Elements of cold hardiness in a littoral population of the land snail Helix aspersa (Gastropoda: Pulmonata)

The land snail Helix aspersa can be considered partially tolerant to freezing, in the sense it can survive some ice formation within its body for a limited time, and possesses a limited ability to supercool. This study aimed at understanding what factors are responsible for the variation of the temperature of crystallization ( Tc) in a littoral temperate population. The ability to supercool was maximal (ca. -5 degrees C) during dormancy periods (hibernation and aestivation) and minimal (ca. -3 degrees C) during spring and autumn, in relation with the decrease of water mass and the increase of osmolality. Tc decreased in October to remain stable through late autumn and winter; it increased quickly with the awakening of animals in April. Snails with an epiphragm had a significantly higher ability to supercool (ca. -4.8 degrees C) than snails which did not form an epiphragm (ca. -4.2 degrees C). The animals' size had a weak but significant influence on the realization of the Tc. It appeared that there was not a real cold-hardiness strategy in this population; rather a sum of parameters, varying in consequences of the external conditions and of the activity cycle, which are responsible for the enhancement of the supercooling ability during winter.     

Intracellular freezing and survival in the freeze tolerant alpine cockroach Celatoblatta quinquemaculata

The alpine cockroach Celatoblatta quinquemaculata is common at altitudes of around 1500 m on the Rock and Pillar range of Central Otago, New Zealand where it experiences freezing conditions in the winter. The cockroach is freeze tolerant, but only to c. -9 degrees C. The cause of death at temperatures below this is unknown but likely to be due to osmotic damage to cells (shrinkage). This study compared the effect of different ice nucleation temperatures (-2 and -4 degrees C) on the viability of three types of cockroach tissue (midgut, Malpighian tubules and fat body cells) and cooling to three different temperatures (-5, -8, -12 degrees C). Two types of observations were made (i) cryomicroscope observations of ice formation and cell shrinkage (ii) cell integrity (viability) using vital stains. Cell viability decreased with lower treatment temperatures but ice nucleation temperature had no significant effect. Cryomicroscope observations showed that ice spread through tissue faster at -4 than -2 degrees C and that intracellular freezing only occurred when nucleated at -4 degrees C. From temperature records during cooling, it was observed that when freezing occurred, latent heat immediately increased the insect's body temperature close to its melting point (c. -0.3 degrees C). This "rebound" temperature was independent of nucleation temperature. Some tissues were more vulnerable to damage than others. As the gut is thought to be the site of freezing, it is significant that this tissue was the most robust. The ecological importance of the effect of nucleation temperature on survival of whole animals under field conditions is discussed.     

Influence of temperature on survival, growth and fecundity of the freshwater snail Indoplanorbis exustus (Deshayes).

To note the effect of temperature on survival, growth and fecundity, newly hatched (zero day old) snails Indoplanorbis exustus were cultured at 10 degrees, 15 degrees, 20 degrees, 25 degrees, 30 degrees and 35 degrees C constant temperatures and room temperature (17.5 degrees-32.5 degrees C). Individuals exposed to 10 degrees C died within 3 days while those reared at 15 degrees, 20 degrees, 25 degrees, 30 degrees, 35 degrees C and room temperature survived for a period of 6, 27, 18, 16, 12 and 17 weeks respectively. An individual added on an average 0.21 mm and 0.45 mg, 0.35 mm and 7.94 mg, 0.63 mm and 15.5 mg, 0.81 mm and 27.18 mg, 1.07 mm and 41.48 mg and 0.78 mm and 31.2 mg to the shell diameter and body weight respectively at those temperatures per week. The snails cultured at 15 degrees C died prior to attainment of sexual maturity. On an average, an individual produced 31.9 and 582.77, 54.86 and 902.18, 56.01 and 968.45, 49.32 and 798.68 and 62.34 and 1143.97 capsules and eggs respectively at 20 degrees, 25 degrees, 30 degrees, 35 degrees C and room temperature (17.5 degrees-32.5 degrees C).     

The effect of antibiotic treatment on the supercooling ability of the land snail Helix aspersa (Gastropoda: Pulmonata)

The land snail Helix aspersa is a partially freezing tolerant species whose supercooling ability is limited to ca. -3 to -5 degrees C. One hundred adult snails were subjected to the following two experimental conditions: (i) a starved group, provided with water; (ii) an antibiotic-treated group that was provided with a solution containing a mixture of two antibiotics. The antibiotic group exhibited a T(c) significantly lower than the starved group (-3.94 +/- 1.32 degrees C, n = 40 and -3.07 +/- 0.99, n = 30, t test, p < 0.005). This study showed that bacteria of the gut are likely to elevate animal supercooling points. It is also the first report in which a possible ice-nucleating activity of the gut microflora in a land snail has been suggested by the action of antibiotics on the T(c).     

Is cold hardiness size-constrained? A comparative approach in land snails

Body water is a major element of the cold-hardiness strategies observed in ectothermic animals, in particular in freezing avoidant species for which body ice formation is lethal. Here, we investigate the relationships, in terrestrial snails, between the temperature of crystallisation (Tc) and body water (water mass and water content), shell shape, geographic and climatic distribution, taking into account phylogenetic inertia. Phylogenetic relationships among 31 species from 13 different families of terrestrial Gastropods were studied using 28S rRNA nuclear and COI mitochondrial sequence data, together with species-specific traits. Our results provide evidence for clear relationships between Tc and absolute/relative body water: smaller species with lower water content tended to be characterized by colder temperatures of crystallisation, although some exceptions were noticeable. Environmental conditions do not appear to affect Tc significantly, as well as shell shape which is however correlated with water content. This study confirmed that supercooling ability in land snails is size-constrained, with consequences on cold-hardiness strategies.     

Avoidance of intracellular freezing by the freezing-tolerant New Zealand Alpine weta Hemideina maori (Orthoptera: Stenopelmatidae)

Calorimetric analysis indicates that 82% of the body water of Hemideina maori is converted into ice at 10 degrees C. This is a high proportion and led us to investigate whether intracellular freezing occurs in H. maori tissue. Malpighian tubules and fat bodies were frozen in haemolymph on a microscope cold stage. No fat body cells, and 2% of Malpighian tubule cells froze during cooling to -8 degrees C. Unfrozen cells appeared shrunken after ice formed in the extracellular medium. There was no difference between the survival of control tissues and those frozen to -8 degrees C. At temperatures below -15 degrees C (lethal temperatures for weta), there was a decline in survival, which was strongly correlated with temperature, but no change in the appearance of tissue. It is concluded that intracellular freezing is avoided by Hemideina maori through osmotic dehydration and freeze concentration effects, but the reasons for low temperature mortality remain unclear. The freezing process in H. maori appears to rely on extracellular ice nucleation, possibly with the aid of an ice nucleating protein, to osmotically dehydrate the cells and avoid intracellular freezing. The lower lethal temperature of H. maori (-10 degrees C) is high compared to organisms that survive intracellular freezing. This suggests that the category of 'freezing tolerance' is an oversimplification, and that it may encompass at least two strategies: intracellular freezing tolerance and avoidance.     

A comparative study of the morphology and viability of hyphae of Penicillium expansum and Phytophthora nicotianae during freezing and thawing

The changes in morphology of Penicillium expansum Link and Phytophthora nicotianae Van Breda de Haan during freezing and thawing in a growth medium with and without the cryoprotective additive glycerol were examined with a light microscope fitted with a temperature-controlled stage. Viability of 0.5-1.0 mm diameter colonies of both fungi was determined after equivalent rates of cooling to -196 degrees C in the presence or absence of glycerol. In P. expansum shrinkage occurred in all hyphae at rates of cooling of less than 15 degrees C min-1; at faster rates intracellular ice nucleation occurred. The addition of glycerol increased the rate of cooling at which 50% of the hyphae formed intracellular ice from 18 degrees C min-1 to 55 degrees C min-1. This species was particularly resistant to freezing injury and recovery was greater than 60% at all rates of cooling examined. At rapid rates of cooling recovery occurred in hyphae in which intracellular ice had nucleated. In contrast, during the cooling of Ph. nicotianae in the growth medium, shrinkage occurred and no samples survived on thawing from -196 degrees C. However, on the addition of glycerol, shrinkage during freezing decreased and viable hyphae were recovered upon thawing; at rates of cooling over 10 degrees C min-1 the loss of viability was related to glycerol-induced osmotic shrinkage during cooling rather than to the nucleation of intracellular ice.     

Desiccation stress at sub-zero temperatures in polar terrestrial arthropods

Cold tolerant polar terrestrial arthropods have evolved a range of survival strategies which enable them to survive the most extreme environmental conditions (cold and drought) they are likely to encounter. Some species are classified as being freeze tolerant but the majority of those found in the Antarctic survive sub-zero temperatures by avoiding freezing by supercooling. For many arthropods, not just polar species, survival of desiccating conditions is equally important to survival of low temperatures. At sub-zero temperatures freeze avoiding arthropods are susceptible to desiccation and may lose water due to a vapour diffusion gradient between their supercooled body fluids and ice in their surroundings. This process ceases once the body fluids are frozen and so is not a problem for freeze tolerant species. This paper compares five polar arthropods, which have evolved different low temperature survival strategies, and the effects of exposure to sub-zero temperatures on their supercooling points (SCP) and water contents. The Antarctic oribatid mite (Alaskozetes antarcticus) reduced its supercooling point temperature from -6 to -30 degrees C, when exposed to decreasing sub-zero temperatures (cooled from 5 to -10 degrees C over 42 days) with little loss of body water during that period. However, Cryptopygus antarcticus, a springtail which occupies similar habitats in the Antarctic, showed a decrease in both water content and supercooling ability when exposed to the same experimental protocol. Both these Antarctic arthropods have evolved a freeze avoiding survival strategy. The Arctic springtail (Onychiurus arcticus), which is also freeze avoiding, dehydrated (from 2.4 to 0.7 g water g(-1) dry weight) at sub-zero temperatures and its SCP was lowered from c. -3 to below -15 degrees C in direct response to temperature (5 to -5.5 degrees C). In contrast, the freeze tolerant larvae of an Arctic fly (Heleomyza borealis) froze at c. -7 degrees C with little change in water content or SCP during further cold exposure and survived frozen to -60 degrees C. The partially freeze tolerant sub-Antarctic beetle Hydromedion sparsutum froze at c. -2 degrees C and is known to survive frozen to -8 degrees C. During the sub-zero temperature treatment, its water content reduced until it froze and then remained constant. The survival strategies of such freeze tolerant and freeze avoiding arthropods are discussed in relation to desiccation at sub-zero temperatures and the evolution of strategies of cold tolerance.     

Survival and metabolic responses to freezing by the water frog (Rana ridibunda)

We studied the ability of the marsh frog Rana ridibunda to survive freezing exposure and the associated subsequent metabolic variations. This species that typically overwinters under water tolerates the conversion of 55% of its body water into ice. This ice content is attained after a few hours (between 8 and 36 hours depending on the mass of the individual and the environmental temperature) but death occurs at greater than 58% ice. Freezing stimulated a significant increase in blood carnitine and trimethylamine levels (respectively 4.5+/-2.5 and 0.5+/-0.2 micromol.l(-1) for controls versus 27.0+/-18.9 and 3.6+/-4.1 micromol.l(-1) after thawing) but these increases had no significant effect on plasma osmolality which was unchanged between control and freeze exposed frogs (252.6+/-20.3 versus 240.2+/-25.0 mOsmol.l(-1), respectively). Freezing also induced a significant dehydration of heart, liver and muscles (respectively 4.2, 3.2 and 2.8%) but the observed levels are low compared to values found in highly freeze tolerant species. This species could be classified as "partially freeze tolerant" enduring the transformation of a significant part of its body water into ice but not the completion of the exotherm. The existence of freeze tolerance in an aquatic hibernator that does not accumulate cryoprotectant, exhibiting low organ dehydration after freezing and low hypoxia tolerance, raises the possibility that a tolerance of nearly 60% ice within the body is common among anurans.     

Supercooling and freeze-tolerance in the European wall lizard,Podarcis muralis,with a revisional history of the discovery of freeze-tolerance in vertebrates

Summary Wall lizards were collected in the fall of 1988 from a population introduced in 1951 into Cincinnati, OH. They were acclimated to 5 °C for several weeks prior to testing at sub-zero temperatures. Eleven super-cooled lizards were removed from the cooling chamber prior to crystallization after between 15 min and 26 h at body temperatures ranging from -2.2 to -5.9 °C. With the exception of one individual supercooled to-5.0 °C, all lizards recovered fully. The crystallization temperatures of 15 lizards which froze ranged from -0.6 to -6.4 °C. Frozen lizards were stiff with a distinct blue color, which faded upon thawing at 3 °C. The ice contents of frozen lizards were determined calorimetrically and/or estimated from a theoretical model, the two methods being generally in close agreement. Remarkably, five individuals recovered fully from exposures as long as 2 h and with as much as 28% of their body water frozen. Although these animals are not as tolerant as certain other vertebrates they are clearly able to withstand freezing under some circumstances. Failure to survive freezing was attributed either to excessive ice accumulation during a prolonged freeze or to excessive supercooling prior to freezing, which induced a large initial surge of ice formation upon crystallization. Our results accord with those of Weigmann (1929). We accordingly recognize him as the first to demonstrate freeze-tolerance in vertebrates, and we further recognize P. muralis as the first vertebrate known to survive freezing.     

Supercooling ability in two populations of the land snail Helix pomatia (Gastropoda: Helicidae) and ice-nucleating activity of gut bacteria

The land snail Helix pomatia (Gastropoda: Helicidae) is widely distributed in Northern and Central Europe where it may experience subzero temperatures during winter months. Its supercooling ability was studied in two populations of H. pomatia. One population originated from Southern Sweden (Gotaland) and the other from Central France (Auvergne). In the experimental design, they were acclimated, over 2 weeks, to artificial winter conditions (hibernation, T=5 degrees C). The Swedish snails showed a rather limited supercooling ability (temperature of crystallization, T(c)=-6.4+/-0.8 degrees C), significantly greater, however, than the supercooling capacity of the population from France (T(c)=-4.6+/-1.4 degrees C). In artificial spring conditions (3 months of hibernation followed by a progressive acclimation, over 2 weeks, to activity at T=20 degrees C), both populations exhibited a similar high T(c) (-2.0+/-1.0 degrees C). The lower T(c) of hibernating Swedish snails could be due to a greater loss of body water, accompanied by a higher concentration of solutes in the hemolymph. In both populations, the variation in hemolymph osmolality measured between hibernating (250-270 mOsm kg(-1)) and active (165-215 mOsm kg(-1)) snails may be explained by the variation in body water mass and did not suggest the production of colligative cryoprotectants. Moreover, the three bacterial strains, Buttiauxella sp., Kluyvera sp., and Tatumella sp. (Enterobacteriaceae) which were isolated from fed snails, but absent in starved snails, did not show any ice-nucleating activity at temperatures higher than -9 degrees C. Only the strain Kluyvera sp. initiated nucleation at -9 degrees C. This strain, therefore, is a weak, also termed a Type III or Class C ice-nucleating active bacterium, but with no influence on the supercooling ability of individual snails. In summary, fluctuations in body water mass of hibernating snail populations, triggering changes in osmolyte concentration, rather than the presence of endogenous ice-nucleating-active bacteria, accounts for fluctuations in their T(c).     

Intracellular freezing, viability, and composition of fat body cells from freeze-intolerant larvae of Sarcophaga crassipalpis.

Although it is often assumed that survival of freezing requires that ice formation must be restricted to extracellular compartments, fat body cells from freeze-tolerant larvae of the gall fly, Eurosta solidaginis (Diptera, Tephritidae) survive intracellular freezing. Furthermore, these cells are highly susceptible to inoculative freezing by external ice, undergo extensive lipid coalescence upon thawing, and survive freezing better when glycerol is added to the suspension medium. To determine whether these traits are required for intracellular freeze tolerance or whether they are incidental and possessed by fat body cells in general, we investigated the capacity of fat body cells from nondiapause-destined and diapause-destined (i.e., cold-hardy) larvae of the freeze-intolerant flesh fly Sarcophaga crassipalpis (Diptera, Sarcophagidae) to survive intracellular freezing. Fat body cells from both types of larvae were highly susceptible to inoculative freezing; all cells froze between -3.7 to -6.2 degrees C. The highest rates for survival of intracellular freezing occurred at -5 degrees C. The addition of glycerol to the media markedly increased survival rates. Upon thawing, the fat body cells showed little or no lipid coalescence. Fat body cells from E. solidaginis had a water content of only 35% compared to cells from S. crassipalpis larvae that had 52-55%; cells with less water may be less likely to be damaged by mechanical forces during intracellular freezing.     

A pig model of hepatic cryotherapy. In vivo temperature distribution during freezing and histopathological changes

We aimed to assess the temperature distribution in the cryolesion during hepatic cryotherapy and the association with postoperative histological changes to optimise the technique and allow better preoperative planning. Hepatic cryolesions were produced in 22 pigs following laparotomy using a CMS-cryosystem and 8mm-AccuProbe-Cryoprobes. The temperature was measured in 1 min intervals at different distances from the probe during freezing. The animals were treated in 5 groups: (i) single freezing of 20 min; (ii) double freezing of 20 min each; (iii) single freezing of 40 min; (iv) single freezing of 20 min (n=4), histology at 1 week p.o., and (v) single freezing of 20 min and Pringle manoeuvre; [(i)-(iii) and (v): histology at 24 h p.o.]. The mean diameter of the -38 degrees C isotherm, i.e., the zone of effective treatment for colorectal metastases was 37 mm for group (i) with a mean iceball diameter of 59 mm and about 46 mm for groups (ii, iii, and v) with mean iceball diameters of 78, 75, and 75 mm, respectively. At 7 days postoperatively secondary necrosis was seen in the largest central part of the lesion, wherever temperatures of -15 degrees C or lower were achieved during cryosurgery. Under the hypothesis that -38 degrees C is the effective temperature for the destruction of colorectal liver metastases, a lesion of 37-mm diameter may be effectively treated with a single 8mm-AccuProbe-Cryoprobe and a 20 min single freeze cycle and a lesion of 46 mm may be effectively treated when a double freeze-thaw cycle of 20 min each, a single freeze cycle of 40 min, or a 20 min single freeze cycle with additional Pringle manoeuvre is used, when it is perfectly placed in the lesion.     

Effects of snail size on encystment of Echinostoma caproni in juvenile Biomphalaria glabrata (NMRI strain) and observations on the survival of infected snails

The effects of snail size on encystment of Echinostoma caproni cercariae in neonatal and juvenile Biomphalaria glabrata (NMRI strain) snails were studied. Encystment in neonatal (0.7-1.1 mm shell diameter) and juvenile (2-3 mm shell diameter) snails was compared 24 h post-infection (PI) following individual exposure of snails of each size to 1, 5, 10, 25 and 50 cercariae. Significantly more cysts were recovered from juveniles exposed to 1, 5, 10 and 50 cercariae than from neonatals with comparable exposure. Size of B. glabrata was a major factor in determining cyst burden in this planorbid. Survival of infected versus uninfected neonatals and juveniles was also examined for 7 days. Neonatals exposed to 10 cercariae showed a significant decrease in survival at 3, 6 and 7 days PI when compared to the uninfected controls. There was no significant decrease in the survival of juveniles exposed to 10 cercariae compared to uninfected controls at any time point. Snail size was a factor in mortality associated with echinostome cercarial penetration and encystment.     

Seasonal variation in freeze tolerance and ice content of the tree frog Hyla versicolor

Freeze tolerance and ice content of Hyla versicolor showed pronounced variation between summer (June) and winter (December). Summer frogs survived freezing at -3 degrees C for up to 9 hr and ice accumulation up to 50% of their total body water. A time course of ice formation indicated that an equilibrium level was reached in approximately 15 hr. Thus, the lethal ice content was less than the equilibrium ice content for these conditions (63.1%). A second group was induced to enter an overwintering condition by holding them through the summer and then subjecting them to a progressive reduction in temperature and photoperiod for 2 months. These frogs survived freezing for 48 hr at -3 degrees C. Their equilibrium ice content at this temperature was significantly lower (52.5%) than comparably treated summer animals. In the winter acclimatized group, frozen frogs had substantially higher blood glucose levels than unfrozen frogs (22.7 mumol/ml vs. 1.33 mumol/ml), but glycerol levels were not elevated after freezing. Freezing frogs conditioned for overwintering at -7 degrees C resulted in a higher equilibrium ice content (62.6%), but none survived. It is evident that in preparation for overwintering, frogs reduce the amount of ice formed at a given subzero temperature, but there is little indication of a substantial change in the total amount of ice tolerated.     

Energy and water conservation in frozen vs. supercooled larvae of the goldenrod gall fly, Eurosta solidaginis (fitch) (Diptera: Tephritidae)

Insects that tolerate severe cold during winter may either supercool or tolerate ice forming within the tissues of the body. To compare the relative advantages of freezing and supercooling, we measured rates of CO(2) production and water loss in frozen and supercooled goldenrod gall fly larvae (Eurosta solidaginis). As an important first step, we measured the time required for ice content and metabolic rate to stabilize upon freezing. Ice content stabilized after only three hours of freezing at -5 degrees C, whereas CO(2) production required 12 hours to stabilize. Subsequent experiments found that freezing greatly reduced both water loss and metabolic rate. Comparisons of supercooled and frozen larvae at -5 degrees C indicated that CO(2) production fell 47% with freezing and water loss decreased 35%. As temperature decreased to -10 and -15 degrees C, CO(2) production fell exponentially and was no longer detectable at -20 degrees C with our measurement system. Our results demonstrate that freezing significantly reduces energy consumption during the winter and may therefore improve winter survival and spring fecundity. The advantages of freezing over supercooling would drive selection toward insect freeze tolerance and also toward higher supercooling points to increase the duration of freezing each winter.     

Cryomicroscopic analysis of intracellular ice formation during freezing of mouse oocytes without cryoadditives

Kinetics of intracellular ice formation (IIF) under various freezing conditions was investigated for mouse oocytes at metaphase II obtained from B6D2F1 mice. A new cryostage with improved optical performance and "isothermal" temperature field was used for nucleation experiments. The maximum thermal gradient across the window was less than 0.1 degrees C/10 mm at sample temperatures near 0 degrees C. The dependence of IIF on the initial concentration of the suspending medium was found to be pronounced. The mean IIF temperatures were found to be -9.56, -12.49, -17.63, -22.20 degrees C for freezing at 120 degrees C/min in 200, 285, 510, and 735 mosm phosphate-buffered saline, respectively. For concentrations higher than 735 mosm, the kinetics of IIF showed a break point at approximately -31 degrees C. Below -31 degrees C, all the remaining unfrozen oocytes underwent IIF almost immediately over a temperature range of less than 3 degrees C. This dramatic shift in the kinetics of IIF suggests that there were two distinct mechanisms responsible for IIF during freezing. The effect of the cooling rate on the kinetics of IIF was also investigated in isotonic PBS. At 1 degrees C/min none of the oocytes contained ice, whereas, at 5 degrees C/min all the oocytes contained ice. The mean IIF temperatures for cooling rates between 1 and 120 degrees C/min were almost constant with an average of -12.82 +/- 0.6 degrees C (SEM). In addition, constant temperature experiments were conducted in isotonic PBS. The percentages of oocytes with IIF were 0, 50, 60, and 95% for -3.8, -6.4, -7.72, and -8.85 degrees C. In undercooling experiments, IIF was not observed until approximately -20 degrees C (at which temperature the whole suspension was frozen spontaneously), suggesting the involvement of the external ice in the initiation of IIF between approximately -5 and -31 degrees C during freezing of oocytes.     

Quantitative study of the importance of water permeability in plant cold hardiness

The rate of ice formation was measured for Hedera helix L. cv. Thorndale (English ivy) bark exposed to -10 C. The cooling rate of bark exposed to -10 C was 31 C per minute. The water efflux rate required for ice formation to occur extracellularly was calculated from the rate of ice formation and the average cell diameter. The water potential difference driving the efflux of water to sites of extracellular ice was calculated from the sample temperature, osmotic water potential, and fraction of water frozen at a given freezing temperature. From the water efflux rate and water potential difference, the resistance of the barrier controlling movement of intracellular water to sites of extracellular ice was calculated. Comparison of the resistance of this barrier to water movement with the resistance of the cell membrane revealed that the membrane represented only 0.5% of the barrier resistance. Thus, membrane resistance can have little influence on the rate of water efflux and ice formation when bark is cooled at a rate of 31 C per minute. If ice formation occurred at the same rate in ivy bark as it occurred in a 10 mm MnCl(2) solution, the membrane resistance would still have represented only 1% of the resistance of the barrier to ice formation. Therefore, at a cooling rate of 31 C/minute, heat removal plays a large part in determining the rate of ice formation. At slower cooling rates experienced under natural freezing conditions the ability to remove heat would play an even larger role. It is concluded that under natural freezing conditions membrane resistance does not limit water efflux.     

Mechanisms underlying insect freeze tolerance

Freeze tolerance – the ability to survive internal ice formation – has evolved repeatedly in insects, facilitating survival in environments with low temperatures and/or high risk of freezing. Surviving internal ice formation poses several challenges because freezing can cause cellular dehydration and mechanical damage, and restricts the opportunity to metabolise and respond to environmental challenges. While freeze‐tolerant insects accumulate many potentially protective molecules, there is no apparent ‘magic bullet’ – a molecule or class of molecules that appears to be necessary or sufficient to support this cold‐tolerance strategy. In addition, the mechanisms underlying freeze tolerance have been minimally explored. Herein, we frame freeze tolerance as the ability to survive a process: freeze‐tolerant insects must withstand the challenges associated with cooling (low temperatures), freezing (internal ice formation), and thawing. To do so, we hypothesise that freeze‐tolerant insects control the quality and quantity of ice, prevent or repair damage to cells and macromolecules, manage biochemical processes while frozen/thawing, and restore physiological processes post‐thaw. Many of the molecules that can facilitate freeze tolerance are also accumulated by other cold‐ and desiccation‐tolerant insects. We suggest that, when freezing offered a physiological advantage, freeze tolerance evolved in insects that were already adapted to low temperatures or desiccation, or in insects that could withstand small amounts of internal ice formation. Although freeze tolerance is a complex cold‐tolerance strategy that has evolved multiple times, we suggest that a process‐focused approach (in combination with appropriate techniques and model organisms) will facilitate hypothesis‐driven research to understand better how insects survive internal ice formation.