Hardening trumps acclimation in improving cold tolerance of Drosophila melanogaster larvae

Abstract Chill‐susceptible insects are able to improve their survival of acute cold exposure over both the short term (i.e. hardening at a relatively severe temperature) and longer term (i.e. acclimation responses at milder temperatures over a longer time frame). However, the mechanistic overlap of these responses is not clear. Four larval stages of four different strains of Drosophila melanogaster are used to test whether low temperature acclimation (10 °C for 48 h) improves the acute cold tolerance (LT₉₀, ～2 h) of larvae, and whether acclimated larvae still show hardening responses after brief exposures to nonlethal cold or heat, or a combination of the two. Acclimation results in increased cold tolerance in three of four strains, with variation among instars. However, if acclimation is followed by hardening pre‐treatments, there is no improvement in acute cold survival. It is concluded that short‐term thermal responses (e.g. hardening) may be of more ecological relevance to short‐lived life stages such as larvae, and that the mechanisms of low temperature hardening and acclimation in D. melanogaster may be antagonistic, rather than complementary.     

Supercooling ability and cold hardiness of the pollen beetle Meligethes aeneus

Supercooling point (SCP) and cold‐hardiness of the pollen beetle Meligethes aeneus (Fabricius) (Coleoptera: Nitidulidae) were investigated. Mature eggs from the oviduct were supercooled on average to ?28.0 °C and from oilseed rape buds to ?24.4 °C; first instars were supercooled to ?21.0 °C and second instars to ?16.8 °C. Despite their high supercooling ability, none of the eggs survived 24 h exposure to ?2.5 °C. The supercooling ability of adults varied significantly among feeding and non‐feeding beetles: high SCPs prevailed during the whole warm period, being about ?12 °C; low values of SCP of ?20 °C dominated in non‐feeding beetles. In spring and autumn, beetles displayed the same acclimation efficiency: after 1 week of exposure at 2.0 °C with no access to food their SCPs were depressed equally by about 3 °C. Meligethes aeneus beetles have a different response to low temperatures depending on the season. The lowest tolerance was found in reproductively active beetles after emergence from overwintering sites; the time needed to kill 50% of individuals (Ltime₅₀) was 56.2 h at ?7 °C and the lower lethal temperature needed to kill 50% (Ltemp₅₀) after 24 h exposure was ?8.6 °C. Cold hardiness increased from midsummer to midwinter; Ltime₅₀ was 80 h in August, 182.8 h in September, and 418.1 h in January. Lethal temperature after 24 h exposure was ?9.1 °C in August and ?9.8 °C in September. In February, after diapause, the beetles started to loose their cold tolerance, and Ltemp₅₀ was slightly increased to ?9.5 °C. Hibernating beetles tolerated long exposure at ?7 °C well, but mortality was high after short exposure if the temperature dropped below ?9 °C for 24 h. Despite the season, the beetles died at temperatures well above their mean SCP; consequently, SCP is not a suitable index for cold hardiness of M. aeneus.     

Long‐term after‐effects of cold exposure in adult Alphitobius diaperinus (Tenebrionidae): the need to link survival ability with subsequent reproductive success

1. The effects of cold acclimation and cold exposure on the survival and reproductive capabilities of Alphitobius diaperinus (Tenebrionidae) adult beetles are examined. 2. First, the impact of temperature on survival duration was assessed by placing beetles in a range of cool temperature treatments. Second, the importance of acclimation duration was assessed. Third, the impact of thermal stress on subsequent reproductive ability was examined for beetles that had no previous cold exposure, and for beetles that had been subjected to previous cold exposure (i.e. acclimated) at various conditions, including fluctuating temperatures. 3. In all groups, the number of recorded survivors was strongly impacted by recovery period duration (i.e. 2 vs. 10 days). Survival of non‐acclimated and 3‐day acclimated beetles, expressed as lethal time for 50% of the samples, was reduced significantly when the insects were re‐assessed for survival at 10 days after being returned to optimal growth conditions (7.9 ± 0.4 vs. 5.1 ± 0.6 days and 8.8 ± 0.5 vs. 6.8 ± 0.6 days, respectively). 4. Insects that had been subject to cold acclimation expressed better subsequent reproduction success than non‐acclimated beetles. This beneficial impact increased when the acclimation period was prolonged, but some longer acclimation periods had no significant impact on survival. 5. Our results indicate that cold exposure has the capacity to irreversibly damage the reproductive system and that insect survival depends on the duration of the recovery period. Both the survival ability and subsequent reproductive output have to be examined to objectively determine insect cold resistance.     

Acclimation responses to short‐term temperature treatments during early life stages causes long lasting changes in spontaneous activity of adult Drosophila melanogaster

Ecotherms adjust their physiology to environmental temperatures. Long‐term exposures to heat or cold typically induce acclimation responses that generate directional, but reversible shifts in thermal tolerance and performance. However, less is known about how short exposure in different life stages will affect the adult phenotype. In the present study, we compared the effects of long‐term temperature exposure to 15, 19 and 31 °C with that of brief (16 h) exposure periods at the same temperatures in Drosophila melanogaster eggs, larvae, pupae, or adults, respectively. The acclimation responses are evaluated using activity measurements at 11, 15, 19, 27, 31 and 33 °C and by measuring upper and lower thermal limits (CT_max and CT_min) in 5‐day‐old adult males. As expected, long‐term cold exposure reduces relative CT_min, whereas long‐term heat exposure increases relative CT_max. By contrast, we find little effect on thermal limits when using short‐term exposures at different life stages. Long‐term exposures to 31 and 15 °C both suppressed activity relative to the 19 °C control, suggesting that development at high and low temperatures may lead to reduced activity later in life. Short‐term cold exposure early in development reduces activity in the adult stage, whereas the effects of short‐term heat exposure on behaviour are dependent on life stage and test temperature. Together, our results highlight how the thermal sensitivity of the trait measured determines the ability to detect acclimation responses.     

Fitness costs of rapid cold-hardening in Ceratitis capitata

Rapid cold‐hardening (RCH) is a unique form of phenotypic plasticity which confers survival advantages at low temperature. The fitness costs of RCH are generally poorly elucidated and are important to understanding the evolution of plastic physiology. This study examined whether RCH responses, induced by ecologically relevant diel temperature fluctuations, carry metabolic, survival, or fecundity costs. We predicted that potential costs in RCH would be manifested as differences in metabolic rate, fecundity, or survival in flies which have hardened versus those which have not, or flies that have experienced more RCH events would show greater costs than those which have experienced fewer events. One group of flies cooled to 10°C for 2 h for 11 consecutive days experienced daily RCH (Hardened), whereas the other group exposed to 15°C for the same 2‐h period each day formed a Control group. Hardened flies had higher survival at –5°C for 2 h than control flies (69 ± 9% vs. 44 ± 19%, P = 0.04). Hardened flies showed no metabolic or fecundity costs, but had reduced average survival (P = 0.0403). Thus, a major cost to repeated low temperature exposures in Ceratitis capitata is through direct mortality caused by chilling injury, although this appears not to be a direct cost of RCH.     

Low temperature tolerance,cold hardening and acclimation in tadpoles of the neotropical túngara frog (Engystomops pustulosus)

Many frogs from temperate climates can tolerate low temperatures and increase their thermal tolerance through hardening and acclimation. Most tropical frogs, on the other hand, fail to acclimate to low temperatures. This lack of acclimation ability is potentially due to lack of selection pressure for acclimation because cold weather is less common in the tropics. We tested the generality of this pattern by characterizing the critical temperature minimum (CTMin), hardening, and acclimation responses of túngara frogs (Engystomops pustulosus). These frogs belong to a family with unknown thermal ecology. They are found in a tropical habitat with a highly constant temperature regime. The CTMin of the tadpoles was on average 12.5 °C. Pre-metamorphic tadpoles hardened by 1.18 °C, while metamorphic tadpoles hardened by 0.36 °C. When raised at 21 °C, tadpoles acclimated expanding their cold tolerance by 1.3 °C in relation to larvae raised at 28 °C. These results indicate that the túngara frog has a greatly reduced cold tolerance when compared to species from temperate climates, but it responds to cold temperatures with hardening and acclimation comparable to those of temperate-zone species. Cold tolerance increased with body length but cold hardening was more extensive in pre-metamorphic tadpoles than in metamorphic ones. This study shows that lack of acclimation ability is not general to the physiology of tropical anurans.     

Chill-tolerant Gryllus crickets maintain ion balance at low temperatures

Insect cold tolerance is both phenotypically-plastic and evolutionarily labile, but the mechanisms underlying this variation are uncertain. Chill-susceptible insects lose ion and water homeostasis in the cold, which contributes to the development of injuries and eventually death. We thus hypothesized that more cold-tolerant insects will better maintain ion and water balance at low temperatures. We used rapid cold-hardening (RCH) and cold acclimation to improve cold tolerance of male Gryllus pennsylvanicus, and also compared this species to its cold-tolerant relative (Gryllus veletis). Cold acclimation and RCH decreased the critical thermal minimum (CT_min) and chill coma recovery time (CCR) in G. pennsylvanicus, but while cold acclimation improved survival of 0 °C, RCH did not; G. veletis was consistently more cold-tolerant (and had lower CCR and CT_min) than G. pennsylvanicus. During cold exposure, hemolymph water and Na⁺ migrated to the gut of warm-acclimated G. pennsylvanicus, which increased hemolymph [K⁺] and decreased muscle K⁺ equilibrium potentials. By contrast, cold-acclimated G. pennsylvanicus suffered a smaller loss of ion and water homeostasis during cold exposure, and this redistribution did not occur at all in cold-exposed G. veletis. The loss of ion and water balance was similar between RCH and warm-acclimated G. pennsylvanicus, suggesting that different mechanisms underlie decreased CCR and CT_min compared to increased survival at 0 °C. We conclude that increased tolerance of chilling is associated with improved maintenance of ion and water homeostasis in the cold, and that this is consistent for both phenotypic plasticity and evolved cold tolerance.     

Effect of cold acclimation and rapid cold-hardening on the survival of Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) under cold stress

Spodoptera frugiperda is a highly invasive pest species that recently invaded Africa and Asia causing severe economic losses, primarily related to corn and rice crops. Temperature is one of the most important environmental factors that influence the invasion of pests into new habitats. However, little is known regarding the thermal tolerance characteristics of invasive S. frugiperda. Thus, we investigated the response of four developmental stages of S. frugiperda (i.e., eggs, third and sixth instar larvae, and pupae) to cold acclimation (CA) and rapid cold-hardening (RCH). All individuals suffered high mortality with 24-h temperature treatments at <?5°C and >35 °C. The CA treatment significantly increased the survival rate of the eggs and third instar larvae, although it did not affect the sixth instar larvae and it decreased the pupation rate. The RCH treatment at 5 °C for 5 h or 2 °C for 2 h increased the cold tolerance capabilities of the third and sixth instar larvae, respectively. Thus, the larval stage appears to be crucial for the cold tolerance of S. frugiperda. Our findings improve the current understanding of the cold tolerance characteristics of S. frugiperda and indicate its potential for survival in the newly invaded temperate regions of Asia.     

Thermal tolerance and geographical range size in the Agabus brunneus group of European diving beetles (Coleoptera: Dytiscidae)

Aim Within clades, most taxa are rare, whilst few are common, a general pattern for which the causes remain poorly understood. Here we investigate the relationship between thermal performance (tolerance and acclimation ability) and the size of a species’ geographical range for an assemblage of four ecologically similar European diving beetles (the Agabus brunneus group) to examine whether thermal physiology relates to latitudinal range extent, and whether Brown’s hypothesis and the environmental variability hypothesis apply to these taxa. Location Europe. Methods In order to determine the species tolerances to either low or high temperatures we measured the lethal thermal limits of adults, previously acclimated at one of two temperatures, by means of thermal ramping experiments (± 1°C min^?1). These measures of upper and lower thermal tolerances (UTT and LTT respectively) were then used to estimate each species’ thermal tolerance range, as total thermal tolerance polygons and marginal UTT and LTT thermal polygons. Results Overall, widespread species have higher UTTs and lower LTTs than restricted ones. Mean upper lethal limits of the Agabus brunneus group (43 to 46°C), are similar to those of insects living at similar latitudes, whilst mean lower lethal limits (?6 to ?9°C) are relatively high, suggesting that this group is not particularly cold‐hardy compared with other mid‐temperate‐latitude insects. Widespread species possess the largest thermal tolerance ranges and have a relatively symmetrical tolerance to both high and low temperatures, when compared with range‐restricted relatives. Over the temperature range employed, adults did not acclimate to either high or low temperatures, contrasting with many insect groups, and suggesting that physiological plasticity has a limited role in shaping distribution. Main conclusions Absolute thermal niche appears to be a good predictor of latitudinal range, supporting both Brown’s hypothesis and the environmental variability hypothesis. Restricted‐range species may be more susceptible to the direct effect of climate change than widespread species, notwithstanding the possibility that even ‘thermally‐hardy’, widespread species may be influenced by the indirect effects of climate change such as reduction in habitat availability in Mediterranean areas.     

Phenotypic plasticity of thermal tolerance contributes to the invasion potential of Mediterranean fruit flies (Ceratitis capitata)

1. The invasion success of Ceratitis capitata probably stems from physiological, morphological, and behavioural adaptations that enable them to survive in different habitats. However, it is generally poorly understood if variation in acute thermal tolerance and its phenotypic plasticity might be important in facilitating survival of C. capitata upon introduction to novel environments. 2. Here, by comparison of widely distributed C. capitata with a narrowly distributed congener, C. rosa, we show that both species have similar levels of survival to acute high and low temperature exposures under common rearing conditions. However, these species differ dramatically in the time‐course of plastic responses to acute low temperature treatments. 3. The range of temperatures that induce rapid cold hardening (RCH) are similar for both species. However, C. capitata has two distinct advantages over C. rosa. First, at 5°C C. capitata develops RCH significantly faster than C. rosa. Second, C. capitata maintains a RCH response longer than C. rosa (8 vs. 0.5 h). 4. A simple population survival model, based on the estimated time‐course of RCH responses determined for both species, was undertaken to simulate time to extinction for both species introduced into a similar thermally variable environment. The model showed that time to extinction is greater for C. capitata than for C. rosa, especially in habitats where temperatures frequently drop below 10°C. 5. Thus, variation in RCH responses may translate into significant variation in survival upon introduction to novel thermal habitats for C. capitata, particularly in cooler and more thermally variable geographic regions, and may contribute to their ongoing invasion success relative to other, more geographically constrained Ceratitis species.     

The oatmeal nematode <Emphasis Type="Italic">Panagrellus redivivus</Emphasis> survives moderately low temperatures by freezing tolerance and cryoprotective dehydration

The cold tolerance abilities of only a few nematode species have been determined. This study shows that the oatmeal nematode, Panagrellus redivivus, has modest cold tolerance with a 50% survival temperature (S ₅₀) of −2.5°C after cooling at 0.5°C min⁻¹ and freezing for 1 h. It can survive low temperatures by freezing tolerance and cryoprotective dehydration; although freezing tolerance appears to be the dominant strategy. Freezing survival is enhanced by low temperature acclimation (7 days at 5°C), with the S ₅₀ being lowered by a small but significant amount (0.42°C). There is no cold shock or rapid cold hardening response under the conditions tested. Cryoprotective dehydration enhances the ability to survive freezing (the S ₅₀ is lowered by 0.55°C, compared to the control, after 4 h freezing at −1°C) and this effect is in addition to that produced by acclimation. Breeding from survivors of a freezing stress did not enhance the ability to survive freezing. The cold tolerance abilities of this nematode are modest, but sufficient to enable it to survive in the cold temperate environments it inhabits.     

Effects of acclimation on the thermal tolerance of the brown planthopper Nilaparvata lugens (Stål)

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The importance of fluctuating thermal regimes for repairing chill injuries in the tropical beetle Alphitobius diaperinus (Coleoptera: Tenebrionidae) during exposure to low temperature

Abstract. In this study, the impact of acclimation (1 month at 15 °C vs. breeding at 30 °C) and fluctuating thermal regimes (daily transfers from low temperatures to various higher temperatures for 2 h) on the cold tolerance of the tropical beetle, Alphitobius diaperinus Panzer (Coleoptera: Tenebrionidae) was examined. Acclimation increased significantly the duration of survival (Lt₅₀) at a constant 5 °C (7.7 ± 0.3 days to 9.7 ± 0.5 days). Survival of acclimated and nonacclimated beetles increased slightly at alternating temperatures of 5 °C/10 °C or 5 °C/15 °C. When daily transfer to 20 °C was applied, survival (Lt₅₀) was improved markedly (nonacclimated: 15.5 ± 0.7 days, acclimated: 19.6 ± 0.6 days). The higher temperatures may allow progressive repair of injuries, and the effects of chilling may be repaired completely at 25 and 30 °C, a phenomenon recorded here for the first time. It is estimated that the theoretical upper threshold of chill injury (Th) of nonacclimated beetles is 15.1 °C whereas it is shifted down to 11.2 °C in acclimated beetles, which might enable this temperature to allow effective repair of injury.     

Seasonal variation in basal and plastic cold tolerance: Adaptation is influenced by both long‐ and short‐term phenotypic plasticity

Understanding how thermal selection affects phenotypic distributions across different time scales will allow us to predict the effect of climate change on the fitness of ectotherms. We tested how seasonal temperature variation affects basal levels of cold tolerance and two types of phenotypic plasticity in Drosophila melanogaster. Developmental acclimation occurs as developmental stages of an organism are exposed to seasonal changes in temperature and its effect is irreversible, while reversible short‐term acclimation occurs daily in response to diurnal changes in temperature. We collected wild flies from a temperate population across seasons and measured two cold tolerance metrics (chill‐coma recovery and cold stress survival) and their responses to developmental and short‐term acclimation. Chill‐coma recovery responded to seasonal shifts in temperature, and phenotypic plasticity following both short‐term and developmental acclimation improved cold tolerance. This improvement indicated that both types of plasticity are adaptive, and that plasticity can compensate for genetic variation in basal cold tolerance during warmer parts of the season when flies tend to be less cold tolerant. We also observed a significantly stronger trade‐off between basal cold tolerance and short‐term acclimation during warmer months. For the longer‐term developmental acclimation, a trade‐off persisted regardless of season. A relationship between the two types of plasticity may provide additional insight into why some measures of thermal tolerance are more sensitive to seasonal variation than others.     

Survivorship of geographic Pomacea canaliculata populations in responses to cold acclimation

1. Pomacea canaliculata, a freshwater snail from South America, has rapidly established natural populations from south to north subtropical region in China, since its original introductions in the 1980s. Low temperature in winter is a limiting factor in the geographic expansion and successfully establishment for apple snail populations. There have been some studies on population level of low temperature tolerance for P. canaliculata, yet little is quantified about its life‐history traits in responses to cold temperatures. Whether these responses vary with the acclimation location is also unclear. We investigated the survivorship and longevity of P. canaliculata in responses to cold temperatures and examine whether these responses vary with the location and snail size. We hypothesized that survival of the snails depends on their shell height and the level of low temperature, and P. canaliculata population from the mid-subtropical zone may exhibit the highest viability over the cold thermal range.
2. We sampled P. canaliculata populations from five latitude and longitude ranges of subtropical China: Guangzhou population in southernmost (SM‐GZ), three populations of Yingtan (MR‐YT), Ningbo (MR‐NB), Ya'an (MR‐YA) in midrange, and Huanggang population in northernmost (NM‐HG) subtropical zone. For each P. canaliculata population, survival and longevity at six cold acclimation temperature levels (12, 9, 6, 3, 0, and ?3°C) were quantified, and the effects of location and shell height were examined.
3. The MR‐YA population from mid-subtropical zone of China exhibited the highest survival rate and prolonged survival time regardless of the temperature acclimation treatments, whereas the SM‐GZ population from southern subtropical was the most sensitive to cold temperatures, particular temperatures below 9°C. No individuals of the SM‐GZ population could survive after stressed for 30 days (3°C), 5 days (0°C) and 2 days (?3°C), respectively. For each experimental P. canaliculata population held at 3, 0, and ?3°C, individuals with intermediate shell height of 15.0–25.0 mm had significantly higher survivals.
4. The results highlight a request of a more thorough investigation on acclimation responses in each of the life table demographic parameters for P. canaliculata, and pose the question of whether natural selection or some genetic changes may have facilitated adaptation in invasive locations.

     

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.     

Fall field crickets did not acclimate to simulated seasonal changes in temperature

In nature, many organisms alter their developmental trajectory in response to environmental variation. However, studies of thermal acclimation have historically involved stable, unrealistic thermal treatments. In our study, we incorporated ecologically relevant treatments to examine the effects of environmental stochasticity on the thermal acclimation of the fall field cricket (Gryllus pennsylvanicus). We raised crickets for 5 weeks at either a constant temperature (25°C) or at one of three thermal regimes mimicking a seasonal decline in temperature (from 25 to 12°C). The latter three treatments differed in their level of thermal stochasticity: crickets experienced either no diel cycle, a predictable diel cycle, or an unpredictable diel cycle. Following these treatments, we measured several traits considered relevant to survival or reproduction, including growth rate, jumping velocity, feeding rate, metabolic rate, and cold tolerance. Contrary to our predictions, the acclimatory responses of crickets were unrelated to the magnitude or type of thermal variation. Furthermore, acclimation of performance was not ubiquitous among traits. We recommend additional studies of acclimation in fluctuating environments to assess the generality of these findings.     

Adult plasticity of cold tolerance in a continental-temperate population of Drosophila suzukii

Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) is a worldwide emerging pest of soft fruits, but its cold tolerance has not been thoroughly explored. We determined the cold tolerance strategy, low temperature thermal limits, and plasticity of cold tolerance in both male and female adult D. suzukii. We reared flies under common conditions (long days, 21 °C; control) and induced plasticity by rapid cold-hardening (RCH, 1 h at 0 °C followed by 1 h recovery), cold acclimation (CA, 5 days at 6 °C) or acclimation under fluctuating temperatures (FA). D. suzukii had supercooling points (SCPs) between −16 and −23 °C, and were chill-susceptible. 80% of control flies were killed after 1 h at −7.2 °C (males) or −7.5 °C (females); CA and FA improved survival of this temperature in both sexes, but RCH did not. 80% of control flies were killed after 70 h (male) or 92 h (female) at 0 °C, and FA shifted this to 112 h (males) and 165 h (females). FA flies entered chill coma (CT_min) at approximately −1.7 °C, which was ca. 0.5 °C colder than control flies; RCH and CA increased the CT_min compared to controls. Control and RCH flies exposed to 0 °C for 8 h took 30–40 min to recover movement, but this was reduced to <10 min in CA and FA. Flies placed outside in a field cage in London, Ontario, were all killed by a transient cold snap in December. We conclude that adult phenotypic plasticity is not sufficient to allow D. suzukii to overwinter in temperate habitats, and suggest that flies could overwinter in association with built structures, or that there may be additional cold tolerance imparted by developmental plasticity.     

RAPID COLD HARDENING PROVIDING HIGHER COLD TOLERANCE THAN COLD ACCLIMATION IN THE PINE NEEDLE GALL MIDGE THECODIPLOSIS JAPO‐NENSIS LARVAE*

Abstract The responses of overwintering larvae of the pine needle gall midge Thecodiplosis japonensis Uchida et Inouye to rapid cold hardening and cold acclimation were studied. A rapid cold hardening response is found in the 3rd instar larvae of T. japonensis. When overwintering larvae are transferred directly from 27°C to ‐ 15°C for 3 h, there is only 17.9% survival, whereas exposure to 4°C for 2 h prior to transfer to ‐ 15°C increases survival to 40.0%. The acquired cold tolerance is transient and is rapidly lost (after 15 min at 27°C). Rapid cold hardening is more effective in maintaining larval survival than cold acclimation. Different mechanisms are suggested to regulate the insect's cold hardiness under rapid cold hardening and cold acclimation.     

Experimental evolution for generalists and specialists reveals multivariate genetic constraints on thermal reaction norms

Theory predicts the emergence of generalists in variable environments and antagonistic pleiotropy to favour specialists in constant environments, but empirical data seldom support such generalist–specialist trade‐offs. We selected for generalists and specialists in the dung fly Sepsis punctum (Diptera: Sepsidae) under conditions that we predicted would reveal antagonistic pleiotropy and multivariate trade‐offs underlying thermal reaction norms for juvenile development. We performed replicated laboratory evolution using four treatments: adaptation at a hot (31 °C) or a cold (15 °C) temperature, or under regimes fluctuating between these temperatures, either within or between generations. After 20 generations, we assessed parental effects and genetic responses of thermal reaction norms for three correlated life‐history traits: size at maturity, juvenile growth rate and juvenile survival. We find evidence for antagonistic pleiotropy for performance at hot and cold temperatures, and a temperature‐mediated trade‐off between juvenile survival and size at maturity, suggesting that trade‐offs associated with environmental tolerance can arise via intensified evolutionary compromises between genetically correlated traits. However, despite this antagonistic pleiotropy, we found no support for the evolution of increased thermal tolerance breadth at the expense of reduced maximal performance, suggesting low genetic variance in the generalist–specialist dimension.