Effects of acclimation temperature on thermal tolerance, locomotion performance and respiratory metabolism in Acheta domesticus L. (Orthoptera: Gryllidae)

The effects of acclimation temperature on insect thermal performance curves are generally poorly understood but significant for understanding responses to future climate variation and the evolution of these reaction norms. Here, in Acheta domesticus, we examine the physiological effects of 7-9 days acclimation to temperatures 4 °C above and below optimum growth temperature of 29 °C (i.e. 25, 29, 33 °C) for traits of resistance to thermal extremes, temperature-dependence of locomotion performance (jumping distance and running speed) and temperature-dependence of respiratory metabolism. We also examine the effects of acclimation on mitochondrial cytochrome c oxidase (CCO) enzyme activity. Chill coma recovery time (CRRT) was significantly reduced from 38 to 13 min with acclimation at 33-25 °C, respectively. Heat knockdown resistance was less responsive than CCRT to acclimation, with no significant effects of acclimation detected for heat knockdown times (25 °C: 18.25, 29 °C: 18.07, 33 °C: 25.5 min). Thermal optima for running speed were higher (39.4-40.6 °C) than those for jumping performance (25.6-30.9 °C). Acclimation temperature affected jumping distance but not running speed (general linear model, p = 0.0075) although maximum performance (U_MAX) and optimum temperature (T_OPT) of the performance curves showed small or insignificant effects of acclimation temperature. However, these effects were sensitive to the method of analysis since analyses of T_OPT, U_MAX and the temperature breadth (T_BR) derived from non-linear curve-fitting approaches produced high inter-individual variation within acclimation groups and reduced variation between acclimation groups. Standard metabolic rate (SMR) was positively related to body mass and test temperature. Acclimation temperature significantly influenced the slope of the SMR-temperature reaction norms, whereas no variation in the intercept was found. The CCO enzyme activity remained unaffected by thermal acclimation. Finally, high temperature acclimation resulted in significant increases in mortality (60-70% at 33 °C vs. 20-30% at 25 and 29 °C). These results suggest that although A. domesticus may be able to cope with low temperature extremes to some degree through phenotypic plasticity, population declines with warmer mean temperatures of only a few degrees are likely owing to the limited plasticity of their performance curves.     

Effects of acclimation and latitude on the activity thresholds of the aphid Myzus persicae in Europe

Activity thresholds were measured in nine anholocyclic clones of the peach‐potato aphid Myzus persicae collected along a latitudinal cline of its European distribution from Sweden to Spain. The effects of collection origin and intra‐ and intergenerational acclimation on these thresholds were investigated. Low‐temperature (10°C) acclimation for one generation depressed the movement threshold and chill coma temperatures, with the largest reduction in movement threshold recorded for clone UK 1 (8.8–2.5°C) and in chill coma for UK 2 (4.8–2.0°C). High‐temperature (25°C) acclimation for one generation increased the heat movement threshold and heat coma temperature with the largest increase in the movement threshold (40.1–41.1°C) and heat coma (41.4–42.3°C) recorded for clone Swed 1. There was no further intergenerational acclimation over three generations. High‐temperature activity thresholds were less plastic than low‐temperature thresholds, and, consequently, thermal activity ranges were expanded following low‐temperature acclimation. No constant affect of acclimation was observed on chill coma recovery, although clonal differences were observed with Swed 1 and 3 requiring some of the longest complete recovery times. There was no relationship between latitude and activity thresholds with the exception of heat coma data where Scandinavian clones Swed 2 and 3 consistently displayed some of the lowest heat coma temperatures (e.g. 41.3°C for both clones at 20°C) and Mediterranean clones Span 1, 2 and 3 displayed some of the highest (e.g. 42.1, 41.9 and 42.5°C, respectively, at 20°C). These data suggest that clonal mixing could occur over a large scale across Europe, limiting local adaptation to areas where conditions enable long‐term persistence of populations, e.g. adaptation to higher temperatures in the Mediterranean region. It is suggested that aphid thermal tolerance could be governed more by clonal type than the latitudinal origin.     

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

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

Complexity of the cold acclimation response in Drosophila melanogaster

Insects can increase their resistance to cold stress when they are exposed to non-lethal conditions prior to the stress; these plastic responses are normally described only in terms of immediate effects on mortality. Here we examine in Drosophila melanogaster the short- and longer-term effects of different conditions on several measures of cold resistance, but particularly chill coma recovery. Short-term exposure to sublethal temperature (cold hardening) did not decrease chill coma recovery times even though it decreased mortality. Exposure to 12 degrees C for 2 days (acclimation) decreased chill coma recovery times for a range of stressful temperatures when flies were cultured at 25 degrees C, but did not usually affect recovery times when flies were cultured at 19 degrees C. In contrast, 2-day exposure to 12 degrees C decreased mortality regardless of rearing temperature. Rearing at 19 degrees C decreased mortality and chill coma recovery time relative to rearing at 25 degrees C. Acclimation increased the eclosion rate of eggs from stressed females, but did not affect development time or size of the offspring. These results indicate that plastic responses to cold in D. melanogaster are complex when resistance is scored in different ways, and that effects can extend across generations.     

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.     

Fatty acid composition and extreme temperature tolerance following exposure to fluctuating temperatures in a soil arthropod

Ectotherms commonly adjust their lipid composition to ambient temperature to counteract detrimental thermal effects on lipid fluidity. However, the extent of lipid remodeling and the associated fitness consequences under continuous temperature fluctuations are not well-described. The objective of this study was to investigate the effect of repeated temperature fluctuations on fatty acid composition and thermal tolerance. We exposed the springtail Orchesella cincta to two constant temperatures of 5 and 20 °C, and a continuously fluctuating treatment between 5 and 20 °C every 2 days. Fatty acid composition differed significantly between constant low and high temperatures. As expected, animals were most cold tolerant in the low temperature treatment, while heat tolerance was highest under high temperature. Under fluctuating temperatures, fatty acid composition changed with temperature initially, but later in the experiment fatty acid composition stabilized and closely resembled that found under constant warm temperatures. Consistent with this, heat tolerance in the fluctuating temperature treatment was comparable to the constant warm treatment. Cold tolerance in the fluctuating temperature treatment was intermediate compared to animals acclimated to constant cold or warmth, despite the fact that fatty acid composition was adjusted to warm conditions. This unexpected finding suggests that in animals acclimated to fluctuating temperatures an additional underlying mechanism is involved in the cold shock response. Other aspects of homeoviscous adaptation may protect animals during extreme cold. This paper forms a next step to fully understand the functioning of ectotherms in more thermally variable environments.     

Assessing population and environmental effects on thermal resistance in Drosophila melanogaster using ecologically relevant assays

To make laboratory studies of thermal resistance in ectotherms more ecologically relevant, temperature changes that reflect conditions experienced by individuals in nature should be used. Here we describe an assay that is useful for quantifying multiple measures of thermal resistance of individual adult flies. We use this approach to assess upper and lower thermal limits and functional thermal scope for Drosophila melanogaster and also show that the method can be used to (1) detect a previously described latitudinal cline for cold tolerance in D. melanogaster populations collected along the east coast of Australia, (2) demonstrate that acclimation at variable temperatures during development increases tolerance to both low and high thermal stresses and therefore increases thermal scope compared to acclimation at a constant temperature, (3) show that temperate populations adapted to variable thermal environments have wider thermal limits compared to those from the less variable tropics, at least when flies were reared under constant temperature conditions and (4) demonstrate that different measures of cold resistance are often not strongly correlated. Based on our findings, we suggest that the method could be routinely used in evaluating thermal responses potentially linked to ecological processes and evolutionary adaptation.     

A method for the rapid measurement of thermal tolerance traits in studies of small insects

Abstract A technique for rapidly measuring non‐lethal thermal tolerance traits in small insects and terrestrial arthropods of similar size is described. Single or multiple individuals are heated or cooled in an arena milled into a temperature‐controlled aluminium block and their behaviour recorded continuously using a digital video camera. Data are collected retrospectively by playback of the stored images. To illustrate this technique measurement of six thermal tolerance traits using this method is described using first instar nymphs (body length = 0.66 mm) of the aphid Myzus persicae. These traits are high and low temperatures at which individual aphids cease walking, high and low temperatures at which aphids move for the final time, the temperature at which aphids begin to recover from chill coma, and the temperature at which they begin to walk again. The method is validated by comparing the results of multiple low temperature assays. No significant differences are detected between assays. Potential applications, limitations and technical problems are discussed.     

The effect of selection for desiccation resistance on cold tolerance of Drosophila melanogaster

Abstract.  Low temperature and desiccation stress are thought to be mechanistically similar in insects, and several studies indicate that there is a degree of cross-tolerance between them, such that increased cold tolerance results in greater desiccation tolerance and vice versa . This assertion is tested at an evolutionary scale by examining basal cold tolerance, rapid cold-hardening (RCH) and chill coma recovery in replicate populations of Drosophila melanogaster selected for desiccation resistance (with controls for both selection and concomitant starvation) for over 50 generations. All of the populations display a RCH response, and there is no effect of selection regime on RCH or basal cold tolerance, although there are differences in basal cold tolerance between sampling dates, apparently related to inter-individual variation in development time. Flies selected for desiccation tolerance recover from chill coma slightly, but significantly, faster than control and starvation-control flies. These findings provide little support for cross-tolerance between survival of near-lethal cold and desiccation stress in D. melanogaster .     

The effect of temperature and thermal acclimation on the sustainable performance of swimming scup

There is a significant reduction in overall maximum power output of muscle at low temperatures due to reduced steady-state (i.e. maximum activation) power-generating capabilities of muscle. However, during cyclical locomotion, a further reduction in power is due to the interplay between non-steady-state contractile properties of muscle (i.e. rates of activation and relaxation) and the stimulation and the length-change pattern muscle undergoes in vivo. In particular, even though the relaxation rate of scup red muscle is slowed greatly at cold temperatures (10°C), warm-acclimated scup swim with the same stimulus duty cycles at cold as they do at warm temperature, not affording slow-relaxing muscle any additional time to relax. Hence, at 10°C, red muscle generates extremely low or negative work in most parts of the body, at all but the slowest swimming speeds.Do scup shorten their stimulation duration and increase muscle relaxation rate during cold acclimation? At 10°C, electromyography (EMG) duty cycles were 18% shorter in cold-acclimated scup than in warm-acclimated scup. But contrary to the expectations, the red muscle did not have a faster relaxation rate, rather, cold-acclimated muscle had an approximately 50% faster activation rate. By driving cold- and warm-acclimated muscle through cold- and warm-acclimated conditions, we found a very large increase in red muscle power during swimming at 10°C. As expected, reducing stimulation duration markedly increased power output. However, the increased rate of activation alone produced an even greater effect. Hence, to fully understand thermal acclimation, it is necessary to examine the whole system under realistic physiological conditions.     

The temperature acclimation potential of tropical bryophytes

Bryophyte biomass and diversity in tropical moist forests decrease dramatically from higher altitudes towards the lowlands. High respiratory carbon losses at high temperatures may partly explain this pattern, if montane species are unable to acclimatise their metabolic rates to lowland temperatures. We transplanted ten bryophyte species from two altitudes (1200 and 500 m a.s.l.) to lower (warmer) altitudes (500 m and sea level) in Panama. We studied short‐term temperature acclimation of CO₂ exchange for 2.5 months, and survival and growth for 21 months following transplantation. Short‐term acclimation did not occur, and on a longer time scale mortality was highest and growth lowest in the transplanted samples. A few transplanted samples of most species, however, survived the whole experiment and finished with growth rates similar to controls. This recovery of growth rate suggests temperature acclimation, in spite of no measurable metabolic changes in smaller random samples. This acclimation even compensated for shorter periods of CO₂ uptake due to more rapid drying. Nevertheless, these species are not abundant in lowland forests, perhaps due to dispersal or establishment limitation. The apparent heterogeneity of the acclimation potential within species may allow populations to adapt locally and avoid being forced uphill under climatic warming.     

温度驯化对五种鲤科鱼类糖代谢酶活性的影响

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Exceptional thermal tolerance and water resistance in the mite Paratarsotomus macropalpis (Erythracaridae) challenge prevailing explanations of physiological limits

Physiological performance and tolerance limits in metazoans have been widely studied and have informed our understanding of processes such as extreme heat and cold tolerance, and resistance to water loss. Because of scaling considerations, very small arthropods with extreme microclimatic niches provide promising extremophiles for testing predictive physiological models. Corollaries of small size include rapid heating and cooling (small thermal time constants) and high mass-specific metabolic and water exchange rates. This study examined thermal tolerance and water loss in the erythracarid mite Paratarsotomus macropalpis (Banks, 1916), a species that forages on the ground surface of the coastal sage scrub habitat of Southern California, USA. Unlike most surface-active diurnal arthropods, P. macropalpis remains active during the hottest parts of the day in midsummer. We measured water-loss gravimetrically and estimated the critical thermal maximum (CT_max) by exposing animals to a given temperature for 1 h and then increasing temperature sequentially. The standardized water flux of 4.4 ng h⁻¹ cm⁻² Pa⁻¹, averaged for temperatures between 22 and 40 °C, is among the lowest values reported in the literature. The CT_max of 59.4 °C is, to our knowledge, the highest metazoan value reported for chronic (1-h) exposure, and closely matches maximum field substrate temperatures during animal activity. The extraordinary physiological performance seen in P. macropalpis likely reflects extreme selection resulting from its small size and resultant high mass-specific water loss rate and low thermal time-constant. Nevertheless, the high water resistance attained with a very thin lipid barrier, and the mite’s exceptional thermal tolerance, challenge existing theories seeking to explain physiological limits.     

The effect of heat acclimation on maximal urine osmolality in humans

The purpose of the current study was to determine the effect of 9 days of active heat acclimation on maximal urine osmolality (MUO) in humans. Eight subjects completed 9 days of heat acclimation, which consisted of a daily 90 min exercise session in a heated environmental chamber. Before and following heat acclimation the subjects abstained from drinking fluids for 15 h, and urine samples were collected to measure MUO. The subjects successfully heat acclimated as evidenced by a significant (P<0.05) decrease in the mean±SD exercise core temperature (37.7±0.3 °C on day 1 to 37.4±0.3 °C on day 9) and heart rate (143±17 bpm on day 1 to 128±14 bpm on day 9). The mean pre- and post-heat acclimation MUOs were 868±117 and 846±89 mmol kg⁻¹, respectively, which were not significantly different (P>0.05). Previous studies have shown that prolonged dehydration can increase the MUO in various mammalian species, including humans. In contrast, the results of the current study suggest that 9 days of active heat acclimation, without significant dehydration, does not affect the MUO in humans.     

Effects of acclimation and diapause on the thermal tolerance of the two-spotted spider mite Tetranychus urticae

The two-spotted spider mite, Tetranychus urticae, is a worldwide pest species that overwinters as diapausing females. Cold hardening is presumed to start during diapause development to ensure the successful overwintering of this species. To address this hypothesis, we compared cold tolerance between non-diapausing and diapausing females. We measured supercooling point (SCP) and survival to acute cold stress by exposing the mites at a range of sub-zero temperatures (from −4 to −28 °C for 2 h). The mean SCPs of non-diapausing and diapausing females were −19.6±0.5 and −24.7±0.3 °C respectively, and freezing killed the mites. Diapausing females were significantly more cold tolerant than non-diapausing ones, with LT₅₀ of −19.7 and −13.3 °C, respectively. Further, we also examined the effects of cold acclimation (10 d at 0 or 5 °C) in non-diapausing and diapausing females. Our findings indicated that diapause decreased SCP significantly, while cold acclimation had no effect on the SCP except for non-diapausing females that were acclimated at 5 °C. Acclimation at 5 °C enhanced survival to acute cold stress in diapausing and non-diapausing females, with LT₅₀ of −22.0 and −17.1 °C, respectively. Altogether, our results indicate that T. urticae is a chill tolerant species, and that diapause and cold acclimation elevate cold hardiness in this species.     

The effects of carbon dioxide anesthesia and anoxia on rapid cold-hardening and chill coma recovery in Drosophila melanogaster

Carbon dioxide gas is used as an insect anesthetic in many laboratories, despite recent studies which have shown that CO(2) can alter behavior and fitness. We examine the effects of CO(2) and anoxia (N(2)) on cold tolerance, measuring the rapid cold-hardening (RCH) response and chill coma recovery in Drosophila melanogaster. Short exposures to CO(2) or N(2) do not significantly affect RCH, but 60 min of exposure negates RCH. Exposure to CO(2) anesthesia increases chill coma recovery time, but this effect disappears if the flies are given 90 min recovery in air before chill coma induction. Flies treated with N(2) show a similar pattern, but require significantly longer chill coma recovery times even after 90 min of recovery from anoxia. Our results suggest that CO(2) anesthesia is an acceptable way to manipulate flies before cold tolerance experiments (when using RCH or chill coma recovery as a measure), provided exposure duration is minimized and recovery is permitted before chill coma induction. However, we recommend that exposure to N(2) not be used as a method of anesthesia for chill coma studies.     

Life stage-related differences in hardening and acclimation of thermal tolerance traits in the kelp fly, Paractora dreuxi (Diptera, Helcomyzidae)

It is widely appreciated that physiological tolerances differ between life stages. However, few studies have examined stage-related differences in acclimation and hardening. In addition, the behavioural responses involved in determining the form and extent of the short-term phenotypic response are rarely considered. Here, we investigate life stage differences in the acclimation and hardening responses of the survival of a standard heat shock (SHS) and standard low temperature (or cold) shock (SCS), and the crystallization temperature (or supercooling point, SCP) of adults and larvae of the sub-Antarctic kelp fly, Paractora dreuxi. These stages live in the same habitat, but differ substantially in their mobility and thus environmental temperatures experienced. Results showed that neither acclimation nor hardening affected the lower lethal limits in larvae or adults. Adults showed an increase in survival of upper lethal limits after low temperature acclimation, whilst larvae showed a consistent lack of response. The acclimation × hardening interaction significantly affected the SCP in adults, but no response to either acclimation or hardening was found in the larvae. This study further demonstrates the complexities of thermal tolerance responses in P. dreuxi.     

The effect of thermal acclimation on the activity of arylhydrocarbon hydroxylase in rainbow trout (Oncorhynchus mykiss)

1. The possibility that temperature acclimation (to 10 or 18 degrees C for 28 days) would alter the cytochromes P-450 of rainbow trout was addressed. 2. The specific content of LM4b (P-450 IA1), the trout isozyme responsible for activation of polynuclear aromatic hydrocarbons, was lower in 18 degrees C fish than it was in 10 degrees C fish. 3. Kinetic analysis of aryl hydrocarbon hydroxylase indicated that, while thermal acclimation caused no change in Vmax, it lowered the apparent Km of this enzyme for benzo[a]pyrene when assayed at acutely shifted temperatures. 4. Thermal acclimation of fish may have significance when feral populations are subjected to acute temperature shifts.