Knockdown resistance to heat stress and slow recovery from chill coma are genetically associated in a quantitative trait locus region of chromosome 2 inDrosophila melanogaster

Naturally segregating genetic variants contribute to thermal tolerance in a Drosophila melanogaster model system

Thermal tolerance is a fundamental physiological complex trait for survival in many species. For example, everyday tasks such as foraging, finding a mate, and avoiding predation are highly dependent on how well an organism can tolerate extreme temperatures. Understanding the general architecture of the natural variants within the genes that control this trait is of high importance if we want to better comprehend thermal physiology. Here, we take a multipronged approach to further dissect the genetic architecture that controls thermal tolerance in natural populations using the Drosophila Synthetic Population Resource as a model system. First, we used quantitative genetics and Quantitative Trait Loci mapping to identify major effect regions within the genome that influences thermal tolerance, then integrated RNA-sequencing to identify differences in gene expression, and lastly, we used the RNAi system to (1) alter tissue-specific gene expression and (2) functionally validate our findings. This powerful integration of approaches not only allows for the identification of the genetic basis of thermal tolerance but also the physiology of thermal tolerance in a natural population, which ultimately elucidates thermal tolerance through a fitness-associated lens.

Metabolomes of bumble bees reared in common garden conditions suggest constitutive differences in energy and toxin metabolism across populations

Cold tolerance of ectotherms can vary strikingly among species and populations. Variation in cold tolerance can reflect differences in genomes and transcriptomes that confer cellular-level protection from cold; additionally, shifts in protein function and abundance can be altered by other cellular constituents as cold-exposed insects often have shifts in their metabolomes. Even without a cold challenge, insects from different populations may vary in cellular composition that could alter cold tolerance, but investigations of constitutive differences in metabolomes across wild populations remain rare. To address this gap, we reared Bombus vosnesenskii queens collected from Oregon and California (USA) that differ in cold tolerance (CTmin = -6 °C and 0 °C, respectively) in common garden conditions, and measured offspring metabolomes using untargeted LC-MS/MS. Oregon bees had higher levels of metabolites associated with carbohydrate (sorbitol, lactitol, maltitol, and sorbitol-6-phosphate) and amino acid (hydroxyproline, ornithine, and histamine) metabolism. Exogenous metabolites, likely derived from the diet, also varied between Oregon and California bees, suggesting population-level differences in toxin metabolism. Overall, our results reveal constitutive differences in metabolomes for bumble bees reared in common garden conditions from queens collected in different locations despite no previous cold exposure.

Naturally segregating genetic variants contribute to thermal tolerance in a D. melanogaster model system

Thermal tolerance is a fundamental physiological complex trait for survival in many species. For example, everyday tasks such as foraging, finding a mate, and avoiding predation, are highly dependent on how well an organism can tolerate extreme temperatures. Understanding the general architecture of the natural variants of the genes that control this trait is of high importance if we want to better comprehend how this trait evolves in natural populations. Here, we take a multipronged approach to further dissect the genetic architecture that controls thermal tolerance in natural populations using the Drosophila Synthetic Population Resource (DSPR) as a model system. First, we used quantitative genetics and Quantitative Trait Loci (QTL) mapping to identify major effect regions within the genome that influences thermal tolerance, then integrated RNA-sequencing to identify differences in gene expression, and lastly, we used the RNAi system to 1) alter tissue-specific gene expression and 2) functionally validate our findings. This powerful integration of approaches not only allows for the identification of the genetic basis of thermal tolerance but also the physiology of thermal tolerance in a natural population, which ultimately elucidates thermal tolerance through a fitness-associated lens.

Trait divergence and trade-offs among Brassicaceae species differing in elevational distribution

Species have restricted geographic distributions and the causes are still largely unknown. Temperature has long been associated with distribution limits, suggesting that there are ubiquitous constraints to the evolution of the climate niche. Here, we investigated the traits involved in such constraints by macroevolutionary comparisons involving 100 Brassicaceae species differing in elevational distribution. Plants were grown under three temperature treatments (regular frost, mild, regular heat) and phenotyped for phenological, morphological, and thermal resistance traits. Trait values were analyzed by assessing the effect of temperature and elevational distribution, by comparing models of evolutionary trajectories, and by correlative approaches to identify trade-offs. Analyses pointed to size, leaf morphology, and growth under heat as among the most discriminating traits between low- and high-elevation species, with high-elevation species growing faster under the occurrence of regular heat bouts, at the cost of reduced size. Mixed models and evolutionary models supported adaptive divergence for these traits, and correlation analysis indicated their involvement in moderate trade-offs. Finally, we found asymmetry in trait evolution, with evolvability across traits being 50% less constrained under regular frost. Overall, results suggest that trade-offs between traits under adaptive divergence contribute to the disparate distribution of species along the elevational gradient.

Genetic variation in the heat-stress survival of embryos is largely decoupled from adult thermotolerance in an intercontinental set of recombinant lines of Drosophila melanogaster

In insects, thermal adaptation works on the genetic variation for thermotolerance of not only larvae and adults but also of the immobile stages of the life cycle including eggs. In contrast to adults and larvae, the genetic basis for thermal adaptation in embryos (eggs) remains to be tested in the model insect Drosophila melanogaster. Quantitative-trait loci (QTL) for heat-stress resistance in embryos could largely differ from previously identified QTL for larvae and adults. Here we used an intercontinental set of recombinant inbred lines (RIL), which were previously used to identify thermotolerance-QTLs in adults and larvae because of their high variation segregating for adult thermotolerance. Eggs appeared to be more heat resistant than larvae and adults from previous studies on these RIL, though different heat-shock assays were used in previous studies. We found that variation in thermotolerance in embryos can be, at least partially, genetically decoupled from thermotolerance in the adult insect. Some RIL that are heat resistant in the adult and larvae can be heat susceptible in embryos. Only one small-effect QTL out of five autosomal QTL co-localized between embryo and other ontogenetic stages. These results suggest that selection for thermal adaptation in adult flies and larvae is predicted to have only a small impact on embryo thermotolerance. In addition, heat-stress tolerance of insects can be measured across ontogenetic stages including embryos in order to better predict thermal adaptive limits of populations and species.

Cardiac performance in heat-stressed flies of heat-susceptible and heat-resistant Drosophila melanogaster

Thermotolerance is a complex trait that can greatly differ between heat-susceptible (HS) and heat-adapted populations of small insects including Drosophila, with short-term effects after a sub-lethal level of heat stress on many physiological functions. Cardiac performance could accordingly be more robust in heat-resistant (HR) than in HS individuals under heat stress. Here, we tested heart performance under heat-stress effects in two recombinant inbred lines (RIL) of Drosophila melanogaster that dramatically differ in heat knockdown resistance. Heart rate did not strongly differ between heat-susceptible and heat-tolerant flies after a sub-lethal heat stress. Instead, heat-susceptible flies showed a much higher arrhythmia incidence, a longer duration of each heartbeat, and a larger amount of bradycardia than heat-tolerant flies. The highly conserved cardiac proteins SERCA, RyR and NCX that participate in the excitation/contraction coupling, did not differ in activity level between HR and HS flies. Available information for both RIL suggests that heart performance under heat stress may be linked, at least partially, to candidate genes of previously identified quantitative trait loci (QTL) for thermotolerance. This study indicates that HR flies can be genetically more robust in their heart performance than HS flies under even sub-lethal levels of heat stress.

Time course of acclimation of critical thermal limits in two springtail species (Collembola)

Critical thermal limits are one of the most important sources of information on the possible impacts of climate change on soil microarthropods. The extent of plasticity of tolerance limits can provide valuable insights about the likely responses of ectotherms to environmental change. Although many studies have investigated various aspects of the acclimatory response of thermal limits to temperature changes in arthropods, the number of studies focusing on the temporal dynamics of this plastic response is relatively small. Collembola, one of the key microarthropods groups in almost all soil ecosystems around the world, have been the focus of several thermal acclimation studies. Yet the time course of acclimation and its reversal have not been widely studied in this group. Here we investigated the time course of acclimation of critical thermal maxima (CT_max) and minima (CT_min) of two springtail species. We exposed a Cryptopygus species from temperate southern Australia to high and low temperature conditions and Mucrosomia caeca from Sub-Antarctic Macquarie Island to high temperature conditions. Upper thermal limits in both species were found to be highly constrained, as CT_max did not show substantial response to high and low temperature acclimation both in the Cryptopygus species and M. caeca, whereas CT_min showed significant responses to high and low temperature conditions. The acclimation begins to stabilize in approximately seven days in all treatments except for the acclimation of CT_min under high temperature conditions, where the pattern of change suggests that this acclimation might take longer to be completed. Although reversal of this acclimation also begins to stabilize under 7 days, re-acclimation was relatively slow as we did not observe a very clear settling point in 2 of the 3 re-acclimation treatments. The observed limits on the plasticity of CT_max indicate that both of these species may be very limited in their ability to respond plastically to short-term rapid changes in temperature (i.e temperature extremes).

Heat knockdown resistance and chill-coma recovery as correlated responses to selection on mating success at high temperature in Drosophila buzzatii

Reproduction and related traits such as mating success are strongly affected by thermal stress. We tested direct and correlated responses to artificial selection in replicated lines of Drosophila buzzatii that were selected for mating success at high temperature. Knockdown resistance at high temperature (KRHT) and chill-coma recovery (CCR) were tested as correlated selection responses. Virgin flies were allowed to mate for four hours at 33°C in three replicated lines (S lines) to obtain the selected flies and then returned at 25°C to lay eggs. Other three replicated lines were maintained at 25°C without any selection as control (C lines). After 15 selection generations, KRHT and CCR were measured. Both traits were assessed in flies that did not receive any hardening pretreatments as well as in flies that were either heat or cold hardened. Thermotolerance traits showed significant correlated responses with higher KRHT in S than in C lines, both with a heat-hardening pretreatment and without a heat-hardening pretreatment. CCR time was longer in S than in C lines both with a cold-hardening pretreatment and without a cold-hardening pretreatment. Hardening treatments improved both KRHT and CCR in all cases excepting KRHT in C lines. Overall, KRHT and CCR showed an antagonistic pattern of correlated responses to our selection regime, suggesting either pleiotropy or tightly linked trait-specific genes partially affecting KRHT and CCR.

Transcript-Level Analysis in Combination with Real-Time PCR Elucidates Heat Adaptation Mechanism of Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) Larvae

Tribolium castaneum (Herbst) ranks as one of the most prevalent insects in food processing and storage facilities worldwide. Heat treatment has been revisited to disinfest food processing and storage facilities due to increasingly strict regulation on chemicals. The effect of acclimation of T. castaneum larvae to sublethal high temperatures of 36 and 42℃ for 10 h on their heat adaptation was investigated, and transcript-level analysis combinating with real-time PCR (RT-qPCR) was applied for elucidating the heat adaptation mechanism of T. castaneum larvae. Short-term sublethal high temperature acclimation could greatly enhance the thermal adaptability in T. castaneum larvae. In total, 575, 875, and 1017 differentially expressed genes (DEGs) were, respectively, determined in comparisons between the 28 and 36℃ treatments, the 28 and 42℃ treatments, and the 36 and 42℃ treatments. Fifty-three and 96 genes were commonly up- and down-regulated in both the 36 and 42℃ treatments relative to 28℃, respectively. The results of RT-qPCR analysis further confirmed the RNA-seq analysis. The current results are in favor of enhancing the insecticidal effectiveness of extreme high temperature treatment and elucidating the heat adaptation mechanism in T. castaneum larvae.

Adaptation to Low Temperature Exposure Increases Metabolic Rates Independently of Growth Rates

Metabolic cold adaptation is a pattern where ectotherms from cold, high-latitude, or -altitude habitats have higher metabolic rates than ectotherms from warmer habitats. When found, metabolic cold adaptation is often attributed to countergradient selection, wherein short, cool growing seasons select for a compensatory increase in growth rates and development times of ectotherms. Yet, ectotherms in high-latitude and -altitude environments face many challenges in addition to thermal and time constraints on lifecycles. In addition to short, cool growing seasons, high-latitude and - altitude environments are characterized by regular exposure to extreme low temperatures, which cause ectotherms to enter a transient state of immobility termed chill coma. The ability to resume activity quickly after chill coma increases with latitude and altitude in patterns consistent with local adaptation to cold conditions. We show that artificial selection for fast and slow chill coma recovery among lines of the fly Drosophila melanogaster also affects rates of respiratory metabolism. Cold-hardy fly lines, with fast recovery from chill coma, had higher respiratory metabolic rates than control lines, with cold-susceptible slow-recovering lines having the lowest metabolic rates. Fast chill coma recovery was also associated with higher respiratory metabolism in a set of lines derived from a natural population. Although their metabolic rates were higher than control lines, fast-recovering cold-hardy lines did not have faster growth rates or development times than control lines. This suggests that raised metabolic rates in high-latitude and -altitude species may be driven by adaptation to extreme low temperatures, illustrating the importance of moving "Beyond the Mean".

Cold acclimation wholly reorganizes the Drosophila melanogaster transcriptome and metabolome

Cold tolerance is a key determinant of insect distribution and abundance, and thermal acclimation can strongly influence organismal stress tolerance phenotypes, particularly in small ectotherms like Drosophila. However, there is limited understanding of the molecular and biochemical mechanisms that confer such impressive plasticity. Here, we use high-throughput mRNA sequencing (RNA-seq) and liquid chromatography – mass spectrometry (LC-MS) to compare the transcriptomes and metabolomes of D. melanogaster acclimated as adults to warm (rearing) (21.5 °C) or cold conditions (6 °C). Cold acclimation improved cold tolerance and led to extensive biological reorganization: almost one third of the transcriptome and nearly half of the metabolome were differentially regulated. There was overlap in the metabolic pathways identified via transcriptomics and metabolomics, with proline and glutathione metabolism being the most strongly-supported metabolic pathways associated with increased cold tolerance. We discuss several new targets in the study of insect cold tolerance (e.g. dopamine signaling and Na⁺-driven transport), but many previously identified candidate genes and pathways (e.g. heat shock proteins, Ca²⁺ signaling, and ROS detoxification) were also identified in the present study, and our results are thus consistent with and extend the current understanding of the mechanisms of insect chilling tolerance.

Artificial selection on chill-coma recovery time in Drosophila melanogaster: Direct and correlated responses to selection

Artificial selection can be used to create populations with extreme phenotypic responses to environmental stressors. When artificial selection is applied to a single component of a stress response, this selection may result in correlated responses in other stress responses, a phenomenon called cross-tolerance, which is ultimately controlled by the genetic correlations among traits. We selected for extreme responses to cold tolerance by selecting for chill-coma recovery time from a single temperate population of Drosophila melanogaster. Chill-coma recovery time is a common metric of low, but non-lethal, cold temperature tolerance. Replicated divergent artificial selection was applied to a genetically variable base population for 31 generations, resulting in two cold resistant, two cold susceptible, and two unselected control lines. To quantify the relationship between selection on chill-coma recovery and other metrics of thermal performance, we also measured survivorship after acute cold exposure, survivorship after chronic cold exposure, survivorship after cold exposure following a pre-treatment period (rapid cold hardening), starvation tolerance, and heat tolerance. We find that chill-coma recovery time is heritable within this population and that there is an asymmetric response to increased and decreased chill-coma recovery time. Surprisingly, we found no cross-tolerances between selection on chill-coma recovery time and the other environmental stress response traits. These results suggest that although artificial selection has dramatically altered chill-coma recovery time, the correlated response to selection on other stress response phenotypes has been negligible. The lack of a correlated response suggests that chill-coma recovery time in these selection lines is likely genetically independent from measures of cold survivorship tested here.

Population genetic evidence for cold adaptation in European Drosophila melanogaster populations

We studied Drosophila melanogaster populations from Europe (the Netherlands and France) and Africa (Rwanda and Zambia) to uncover genetic evidence of adaptation to cold. We present here four lines of evidence for genes involved in cold adaptation from four perspectives: (i) the frequency of SNPs at genes previously known to be associated with chill-coma recovery time (CCRT), startle reflex (SR) and resistance to starvation stress (RSS) vary along environmental gradients and therefore among populations; (ii) SNPs of genes that correlate significantly with latitude and altitude in African and European populations overlap with SNPs that correlate with a latitudinal cline from North America; (iii) at the genomewide level, the top candidate genes are enriched in gene ontology (GO) terms that are related to cold tolerance; (iv) GO enriched terms from North American clinal genes overlap significantly with those from Africa and Europe. Each SNP was tested in 10 independent runs of Bayenv2, using the median Bayes factors to ascertain candidate genes. None of the candidate genes were found close to the breakpoints of cosmopolitan inversions, and only four candidate genes were linked to QTLs related to CCRT. To overcome the limitation that we used only four populations to test correlations with environmental gradients, we performed simulations to estimate the power of our approach for detecting selection. Based on our results, we propose a novel network of genes that is involved in cold adaptation.

Localization of quantitative trait loci for diapause and other photoperiodically regulated life history traits important in adaptation to seasonally varying environments

Seasonally changing environments at high latitudes present great challenges for the reproduction and survival of insects, and photoperiodic cues play an important role in helping them to synchronize their life cycle with prevalent and forthcoming conditions. We have mapped quantitative trait loci (QTL) responsible for the photoperiodic regulation of four life history traits, female reproductive diapause, cold tolerance, egg-to-eclosion development time and juvenile body weight in Drosophila montana strains from different latitudes in Canada and Finland. The F2 progeny of the cross was reared under a single photoperiod (LD cycle 16:8), which the flies from the Canadian population interpret as early summer and the flies from the Finnish population as late summer. The analysis revealed a unique QTL for diapause induction on the X chromosome and several QTL for this and the other measured traits on the 4th chromosome. Flies' cold tolerance, egg-to-eclosion development time and juvenile body weight had several QTL also on the 2nd, 3rd and 5th chromosome, some of the peaks overlapping with each other. These results suggest that while the downstream output of females' photoperiodic diapause response is partly under a different genetic control from that of the other traits in the given day length, all traits also share some QTL, possibly involving genes with pleiotropic effects and/or multiple tightly linked genes. Nonoverlapping QTL detected for some of the traits also suggest that the traits are potentially capable of independent evolution, even though this may be restricted by epistatic interactions and/or correlations and trade-offs between the traits.

Increased abundance of frost mRNA during recovery from cold stress is not essential for cold tolerance in adult Drosophila melanogaster

Frost (Fst) is a candidate gene associated with the response to cold in Drosophila melanogaster because Fst mRNA accumulation increases during recovery from low temperature exposure. We investigated the contribution of Fst expression to chill-coma recovery time, acute cold tolerance and rapid cold hardening (RCH) in adult D. melanogaster by knocking down Fst mRNA expression using GAL4/UAS-mediated RNA interference. In this experiment, four UAS-Fst and one tubulin-GAL4 lines were used. We predicted that if Fst is essential for cold tolerance phenotypes, flies with low Fst mRNA levels should be less cold tolerant than flies with normal levels of cold-induced Fst mRNA. Cold-induced Fst abundance and recovery time from chill-coma were not negatively correlated in male or female flies. Survival of 2 h exposures to sub-zero temperatures in Fst knockdown lines was not lower than that in a control line. Moreover, a low temperature pretreatment increased survival of severe cold exposure in flies regardless of Fst abundance level during recovery from cold stress, suggesting that Fst expression is not essential for RCH. Thus, cold-induced Fst accumulation is not essential for cold tolerance measured as chill-coma recovery time, survival to acute cold stress and RCH response in adult D. melanogaster.

Heat stress survival in the pre-adult stage of the life cycle in an intercontinental set of recombinant inbred lines of Drosophila melanogaster

In insects, pre-adult stages of the life cycle are exposed to variation in temperature that may differ from that in adults. However, the genetic basis for adaptation to environmental temperature could be similar between the pre-adult and the adult stages of the life cycle. Here, we tested quantitative trait loci (QTL) for heat-stress survival in larvae of D. melanogaster, with and without a mild-heat-stress pre-treatment. Two sets of recombinant inbred lines derived from lines artificially selected for high and low levels of knockdown resistance to high temperature in young flies were used as mapping population. There was no apparent increase in heat-shock survival between heat-pretreated and non-pretreated larvae. There was a positive correlation between the two experimental conditions of heat-shock survival (with and without a heat pre-treatment) except for males from one set of lines. Several QTL were identified involving all three major chromosomes. Many QTL for larval thermotolerance overlapped with thermotolerance-QTL identified in previous studies for adults. One new thermotolerance-QTL was found but these QTL explained only a small fraction of the phenotypic variance and were only significant in larvae that received no heat pre-treatment. Several candidate genes mapped within QTL ranges. We discuss an overall co-localization for thermotolerance-QTL between the adult fly in previous studies and the pre-adult stage of the life cycle in this study.

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.

Reestablishment of ion homeostasis during chill-coma recovery in the cricket Gryllus pennsylvanicus

The time required to recover from cold-induced paralysis (chill-coma) is a common measure of insect cold tolerance used to test central questions in thermal biology and predict the effects of climate change on insect populations. The onset of chill-coma in the fall field cricket (Gryllus pennsylvanicus, Orthoptera: Gryllidae) is accompanied by a progressive drift of Na(+) and water from the hemolymph to the gut, but the physiological mechanisms underlying recovery from chill-coma are not understood for any insect. Using a combination of gravimetric methods and atomic absorption spectroscopy, we demonstrate that recovery from chill-coma involves a reestablishment of hemolymph ion content and volume driven by removal of Na(+) and water from the gut. Recovery is associated with a transient elevation of metabolic rate, the time span of which increases with increasing cold exposure duration and closely matches the duration of complete osmotic recovery. Thus, complete recovery from chill-coma is metabolically costly and encompasses a longer period than is required for the recovery of muscle potentials and movement. These findings provide evidence that physiological mechanisms of hemolymph ion content and volume regulation, such as ion-motive ATPase activity, are instrumental in chill-coma recovery and may underlie natural variation in insect cold tolerance.

Cold hardening modulates K+ homeostasis in the brain of Drosophila melanogaster during chill coma

Environmental temperature is one of the most important abiotic factors affecting insect behaviour; virtually all physiological processes, including those which regulate nervous system function, are affected. At both low and high temperature extremes insects enter a coma during which individuals do not display behaviour and are unresponsive to stimulation. We investigated neurophysiological correlates of chill and hyperthermic coma in Drosophila melanogaster. Coma resulting from anoxia causes a profound loss of K(+) homeostasis characterized by a surge in extracellular K(+) concentration ([K(+)](o)) in the brain. We recorded [K(+)](o) in the brain during exposure to both low and high temperatures and observed a similar surge in [K(+)](o) which recovered to baseline concentrations following return to room temperature. We also found that rapid cold hardening (RCH) using a cold pretreatment (4°C for 2h; 2h recovery at room temperature) increased the peak brain [K(+)](o) reached during a subsequent chill coma and increased the rates of accumulation and clearance of [K(+)](o). We conclude that RCH preserves K(+) homeostasis in the fly brain during exposure to cold by reducing the temperature sensitivity of the rates of homeostatic processes.

QTL for survival to UV-C radiation in Drosophila melanogaster

PURPOSE

The aim of this study was to investigate tolerance to UV-C (ultraviolet C, 280-100 nm) radiation in Drosophila melanogaster, implementing a quantitative trait locus (QTL) mapping approach. This is of interest to test for genetic variation in survival to UV (ultraviolet) radiation.

MATERIALS AND METHODS

We performed a QTL scan in D. melanogaster recombinant inbred lines (RIL) constructed from parental stocks derived from a crossing between northern and southern hemisphere populations that segregated substantial genetic variation in thermal resistance in a previous study. Here, two experimental treatments were implemented: Continuous and cyclic UV-C radiation.

RESULTS

Significant QTL were detected on all three major chromosomes. Among these, multiple trait composite interval mapping revealed a significant QTL in the pericentromeric region of chromosome 2, a genome region consistently implicated in thermotolerance in previous studies.

CONCLUSIONS

This study shows substantial genetic variation for UV-C radiation resistance in D. melanogaster, with QTL for survival to UV-C radiation generally overlapping with major thermotolerance QTL. The genetic architecture of UV-C radiation resistance appears to be more complex in continuously irradiated individuals.

Survival of heat stress with and without heat hardening in Drosophila melanogaster: interactions with larval density

Survival of a potentially lethal high temperature stress is a genetically variable thermal adaptation trait in many organisms. Organisms cope with heat stress by basal or induced thermoresistance. Here, we tested quantitative trait loci (QTL) for heat stress survival (HSS) in Drosophila melanogaster, with and without a cyclic heat-hardening pre-treatment, for flies that were reared at low (LD) or high (HD) density. Mapping populations were two panels of recombinant inbred lines (RIL), which were previously constructed from heat stress-selected stocks: RIL-D48 and RIL-SH2, derived from backcrosses to stocks of low and high heat resistance, respectively. HSS increased with heat hardening in both LD and HD flies. In addition, HSS increased consistently with density in non-hardened flies. There was a significant interaction between heat hardening and density effects in RIL-D48. Several QTL were significant for both density and hardening treatments. Many QTL overlapped with thermotolerance QTL identified for other traits in previous studies based on LD cultures only. However, three new QTL were found in HD only (cytological ranges: 12E–16F6; 30A3–34C2; 49C–50C). Previously found thermotolerance QTL were also significant for flies from HD cultures.

Cold tolerance and cold-induced modulation of gene expression in two Drosophila virilis group species with different distributions

The importance of high and low temperature tolerance in adaptation to changing environmental conditions has evoked new interest in modulations in gene expression and metabolism linked with stress tolerance. We investigated the effects of rapid cold hardening and cold acclimatization on the chill coma recovery times of two Drosophila virilis group species, Drosophila montana and D. virilis, with different distributions and utilized a candidate gene approach to trace changes in their gene expression during and after the cold treatments. The study showed that cold acclimatization clearly decreases chill coma recovery times in both species, whereas rapid cold hardening did not have a significant effect. Microarray analysis revealed several genes showing expression changes during different stages of cold response. Amongst the 219 genes studied, two genes showed rather consistent expression changes: hsr-omega, which was up-regulated in both study species during cold acclimatization, and Eip71CD, which was down-regulated in nearly all of the cold treatments. In addition, 29 genes showed expression changes that were more treatment- and/or species specific. Overall, different stages of cold response elicited changes mainly in genes involved in heat shock response, circadian rhythm and metabolism.

Consistent effects of a major QTL for thermal resistance in field-released Drosophila melanogaster

Molecular genetic markers can be used to identify quantitative trait loci (QTL) for thermal resistance and this has allowed characterization of a major QTL for knockdown resistance to high temperature in Drosophila melanogaster. The QTL showed trade-off associations with cold resistance under laboratory conditions. However, assays of thermal tolerance conducted in the laboratory may not necessarily reflect performance at varying temperatures in the field. Here we tested if lines with different genotypes in this QTL show different thermal performance under high and low temperatures in the field using a release recapture assay. We found that lines carrying the QTL genotype for high thermal tolerance were significantly better at locating resources in the field releases under hot temperatures while the QTL line carrying the contrasting genotype were superior at cold temperatures. Further, we studied copulatory success between the different QTL genotypes at different temperatures. We found higher copulatory success in males of the high tolerance QTL genotype under hot temperature conditions, while there was no difference in females at cold temperatures. The results allow relating components of field fitness at different environmental temperatures with genotypic variation in a QTL for thermal tolerance.

Knocking down expression of Hsp22 and Hsp23 by RNA interference affects recovery from chill coma in Drosophila melanogaster

To protect cells from the damaging effects of environmental stresses, all organisms possess a universal stress response involving upregulation of heat shock proteins (Hsps). The mechanisms underlying chilling injuries and the subsequent recovery phase are only beginning to be understood in insects. Hsp22 and Hsp23 are both upregulated during the recovery from prolonged chill coma in Drosophila melanogaster. This prompted us to investigate the functional significance of these modulations by testing whether expression of these two small Hsps is necessary for recovery after cold stress. We used the GAL4/UAS system to separately knock down expression of Hsp22 and Hsp23, and assayed three aspects of recovery performance in transgenic adults that had undergone 12 h of chill coma at 0°C. The time to recover (short-term recovery) and mobility parameters (medium-term recovery) were significantly impaired in the transgenic flies in which Hsp22 or Hsp23 was suppressed. Our findings show that both Hsp22 and Hsp23 play important roles in the recovery from chill coma in adult males, and suggest that these contribute to adaptive responses to fluctuating thermal conditions.

Mechanisms underlying insect chill-coma

At their critical thermal minimum (CT(min)) insects enter chill-coma, a reversible state where neuromuscular transmission and movement cease. The physiological mechanisms responsible for the insect CT(min) remain poorly understood despite the regular use of chill-coma onset and recovery as a means to assess evolved or acquired variation in low temperature tolerance. In this review, we summarize the use of chill-coma as a metric of thermal tolerance to date, and synthesise current knowledge on the nature and plasticity of lower thermal limits to present probable physiological mechanisms of cold-induced failure. Chill-coma is likely to be driven by an inability to maintain ionic homeostasis through the effects of temperature on ion-motive ATPases, ion channel gating mechanisms, and/or the lipid membrane, leading to a loss of nerve and muscle excitability.

Identification of X-linked quantitative trait loci affecting cold tolerance in Drosophila melanogaster and fine mapping by selective sweep analysis

Drosophila melanogaster is a cosmopolitan species that colonizes a great variety of environments. One trait that shows abundant evidence for naturally segregating genetic variance in different populations of D. melanogaster is cold tolerance. Previous work has found quantitative trait loci (QTL) exclusively on the second and the third chromosomes. To gain insight into the genetic architecture of cold tolerance on the X chromosome and to compare the results with our analyses of selective sweeps, a mapping population was derived from a cross between substitution lines that solely differed in the origin of their X chromosome: one originates from a European inbred line and the other one from an African inbred line. We found a total of six QTL for cold tolerance factors on the X chromosome of D. melanogaster. Although the composite interval mapping revealed slightly different QTL profiles between sexes, a coherent model suggests that most QTL overlapped between sexes, and each explained around 5-14% of the genetic variance (which may be slightly overestimated). The allelic effects were largely additive, but we also detected two significant interactions. Taken together, this provides evidence for multiple QTL that are spread along the entire X chromosome and whose effects range from low to intermediate. One detected transgressive QTL influences cold tolerance in different ways for the two sexes. While females benefit from the European allele increasing their cold tolerance, males tend to do better with the African allele. Finally, using selective sweep mapping, the candidate gene CG16700 for cold tolerance colocalizing with a QTL was identified.

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.

Genetic variation in senescence marker protein-30 is associated with natural variation in cold tolerance in Drosophila

A comprehensive understanding of the genetic basis of phenotypic adaptation in nature requires the identification of the functional allelic variation underlying adaptive phenotypes. The manner in which organisms respond to temperature extremes is an adaptation in many species. In the current study, we investigate the role of molecular variation in senescence marker protein-30 (Smp-30) on natural phenotypic variation in cold tolerance in Drosophila melanogaster. Smp-30 encodes a product that is thought to be involved in the regulation of Ca2+ ion homeostasis and has been shown previously to be differentially expressed in response to cold stress. Thus, we sought to assess whether molecular variation in Smp-30 was associated with natural phenotypic variation in cold tolerance in a panel of naturally derived inbred lines from a population in Raleigh, North Carolina. We identified four non-coding polymorphisms that were strongly associated with natural phenotypic variation in cold tolerance. Interestingly, two polymorphisms that were in close proximity to one another (2 bp apart) exhibited opposite phenotypic effects. Consistent with the maintenance of a pair of antagonistically acting polymorphisms, tests of molecular evolution identified a significant excess of maintained variation in this region, suggesting balancing selection is acting to maintain this variation. These results suggest that multiple mutations in non-coding regions can have significant effects on phenotypic variation in adaptive traits within natural populations, and that balancing selection can maintain polymorphisms with opposite effects on phenotypic variation.

Association mapping of quantitative disease resistance in a natural population of loblolly pine (Pinus taeda L.)

Genetic resistance to disease incited by necrotrophic pathogens is not well understood in plants. Whereas resistance is often quantitative, there is limited information on the genes that underpin quantitative variation in disease resistance. We used a population genomic approach to identify genes in loblolly pine (Pinus taeda) that are associated with resistance to pitch canker, a disease incited by the necrotrophic pathogen Fusarium circinatum. A set of 498 largely unrelated, clonally propagated genotypes were inoculated with F. circinatum microconidia and lesion length, a measure of disease resistance, data were collected 4, 8, and 12 weeks after inoculation. Best linear unbiased prediction was used to adjust for imbalance in number of observations and to identify highly susceptible and highly resistant genotypes ("tails"). The tails were reinoculated to validate the results of the full population screen. Significant associations were detected in 10 single nucleotide polymorphisms (SNPs) (out of 3938 tested). As hypothesized for genes involved in quantitative resistance, the 10 SNPs had small effects and proposed roles in basal resistance, direct defense, and signal transduction. We also discovered associated genes with unknown function, which would have remained undetected in a candidate gene approach constrained by annotation for disease resistance or stress response.

Temporal expression of heat shock genes during cold stress and recovery from chill coma in adult Drosophila melanogaster

A common physiological response of organisms to environmental stresses is the increase in expression of heat shock proteins (Hsps). In insects, this process has been widely examined for heat stress, but the response to cold stress has been far less studied. In the present study, we focused on 11 Drosophila melanogaster Hsp genes during the stress exposure and recovery phases. The temporal gene expression of adults was analyzed during 9 h of cold stress at 0 degrees C and during 8 h of recovery at 25 degrees C. Increased expression of some, but not all, Hsp genes was elicited in response to cold stress. The transcriptional activity of Hsp genes was not modulated during the cold stress, and peaks of expression occurred during the recovery phase. On the basis of their response, we consider that Hsp60, Hsp67Ba and Hsc70-1 are not cold-inducible, whereas Hsp22, Hsp23, Hsp26, Hsp27, Hsp40, Hsp68, Hsp70Aa and Hsp83 are induced by cold. This study suggests the importance of the recovery phase for repairing chilling injuries, and highlights the need to further investigate the contributions of specific Hsp genes to thermal stress responses. Parallels are drawn between the stress response networks resulting from heat and cold stress.

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.

Hsps and aging

Heat-shock proteins (Hsps) are increasingly being implicated in aging phenotypes and control of life span across species. They are targets of the conserved heat-shock factor and insulin/IGF1-like signaling pathways that affect life span and aging phenotypes. Hsps are expressed in tissue-specific and disease-specific patterns during aging, and their level of expression and induction by stress correlates with and, in some instances, predicts life span. In model organisms, Hsps have been shown to increase life span and ameliorate aging-associated proteotoxicity. Finally, Hsps have emerged as key components in regulating aging-related cellular phenotypes, including cell senescence, apoptosis and cancer. The Hsps, therefore, provide a metric of individual stress and aging and are potential targets for interventions in aging and aging-related diseases.

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.

QTL for the thermotolerance effect of heat hardening, knockdown resistance to heat and chill-coma recovery in an intercontinental set of recombinant inbred lines of Drosophila melanogaster

The thermotolerance effect of heat hardening (also called short-term acclimation), knockdown resistance to high temperature (KRHT) with and without heat hardening and chill-coma recovery (CCR) are important phenotypes of thermal adaptation in insects and other organisms. Drosophila melanogaster from Denmark and Australia were previously selected for low and high KRHT, respectively. These flies were crossed to construct recombinant inbred lines (RIL). KRHT was higher in heat-hardened than in nonhardened RIL. We quantify the heat-hardening effect (HHE) as the ratio in KRHT between heat-hardened and nonhardened RIL. Composite interval mapping revealed a more complex genetic architecture for KRHT without heat-hardening than for KRHT in heat-hardened insects. Five quantitative trait loci (QTL) were found for KRHT, but only two of them were significant after heat hardening. KRHT and CCR showed trade-off associations for QTL both in the middle of chromosome 2 and the right arm of chromosome 3, which should be the result of either pleiotropy or linkage. The major QTL on chromosome 2 explained 18% and 27-33% of the phenotypic variance in CCR and KRHT in nonhardened flies, respectively, but its KRHT effects decreased by heat hardening. We discuss candidate loci for each QTL. One HHE-QTL was found in the region of small heat-shock protein genes. However, HHE-QTL explained only a small fraction of the phenotypic variance. Most heat-resistance QTL did not colocalize with CCR-QTL. Large-effect QTL for CCR and KRHT without hardening (basal thermotolerance) were consistent across continents, with apparent transgressive segregation for CCR. HHE (inducible thermotolerance) was not regulated by large-effect QTL.

QTL mapping of inbreeding-related cold sensitivity and conditional lethality in Drosophila melanogaster

Inbreeding depression is a central theme within genetics, and is of specific interest for researchers within evolutionary and conservation genetics and animal and plant breeding. Inbreeding effects are thought to be caused by the joint expression of conditional and unconditional deleterious alleles. Whenever the expression of deleterious alleles is conditional, this can result in extreme environmental sensitivity in certain inbred lineages. Analysis of conditional lethal effects can reveal some of the loci that are sensitive to inbreeding. We performed a QTL (quantitative trait locus) mapping study of inbreeding-related and conditionally expressed lethality in Drosophila melanogaster. The lethal effect was triggered by exposure to a cold shock. We used a North Carolina crossing Design 3 to establish the mapping population, as well as to estimate the average dominance ratio and heritability. We found two QTL on the major autosomes carrying recessive lethals that caused male mortality, one of which also affected female mortality. More detailed study of these loci will provide information on the mechanistic basis and environmental sensitivity of inbreeding depression.

Detecting genetic responses to environmental change

Changes in environmental conditions can rapidly shift allele frequencies in populations of species with relatively short generation times. Frequency shifts might be detectable in neutral genetic markers when stressful conditions cause a population decline. However, frequency shifts that are diagnostic of specific conditions depend on isolating sets of genes that are involved in adaptive responses. Shifts at candidate loci underlying adaptive responses and DNA regions that control their expression have now been linked to evolutionary responses to pollution, global warming and other changes. Conversely, adaptive constraints, particularly in physiological traits, are recognized through DNA decay in candidate genes. These approaches help researchers and conservation managers understand the power and constraints of evolution.