Selection on knockdown performance in Drosophila melanogaster impacts thermotolerance and heat-shock response differently in females and males

Genetic Architecture of the Thermal Tolerance Landscape in Drosophila melanogaster

Increased environmental temperatures associated with global warming strongly impact natural populations of ectothermic species. Therefore, it is crucial to understand the genetic basis and evolutionary potential of heat tolerance. However, heat tolerance and its genetic components depend on the methodology, making it difficult to predict the adaptive responses to global warming. Here, we measured the knockdown time for 100 lines from the Drosophila Genetic Reference Panel (DGRP) at four different static temperatures, and we estimated their thermal-death-time (TDT) curves, which incorporate the magnitude and the time of exposure to thermal stress, to determine the genetic basis of the thermal tolerance landscape. Through quantitative genetic analyses, the knockdown time showed a significant heritability at different temperatures and that its genetic correlations decreased as temperatures differences increased. Significant genotype-by-sex and genotype-by-environment interactions were noted for heat tolerance. We also discovered genetic variability for the two parameters of TDT: CTmax and thermal sensitivity. Taking advantage of the DGRP, we performed a GWAS and identified multiple variants associated with the TDT parameters, which mapped to genes related to signalling and developmental functions. We performed functional validations for some candidate genes using RNAi, which revealed that genes such as mam, KNCQ, or robo3 affect the knockdown time at a specific temperature but are not associated with the TDT parameters. In conlusion, the thermal tolerance landscape display genetic variation and plastic responses, which may facilitate the adaptation of Drosophila populations to a changing world.

"A comparison of thermal stress response between Drosophila melanogaster and Drosophila pseudoobscura reveals differences between species and sexes"

The environment is changing faster than anticipated due to climate change, making species more vulnerable to its impacts. The level of vulnerability of species is influenced by factors such as the degree and duration of exposure, as well as the physiological sensitivity of organisms to changes in their environments, which has been shown to vary among species, populations, and individuals. Here, we compared physiological changes in fecundity, critical thermalmaximum (CTmax), respiratory quotient (RQ), and DNA damage in ovaries in response to temperature stress in two species of fruit fly, Drosophila melanogaster (25 vs. 29.5 °C) and Drosophila pseudoobscura (20.5 vs. 25 °C). The fecundity of D. melanogaster was more affected by high temperatures when exposed during egg through adult development, while D. pseudoobscura was most significantly affected when exposed to high temperatures exclusively during egg through pupal development. Additionally, D. melanogaster males exhibited a decrease of CTmax under high temperatures, while females showed an increase of CTmax when exposed to high temperatures during egg through adult development. while D. pseudoobscura females and males showed an increased CTmax only when reared at high temperatures during egg through pupae development. Moreover, both species showed an acceleration in oogenesis and an increase in apoptosis due to heat stress. These changes can likely be attributed to key differences in the geographic range, thermal range, development time, and other different factors between these two systems. Through this comparison of variation in physiology and developmental response to thermal stress, we found important differences between species and sexes that suggest future work needs to account for these factors separately in understanding the effects of constant increased temperatures.

Loss of neurexin-1 in Drosophila melanogaster results in altered energy metabolism and increased seizure susceptibility

Although autism is typically characterized by differences in language, social interaction and restrictive, repetitive behaviors, it is becoming more well known in the field that alterations in energy metabolism and mitochondrial function are comorbid disorders in autism. The synaptic cell adhesion molecule, neurexin-1 (NRXN1), has previously been implicated in autism, and here we show that in Drosophila melanogaster, the homologue of NRXN1, called Nrx-1, regulates energy metabolism and nutrient homeostasis. First, we show that Nrx-1-null flies exhibit decreased resistance to nutrient deprivation and heat stress compared to controls. Additionally, Nrx-1 mutants exhibit a significantly altered metabolic profile characterized by decreased lipid and carbohydrate stores. Nrx-1-null Drosophila also exhibit diminished levels of nicotinamide adenine dinucleotide (NAD⁺), an important coenzyme in major energy metabolism pathways. Moreover, loss of Nrx-1 resulted in striking abnormalities in mitochondrial morphology in the flight muscle of Nrx-1-null Drosophila and impaired flight ability in these flies. Further, following a mechanical shock Nrx-1-null flies exhibited seizure-like activity, a phenotype previously linked to defects in mitochondrial metabolism and a common symptom of patients with NRXN1 deletions. The current studies indicate a novel role for NRXN1 in the regulation of energy metabolism and uncover a clinically relevant seizure phenotype in Drosophila lacking Nrx-1.

Identification and Characterization of Antioxidant Enzyme Genes in Parasitoid Aphelinus asychis (Hymenoptera: Aphelinidae) and Expression Profiling Analysis under Temperature Stress

Simple Summary

High temperature affects the control efficiency of Aphelinus asychis, an important parasitic natural enemy of aphids. Antioxidative enzymes can protect organisms against oxidative damage by eliminating excess reactive oxygen species (ROS). This study identified 14 genes belonging to four classes of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), peroxidases (POD), and glutathione-S-transferase (GST). The results showed that the expression levels and enzymatic activities of most antioxidant enzyme genes are significantly induced by high temperature, which indicates that antioxidant enzymes increase the resistance of A. asychis to lethal high temperature. Taken together, this study enriches the understanding of the molecular mechanisms of resistance of A. asychis to environmental high temperatures.

Abstract

It is well known that high temperature, a typically negative environmental factor, reduces the parasitism of a parasitoid. Generally, high temperature causes the rapid overproduction of reactive oxygen species (ROS) in organisms, and antioxidative enzymes participate in the process of resisting environmental stress by eliminating excess ROS. In the present study, we identify two superoxide dismutase (SOD), one catalase (CAT), six peroxidases (POD), and five glutathione-S-transferase (GST) genes; and the survival rate and antioxidative enzyme patterns under short-term high temperature exposure of the parasitoid wasp, A.asychis, are examined. Survival results of A.asychis reveal that females show higher thermal tolerance than males. Under short-term high-temperature exposure, in females, the expression levels of most antioxidant enzyme genes decreased first and then increased to a peak at 41 °C, while only the expression of AasyGST4 showed a continuous increase. In males, the expression patterns of most antioxidant enzyme genes fluctuated and reached a maximum at 41 °C. Moreover, the expression levels of the majority of antioxidant enzyme genes were higher in females than in males. In addition, at temperatures of and above 35 °C, the activities of these four antioxidant enzymes were induced. The results show that the antioxidant enzymes confer thermo-tolerance to A. asychis against lethal thermal stress. Our observations enrich the understanding of the response mechanism to high-temperature assaults of A. asychis.

Hsian-Tsao (Mesona chinensis Benth.) Extract Improves the Thermal Tolerance of Drosophila melanogaster

Global warming has prompted scientific communities to consider how to alleviate thermal stress in humans and animals. The present study assessed the supplementation of hsian-tsao extract (HTE) on thermal stress in Drosophila melanogaster and preliminarily explicated its possible physiological and molecular mechanisms. Our results indicated that the lethal time for 50% of female flies fed on HTE was significantly longer than that of male flies at the same heat stress temperature. Under thermal stress, the survival time of females was remarkably increased in the HTE addition groups compared to the non-addition group. Thermal hardening by acute exposure to 36°C for 30 min (9:00 to 9:30 a.m.) every day could significantly prolong the longevity of females. Without thermal hardening, HTE increased the antioxidant capacity of females under heat stress, accompanied by an increment of catalase (CAT) activity, and the inhibition for hydroxyl radicals (OH⋅) and superoxide anions (⋅O2 -). Superoxide dismutase (SOD) activity and the inhibition for ⋅O2 - was significantly affected by thermal hardening in the non-HTE addition groups, and significant differences were shown in CAT and SOD activities, and the inhibition for ⋅O2 - among groups with thermal hardening. After heat exposure, heat shock protein 70 (Hsp70) was only up-regulated in the group with high levels of added HTE compared with the group without and this was similar in the thermal hardening group. It was concluded that the heat stress-relieving ability of HTE might be partly due to the enhancement of enzymatic activities of SOD and CAT, and the inhibition for OH⋅ and ⋅O2 -. However, the expression levels of Hsp70 were not well related to thermal tolerance or heat survival.

Effects of periodically repeated high-temperature exposure on the immediate and subsequent fitness of different developmental stages of Propylaea japonica

BACKGROUND

Repeated extreme high temperature occurs frequently in summer. Propylaea japonica is a predominant predator in South-East Asia and has been considered as a successful natural enemy to control aphids. However, how repeated extreme high temperature affects the fitness of P. japonica remains unclear. This study evaluated the immediate and subsequent fitness of P. japonica when egg, larva, pupa, and adult were exposed to repeated high temperatures (39, 41, or 43 °C for 3 h exposure duration per day) during several days.

RESULTS

The effect of repeated high temperatures on P. japonica fitness was stage-specific: the egg stage was the most sensitive, the larval and pupal stages were moderately resistant and the adult stage was the most resistant to heat. Repeated high temperatures extended the immature developmental time and decreased the sex ratio of eggs treated with these temperatures, compared to control eggs. A temperature of 39 °C had no significant effect on the pre-oviposition period, oviposition period, fecundity (except stress pupa), or longevity compared with the control, but negative carry-over effects above 39 °C on subsequent stages were found.

CONCLUSION

Repeated high temperature for consecutive days not only had a significant effect on the immediate survival and developmental time, but also had deleterious effects on the subsequent development and performance of P. japonica. The present study provides valuable information for understanding and utilizing P. japonica to control aphids in challenging environments. © 2022 Society of Chemical Industry.

Alternative models for transgenerational epigenetic inheritance: Molecular psychiatry beyond mice and man

Mental illness remains the greatest chronic health burden globally with few in-roads having been made despite significant advances in genomic knowledge in recent decades. The field of psychiatry is constantly challenged to bring new approaches and tools to address and treat the needs of vulnerable individuals and subpopulations, and that has to be supported by a continuous growth in knowledge. The majority of neuropsychiatric symptoms reflect complex gene-environment interactions, with epigenetics bridging the gap between genetic susceptibility and environmental stressors that trigger disease onset and drive the advancement of symptoms. It has more recently been demonstrated in preclinical models that epigenetics underpins the transgenerational inheritance of stress-related behavioural phenotypes in both paternal and maternal lineages, providing further supporting evidence for heritability in humans. However, unbiased prospective studies of this nature are practically impossible to conduct in humans so preclinical models remain our best option for researching the molecular pathophysiologies underlying many neuropsychiatric conditions. While rodents will remain the dominant model system for preclinical studies (especially for addressing complex behavioural phenotypes), there is scope to expand current research of the molecular and epigenetic pathologies by using invertebrate models. Here, we will discuss the utility and advantages of two alternative model organisms-Caenorhabditis elegans and Drosophila melanogaster-and summarise the compelling insights of the epigenetic regulation of transgenerational inheritance that are potentially relevant to human psychiatry.

Heat hardening in a pair of Anolis lizards: constraints, dynamics and ecological consequences

Heat tolerance plasticity is predicted to be an important buffer against global warming. Nonetheless, basal heat tolerance often correlates negatively with tolerance plasticity ('trade-off hypothesis'), a constraint that could limit plasticity benefits. We tested the trade-off hypothesis at the individual level with respect to heat hardening in two lizard species, Anolis carolinensis and Anolis sagrei. Heat hardening is a rapid increase in heat tolerance after heat shock that is rarely measured in reptiles but is generally considered to be a first line of physiological defense against heat. We also employed a biophysical model of operative habitat temperatures to estimate the performance consequences of hardening under ecologically relevant conditions. Anolis carolinensis hardened by 2 h post-heat shock and maintained hardening for several hours. However, A. sagrei did not harden. Biophysical models showed that hardening in A. carolinensis reduces their overheating risk in the field. Therefore, while not all lizards heat harden, hardening has benefits for species that can. We initially found a negative relationship between basal tolerance and hardening within both species, consistent with the trade-off hypothesis. However, permutation analyses showed that the apparent trade-offs could not be differentiated from statistical artifact. We found the same result when we re-analyzed published data supporting the trade-off hypothesis in another lizard species. Our results show that false positives may be common when testing the trade-off hypothesis. Statistical approaches that account for this are critical to ensure that the hypothesis, which has broad implications for thermal adaptation and responses to warming, is assessed appropriately.

Experimental evolution on heat tolerance and thermal performance curves under contrasting thermal selection in Drosophila subobscura

Ectotherms can respond to global warming via evolutionary change of their upper thermal limits (CT_max ). Thus, the estimation of CT_max and its evolutionary potential is crucial to determine their vulnerability to global warming. However, CT_max estimations depend on the thermal stress intensity, and it is not completely clear whether its evolutionary capacity can be affected. Here, we performed an artificial selection experiment to increase heat tolerance using fast- and slow-ramping selection protocols in Drosophila subobscura. We found that heat tolerance evolved in both selection protocols, exhibiting similar evolutionary change rates and realized heritabilities. Additionally, we estimated the thermal performance curves (TPC) to evaluate correlated responses to selection on heat tolerance. We detected that thermal optimum increased in fast-ramping selection lines, but with a cost at the thermal performance breadth. Conversely, we did not detect changes in the TPC for the slow-ramping selection lines, indicating that thermal stress intensity has important effects on the evolution of thermal physiology of ectotherms. These findings, together with previous studies in D. subobscura reporting interpopulation variability and significant heritabilities for heat tolerance, suggest that evolutionary change can contribute to insect persistence in thermally changing environments and adaptation to global warming conditions.

Survive a Warming Climate: Insect Responses to Extreme High Temperatures

Global change includes a substantial increase in the frequency and intensity of extreme high temperatures (EHTs), which influence insects at almost all levels. The number of studies showing the ecological importance of EHTs has risen in recent years, but the knowledge is rather dispersed in the contemporary literature. In this article, we review the biological and ecological effects of EHTs actually experienced in the field, i.e., when coupled to fluctuating thermal regimes. First, we characterize EHTs in the field. Then, we summarize the impacts of EHTs on insects at various levels and the processes allowing insects to buffer EHTs. Finally, we argue that the mechanisms leading to positive or negative impacts of EHTs on insects can only be resolved from integrative approaches considering natural thermal regimes. Thermal extremes, perhaps more than the gradual increase in mean temperature, drive insect responses to climate change, with crucial impacts on pest management and biodiversity conservation.

Sex differences in the response to oxidative and proteolytic stress

Sex differences in diseases involving oxidative and proteolytic stress are common, including greater ischemic heart disease, Parkinson disease and stroke in men, and greater Alzheimer disease in women. Sex differences are also observed in stress response of cells and tissues, where female cells are generally more resistant to heat and oxidative stress-induced cell death. Studies implicate beneficial effects of estrogen, as well as cell-autonomous effects including superior mitochondrial function and increased expression of stress response genes in female cells relative to male cells. The p53 and forkhead box (FOX)-family genes, heat shock proteins (HSPs), and the apoptosis and autophagy pathways appear particularly important in mediating sex differences in stress response.

Changes in lipid classes of Drosophila melanogaster in response to selection for three stress traits

Laboratory selection on environmental stress traits is an evolutionary approach that is informative in the context of understanding stress adaptation. Here we characterize changes in a lipidome of Drosophila melanogaster in lines selected for increased heat (elevated heat knockdown refractoriness), cold (decreased time to recover from chill-coma) and desiccation survival. Selection for desiccation resistance resulted in changes in multiple lipid classes used to characterize a lipidome. This included a decrease in triacylglycerols (TAGs) which is relevant to interpretation of storage lipid levels in previous D. melanogaster desiccation survival selection experiments. Chill-coma recovery rate selection was expected to show extensive changes in lipid classes, but only phosphatidic acids exhibited significant change. Selection for increased heat knockdown resistance resulted in a substantial change in the abundance of a class of lipids (diacylglycerols) which could play a role in mediating the heat shock response or result in an increase in neutral lipid mobilization.

The importance of timing of heat events for predicting the dynamics of aphid pest populations

BACKGROUND

Heatwaves are increasing in frequency and there is growing interest in their impact on pest organisms. Previous work indicates that effects depend on the timing of the stress event, whose impact needs to be characterized across the full set of developmental stages and exposure periods of an organism. Here, we undertake such a detailed assessment using heat stress (20-35 °C diurnal cycle) across the nymph and adult stages of the English grain aphid, Sitobion avenae (Fabricius).

RESULTS

Stress-related mortality increased with stress duration at all stages; effects were less severe at the late nymphal stage. Effects on longevity adults after stress showed a complex pattern with nymphal heat stress, increasing with stress duration at the late nymphal stage, but decreasing with duration at the early nymphal stage. Longevity was also reduced by adult stress although to a lesser extent, and patterns were not connected to duration. Post-stress productivity decreased following adult and nymphal stress and the decrease tended to be correlated with stress duration. The rate of offspring production was more affected by adult stress than nymphal stress. Productivity and longevity effects, when combined, showed that the largest effect of heat stress occurred at the early nymphal stage.

CONCLUSION

These findings highlight the complex ways in which heat stress at a particular life stage influences later fitness and they also emphasize the importance of considering multiple fitness components when assessing stress effects. © 2019 Society of Chemical Industry.

Heat shock protein 70 from a thermotolerant Diptera species provides higher thermoresistance to Drosophila larvae than correspondent endogenous gene

Heat shock proteins (Hsp70s) from two Diptera species that drastically differ in their heat shock response and longevity were investigated. Drosophila melanogaster is characterized by the absence of Hsp70 and other hsps under normal conditions and the dramatic induction of hsp synthesis after temperature elevation. The other Diptera species examined belongs to the Stratiomyidae family (Stratiomys singularior) and exhibits high levels of inducible Hsp70 under normal conditions coupled with a thermotolerant phenotype and much longer lifespan. To evaluate the impact of hsp70 genes on thermotolerance and longevity, we made use of a D. melanogaster strain that lacks all hsp70 genes. We introduced single copies of either S. singularior or D. melanogaster hsp70 into this strain and monitored the obtained transgenic flies in terms of thermotolerance and longevity. We developed transgenic strains containing the S. singularior hsp70 gene under control of a D. melanogaster hsp70 promoter. Although these adult flies did synthesize the corresponding mRNA after heat shock, they were not superior to the flies containing a single copy of D. melanogaster hsp70 in thermotolerance and longevity. By contrast, Stratiomyidae Hsp70 provided significantly higher thermotolerance at the larval stage in comparison with endogenous Hsp70.

The Potential Coordination of the Heat-Shock Proteins and Antioxidant Enzyme Genes of Aphidius gifuensis in Response to Thermal Stress

Aphidius gifuensis is one of the most important aphid natural enemies and has been successfully used to control Myzys persicae and other aphid species. High temperature in summer is one of the key barriers for the application of A. gifuensis in the field and greenhouse. In this work, we investigated the biological performance of A. gifuensis and the response of heat-shock proteins and antioxidant enzymes under high temperature. The results showed that A. gifuensis could not survive at 40°C and female exhibited a higher survival in 35°C. Furthermore, the short term exposure to high temperature negatively affected the performance of A. gifuensis especially parasitism efficiency. Under short-term heating, the expression of AgifsHSP, Agifl(2)efl, AgifHSP70, AgifHSP70-4 and AgifHSP90 showed an increased trend, whereas AgifHSP10 initially increased and then decreased. In 35°C, the expressions of Agifl(2)efl, AgifHSP70-4 and AgifHSP90 in female were higher than those in male, whereas the expression of AgifHSP70 exhibited an opposite trend. Besides the HSPs, we also quantified the expression levels of 11 antioxidant enzyme genes: AgifPOD, AgifSOD1, AgifSOD2, AgifSOD3, AgifCAT1, AgifCAT2, AgifGST1, AgifGST2, AgifGST3, AgifGST4 and AgifGST5. We found that the sex-specific expression of AgifSOD2, AgifSOD3, AgifPOD, AgifGST1 and AgifGST3 were highly consistent with sex-specific heat shock survival rates at 35°C. Furthermore, when the temperature was above 30°C, the activities of GST, SOD, CAT and POD were significantly increased; however, there was no significant difference of the CAT activity between the male and female at 35°C. Collectively, all of these results suggested that the protection of thermal damage is coordinated by HSPs and antioxidant enzymes in A. gifuensis. Based on the heat tolerance abilities of many aphid natural enemies, we also discussed an integrated application strategy of many aphid enemies in summer.

A visual screen for diet-regulated proteins in the Drosophila ovary using GFP protein trap lines

The effect of diet on reproduction is well documented in a large number of organisms; however, much remains to be learned about the molecular mechanisms underlying this connection. The Drosophila ovary has a well described, fast and largely reversible response to diet. Ovarian stem cells and their progeny proliferate and grow faster on a yeast-rich diet than on a yeast-free (poor) diet, and death of early germline cysts, degeneration of early vitellogenic follicles and partial block in ovulation further contribute to the ∼60-fold decrease in egg laying observed on a poor diet. Multiple diet-dependent factors, including insulin-like peptides, the steroid ecdysone, the nutrient sensor Target of Rapamycin, AMP-dependent kinase, and adipocyte factors mediate this complex response. Here, we describe the results of a visual screen using a collection of green fluorescent protein (GFP) protein trap lines to identify additional factors potentially involved in this response. In each GFP protein trap line, an artificial GFP exon is fused in frame to an endogenous protein, such that the GFP fusion pattern parallels the levels and subcellular localization of the corresponding native protein. We identified 53 GFP-tagged proteins that exhibit changes in levels and/or subcellular localization in the ovary at 12-16 hours after switching females from rich to poor diets, suggesting them as potential candidates for future functional studies.

Brief Heat Stress Negatively Affects the Population Fitness and Host Feeding of Aphelinus asychis (Hymenoptera: Aphelinidae) Parasitizing Myzus persicae (Hemiptera: Aphididae)

Aphelinus asychis Walker (Hymenoptera: Aphelinidae), a polyphagous parasitoid, has been widely used as a biological control agent against Myzus persicae (Sulzer) (Hemiptera: Aphididae) and other aphid species. In order to ensure the successful biological control of M. persicae, we evaluated the influence of brief heat stresses (32.5, 35.0, 37.5, 40.0, and 41.5°C for 1 h), which occur frequently during the summer or under greenhouse conditions, on survival, longevity, host feeding, and parasitism of A. asychis that were reared on chili pepper at 25°C in the laboratory. Our results showed several major consequences on the parasitoid adults after brief heat stress. First, the survival of briefly heat stress-treated A. asychis female and male adults decreased significantly at ≥37.5°C and their longevity decreased as well. Second, the number of M. persicae nymphs infesting chili pepper killed through host feeding by the treated A. asychis females decreased significantly. Third, the cumulative number of mummified aphids and female progeny produced by the treated A. asychis also decreased significantly at ≥37.5°C. Our results indicated that a brief heat stress on A. asychis adults caused severe effects on major life history traits and total numbers of hosts killed by host feeding and parasitism, suggesting that high temperatures should be avoided to ensure the success of biological control of M. persicae and other aphid species using this parasitoid species.

Plastic and evolutionary responses to heat stress in a temperate dung fly: negative correlation between basal and induced heat tolerance?

Extreme weather events such as heat waves are becoming more frequent and intense. Populations can cope with elevated heat stress by evolving higher basal heat tolerance (evolutionary response) and/or stronger induced heat tolerance (plastic response). However, there is ongoing debate about whether basal and induced heat tolerance are negatively correlated and whether adaptive potential in heat tolerance is sufficient under ongoing climate warming. To evaluate the evolutionary potential of basal and induced heat tolerance, we performed experimental evolution on a temperate source population of the dung fly Sepsis punctum. Offspring of flies adapted to three thermal selection regimes (Hot, Cold and Reference) were subjected to acute heat stress after having been exposed to either a hot-acclimation or non-acclimation pretreatment. As different traits may respond differently to temperature stress, several physiological and life history traits were assessed. Condition dependence of the response was evaluated by exposing juveniles to different levels of developmental (food restriction/rearing density) stress. Heat knockdown times were highest, whereas acclimation effects were lowest in the Hot selection regime, indicating a negative association between basal and induced heat tolerance. However, survival, adult longevity, fecundity and fertility did not show such a pattern. Acclimation had positive effects in heat-shocked flies, but in the absence of heat stress hot-acclimated flies had reduced life spans relative to non-acclimated ones, thereby revealing a potential cost of acclimation. Moreover, body size positively affected heat tolerance and unstressed individuals were less prone to heat stress than stressed flies, offering support for energetic costs associated with heat tolerance. Overall, our results indicate that heat tolerance of temperate insects can evolve under rising temperatures, but this response could be limited by a negative relationship between basal and induced thermotolerance, and may involve some but not other fitness-related traits.

Does metabolic rate and evaporative water loss reflect differences in migratory strategy in sexually dimorphic hoverflies?

A typical explanation for ecologically stable strategies that apply to only a proportion of a population, is bet hedging, where increased reproductive success offsets reduced reproductive rate. One such is partial migration, where only a proportion of a population moves seasonally to avoid inclement climatic conditions. Bet hedging may overlook unseen costs to maintain broad physiological resilience, implied by encountering a breadth of environmental conditions. We investigated the physiological correlates of partial migration by measuring standard metabolic rates, and rates of evaporative water loss, and then estimating upper and lower thermal tolerance in males and females of two hoverfly species, Episyrphus balteatus and Eristalis tenax. In central Europe, females of these species may either migrate or overwinter, whereas males may migrate south to the Mediterranean, but have not been found overwintering. Both species were sexually dimorphic; female Ep. balteatus were lighter than males, but female Er. tenax were heavier than males. While allometrically- corrected metabolic rate in both species increased with temperature, the most parsimonious models included no sex-specific differences in metabolic rate for either species. Evaporative water loss of both species also increased with temperature, but was higher for females of both species than males. Assuming that resting metabolism is congruent with the activity requirements of migration, highly consistent thermal tolerance and metabolic rate suggests that any given fly could migrate, although water loss patterns suggest that females may be less well-adapted to Mediterranean climates. We infer that partial migration probably results from the imperatives of their reproductive strategies.

Inducing extra copies of the Hsp70 gene in Drosophila melanogaster increases energetic demand

Background

Mutations that increase gene expression are predicted to increase energy allocation to transcription, translation and protein function. Despite an appreciation that energetic tradeoffs may constrain adaptation, the energetic costs of increased gene expression are challenging to quantify and thus easily ignored when modeling the evolution of gene expression, particularly for multicellular organisms. Here we use the well-characterized, inducible heat-shock response to test whether expressing additional copies of the Hsp70 gene increases energetic demand in Drosophila melanogaster.

Results

We measured metabolic rates of larvae with different copy numbers of the Hsp70 gene to quantify energy expenditure before, during, and after exposure to 36°C, a temperature known to induce robust expression of Hsp70. We observed a rise in metabolic rate within the first 30 minutes of 36°C exposure above and beyond the increase in routine metabolic rate at 36°C. The magnitude of this increase in metabolic rate was positively correlated with Hsp70 gene copy number and reflected an increase as great as 35% of the 22°C metabolic rate. Gene copy number also affected Hsp70 mRNA levels as early as 15 minutes after larvae were placed at 36°C, demonstrating that gene copy number affects transcript abundance on the same timescale as the metabolic effects that we observed. Inducing Hsp70 also had lasting physiological costs, as larvae had significantly depressed metabolic rate when returned to 22°C after induction.

Conclusions

Our results demonstrate both immediate and persistent energetic consequences of gene copy number in a multicellular organism. We discuss these consequences in the context of existing literature on the pleiotropic effects of variation in Hsp70 copy number, and argue that the increased energetic demand of expressing extra copies of Hsp70 may contribute to known tradeoffs in physiological performance of extra-copy larvae. Physiological costs of mutations that greatly increase gene expression, such as these, may constrain their utility for adaptive evolution.

Direct and correlated responses to laboratory selection for body melanisation in Drosophila melanogaster: support for the melanisation-desiccation resistance hypothesis

For Drosophila melanogaster, cuticular melanisation is a quantitative trait, varying from no melanin to completely dark. Variation in melanisation has been linked with stress resistance, especially desiccation, in D. melanogaster and other species. As melanism has a genetic component, we selected melanic and non-melanic phenotypes of D. melanogaster in order to confirm the association of desiccation resistance and rate of water loss with cuticular melanisation previously reported for this species. A bidirectional selection experiment for dark (D1-D4) and light (L1-L4) body colour in D. melanogaster was conducted for 60 generations. We observed a 1.6-fold increase in abdominal melanisation in selected dark strains and a 14-fold decrease in selected light strains compared with control populations. Desiccation resistance increased significantly in the dark-selected morphs as compared with controls. The observed increase in desiccation resistance appeared as a consequence of a decrease in cuticular permeability. Our results show that traits related to water balance were significantly correlated with abdominal melanisation and were simultaneously selected bidirectionally along with melanisation.

Keeping pace with climate change: what is wrong with the evolutionary potential of upper thermal limits?

The potential of populations to evolve in response to ongoing climate change is partly conditioned by the presence of heritable genetic variation in relevant physiological traits. Recent research suggests that Drosophila melanogaster exhibits negligible heritability, hence little evolutionary potential in heat tolerance when measured under slow heating rates that presumably mimic conditions in nature. Here, we study the effects of directional selection for increased heat tolerance using Drosophila as a model system. We combine a physiological model to simulate thermal tolerance assays with multilocus models for quantitative traits. Our simulations show that, whereas the evolutionary response of the genetically determined upper thermal limit (CTmax) is independent of methodological context, the response in knockdown temperatures varies with measurement protocol and is substantially (up to 50%) lower than for CTmax. Realized heritabilities of knockdown temperature may grossly underestimate the true heritability of CTmax. For instance, assuming that the true heritability of CTmax in the base population is h(2) = 0.25, realized heritabilities of knockdown temperature are around 0.08-0.16 depending on heating rate. These effects are higher in slow heating assays, suggesting that flawed methodology might explain the apparently limited evolutionary potential of cosmopolitan D. melanogaster.

Ecologically relevant measures of the physiological tolerance of light brown apple moth, Epiphyas postvittana, to high temperature extremes

Invasive ectothermic species are limited in their geographic range expansion primarily by their capacity to withstand temperature extremes. Epiphyas postvittana is a highly polyphagous invasive leafroller that was discovered in California in 2006. To predict its potential range and future response to climate change, high temperature tolerance of this species was determined for all life stages and larval instars. Using the static method to estimate high temperature tolerance with response to probing as an endpoint, the mean time leading to 50% mortality (LT(50)) ranged from 45 to 187h at 32.3°C, 34 to 68h at 36°C, 11 to 21h at 38°C, and 1.2 to 5.6h at 40.4°C. There was no clear pattern in the relative tolerance of the life stages across the range of temperatures tested. For pupae and adults, gender did not influence the LT(50) values at any of the temperatures tested. For the larval instars, LT(50) values increased with increasing larval instar at the highest three temperatures while this trend was reversed for the lowest temperature (32.3°C). An analysis of LT(50) values obtained from acute responses to probing compared to subsequent survival to adult emergence, showed that chronic mortality severely affected all larval instars at three out of the four constant temperatures and resulted in 64-85% reduction in LT(50) values. No difference in acute and chronic mortality was found for exposure of the egg stage to high temperatures. These findings have important implications for predicting thermal limits and range expansions of insect species, since upper thermal tolerance could readily be overestimated from the use of ad hoc rather than ecologically relevant endpoint measurements such as survival to adult emergence.

Complex patterns of local adaptation in heat tolerance in Drosophila simulans from eastern Australia

Latitudinal clines are considered a powerful means of investigating evolutionary responses to climatic selection in nature. However, most clinal studies of climatic adaptation in Drosophila have involved species that contain cosmopolitan inversion polymorphisms that show clinal patterns themselves, making it difficult to determine whether the traits or inversions are under selection. Further, although climatic selection is unlikely to act on only one life stage in metamorphic organisms, a few studies have examined clinal patterns across life stages. Finally, clinal patterns of heat tolerance may also depend on the assay used. To unravel these potentially confounding effects on clinal patterns of thermal tolerance, we examined adult and larval heat tolerance traits in populations of Drosophila simulans from eastern Australia using static and dynamic (ramping 0.06 °C min(-1)) assays. We also used microsatellites markers to clarify whether demographic factors or selection are responsible for population differentiation along clines. Significant cubic clinal patterns were observed for adult static basal, hardened and dynamic heat knockdown time and static basal heat survival in larvae. In contrast, static, hardened larval heat survival increased linearly with latitude whereas no clinal association was found for larval ramping survival. Significant associations between adult and larval traits and climatic variables, and low population differentiation at microsatellite loci, suggest a role for climatic selection, rather than demographic processes, in generating these clinal patterns. Our results suggest that adaptation to thermal stress may be species and life-stage specific, complicating our efforts to understand the evolutionary responses to selection for increasing thermotolerance.

Hsp70 protein levels and thermotolerance in Drosophila subobscura: a reassessment of the thermal co-adaptation hypothesis

Theory predicts that geographic variation in traits and genes associated with climatic adaptation may be initially driven by the correlated evolution of thermal preference and thermal sensitivity. This assumes that an organism's preferred body temperature corresponds with the thermal optimum in which performance is maximized; hence, shifts in thermal preferences affect the subsequent evolution of thermal-related traits. Drosophila subobscura evolved worldwide latitudinal clines in several traits including chromosome inversion frequencies, with some polymorphic inversions being apparently associated with thermal preference and thermal tolerance. Here we show that flies carrying the warm-climate chromosome arrangement O(3+4) have higher basal protein levels of Hsp70 than their cold-climate O(st) counterparts, but this difference disappears after heat hardening. O(3+4) carriers are also more heat tolerant, although it is difficult to conclude from our results that this is causally linked to their higher basal levels of Hsp70. The observed patterns are consistent with the thermal co-adaptation hypothesis and suggest that the interplay between behaviour and physiology underlies latitudinal and seasonal shifts in inversion frequencies.

Survival rate and expression of Heat-shock protein 70 and Frost genes after temperature stress in Drosophila melanogaster lines that are selected for recovery time from temperature coma

In this study, we investigated the physiological mechanisms underlying temperature tolerance using Drosophila melanogaster lines with rapid, intermediate, or slow recovery from heat or chill coma that were established by artificial selection or by free recombination without selection. Specifically, we focused on the relationships among their recovery from heat or chill coma, survival after severe heat or cold, and survival enhanced by rapid cold hardening (RCH) or heat hardening. The recovery time from heat coma was not related to the survival rate after severe heat. The line with rapid recovery from chill coma showed a higher survival rate after severe cold exposure, and therefore the same mechanisms are likely to underlie these phenotypes. The recovery time from chill coma and survival rate after severe cold were unrelated to RCH-enhanced survival. We also examined the expression of two genes, Heat-shock protein 70 (Hsp70) and Frost, in these lines to understand the contribution of these stress-inducible genes to intraspecific variation in recovery from temperature coma. The line showing rapid recovery from heat coma did not exhibit higher expression of Hsp70 and Frost. In addition, Hsp70 and Frost transcription levels were not correlated with the recovery time from chill coma. Thus, Hsp70 and Frost transcriptional regulation was not involved in the intraspecific variation in recovery from temperature coma.

How does heat shock affect the life history traits of adults and progeny of the aphid parasitoid Aphidius avenae (Hymenoptera: Aphidiidae)?

Because insects are ectotherms, their physiology, behaviour and fitness are influenced by the ambient temperature. Any changes in environmental temperatures may impact the fitness and life history traits of insects and, thus, affect population dynamics. Here, we experimentally tested the impact of heat shock on the fitness and life history traits of adults of the aphid parasitoid Aphidius avenae and on the later repercussions for their progeny. Our results show that short exposure (1 h) to an elevated temperature (36 degrees C), which is frequently experienced by parasitoids during the summer, resulted in high mortality rates in a parasitoid population and strongly affected the fitness of survivors by drastically reducing reproductive output and triggering a sex-dependent effect on lifespan. Heat stress resulted in greater longevity in surviving females and in shorter longevity in surviving males in comparison with untreated individuals. Viability and the developmental rates of progeny were also affected in a sex-dependent manner. These results underline the ecological importance of the thermal stress response of parasitoid species, not only for survival, but also for maintaining reproductive activities.

Three selections are better than one: clinal variation of thermal QTL from independent selection experiments in Drosophila

We report the results of two independent selection experiments that have exposed distinct populations of Drosophila melanogaster to different forms of thermal selection. A recombinant population derived from Arvin California and Zimbabwe isofemale lines was exposed to laboratory natural selection at two temperatures (T(AZ): 18°C and 28°C). Microsatellite mapping identified quantitative trait loci (QTL) on the X-chromosome between the replicate "Hot" and "Cold" populations. In a separate experiment, disruptive selection was imposed on an outbred California population for the "knockdown" temperature (T(KD)) in a thermal column. Microsatellite mapping of the "High" and "Low" populations also uncovered primarily X-linked QTL. Notably, a marker in the shaggy locus at band 3A was significantly differentiated in both experiments. Finer scale mapping of the 3A region has narrowed the QTL to the shaggy gene region, which contains several candidate genes that function in circadian rhythms. The same allele that was increased in frequency in the High T(KD) populations is significantly clinal in North America and is more common at the warm end of the cline (Florida vs. Maine; however, the cline was not apparent in Australia). Together, these studies show that independent selection experiments can uncover the same target of selection and that evolution in the laboratory can recapitulate putatively adaptive clinal variation in nature.

Adult heat tolerance variation in Drosophila melanogaster is not related to Hsp70 expression

Expression of heat-inducible Hsp70 is considered closely linked to thermotolerance in Drosophila melanogaster and other ectotherms. However, intra-specific variation of Hsp70 expression levels and its relationship to heat resistance has only been investigated in a few studies. Although in Drosophila larvae Hsp70 expression may be a key determinant of heat tolerance, the evidence for this in adults is equivocal. We therefore examined heat-induced Hsp70 expression and several measurements of adult heat tolerance in three independent collections of D. melanogaster, measured in three laboratories and using slightly different protocols. Expression levels of Hsp70 were quantified using ELISA or Western blots on extracts from adult females. Both Hsp70 and heat tolerance exhibited substantial within-population variation as previously reported. However, in all experiments there were no significant correlation between Hsp70 expression and laboratory assays of adult heat tolerance commonly used in Drosophila. When combining data across three studies we had high power to detect associations but the results showed that variation in Hsp70 expression is only likely to explain a small proportion of variation in adult heat tolerance. Therefore, although Hsp70 expression is a major component of the cellular heat stress response, its influence on intra-specific heat tolerance variation may be life-stage specific.

Combined expression patterns of QTL-linked candidate genes best predict thermotolerance in Drosophila melanogaster

Knockdown resistance to high temperature (KRHT) is a thermal adaptation trait in Drosophila melanogaster. Here we used quantitative real-time PCR (qRT-PCR) to test for possible associations between KRHT and the expression of candidate genes within quantitative trait loci (QTL) in eight recombinant inbred lines (RIL). hsp60 and hsc70-3 map within an X-linked QTL, while CG10383, catsup, ddc, trap1, and cyp6a13 are linked in a KRHT-QTL on chromosome 2. hsc70-3 expression increased by heat-hardening. Principal Components analysis revealed that catsup, ddc and trap1 were either co-expressed or combined in their expression levels. This composite expression variable (e-PC1) was positively associated to KRHT in non-hardened RIL. In heat-hardened flies, hsp60 was negatively related to hsc70-3 on e-PC2, with effects on KRHT. These results are consistent with the notion that QTL can be shaped by expression variation in combined candidate loci. We found composite variables of gene expression (e-PCs) that best correlated to KRHT. Network effects with other untested linked loci are apparent because, in spite of their associations with KRHT phenotypes, e-PCs were sometimes uncorrelated with their QTL genotype.

Insect thermal tolerance: what is the role of ontogeny, ageing and senescence?

Temperature has dramatic evolutionary fitness consequences and is therefore a major factor determining the geographic distribution and abundance of ectotherms. However, the role that age might have on insect thermal tolerance is often overlooked in studies of behaviour, ecology, physiology and evolutionary biology. Here, we review the evidence for ontogenetic and ageing effects on traits of high- and low-temperature tolerance in insects and show that these effects are typically pronounced for most taxa in which data are available. We therefore argue that basal thermal tolerance and acclimation responses (i.e. phenotypic plasticity) are strongly influenced by age and/or ontogeny and may confound studies of temperature responses if unaccounted for. We outline three alternative hypotheses which can be distinguished to propose why development affects thermal tolerance in insects. At present no studies have been undertaken to directly address these options. The implications of these age-related changes in thermal biology are discussed and, most significantly, suggest that the temperature tolerance of insects should be defined within the age-demographics of a particular population or species. Although we conclude that age is a source of variation that should be carefully controlled for in thermal biology, we also suggest that it can be used as a valuable tool for testing evolutionary theories of ageing and the cellular and genetic basis of thermal tolerance.

Inducible and constitutive heat shock gene expression responds to modification of Hsp70 copy number in Drosophila melanogaster but does not compensate for loss of thermotolerance in Hsp70 null flies

Background

The heat shock protein Hsp70 promotes inducible thermotolerance in nearly every organism examined to date. Hsp70 interacts with a network of other stress-response proteins, and dissecting the relative roles of these interactions in causing thermotolerance remains difficult. Here we examine the effect of Hsp70 gene copy number modification on thermotolerance and the expression of multiple stress-response genes in Drosophila melanogaster, to determine which genes may represent mechanisms of stress tolerance independent of Hsp70.

Results

Hsp70 copy number in four strains is positively associated with Hsp70 expression and inducible thermotolerance of severe heat shock. When assayed at carefully chosen temperatures, Hsp70 null flies are almost entirely deficient in thermotolerance. In contrast to expectations, increasing Hsp70 expression levels induced by thermal pretreatment are associated with increasing levels of seven other inducible Hsps across strains. In addition, complete Hsp70 loss causes upregulation of the inducible Hsps and six constitutive stress-response genes following severe heat shocks.

Conclusion

Modification of Hsp70 copy number quantitatively and qualitatively affects the expression of multiple other stress-response genes. A positive association between absolute expression levels of Hsp70 and other Hsps after thermal pretreatment suggests novel regulatory mechanisms. Severe heat shocks induce both novel gene expression patterns and almost total mortality in the Hsp70 null strain: alteration of gene expression in this strain does not compensate for Hsp70 loss but suggests candidates for overexpression studies.

Critical thermal maxima in knockdown-selected Drosophila: are thermal endpoints correlated?

To explore the correlation of traits linked to thermotolerance, we compared three thermal endpoints (knockdown temperature and two critical thermal maxima) among replicate populations of Drosophila melanogaster selected for high, or low, knockdown temperature. The high knockdown flies maintain normal posture and locomotor ability within a knockdown column at temperatures >or=40 degrees C, whereas the low knockdown flies fall out of the column at much cooler temperatures (approximately 35 degrees C, on average). The critical thermal maximum (CT(max)) for respiratory control in the selected knockdown populations was determined by analyzing CO(2) output of individuals during exposure to a temperature ramp (from 30 degrees C to >45 degrees C) and was indicated by an abrupt alteration in the pattern of CO(2) release. The CT(max) for locomotor function was determined by monitoring activity (concurrent with CO(2) analysis) during the temperature ramp and was marked by the abrupt cessation of activity. We hypothesized that selection for high knockdown temperature may cause an upward shift in CT(max), whereas selection for low knockdown may lower CT(max). Correlations among the three thermal endpoints varied between the high and low knockdown flies. Finally, we compared metabolic profiles, as well as Q(10) values, among the high and low knockdown males and females during the temperature ramp.