Multiple stress tolerance in an antarctic terrestrial arthropod: 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.

Fine-scale variation in microhabitat conditions influences physiology and metabolism in an Antarctic insect

Microhabitats with distinct biotic and abiotic properties exist within landscapes, and this microhabitat variation can have dramatic impacts on the phenology and physiology of the organisms occupying them. The Antarctic midge Belgica antarctica inhabits diverse microhabitats along the Western Antarctic Peninsula that vary in macrophyte composition, hygric qualities, nutrient input, and thermal patterns. Here, we compare seasonal physiological changes in five populations of B. antarctica living in close proximity but in different microhabitats in the vicinity of Palmer Station, Antarctica. Thermal regimes among our sample locations differed in both mean temperature and thermal stability. Between the warmest and coldest sites, seasonal mean temperatures differed by 2.6˚C and degree day accumulations above freezing differed by a factor of 1.7. Larval metabolic and growth rates varied among the sites, and adult emergence occurred at different times. Distinct microhabitats also corresponded with differences in body composition, as lipid and carbohydrate content of larvae differed across sites. Further, seasonal changes in carbohydrate and protein content were dependent on site, indicating fine-scale variation in the biochemical composition of larvae as they prepare for winter. Together, these results demonstrate that variation in microhabitat properties influences the ontogeny, phenology, physiology, and biochemical makeup of midge populations living in close proximity. These results have implications for predicting responses of Antarctic ecosystems to environmental change.

Expression of aquaporins in response to distinct dehydration stresses that confer stress tolerance in the Antarctic midge Belgica antarctica

Larvae of the Antarctic midge Belgica antarctica Jacobs (Diptera: Chironomidae) are highly tolerant of diverse environmental stresses, including freezing, severe desiccation, and osmotic extremes. Furthermore, dehydration confers subsequent desiccation and freeze tolerance. While a role for aquaporins-channels for water and other solutes-has been proposed in these dehydration processes, the types of aquaporins involved in dehydration-driven stress tolerance remain unknown. In the present study, we investigated expression of six aquaporins (Drip, Prip, Eglp1, Eglp2, Aqp12L, and Bib) in larvae of B. antarctica subjected to three different dehydration conditions: desiccation, cryoprotective dehydration, and osmotic dehydration. The expression of Drip and Prip was up-regulated under desiccation and cryoprotective dehydration, suggesting a role for these aquaporins in efficient water loss under these dehydration conditions. Conversely, expression of Drip and Prip was down-regulated under osmotic dehydration, suggesting that their expression is suppressed in larvae to combat dehydration. Larval water content was similarly decreased under all three dehydration conditions. Differences in responses of the aquaporins to the three forms of dehydration suggests distinct water management strategies associated with different forms of dehydration stress.

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.

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.

Aquaporins in the antarctic midge, an extremophile that relies on dehydration for cold survival

The terrestrial midge Belgica antarctica relies extensively on dehydration to survive the low temperatures and desiccation stress that prevail in its Antarctic habitat. The loss of body water is thus a critical adaptive mechanism employed at the onset of winter to prevent injury from internal ice formation; a rapid mechanism for rehydration is equally essential when summer returns and the larva resumes the brief active phase of its life. This important role for water movement suggests a critical role for aquaporins (AQPs). Recent completion of the genome project on this species revealed the presence of AQPs in B. antarctica representing the DRIP, PRIP, BIB, RPIP, and LHIP families. Treatment with mercuric chloride to block AQPs also blocks water loss, thereby decreasing cell survival at low temperatures. Antibodies directed against mammalian or Drosophila AQPs suggest a wide tissue distribution of AQPs in the midge and changes in protein abundance in response to dehydration, rehydration, and freezing. Thus far, functional studies have been completed only for PRIP1. It appears to be a water-specific AQP, but expression levels are not altered by dehydration or rehydration. Functional assays remain to be completed for the additional AQPs.

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.

The protective effect of rapid cold-hardening develops more quickly in frozen versus supercooled larvae of the Antarctic midge, Belgica antarctica

During the austral summer, larvae of the terrestrial midge, Belgica antarctica (Diptera: Chironomidae), experience highly variable and often unpredictable thermal conditions. In addition to remaining freeze tolerant year-round, larvae are capable of swiftly increasing their cold tolerance through the rapid cold-hardening (RCH) response. The present study compared the induction of RCH in frozen versus supercooled larvae. At the same induction temperature, RCH occurred more rapidly and conferred a greater level of cryoprotection in frozen versus supercooled larvae. Furthermore, RCH in frozen larvae could be induced at temperatures as low as -12°C, which is the lowest temperature reported to induce RCH. Remarkably, as little as 15 min at -5°C significantly enhanced larval cold tolerance. Not only is protection from RCH acquired swiftly, but it is also quickly lost after thawing for 2 h at 2°C. Because the primary difference between frozen and supercooled larvae is cellular dehydration caused by freeze concentration of body fluids, we also compared the effects of acclimation in dehydrated versus frozen larvae. Since slow dehydration without chilling significantly increased larval survival to a subsequent cold exposure, we hypothesize that cellular dehydration caused by freeze concentration promotes the rapid acquisition of cold tolerance in frozen larvae.

Function and immuno-localization of aquaporins in the Antarctic midge Belgica antarctica

Aquaporin (AQP) water channel proteins play key roles in water movement across cell membranes. Extending previous reports of cryoprotective functions in insects, this study examines roles of AQPs in response to dehydration, rehydration, and freezing, and their distribution in specific tissues of the Antarctic midge, Belgica antarctica (Diptera, Chironomidae). When AQPs were blocked using mercuric chloride, tissue dehydration tolerance increased in response to hypertonic challenge, and susceptibility to overhydration decreased in a hypotonic solution. Blocking AQPs decreased the ability of tissues from the midgut and Malpighian tubules to tolerate freezing, but only minimal changes were noted in cellular viability of the fat body. Immuno-localization revealed that a DRIP-like protein (a Drosophila aquaporin), AQP2- and AQP3 (aquaglyceroporin)-like proteins were present in most larval tissues. DRIP- and AQP2-like proteins were also present in the gut of adult midges, but AQP4-like protein was not detectable in any tissues we examined. Western blotting indicated that larval AQP2-like protein levels were increased in response to dehydration, rehydration and freezing, whereas, in adults DRIP-, AQP2-, and AQP3-like proteins were elevated by dehydration. These results imply a vital role for aquaporin/aquaglyceroporins in water relations and freezing tolerance in B. antarctica.

Survival and energetic costs of repeated cold exposure in the Antarctic midge, Belgica antarctica: a comparison between frozen and supercooled larvae

In this study, we examined the effects of repeated cold exposure (RCE) on the survival, energy content and stress protein expression of larvae of the Antarctic midge, Belgica antarctica (Diptera: Chironomidae). Additionally, we compared results between larvae that were frozen at –5°C in the presence of water during RCE and those that were supercooled at –5°C in a dry environment. Although >95% of larvae survived a single 12 h bout of freezing at –5°C, after five cycles of RCE survival of frozen larvae dropped below 70%. Meanwhile, the survival of control and supercooled larvae was unchanged, remaining around 90% for the duration of the study. At the tissue level, frozen larvae had higher rates of cell mortality in the midgut than control and supercooled larvae. Furthermore, larvae that were frozen during RCE experienced a dramatic reduction in energy reserves; after five cycles, frozen larvae had 25% less lipid, 30% less glycogen and nearly 40% less trehalose than supercooled larvae. Finally, larvae that were frozen during RCE had higher expression of hsp70 than those that were supercooled, indicating a higher degree of protein damage in the frozen group. Results were similar between larvae that had accumulated 60 h of freezing at –5°C over five cycles of RCE and those that were frozen continuously for 60 h, suggesting that the total time spent frozen determines the physiological response. Our results suggest that it is preferable, both from a survival and energetic standpoint, for larvae to seek dry microhabitats where they can avoid inoculative freezing and remain unfrozen during RCE.

Osmoregulation and salinity tolerance in the Antarctic midge, Belgica antarctica: seawater exposure confers enhanced tolerance to freezing and dehydration

Summer storms along the Antarctic Peninsula can cause microhabitats of the terrestrial midge Belgica antarctica to become periodically inundated with seawater from tidal spray. As microhabitats dry, larvae may be exposed to increasing concentrations of seawater. Alternatively, as a result of melting snow or following rain, larvae may be immersed in freshwater for extended periods. The present study assessed the tolerance and physiological response of B. antarctica larvae to salinity exposure, and examined the effect of seawater acclimation on their subsequent tolerance of freezing, dehydration and heat shock. Midge larvae tolerated extended exposure to hyperosmotic seawater; nearly 50% of larvae survived a 10-day exposure to 1000 mOsm kg(-1) seawater and approximately 25% of larvae survived 6 days in 2000 mOsm kg(-1) seawater. Exposure to seawater drastically reduced larval body water content and increased hemolymph osmolality. By contrast, immersion in freshwater did not affect water content or hemolymph osmolality. Hyperosmotic seawater exposure, and the accompanying osmotic dehydration, resulted in a significant correlation between the rate of oxygen consumption and larval water content and induced the de novo synthesis and accumulation of several organic osmolytes. A 3-day exposure of larvae to hyperosmotic seawater increased freezing tolerance relative to freshwater-acclimated larvae. Even after rehydration, the freezing survival of larvae acclimated to seawater was greater than freshwater-acclimated larvae. Additionally, seawater exposure increased the subsequent tolerance of larvae to dehydration. Our results further illustrate the similarities between these related, yet distinct, forms of osmotic stress and add to the suite of physiological responses used by larvae to enhance survival in the harsh and unpredictable Antarctic environment.

Cryoprotective dehydration and the resistance to inoculative freezing in the Antarctic midge, Belgica antarctica

During winter, larvae of the Antarctic midge, Belgica antarctica (Diptera, Chironomidae), must endure 7-8 months of continuous subzero temperatures, encasement in a matrix of soil and ice, and severely desiccating conditions. This environment, along with the fact that larvae possess a high rate of water loss and are extremely tolerant of desiccation, may promote the use of cryoprotective dehydration as a strategy for winter survival. This study investigates the capacity of larvae to resist inoculative freezing and undergo cryoprotective dehydration at subzero temperatures. Slow cooling to -3 degrees C in an environment at equilibrium with the vapor pressure of ice reduced larval water content by approximately 40% and depressed the body fluid melting point more than threefold to -2.6 degrees C. This melting point depression was the result of the concentration of existing solutes (i.e. loss of body water) and the de novo synthesis of osmolytes. By day 14 of the subzero exposure, larval survival was still >95%, suggesting larvae have the capacity to undergo cryoprotective dehydration. However, under natural conditions the use of cryoprotective dehydration may be constrained by inoculative freezing as result of the insect's intimate contact with environmental ice. During slow cooling within a substrate of frozen soil, the ability of larvae to resist inoculative freezing and undergo cryoprotective dehydration was dependent upon the moisture content of the soil. As detected by a reduction of larval water content, the percentage of larvae that resisted inoculative freezing increased with decreasing soil moisture. These results suggest that larvae of the Antarctic midge have the capacity to resist inoculative freezing at relatively low soil moisture contents and likely undergo cryoprotective dehydration when exposed to subzero temperatures during the polar winter.

Metabolomics reveals unique and shared metabolic changes in response to heat shock, freezing and desiccation in the Antarctic midge, Belgica antarctica

The midge, Belgica antarctica Jacobs, is subjected to numerous environmental stressors during its 2-year life cycle on the Antarctic Peninsula, and in response it has evolved a suite of behavioral, physiological, and life-cycle modifications to counter these stressors, but thus far only a limited number of biochemical adaptations have been identified. In this study, we use a metabolomics approach to obtain a broad overview of changes in energy metabolism, amino acids, and polyols in response to three of the midge's major stresses: heat, freezing, and desiccation. Using GC-MS analysis, a total of 75 compounds were identified. Desiccation (50% water loss) elicited the greatest physiological response (as determined by principal components analysis) when compared to untreated controls, with many elevated metabolites from pathways of central carbohydrate metabolism and a decrease in free amino acids. When larvae were frozen (6h at -10 degrees C), alanine and aspartate increased as well as urea. Freezing also increased three polyols (glycerol, mannitol, erythritol), while desiccation increased only two polyols (glycerol, erythritol). Heating the midges for 1h at 30 degrees C elevated alpha-ketoglutarate and putrescine while suppressing glycerol, glucose, and serine levels. Freezing and desiccation elicited elevation of four shared metabolites, whereas no shared metabolites were elevated by heat. All three treatments resulted in a reduction in serine, potentially identifying this amino acid as a marker for stress in this species. A number of metabolic changes, especially those in the sugar and polyol pools, are adaptations that have potential to enhance survival during both cold and desiccation.

Slow dehydration promotes desiccation and freeze tolerance in the Antarctic midge Belgica antarctica

Adaptations to low moisture availability are arguably as important as cold resistance for polar terrestrial invertebrates, especially because water, in the form of ice, is biologically inaccessible for much of the year. Desiccation responses under ecologically realistic soil humidity conditions--those close to the wilting points of plants [98.9% relative humidity (RH)]--have not previously been examined in polar insect species. In the current study we show that, when desiccated at 98.2% RH, larvae of the Antarctic midge Belgica antarctica are more tolerant of dehydration than larvae desiccated at lower humidities (75% RH), and develop an increased tolerance to freezing. The slow rate of desiccation at this high RH enabled more than 50% of larvae to survive the loss of >75% of their osmotically active water (OAW). Survival rates were further increased when rehydration was performed at 100% RH, rather than by direct contact with water. Two days at 98.2% RH resulted in a approximately 30% loss of OAW, and dramatically increased the freeze tolerance of larvae to -10 and -15 degrees C. The supercooling point of animals was not significantly altered by this desiccation treatment, and all larvae were frozen at -10 degrees C. This is the first evidence of desiccation increasing the freeze tolerance of a polar terrestrial arthropod. Maximum water loss and body fluid osmolality were recorded after 5 days at 98.2% RH, but osmolality values returned to predesiccated levels following just 1 h of rehydration in water, well before all the water lost through desiccation had been replenished. This suggests active removal of osmolytes from the extracellular fluids during the desiccation process, presumably to intracellular compartments. Heat-shock proteins appear not to contribute to the desiccation tolerance we observed in B. antarctica. Instead, we suggest that metabolite synthesis and membrane phospholipid adaptation are likely to be the underpinning physiological mechanisms enhancing desiccation and cold tolerance in this species.

Continuous up-regulation of heat shock proteins in larvae, but not adults, of a polar insect

Antarctica's terrestrial environment is a challenge to which very few animals have adapted. The largest, free-living animal to inhabit the continent year-round is a flightless midge, Belgica antarctica. Larval midges survive the lengthy austral winter encased in ice, and when the ice melts in summer, the larvae complete their 2-yr life cycle, and the wingless adults form mating aggregations while subjected to surprisingly high substrate temperatures. Here we report a dichotomy in survival strategies exploited by this insect at different stages of its life cycle. Larvae constitutively up-regulate their heat shock proteins (small hsp, hsp70, and hsp90) and maintain a high inherent tolerance to temperature stress. High or low temperature exposure does not further up-regulate these genes nor does it further enhance thermotolerance. Such "preemptive" synthesis of hsps is sufficient to prevent irreversible protein aggregation in response to a variety of common environmental stresses. Conversely, adults exhibit no constitutive up-regulation of their hsps and have a lower intrinsic tolerance to high temperatures, but their hsps can be thermally activated, resulting in enhanced thermotolerance. Thus, the midge larvae, but not the adults, have adopted the unusual strategy of expressing hsps continuously, possibly to facilitate proper protein folding in a cold habitat that is more thermally stable than that of the adults but a habitat subjected frequently to freeze-thaw episodes and bouts of pH, anoxic, and osmotic stress.

Rapid cold-hardening increases the freezing tolerance of the Antarctic midge Belgica antarctica

Rapid cold-hardening (RCH) is well known to increase the tolerance of chilling or cold shock in a diverse array of invertebrate systems at both organismal and cellular levels. Here, we report a novel role for RCH by showing that RCH also increases freezing tolerance in an Antarctic midge, Belgica antarctica (Diptera, Chironomidae). The RCH response of B. antarctica was investigated under two distinct physiological states: summer acclimatized and cold acclimated. Summer-acclimatized larvae were less cold tolerant, as indicated by low survival following exposure to -10 degrees C for 24 h; by contrast, nearly all cold-acclimated larvae survived -10 degrees C, and a significant number could survive -15 degrees C. Cold-acclimated larvae had higher supercooling points than summer larvae. To evaluate the RCH response in summer-acclimatized midges, larvae and adults, maintained at 4 degrees C, were transferred to -5 degrees C for 1 h prior to exposures to -10, -15 or -20 degrees C. RCH significantly increased survival of summer-acclimatized larvae frozen at -10 degrees C for 1 h compared with larvae receiving no cold-hardening treatment, but adults, which live for only a week or so in the austral summer, lacked the capacity for RCH. In cold-acclimated larvae, RCH significantly increased freeze tolerance to both -15 and -20 degrees C. Similarly, RCH significantly increased cellular survival of fat body, Malpighian tubules and gut tissue from cold-acclimated larvae frozen at -20 degrees C for 24 h. These results indicate that RCH not only protects against non-freezing injury but also increases freeze tolerance.