viability of the chironomid. Major findings suggest that, in addition to its known anhydroprotectant role, trehalose acts as a major source of energy for rehydration. Citrate and adenosine monophosphate, accumulated in the dry state, allow rapid resumption of metabolism during the recovery phase. Finally, metabolic waste is stored as stable or nontoxic compounds such as allantoin, xanthurenic acid, or ophthalmic acid that may also act as antioxidants.

Some organisms have evolved a survival strategy to withstand severe dehydration in an ametabolic state, called anhydrobiosis. The only known example of anhydrobiosis among insects is observed in larvae of the chironomid Polypedilum vanderplanki. Recent studies have led to a better understanding of the molecular mechanisms underlying anhydrobiosis and the action of specific protective proteins. However, gene regulation alone cannot explain the rapid biochemical reactions and independent metabolic changes that are expected to sustain anhydrobiosis. For this reason, we conducted a comprehensive comparative metabolome–transcriptome analysis in the larvae. We showed that anhydrobiotic larvae adopt a unique metabolic strategy to cope with complete desiccation and, in particular, to allow recovery after rehydration. We argue that trehalose, previously known for its anhydroprotective properties, plays additional vital roles, providing both the principal source of energy and also the restoration of antioxidant potential via the pentose phosphate pathway during the early stages of rehydration. Thus, larval viability might be directly dependent on the total amount of carbohydrate (glycogen and trehalose). Furthermore, in the anhydrobiotic state, energy is stored as accumulated citrate and adenosine monophosphate, allowing rapid reactivation of the citric acid cycle and mitochondrial activity immediately after rehydration, before glycolysis is fully functional. Other specific adaptations to desiccation include potential antioxidants (e.g., ophthalmic acid) and measures to avoid the accumulation of toxic waste metabolites by converting these to stable and inert counterparts (e.g., xanthurenic acid and allantoin). Finally, we confirmed that these metabolic adaptations correlate with unique organization and expression of the corresponding enzyme genes.

Changes in Energy Reserves and Gene Expression Elicited by Freezing and Supercooling in the Antarctic Midge, Belgica antarctica

Freeze-tolerance, or the ability to survive internal ice formation, is relatively rare among insects. Larvae of the Antarctic midge Belgica antarctica are freeze-tolerant year-round, but in dry environments, the larvae can remain supercooled (i.e., unfrozen) at subzero temperatures. In previous work with summer-acclimatized larvae, we showed that freezing is considerably more stressful than remaining supercooled. Here, these findings are extended by comparing survival, tissue damage, energetic costs, and stress gene expression in larvae that have undergone an artificial winter acclimation regime and are either frozen or supercooled at −5 °C. In contrast to summer larvae, winter larvae survive at −5 °C equally well for up to 14 days, whether frozen or supercooled, and there is no tissue damage at these conditions. In subsequent experiments, we measured energy stores and stress gene expression following cold exposure at −5 °C for either 24 h or 14 days, with and without a 12 h recovery period. We observed slight energetic costs to freezing, as frozen larvae tended to have lower glycogen stores across all groups. In addition, the abundance of two heat shock protein transcripts, hsp60 and hsp90, tended to be higher in frozen larvae, indicating higher levels of protein damage following freezing. Together, these results indicate a slight cost to being frozen relative to remaining supercooled, which may have implications for the selection of hibernacula and responses to climate change.

Rapid cold hardening protects against sublethal freezing injury in an Antarctic insect

Rapid cold hardening (RCH) is a type of beneficial phenotypic plasticity that occurs on extremely short time scales (minutes to hours) to enhance insects’ ability to cope with cold snaps and diurnal temperature fluctuations. RCH has a well-established role in extending lower lethal limits, but its ability to prevent sublethal cold injury has received less attention. The Antarctic midge, Belgica antarctica is Antarctica's only endemic insect and has a well-studied RCH response that extends freeze tolerance in laboratory conditions. However, the discriminating temperatures used in previous studies of RCH are far below those ever experienced in the field. Here, we tested the hypothesis that RCH protects against nonlethal freezing injury. Larvae of B. antarctica were exposed to either control (2°C), direct freezing (-9°C for 24 h), or RCH (-5°C for 2 h followed by -9°C for 24 h). All larvae survived both freezing treatments, but RCH larvae recovered more quickly from freezing stress and had significantly higher metabolic rates during recovery. RCH larvae also sustained less damage to fat body and midgut tissue and had lower expression of two heat shock protein transcripts (hsp60 and hsp90), which is consistent with RCH protecting against protein denaturation. The protection afforded by RCH resulted in energy savings; directly frozen larvae experienced a significant depletion in glycogen energy stores that was not observed in RCH larvae. Together, these results provide strong evidence that RCH protects against a variety of sublethal freezing injuries and allows insects to rapidly fine-tune their performance in thermally variable environments.

Heat and humidity induced plastic changes in body lipids and starvation resistance in the tropical Zaprionus indianus of wet - dry seasons

Insects from tropical wet or dry seasons are likely to cope starvation stress through plastic changes (developmental as well as adult acclimation) in energy metabolites. Control and experimental groups of flies of Zaprionus indianus were reared under wet or dry conditions but adults were acclimated at different thermal or humidity conditions. Adult flies of control group were acclimated at 27°C and low (50% RH) or high (60% RH) humidity. For experimental groups, adult flies were acclimated at 32℃ for 1 to 6 days and under low (40% RH) or high (70% RH). For humidity acclimation, adult flies were acclimated at 27°C but under low (40% RH) or high (70% RH) for 1 to 6 days. Plastic changes in experimental groups as compared to control group (developmental as well as adult acclimation) revealed significant accumulation of body lipids due to thermal or humidity acclimation of wet season flies but low humidity acclimation did not change the level of body lipids in dry season flies. Starvation resistance and body lipids were higher in the males of dry season but in the females of wet season. Adult acclimation under thermal or humidity conditions exhibited changes in the rate of utilization of body lipids, carbohydrates and proteins. Adult acclimation of wet or dry season flies revealed plastic changes in mean daily fecundity; and a reduction in fecundity under starvation. Thus, thermal or humidity acclimation of adults revealed plastic changes in energy metabolites to support starvation resistance of wet or dry seasons flies.

Repeated freezing induces a trade-off between cryoprotection and egg production in the goldenrod gall fly, Eurosta solidaginis

Internal ice formation leads to wholesale changes in ionic, osmotic and pH homeostasis, energy metabolism, and mechanical damage, across a small range of temperatures, and is thus an abiotic stressor that acts at a distinct, physiologically-relevant, threshold. Insects that experience repeated freeze-thaw cycles over winter will cross this stressor threshold many times over their lifespan. Here we examine the effect of repeatedly crossing the freezing threshold on short-term physiological parameters (metabolic reserves and cryoprotectant concentration) as well as long-term fitness-related performance (survival and egg production) in the freeze-tolerant goldenrod gall fly Eurosta solidaginis. We exposed overwintering prepupae to a series of low temperatures (-10, -15, or -20 °C) with increasing numbers of freezing events (3, 6, or 10) with differing recovery periods between events (1, 5, or 10 days). Repeated freezing increased sorbitol concentration by about 50% relative to a single freezing episode, and prompted prepupae to modify long chain triacylglycerols to acetylated triacylglycerols. Long-term, repeated freezing did not significantly reduce survival, but did reduce egg production by 9.8% relative to a single freezing event. Exposure temperature did not affect any of these measures, suggesting that threshold crossing events may be more important to fitness than the intensity of stress in E. solidaginis overwintering.

Tropical Drosophila ananassae of wet-dry seasons show cross resistance to heat, drought and starvation

Plastic responses to multiple environmental stressors in wet or dry seasonal populations of tropical Drosophila species have received less attention. We tested plastic effects of heat hardening, acclimation to drought or starvation, and changes in trehalose, proline and body lipids in Drosophila ananassae flies reared under wet or dry season-specific conditions. Wet season flies revealed significant increase in heat knockdown, starvation resistance and body lipids after heat hardening. However, accumulation of proline was observed only after desiccation acclimation of dry season flies while wet season flies elicited no proline but trehalose only. Therefore, drought-induced proline can be a marker metabolite for dry-season flies. Further, partial utilization of proline and trehalose under heat hardening reflects their possible thermoprotective effects. Heat hardening elicited cross-protection to starvation stress. Stressor-specific accumulation or utilization as well as rates of metabolic change for each energy metabolite were significantly higher in wet-season flies than dry-season flies. Energy metabolite changes due to inter-related stressors (heat versus desiccation or starvation) resulted in possible maintenance of energetic homeostasis in wet- or dry-season flies. Thus, low or high humidity-induced plastic changes in energy metabolites can provide cross-protection to seasonally varying climatic stressors.

Summary: In the tropical Drosophila ananassae, low or high humidity-induced plastic changes in energy metabolites provide cross-protection to seasonally varying climatic stressors.

Detecting seasonal variation of antifreeze protein distribution in Rhagium mordax using immunofluorescence and high resolution microscopy

Larvae of the blackspotted pliers support beetle, Rhagium mordax, were collected monthly, for the duration of 2012 and fixed. The larvae were embedded in paraffin wax and sectioned. Using fluorophore-coupled antibodies specific to the R. mordax antifreeze protein, RmAFP1, sections were visualised with UV reflected light microscopy. An automated software analysis method was developed in order to discard autofluorescence, and quantify fluorescence from bound antibodies. The results show that R. mordax cuticle and gut exhibit a higher degree of fluorophore-bound fluorescence during summer, than in the cold months. It is hypothesised that R. mordax stores RmAFP1 in, or near, the fat body during times when freeze avoidance is not needed.

Antarctic Entomology

The Antarctic region comprises the continent, the Maritime Antarctic, the sub-Antarctic islands, and the southern cold temperate islands. Continental Antarctica is devoid of insects, but elsewhere diversity varies from 2 to more than 200 species, of which flies and beetles constitute the majority. Much is known about the drivers of this diversity at local and regional scales; current climate and glacial history play important roles. Investigations of responses to low temperatures, dry conditions, and varying salinity have spanned the ecological to the genomic, revealing new insights into how insects respond to stressful conditions. Biological invasions are common across much of the region and are expected to increase as climates become warmer. The drivers of invasion are reasonably well understood, although less is known about the impacts of invasion. Antarctic entomology has advanced considerably over the past 50 years, but key areas, such as interspecific interactions, remain underexplored.

Stress tolerance in a polyextremophile: the southernmost insect

Since biotic interactions within the simple terrestrial communities on the Antarctic Peninsula are limited compared with tropical and temperate regions, survival is largely dictated by the numerous abiotic challenges. Our research focuses on adaptations to environmental stresses experienced by the Antarctic midge (Belgica antarctica Jacobs, 1900), the southernmost free-living insect. Midge larvae can survive freezing and anoxia year-round. Not only can frozen larvae undergo rapid cold-hardening (RCH) at temperatures as low as –12 °C, but RCH develops more rapidly in frozen compared with supercooled larvae. Whether larvae overwinter in a frozen state or cryoprotectively dehydrated may depend on hydration levels within their hibernacula. Larvae constitutively up-regulate genes encoding heat shock proteins, as well as the antioxidant enzymes superoxide dismutase and catalase. Larvae accumulate osmoprotectants in response to freezing, desiccation, and exposure to seawater; exposure to one of these osmotic stressors confers cross-tolerance to the others. Molecular responses to dehydration stress include extensive genome-wide changes that include differential expression of aquaporins among tissues, upregulation of pathways associated with autophagy, inhibition of apoptosis, and downregulation of metabolism and ATP production.

Metabolic responses to water deprivation in C57BL/6J mice using a proton nuclear magnetic resonance-based metabonomics approach

¹H NMR-based metabonomics approach is effective for elucidating underlying mechanisms response or adaption to water deprivation in mammals. Metabolic relevance of differential compounds in response of C57BL/6J mice to water deprivation was discussed.

Surviving in a frozen desert: environmental stress physiology of terrestrial Antarctic arthropods

Abiotic stress is one of the primary constraints limiting the range and success of arthropods, and nowhere is this more apparent than Antarctica. Antarctic arthropods have evolved a suite of adaptations to cope with extremes in temperature and water availability. Here, we review the current state of knowledge regarding the environmental physiology of terrestrial arthropods in Antarctica. To survive low temperatures, mites and Collembola are freeze-intolerant and rely on deep supercooling, in some cases supercooling below −30°C. Also, some of these microarthropods are capable of cryoprotective dehydration to extend their supercooling capacity and reduce the risk of freezing. In contrast, the two best-studied Antarctic insects, the midges Belgica antarctica and Eretmoptera murphyi, are freeze-tolerant year-round and rely on both seasonal and rapid cold-hardening to cope with decreases in temperature. A common theme among Antarctic arthropods is extreme tolerance of dehydration; some accomplish this by cuticular mechanisms to minimize water loss across their cuticle, while a majority have highly permeable cuticles but tolerate upwards of 50–70% loss of body water. Molecular studies of Antarctic arthropod stress physiology are still in their infancy, but several recent studies are beginning to shed light on the underlying mechanisms that govern extreme stress tolerance. Some common themes that are emerging include the importance of cuticular and cytoskeletal rearrangements, heat shock proteins, metabolic restructuring and cell recycling pathways as key mediators of cold and water stress in the Antarctic.

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.

Expression of genes involved in energy mobilization and osmoprotectant synthesis during thermal and dehydration stress in the Antarctic midge, Belgica antarctica

The Antarctic midge, Belgica antarctica, experiences sub-zero temperatures and desiccating conditions for much of the year, and in response to these environmental insults, larvae undergo rapid shifts in metabolism, mobilizing carbohydrate energy reserves to promote synthesis of low-molecular-mass osmoprotectants. In this study, we measured the expression of 11 metabolic genes in response to thermal and dehydration stress. During both heat and cold stress, we observed upregulation of phosphoenolpyruvate carboxykinase (pepck) and glycogen phosphorylase (gp) to support rapid glucose mobilization. In contrast, there was a general downregulation of pathways related to polyol, trehalose, and proline synthesis during both high- and low-temperature stress. Pepck was likewise upregulated in response to different types of dehydration stress; however, for many of the other genes, expression patterns depended on the nature of dehydration stress. Following fast dehydration, expression patterns were similar to those observed during thermal stress, i.e., upregulation of gp accompanied by downregulation of trehalose and proline synthetic genes. In contrast, gradual, prolonged dehydration (both at a constant temperature and in conjunction with chilling) promoted marked upregulation of genes responsible for trehalose and proline synthesis. On the whole, our data agree with known metabolic adaptations to stress in B. antarctica, although a few discrepancies between gene expression patterns and downstream metabolite contents point to fluxes that are not controlled at the level of transcription.