Myo-inositol as a main metabolite in overwintering flies: seasonal metabolomic profiles and cold stress tolerance in a northern drosophilid fly

Metabolomic signatures associated with cold adaptation and seasonal acclimation of Drosophila: profiling of 43 species

Cold tolerance is a key determinant of poleward colonization in insects. However, the physiological basis underlying interspecific differences in cold tolerance is not fully understood. Here, we analyzed cold tolerance and metabolomic profiles in warm- and cold-acclimated phenotypes of 43 Drosophila species representing a latitudinal gradient from the tropics to the boreal zone. We found a strong positive correlation between cold tolerance and climatic variables associated with habitat seasonality and temperature. Including the effects of cold acclimation, we found most species have similar 'safety margins', measured as the difference between the average environmental temperature and the lower lethal temperature. Searching for metabolomic signatures of cold tolerance, we found that the warm-acclimated flies of cold-hardy species had moderately but significantly higher constitutive signals of putative cryoprotectants such as trehalose, glucose, glycerol and mannitol/sorbitol. Cold acclimation (and the transition to a winter dormant phenotype) resulted in a strong accumulation of myo-inositol, which occurred only in species of the virilis group. Other temperate and boreal species either showed only moderate, idiosyncratic accumulations of sugars/polyols and free amino acids, or did not accumulate any 'classical' cryoprotectant at all. Thus, our results suggest that the colonization of boreal regions by Drosophila does not necessarily depend on the seasonal accumulation of classical cryoprotectants. In contrast, virtually all cold-acclimated species showed a significant increase in products of phospholipid catabolism, suggesting that remodeling of biological membranes is a clear and ubiquitous signature of cold acclimation in Drosophila.

Brief Warm and Aldo-Keto Reductase Family AspiAKR1B1 Contribute to Cold Adaptation of Aleurocanthus spiniferus

Aleurocanthus spiniferus not only damages plant leaves directly but also causes a sooty blotch due to the honeydew secreted by the nymphs and adults. This pest is widespread and seems to be spreading from low latitude to higher latitude areas where winters are typically colder, indicating an increase in its cold tolerance. Changes in temperature help insects to anticipate the arrival of winter, allowing them to take defensive measures in advance. This study examines the impacts of brief warm pulses on the low-temperature tolerance of A. spiniferus, and analyzes the physiological and biochemical mechanisms underlying its cold adaptation, utilizing seasonal differences in cold tolerance. Intermittent training at 25 °C significantly improved the survival rate of overwintering nymphs (third and fourth instar) at -7 °C. Analysis of seasonal differences in the supercooling point (SCP) and freezing point (FP) revealed that overwintering nymph had the highest cold tolerance in November. Seasonal variation in levels of cold-resistant substances were also observed, with moisture decreasing during overwintering, while fat and glycerol levels increased. Conversely, glucose, sorbitol, and trehalose levels rose significantly at the end of the overwintering period. The expression profile of cold-resistant genes indicated that the aldo-keto reductase family 1 member B1 in Aleurocanthus spiniferus (AspiAKR1B1) shows a significant decrease at the end of the overwintering period. Knocking down AspiAKR1B1 led to a marked reduction in the cold tolerance of A. spiniferus. Therefore, brief warm pulses and AspiAKR1B1 are key factors contributing to the enhanced cold tolerance of A. spiniferus. This research provides theoretical support for preventing the further spread of A. spiniferus to higher latitudes, and offers technical guidance for developing effective pest control measures.

Molecular Mechanisms of Temperature Tolerance Plasticity in an Arthropod

How species thrive in a wide range of environments is a major focus of evolutionary biology. For many species, limited genetic diversity or gene flow among habitats means that phenotypic plasticity must play an important role in their capacity to tolerate environmental heterogeneity and to colonize new habitats. However, we have a limited understanding of the molecular components that govern plasticity in ecologically relevant phenotypes. We examined this hypothesis in a spider species (Stegodyphus dumicola) with extremely low species-wide genetic diversity that nevertheless occupies a broad range of thermal environments. We determined phenotypic responses to temperature stress in individuals from four climatic zones using common garden acclimation experiments to disentangle phenotypic plasticity from genetic adaptations. Simultaneously, we created data sets on multiple molecular modalities: the genome, the transcriptome, the methylome, the metabolome, and the bacterial microbiome to determine associations with phenotypic responses. Analyses of phenotypic and molecular associations reveal that acclimation responses in the transcriptome and metabolome correlate with patterns of phenotypic plasticity in temperature tolerance. Surprisingly, genes whose expression seemed to be involved in plasticity in temperature tolerance were generally highly methylated contradicting the idea that DNA methylation stabilizes gene expression. This suggests that the function of DNA methylation in invertebrates varies not only among species but also among genes. The bacterial microbiome was stable across the acclimation period; combined with our previous demonstrations that the microbiome is temporally stable in wild populations, this is convincing evidence that the microbiome does not facilitate plasticity in temperature tolerance. Our results suggest that population-specific variation in temperature tolerance among acclimation temperatures appears to result from the evolution of plasticity in mainly gene expression.

Differences in adult nutritional requirements impact the population growth and survival of two related species of rice leaffolders to produce interspecific differentiation

Nutrition is a limiting feature of species evolution. The differences in nutritional requirements are the evolutionary result of differential adaptations to environmental changes, explaining differences in their ecological traits. Cnaphalocrocis medinalis and Cnaphalocrocis exigua, two related species of rice leaffolders, have similar morphology and feeding properties but different migration and overwintering behaviors. However, it is unclear whether they have evolved adult nutritional differentiation traits to coexist. To explore this issue, this study examined the effects of carbohydrates and amino acids on their reproductive and demographic parameters. The findings indicate that carbohydrate intake prolonged the longevity and population growth of two rice leaffolders, but amino acid intake promoted egg hatching only. However, nutrient deficiency made it impossible for C. medinalis to reproduce successfully and survive, but it did not affect C. exigua. The population expansion and survival of migratory C. medinalis relied on adult nutritional intake. Conversely, the nutrients necessary for C. exigua overwintering activity mostly came from the storage of larvae. The difference in nutritional requirements for population growth and survival between the two rice leaffolders partially explained their differences in migration and overwintering.

Grape Seed Proanthocyanidin Extract Attenuates Cafeteria-Diet-Induced Liver Metabolic Disturbances in Rats: Influence of Photoperiod

This study investigated the influence of photoperiod (day length) on the efficacy of grape seed proanthocyanidin extract (GSPE) in mitigating metabolic disorders in obese rats fed a cafeteria diet. Rats were exposed to standard (L12), long (L18), or short (L6) photoperiods and treated with GSPE or vehicle. In the standard photoperiod, GSPE reduced body weight gain (50.5%), total cholesterol (37%), and triglycerides (34.8%), while increasing the expression of hepatic metabolic genes. In the long photoperiod, GSPE tended to decrease body weight gain, increased testosterone levels (68.3%), decreased liver weight (12.4%), and decreased reverse serum amino acids. In the short photoperiod, GSPE reduced glycemia (~10%) and lowered triglyceride levels (38.5%), with effects modified by diet. The standard photoperiod showed the greatest efficacy against metabolic syndrome-associated diseases. The study showed how day length affects GSPE's benefits and underscores considering biological rhythms in metabolic disease therapies.

Unique and generic crossed metabolism in response to four sub-lethal environmental stresses in the oriental fruit fly, Bactrocera dorsalis Hendel

Bactrocera dorsalis is a well-known invasive pest that causes considerable ecological and economic losses worldwild. Although it has a wide environmental tolerance, few studies have reported its mechanism of adaptation to multiple sub-lethal environmental stresses. In this study, 38, 41, 39 and 34 metabolites changed significantly in B. dorsalis under four sub-lethal stresses (heat, cold, desiccation and hypoxia), as found by the metabolomic method. Therein, lactic acid and pyruvic acid were induced, whereas metabolites in the tricarboxylic acid (TCA) cycle such as citric acid, α-ketoglutarate acid, malic acid and fumaric acid were reduced under at least one of the stresses. Enzyme activity and quantitative polymerase chain reaction (qPCR) analyses verified the repression of pyruvic acid proceeding into the TCA cycle. In addition, the levels of several cryoprotectants and membrane fatty acids in B. dorsalis were altered. The findings indicated that B. dorsalis has evolved shared metabolic pathways to adapt to heat, hypoxia and desiccation stresses, such as reducing energy consumption by activating the anaerobic glycolytic metabolism. Cryoprotectants and membrane fatty acids were produced to improve the efficiency of stress resistance. This study revealed the unique and generic crossed physiological mechanism of insects to adapt to various environmental stresses.

The impact of geography and climate on the population structure and local adaptation in a wild bee

Deciphering processes that contribute to genetic differentiation and divergent selection of natural populations is useful for evaluating the adaptive potential and resilience of organisms faced with various anthropogenic stressors. Insect pollinator species, including wild bees, provide critical ecosystem services but are highly susceptible to biodiversity declines. Here, we use population genomics to infer the genetic structure and test for evidence of local adaptation in an economically important native pollinator, the small carpenter bee (Ceratina calcarata). Using genome-wide SNP data (n = 8302), collected from specimens across the species' entire distribution, we evaluated population differentiation and genetic diversity and identified putative signatures of selection in the context of geographic and environmental variation. Results of the analyses of principal component and Bayesian clustering were concordant with the presence of two to three genetic clusters, associated with landscape features and inferred phylogeography of the species. All populations examined in our study demonstrated a heterozygote deficit, along with significant levels of inbreeding. We identified 250 robust outlier SNPs, corresponding to 85 annotated genes with known functional relevance to thermoregulation, photoperiod, and responses to various abiotic and biotic stressors. Taken together, these data provide evidence for local adaptation in a wild bee and highlight genetic responses of native pollinators to landscape and climate features.

Conservation Genomic Analysis of the Asian Honeybee in China Reveals Climate Factors Underlying Its Population Decline

The Asian honeybee, Apis cerana, is one of the most important native pollinators in Asia. Asian honeybees were believed to be under significant decline in China based on a report in 2005. On the contrary, a recent survey revealed that Asian honeybee populations in China are stable and even slightly increased in some regions. Therefore, the declining status of A. cerana populations in China is still unclear. Taking advantage of the abundant, publicly available genomic data for Asian honeybees in China, we employed conservation genomics methods to understand if Asian honeybee populations in China are declining and what the underlying climate factors are. We reconstructed the changes of effective population size (Ne) within the recent past for 6 population groups of Asian honeybees and found out that only one of them (population in Bomi, Tibet) showed a consistently declining Ne from the last 100 generations to 25 generations. Selective sweep analysis suggests that genes related to the tolerance of low temperatures and strong ultraviolet radiation are under selection in the declining population, indicating that these two climate factors most likely underlie the decline of BM populations during the recent past. Our study provides insights into the dynamic changes of Asian honeybee populations in China and identifies climate factors that underlie its population decline, which is valuable for the conservation of this important pollinator.

Metabolite Profiling of the Social Spider Stegodyphus dumicola Along a Climate Gradient

Animals experience climatic variation in their natural habitats, which may lead to variation in phenotypic responses among populations through local adaptation or phenotypic plasticity. In ectotherm arthropods, the expression of thermoprotective metabolites such as free amino acids, sugars, and polyols, in response to temperature stress, may facilitate temperature tolerance by regulating cellular homeostasis. If populations experience differences in temperatures, individuals may exhibit population-specific metabolite profiles through differential accumulation of metabolites that facilitate thermal tolerance. Such thermoprotective metabolites may originate from the animals themselves or from their associated microbiome, and hence microbial symbionts may contribute to shape the thermal niche of their host. The social spider Stegodyphus dumicola has extremely low genetic diversity, yet it occupies a relatively broad temperature range occurring across multiple climate zones in Southern Africa. We investigated whether the metabolome, including thermoprotective metabolites, differs between populations, and whether population genetic structure or the spider microbiome may explain potential differences. To address these questions, we assessed metabolite profiles, phylogenetic relationships, and microbiomes in three natural populations along a temperature gradient. The spider microbiomes in three genetically distinct populations of S. dumicola showed no significant population-specific pattern, and none of its dominating genera (Borrelia, Diplorickettsia, and Mycoplasma) are known to facilitate thermal tolerance in hosts. These results do not support a role of the microbiome in shaping the thermal niche of S. dumicola. Metabolite profiles of the three spider populations were significantly different. The variation was driven by multiple metabolites that can be linked to temperature stress (e.g., lactate, succinate, or xanthine) and thermal tolerance (e.g., polyols, trehalose, or glycerol): these metabolites had higher relative abundance in spiders from the hottest geographic region. These distinct metabolite profiles are consistent with a potential role of the metabolome in temperature response.

Inositol Improves Cold Tolerance Through Inhibiting CBL1 and Increasing Ca2+ Influx in Rapeseed (Brassica napus L.)

Rapeseed (Brassica napus L.) is an important oilseed crop worldwide. However, its productivity is significantly affected by various abiotic stresses, including cold stress. Among various stresses, cold stress is an important abiotic factor affecting plant growth, yield, and quality. The calcium channels are regarded as key pathways affecting cold tolerance in plants. Thus, improvement in cold tolerance is of great significance for crop improvement. The current study was designed to examine the beneficial role of exogenous inositol in improving cold stress tolerance in rapeseed. From the RNA-seq results, we identified 35 differently expressed genes encoding different inositol enzymes. The results show that inositol (a cyclic polyol) positively regulated cold tolerance by increasing calcium ion (Ca²⁺) influx in rapeseed. Furthermore, we found that the expression of calcineurin B-like (CBL1) gene was inhibited by inositol. On the other hand, overexpressed plant mediated the Ca²⁺ flux under cold stress suggesting the key role of inositol-Ca²⁺ pathway in cold tolerance. Moreover, the overexpression of BnCBL1-2 in Arabidopsis represented that transgenic plants mediated the Ca²⁺ flux highlighting the vital role of the inositol-Ca²⁺ pathway in conferring cold stress. Our study provides new insights into rapeseed cold tolerance mechanism and introduces a feasible method to improve the cold tolerance of rapeseed quickly.

Adaptation and ecological speciation in seasonally varying environments at high latitudes: Drosophila virilis group

Living in high latitudes and altitudes sets specific requirements on species’ ability to forecast seasonal changes and to respond to them in an appropriate way. Adaptation into diverse environmental conditions can also lead to ecological speciation through habitat isolation or by inducing changes in traits that influence assortative mating. In this review, we explain how the unique time-measuring systems of Drosophila virilis group species have enabled the species to occupy high latitudes and how the traits involved in species reproduction and survival exhibit strong linkage with latitudinally varying photoperiodic and climatic conditions. We also describe variation in reproductive barriers between the populations of two species with overlapping distributions and show how local adaptation and the reinforcement of prezygotic barriers have created partial reproductive isolation between conspecific populations. Finally, we consider the role of species-specific chromosomal inversions and the X chromosome in the development of reproductive barriers between diverging lineages.

Biochemical Response to Freezing in the Siberian Salamander Salamandrella keyserlingii

Simple Summary

The Siberian salamander is a unique amphibian that is capable to survive long-term freezing at −55 °C. We used ¹H-NMR analysis to study quantitative changes of multiple metabolites in liver and hindlimb muscle of the Siberian salamander in response to freezing. For the majority of molecules we observed significant changes in concentrations. Glycerol content in frozen organs was as high as 2% w/w, which confirms its role as a cryoprotectant. No other putative cryoprotectants were detected. Freezing resulted in increased concentrations of glycolysis products: lactate and alanine. Unexpectedly, we detected no increase in concentrations of succinate, which accumulates under ischemia in various tetrapods. Freezing proved to be a dramatic stress with high levels of nucleotide degradation products. There was also significant increase in the concentrations of choline and glycerophosphocholine, which may be interpreted as the degradation of biomembranes. Thus, we found that freezing results not only in macroscopical damage due to ice formation, but also to degradation of DNA and biomembranes.

Abstract

The Siberian salamander Salamandrella keyserlingii Dybowski, 1870 is a unique amphibian that is capable to survive long-term freezing at −55 °C. Nothing is known on the biochemical basis of this remarkable freezing tolerance, except for the fact that it uses glycerol as a low molecular weight cryoprotectant. We used ¹H-NMR analysis to study quantitative changes of multiple metabolites in liver and hindlimb muscle of S. keyserlingii in response to freezing. For the majority of molecules we observed significant changes in concentrations. Glycerol content in frozen organs was as high as 2% w/w, which confirms its role as a cryoprotectant. No other putative cryoprotectants were detected. Freezing resulted in ischemia manifested as increased concentrations of glycolysis products: lactate and alanine. Unexpectedly, we detected no increase in concentrations of succinate, which accumulates under ischemia in various tetrapods. Freezing proved to be a dramatic stress with reduced adenosine phosphate pool and high levels of nucleotide degradation products (hypoxanthine, β-alanine, and β-aminoisobutyrate). There was also significant increase in the concentrations of choline and glycerophosphocholine, which may be interpreted as the degradation of biomembranes. Thus, we found that freezing results not only in macroscopical damage due to ice formation, but also to degradation of DNA and biomembranes.

Effect of Defatting and Extraction Solvent on the Antioxidant and Pancreatic Lipase Inhibitory Activities of Extracts from Hermetia illucens and Tenebrio molitor

Simple Summary

The food industry is notably investing more resources on the production of nutritious, healthy, safe and sustainable products derived from edible insects. In this sense, natural extracts (or concentrated forms of compounds from natural sources) are usually food ingredients with added value for human health. This is due to their intrinsic beneficious biological activities; however, bioactive extracts from edible insects have been scarcely explored. Due to that and considering that the bioactivities of extracts might be conditioned by parameters of the technological process, we assessed how different extraction conditions, such as the defatting of the raw insect flours or the extraction solvents employed, affected two bioactivities of the resulting extracts from insects: the blocking of the digestion of fats from the diet by evaluating the inhibition of the responsible enzyme (pancreatic lipase), as well as their antioxidant activity. T. molitor and H. illucens were used, as they are two of the most known edible species for both feed and food. We observed a multibioactivity for all the extracts. Both tested processing factors differentially modulated the bioactivity of extracts from both species. We also analysed the composition of the H. illucens extracts and detected amino acids, lipids, carbohydrates, sterols and organic acids.

Abstract

The production of specific insect extracts with bioactive properties for human health is an emerging and innovative field for the edible insects industry, but there are unexplored extraction factors that might modulate the bioactivity of the extracts. Ultrasound-assisted extracts from T. molitor and H. illucens were produced. Effects of defatting pre-treatment and extraction solvent were evaluated on extraction yield, antioxidant activity and pancreatic lipase inhibitory effect. Chemical characterisation of defatted extracts from H. illucens was performed by GC-MS-FID. Non-defatted extracts showed higher extraction yields. Defatted extracts had similar extraction yields (around 3%). Defatted extracts had higher antioxidant activity, T. molitor being stronger than H. illucens. Antioxidant activity of T. molitor methanol extract was higher than the rest of solvents. Aqueous ethanol improved the antioxidant activity of H. illucens extracts. All extracts inhibited lipase, but no significant effect of defatting and solvent was observed for T. molitor. A significant higher inhibitory activity was observed for H. illucens, the strongest being defatted 100% and 70% ethanol H. illucens extracts. H. illucens extracts contained free amino acids and disaccharides, together with minor fractions of lipids, sterols and organic acids. These results evidence the potential of extracts obtained from edible insects as antioxidants and inhibitors of the pancreatic lipase, a simultaneous multibioactivity that might be favoured by the defatting pre-treatment of the samples and the solvent of extraction.

Cold adaptation drives population genomic divergence in the ecological specialist, Drosophila montana

Detecting signatures of ecological adaptation in comparative genomics is challenging, but analysing population samples with characterised geographic distributions, such as clinal variation, can help identify genes showing covariation with important ecological variation. Here, we analysed patterns of geographic variation in the cold-adapted species Drosophila montana across phenotypes, genotypes and environmental conditions and tested for signatures of cold adaptation in population genomic divergence. We first derived the climatic variables associated with the geographic distribution of 24 populations across two continents to trace the scale of environmental variation experienced by the species, and measured variation in the cold tolerance of the flies of six populations from different geographic contexts. We then performed pooled whole genome sequencing of these six populations, and used Bayesian methods to identify SNPs where genetic differentiation is associated with both climatic variables and the population phenotypic measurements, while controlling for effects of demography and population structure. The top candidate SNPs were enriched on the X and fourth chromosomes, and they also lay near genes implicated in other studies of cold tolerance and population divergence in this species and its close relatives. We conclude that ecological adaptation has contributed to the divergence of D. montana populations throughout the genome and in particular on the X and fourth chromosomes, which also showed highest interpopulation F_ST . This study demonstrates that ecological selection can drive genomic divergence at different scales, from candidate genes to chromosome-wide effects.

Multiple paths to cold tolerance: the role of environmental cues, morphological traits and the circadian clock gene vrille

Background

Tracing the association between insect cold tolerance and latitudinally and locally varying environmental conditions, as well as key morphological traits and molecular mechanisms, is essential for understanding the processes involved in adaptation. We explored these issues in two closely-related species, Drosophila montana and Drosophila flavomontana, originating from diverse climatic locations across several latitudes on the coastal and mountainous regions of North America. We also investigated the association between sequence variation in one of the key circadian clock genes, vrille, and cold tolerance in both species. Finally, we studied the impact of vrille on fly cold tolerance and cold acclimation ability by silencing it with RNA interference in D. montana.

Results

We performed a principal component analysis (PCA) on variables representing bioclimatic conditions on the study sites and used latitude as a proxy of photoperiod. PC1 separated the mountainous continental sites from the coastal ones based on temperature variability and precipitation, while PC2 arranged the sites based on summer and annual mean temperatures. Cold tolerance tests showed D. montana to be more cold-tolerant than D. flavomontana and chill coma resistance (CT_min) of this species showed an association with PC2. Chill coma recovery time (CCRT) of both species improved towards northern latitudes, and in D. flavomontana this trait was also associated with PC1. D. flavomontana flies were darkest in the coast and in the northern mountainous populations, but coloration showed no linkage with cold tolerance. Body size decreased towards cold environments in both species, but only within D. montana populations largest flies showed fastest recovery from cold. Finally, both the sequence analysis and RNAi study on vrille suggested this gene to play an essential role in D. montana cold resistance and acclimation, but not in recovery time.

Conclusions

Our study demonstrates the complexity of insect cold tolerance and emphasizes the need to trace its association with multiple environmental variables and morphological traits to identify potential agents of natural selection. It also shows that a circadian clock gene vrille is essential both for short- and long-term cold acclimation, potentially elucidating the connection between circadian clock system and cold tolerance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01849-y.

Genome Features of Asaia sp. W12 Isolated from the Mosquito Anopheles stephensi Reveal Symbiotic Traits

Asaia bacteria commonly comprise part of the microbiome of many mosquito species in the genera Anopheles and Aedes, including important vectors of infectious agents. Their close association with multiple organs and tissues of their mosquito hosts enhances the potential for paratransgenesis for the delivery of antimalaria or antivirus effectors. The molecular mechanisms involved in the interactions between Asaia and mosquito hosts, as well as Asaia and other bacterial members of the mosquito microbiome, remain underexplored. Here, we determined the genome sequence of Asaia strain W12 isolated from Anopheles stephensi mosquitoes, compared it to other Asaia species associated with plants or insects, and investigated the properties of the bacteria relevant to their symbiosis with mosquitoes. The assembled genome of strain W12 had a size of 3.94 MB, the largest among Asaia spp. studied so far. At least 3585 coding sequences were predicted. Insect-associated Asaia carried more glycoside hydrolase (GH)-encoding genes than those isolated from plants, showing their high plant biomass-degrading capacity in the insect gut. W12 had the most predicted regulatory protein components comparatively among the selected Asaia, indicating its capacity to adapt to frequent environmental changes in the mosquito gut. Two complete operons encoding cytochrome bo₃-type ubiquinol terminal oxidases (cyoABCD-1 and cyoABCD-2) were found in most Asaia genomes, possibly offering alternative terminal oxidases and allowing the flexible transition of respiratory pathways. Genes involved in the production of 2,3-butandiol and inositol have been found in Asaia sp. W12, possibly contributing to biofilm formation and stress tolerance.

Laboratory diet influences cold tolerance in a genotype-dependent manner in Drosophila melanogaster

Cold stress can reduce insect fitness and is an important determinant of species distributions and responses to climate change. Cold tolerance is influenced by genotype and environmental conditions, with factors such as day length and temperature having a particularly strong influence. Recent studies also indicate that diet impacts cold tolerance, but it is unclear whether diet-mediated shifts in cold tolerance are consistent across distinct genotypes. The goal of this study was to determine the extent to which commonly used artificial diets influence cold tolerance in Drosophila melanogaster, and whether these effects are consistent across genetically distinct lines. Specifically, we tested the impact of different fly diets on 1) ability to survive cold stress, 2) critical thermal minimum (CT_min), and 3) the ability to maintain reproduction after cold stress. Experiments were conducted across six isogenic lines from the Drosophila Genetic Reference Panel, and these lines were reared on different fly diets. Cold shock survival, CT_min, and reproductive output pre- and post-cold exposure varied considerably across diet and genotype combinations, suggesting strong genotype by environment interactions shape nutritionally mediated changes in cold tolerance. For example, in some lines cold shock survival remained consistently high or low across diets, while in others cold shock survival ranged from 5% to 75% depending on diet. Ultimately, these results add to a growing literature that cold tolerance is shaped by complex interactions between genotype and environment and inform practical considerations when selecting a laboratory diet for thermal tolerance experiments in Drosophila.

Characterization of Short-Term Heat Stress in Holstein Dairy Cows Using Altered Indicators of Metabolomics, Blood Parameters, Milk MicroRNA-216 and Characteristics

Simple Summary

In this study, we characterize the influence of short-term (4 days) heat stress on Holstein cows during early lactation. The use of indicators, such as production performance, physiological variables, blood parameters, micro-RNA expression, and metabolomes, in heat-stressed cows during early lactation—which is a high-stress phase—may provide insights into how to deal with the level of damage to dairy cows, through appropriate nutritional and management strategies. We identify that short-term heat stress has a negative effect, to some extent, on feed and water intake, rectal temperature, heart rate, blood hematology and metabolites, milk characteristics, miRNA expression in milk, and metabolomics in blood.

Abstract

This study aims to characterize the influence of short-term heat stress (HS; 4 day) in early lactating Holstein dairy cows, in terms of triggering blood metabolomics and parameters, milk yield and composition, and milk microRNA expression. Eight cows (milk yield = 30 ± 1.5 kg/day, parity = 1.09 ± 0.05) were homogeneously housed in environmentally controlled chambers, assigned into two groups with respect to the temperature humidity index (THI) at two distinct levels: approximately ~71 (low-temperature, low-humidity; LTLH) and ~86 (high-temperature, high-humidity; HTHH). Average feed intake (FI) dropped about 10 kg in the HTHH group, compared with the LTLH group (p = 0.001), whereas water intake was only numerically higher (p = 0.183) in the HTHH group than in the LTLH group. Physiological parameters, including rectal temperature (p = 0.001) and heart rate (p = 0.038), were significantly higher in the HTHH group than in the LTLH group. Plasma cortisol and haptoglobin were higher (p < 0.05) in the HTHH group, compared to the LTLH group. Milk yield, milk fat yield, 3.5% fat-corrected milk (FCM), and energy-corrected milk (ECM) were lower (p < 0.05) in the HTHH group than in the LTLH group. Higher relative expression of milk miRNA-216 was observed in the HTHH group (p < 0.05). Valine, isoleucine, methionine, phenylalanine, tyrosine, tryptophan, lactic acid, 3-phenylpropionic acid, 1,5-anhydro-D-sorbitol, myo-inositol, and urea were decreased (p < 0.05). These results suggest that early lactating cows are more vulnerable to short-term (4 day) high THI levels—that is, HTHH conditions—compared with LTLH, considering the enormous negative effects observed in measured blood metabolomics and parameters, milk yield and compositions, and milk miRNA-216 expression.

Changes in Chemical Composition and Accumulation of Cryoprotectants as the Adaptation of Anholocyclic Aphid Cinara tujafilina to Overwintering

One of the consequences of climate change is the expansion of insects' ranges. Colonization of new habitats forces insects to adapt to new conditions, such as low temperatures in winter. Cinara tujafilina is a thermophilic anholocyclic aphid species, which reproduce exclusively parthenogenetic throughout the year, including winter. On the areas where the populations of C. tujafilina had expanded, it demonstrated its adaptation for surviving colder winters. Based on analyses of changes in body chemical composition using Fourier transform infrared (FTIR) and changes in cryoprotectant content using high performance liquid chromatography (HPLC), we showed how aphid C. tujafilina adapted to overwintering as an active stage. In the FTIR spectrum of the winter type of C. tujafilina, higher peak values originating from the carbohydrates, proteins and lipids, were observed. Glucose, trehalose, mannitol, myo-inositol and glycerol were identified in the aphid body in winter as main putative cryoprotectants to increase the insects' tolerance to cold. The complex sugar-polyol cryoprotectant system facilitates aphids' survival in unfavorable low temperatures.

Freeze tolerance and the underlying metabolite responses in the Xizang plateau frog, Nanorana parkeri

The frog Nanorana parkeri (Dicroglossidae) is endemic to the Tibetan Plateau, and overwinters shallow pond within damp caves for up to 6 months. Herein, we investigate the freeze tolerance of this species and profile changes in liver and skeletal muscle metabolite levels using an untargeted LC-MS-based metabolomic approach to investigate molecular mechanisms that may contribute to freezing survival. We found that three of seven specimens of N. parkeri could survive after being frozen for 12 h at - 2.0 °C with 39.91% ± 5.4% (n = 7) of total body water converted to ice. Freezing exposure induced partial dehydration of the muscle, which contributed to decreasing the amount of freezable water within the muscle and could be protective for the myocytes themselves. A comparative metabolomic analysis showed that freezing elicited significant responses, and a total of 33 and 36 differentially expressed metabolites were identified in the liver and muscle, respectively. These metabolites mainly participate in alanine, aspartic acid and glutamic acid metabolism, arginine and proline metabolism, and D-glutamine and D-glutamate metabolism. After freezing exposure, the contents of ornithine, melezitose, and maltotriose rose significantly; these may act as cryoprotectants. Additionally, the content of 8-hydroxy-2-deoxyguanine, 7-Ketocholesterol and hypoxanthine showed a marked increase, suggesting that freezing induced oxidative stress in the frogs. In summary, N. parkeri can tolerate a brief and partial freezing of their body, which was accompanied by substantial changes in metabolomic profiles after freezing exposure.

Evidence for non-colligative function of small cryoprotectants in a freeze-tolerant insect

Freeze tolerance, the ability to survive internal ice formation, facilitates survival of some insects in cold habitats. Low-molecular-weight cryoprotectants such as sugars, polyols and amino acids are hypothesized to facilitate freeze tolerance, but their in vivo function is poorly understood. Here, we use a combination of metabolomics and manipulative experiments in vivo and ex vivo to examine the function of multiple cryoprotectants in the spring field cricket Gryllus veletis. Cold-acclimated G. veletis are freeze-tolerant and accumulate myo-inositol, proline and trehalose in their haemolymph and fat body. Injecting freeze-tolerant crickets with proline and trehalose increases survival of freezing to lower temperatures or for longer times. Similarly, exogenous myo-inositol and trehalose increase ex vivo freezing survival of fat body cells from freeze-tolerant crickets. No cryoprotectant (alone or in combination) is sufficient to confer freeze tolerance on non-acclimated, freeze-intolerant G. veletis. Given that each cryoprotectant differentially impacts survival in the frozen state, we conclude that small cryoprotectants are not interchangeable and likely function non-colligatively in insect freeze tolerance. Our study is the first to experimentally demonstrate the importance of non-colligative cryoprotectant function for insect freeze tolerance both in vivo and ex vivo, with implications for choosing new molecules for cryopreservation.

Cold acclimation triggers lipidomic and metabolic adjustments in the spotted wing drosophila Drosophila suzukii (Matsumara)

Chronic cold exposure is detrimental to chill susceptible insects that may accumulate chill injuries. To cope with deleterious effects of cold temperature, insects employ a variety of physiological strategies and metabolic adjustments, such as production of cryoprotectants, or remodeling of cellular membranes. Cold tolerance is a key element determining the fundamental niche of species. Because Drosophila suzukii is an invasive fruit pest, originating from East Asia, knowledge about its thermal biology is urgently needed. Physiological mechanisms underlying cold tolerance plasticity remain poorly understood in this species. Here, we explored metabolic and lipidomic modifications associated with the acquisition of cold tolerance in D. suzukii using Omics technologies (LC- and GC-MS/MS). In both cold-acclimated males and females, we observed physiological changes consistent with homeoviscous/homeophasic adaptation of membranes: reshuffling of phospholipid head groups and increasing unsaturation rate of fatty acids. Modification of fatty acids unsaturation were also observed in triacylglycerides, which would likely increase accessibility of lipid reserves. At the metabolic level, we observed clear-cut differentiation of metabolic profiles with cold-acclimated metabotypes showing accumulation of several potential cryoprotectants (sugars and amino acids). Metabolic pathway analyses indicated a remodeling of various processes, including purine metabolism and aminoacyl tRNA biosynthesis. These data provide a large-scale characterization of lipid rearrangements and metabolic pathway modifications in D. suzukii in response to cold acclimation and contribute to characterizing the strategies used by this species to modulate cold tolerance.

Cold acclimation triggers major transcriptional changes in Drosophila suzukii

BACKGROUND

Insects have the capacity to adjust their physiological mechanisms during their lifetime to promote cold tolerance and cope with sublethal thermal conditions, a phenomenon referred to as thermal acclimation. The spotted wing drosophila, Drosophila suzukii, is an invasive fruit pest that, like many other species, enhances its thermotolerance in response to thermal acclimation. However, little is known about the underlying mechanisms of this plastic response. Here, we promoted flies' cold tolerance by gradually increasing acclimation duration (i.e. pre-exposure from 2 h to 9 days at 10 °C), and then compared transcriptomic responses of cold hardy versus cold susceptible phenotypes using RNA sequencing.

RESULTS

Cold tolerance of D. suzukii increased with acclimation duration; the longer the acclimation, the higher the cold tolerance. Cold-tolerant flies that were acclimated for 9 days were selected for transcriptomic analyses. RNA sequencing revealed a total of 2908 differentially expressed genes: 1583 were up- and 1325 were downregulated in cold acclimated flies. Functional annotation revealed many enriched GO-terms among which ionic transport across membranes and signaling were highly represented in acclimated flies. Neuronal activity and carbohydrate metabolism were also enriched GO-terms in acclimated flies. Results also revealed many GO-terms related to oogenesis which were underrepresented in acclimated flies.

CONCLUSIONS

Involvement of a large cluster of genes related to ion transport in cold acclimated flies suggests adjustments in the capacity to maintain ion and water homeostasis. These processes are key mechanisms underlying cold tolerance in insects. Down regulation of genes related to oogenesis in cold acclimated females likely reflects that females were conditioned at 10 °C, a temperature that prevents oogenesis. Overall, these results help to understand the molecular underpinnings of cold tolerance acquisition in D. suzukii. These data are of importance considering that the invasive success of D. suzukii in diverse climatic regions relates to its high thermal plasticity.

iTRAQ-based quantitative proteomic analysis of Takifugu fasciatus liver in response to low-temperature stress

Low temperatures profoundly influence the physiological and behavioural processes of ectotherms, especially teleosts, which have made them the subjects of strong interest over time. However, the characteristics of fish cold-tolerance at the protein level remain unclear. Therefore, to shed further light on the molecular mechanisms of low temperature adaptation in fish, we conducted quantitative proteomics on the T. fasciatus liver using iTRAQ. Comparing the proteomic profiles of the T. fasciatus liver at 12 °C and 26 °C, a total of 3741 proteins were identified, and 160 were differentially abundant proteins (DAPs). Among the DAPs, the most significant changes were noted in proteins involved in oxidative stress (nine proteins), mitochondrial enzymes (eleven proteins) and signal transduction (thirteen proteins). The KEGG enrichment analysis indicated significant enhancement of D-arginine and D-ornithine metabolism, MAPK signalling, Wnt signalling and Gap junction pathway. Subsequently, three significantly up-regulated proteins (CIRB, HSP90 and GST) and two significantly down-regulated proteins (FLNB and A2ML1) were validated with parallel reaction monitoring (PRM) assays. Furthermore, the changes in abundance of proteins that are involved in oxidative stress, mitochondrial enzymes and signal transduction were validated at the transcriptional level with qPCR. These verification results show that the experimental data of iTRAQ are reliable. Our results not only deepen the understanding of the mechanisms underlying low-temperature tolerance in fish, but they also may contribute to the enhancement of cold tolerance during its breeding process. SIGNIFICANCE OF THE STUDY: The study focused on a comparative quantitative proteomics analysis of the T. fasciatus liver in response to low temperatures using iTRAQ, which has not yet been reported in the literatures. The results showed that the effect of low temperature on T. fasciatus is significant, including a detoxification of metabolic by-products and oxidative stress, an activation of the mitochondrial enzyme to strengthen energy metabolism, and a negative effect on signal transduction, which result in dysfunction or suboptimal performance. These low-temperature-related changes in the liver proteome of T. fasciatus can facilitate the understanding of the low temperature-related response that takes place in similar conditions in the liver and may contribute to the breeding of cold-resistant strains.

Physiology of crapemyrtle bark scale, Acanthococcus lagerstroemiae (Kuwana), associated with seasonally altered cold tolerance

Temperature is one of the most important abiotic factors influencing the adaptation and diversification of insects. Diverse and complex physiological mechanisms have evolved to help insects adapt to seasonal changes in temperature and prevent cold injury. Although the mechanisms of seasonal adaptation to low temperatures have been studied for insects in different taxa, none of these mechanisms have been investigated in scale insects in the superfamily Coccoidea. The crapemyrtle bark scale, Acanthococcus lagerstroemiae (Kuwana) (Hemiptera: Eriococcidae), is a newly introduced scale pest of crapemyrtles, Lagerstroemia spp. (Myrtales: Lythraceae). Our previous study concerning the cold tolerance of this pest suggested that, from summer to winter, A. lagerstroemiae seasonally adapted to lower temperature with a 5 °C reduction of supercooling points. In addition, time required to achieve the same levels of mortality at lower temperatures also increased. In this study, we used A. lagerstroemiae as a model system to investigate the physiological mechanisms correlated with changes in cold tolerance in scale insects, by measuring water content, lipid content and fatty acid composition, and cryoprotective polyols and sugars every other month. Results suggested that water content was lower in winter and early spring than in summer and early fall (40.8% vs. 63.3%). The proportions of the fatty acids in PL were similar over seasons, but in TAG, shorter chain fatty acids (from C6:0 to C10:0) increased in winter as longer chain fatty acids (from C14:0 to C18:0) decreased. Among all measured polyols and sugars, including glycerol, d-mannitol, myo-inositol, and d-trehalose, the levels of d-mannitol were the highest in January 2016, which were 19-times of those in March 2016 and 4.5-times of those in September 2016. Results from this study provide a better understanding on how A. lagerstroemiae overwinters, which may give insights into the overwintering strategies of other scale insects.

Diapause-associated changes in the lipid and metabolite profiles of the Asian tiger mosquito, Aedes albopictus

Diapause is an alternative life-history strategy that allows organisms to enter developmental arrest in anticipation of unfavorable conditions. Diapause is widespread among insects and plays a key role in enhancing overwinter survival as well as defining the seasonal and geographic distributions of populations. Next-generation sequencing has greatly advanced our understanding of the transcriptional basis for this crucial adaptation but less is known about the regulation of embryonic diapause physiology at the metabolite level. Here, we characterized the lipid and metabolite profiles of embryonic diapause in the Asian tiger mosquito, Aedes albopictus We used an untargeted approach to capture the relative abundance of 250 lipids and 241 metabolites. We observed adjustments associated with increased energy storage, including an accumulation of lipids, the formation of larger lipid droplets and increased lipogenesis, as well as metabolite shifts suggesting reduced energy utilization. We also found changes in neuroregulatory- and insulin-associated metabolites with potential roles in diapause regulation. Finally, we detected a group of unidentified, diapause-specific metabolites which have physical properties similar to those of steroids/steroid derivatives and may be associated with the ecdysteroidal regulation of embryonic diapause in A.albopictus Together, these results deepen our understanding of the metabolic regulation of embryonic diapause and identify key targets for future investigations.

Cold Acclimation Favors Metabolic Stability in Drosophila suzukii

The invasive fruit fly pest, Drosophila suzukii, is a chill susceptible species, yet it is capable of overwintering in rather cold climates, such as North America and North Europe, probably thanks to a high cold tolerance plasticity. Little is known about the mechanisms underlying cold tolerance acquisition in D. suzukii. In this study, we compared the effect of different forms of cold acclimation (at juvenile or at adult stage) on subsequent cold tolerance. Combining developmental and adult cold acclimation resulted in a particularly high expression of cold tolerance. As found in other species, we expected that cold-acclimated flies would accumulate cryoprotectants and would be able to maintain metabolic homeostasis following cold stress. We used quantitative target GC-MS profiling to explore metabolic changes in four different phenotypes: control, cold acclimated during development or at adult stage or during both phases. We also performed a time-series GC-MS analysis to monitor metabolic homeostasis status during stress and recovery. The different thermal treatments resulted in highly distinct metabolic phenotypes. Flies submitted to both developmental and adult acclimation were characterized by accumulation of cryoprotectants (carbohydrates and amino acids), although concentrations changes remained of low magnitude. After cold shock, non-acclimated chill-susceptible phenotype displayed a symptomatic loss of metabolic homeostasis, correlated with erratic changes in the amino acids pool. On the other hand, the most cold-tolerant phenotype was able to maintain metabolic homeostasis after cold stress. These results indicate that cold tolerance acquisition of D. suzukii depends on physiological strategies similar to other drosophilids: moderate changes in cryoprotective substances and metabolic robustness. In addition, the results add to the body of evidence supporting that mechanisms underlying the different forms of acclimation are distinct.

Untargeted metabolic profiling reveals distinct patterns of thermal sensitivity in two related notothenioids

Antarctic marine ectothermal animals may be affected more than temperate species by rising temperatures due to ongoing climate change. Their specialisation on stable cold temperatures makes them vulnerable to even small degrees of warming. Thus, addressing the impacts of warming on Antarctic organisms and identifying their potentially limited capacities to respond is of interest. The objective of the study was to determine changes in metabolite profiles related to temperature acclimation. In a long-term experiment adult fish of two Antarctic sister species Notothenia rossii and Notothenia coriiceps were acclimated to 0°C and 5°C for three months. Impacts and indicators of acclimation at the cellular level were determined from metabolite profiles quantified in gill tissue extracts using nuclear magnetic resonance (NMR) spectroscopy. Furthermore, the metabolite profiles of the two con-generic species were compared. NMR spectroscopy identified 37 metabolites that were present in each sample, but varied in their absolute concentration between species and between treatments. A decrease in amino acid levels indicated an increased amino acid catabolism after incubation to 5°C. In addition, long term warming initiated shifts in organic osmolyte concentrations and modified membrane structure observed by altered levels of phospholipid compounds. Differences in the metabolite profile between the two notothenioid species can be related to their divergent lifestyles, especially their different rates of motor activity. Increased levels of the Krebs cycle intermediate succinate and a higher reduction of amino acid concentrations in warm-acclimated N. rossii showed that N. rossii is more affected by warming than N. coriiceps.

Evolution of the metabolome in response to selection for increased immunity in populations of Drosophila melanogaster

We used NMR-based metabolomics to test two hypotheses–(i) there will be evolved differences in the metabolome of selected and control populations even under un-infected conditions and (ii) post infection, the metabolomes of the selected and control populations will respond differently. We selected replicate populations of Drosophila melanogaster for increased survivorship (I) against a gram-negative pathogen. We subjected the selected (I) and their control populations (S) to three different treatments: (1) infected with heat-killed bacteria (i), (2) sham infected (s), and (3) untreated (u). We performed 1D and 2D NMR experiments to identify the metabolic differences. Multivariate analysis of the metabolic profiles of the untreated (Iu and Su) flies yielded higher concentrations of lipids, organic acids, sugars, amino acids, NAD and AMP in the Iu treatment as compared to the Su treatment, showing that even in the absence of infection, the metabolome of the I and S regimes was different. In the S and I regimes, post infection/injury, concentration of metabolites directly or indirectly associated with energy related pathways (lipids, organic acids, sugars) declined while the concentration of metabolites that are probably associated with immune response (amino acids) increased. However, in most cases, the I regime flies had a higher concentration of such metabolites even under un-infected conditions. The change in the metabolite concentration upon infection/injury was not always comparable between I and S regimes (in case of lactate, alanine, leucine, lysine, threonine) indicating that the I and S regimes had evolved to respond differentially to infection and to injury.

Differences in neurochemical profiles of two gadid species under ocean warming and acidification

Background

Exposure to future ocean acidification scenarios may alter the behaviour of marine teleosts through interference with neuroreceptor functioning. So far, most studies investigated effects of ocean acidification on the behaviour of fish, either isolated or in combination with environmental temperature. However, only few physiological studies on this issue were conducted despite the putative neurophysiological origin of the CO₂-induced behavioural changes. Here, we present the metabolic consequences of long-term exposure to projected ocean acidification (396–548 μatm PCO₂ under control and 915–1272 μatm under treatment conditions) and parallel warming in the brain of two related fish species, polar cod (Boreogadus saida, exposed to 0 °C, 3 °C, 6 °C and 8 °C) and Atlantic cod (Gadus morhua, exposed to 3 °C, 8 °C, 12 °C and 16 °C). It has been shown that B. saida is behaviourally vulnerable to future ocean acidification scenarios, while G. morhua demonstrates behavioural resilience.

Results

We found that temperature alters brain osmolyte, amino acid, choline and neurotransmitter concentrations in both species indicating thermal responses particularly in osmoregulation and membrane structure. In B. saida, changes in amino acid and osmolyte metabolism at the highest temperature tested were also affected by CO₂, possibly emphasizing energetic limitations. We did not observe changes in neurotransmitters, energy metabolites, membrane components or osmolytes that might serve as a compensatory mechanism against CO₂ induced behavioural impairments. In contrast to B. saida, such temperature limitation was not detected in G. morhua; however, at 8 °C, CO₂ induced an increase in the levels of metabolites of the glutamate/GABA-glutamine cycle potentially indicating greater GABAergic activity in G.morhua. Further, increased availability of energy-rich substrates was detected under these conditions.

Conclusions

Our results indicate a change of GABAergic metabolism in the nervous system of Gadus morhua close to the optimum of the temperature range. Since a former study showed that juvenile G. morhua might be slightly more behaviourally resilient to CO₂ at this respective temperature, we conclude that the observed change of GABAergic metabolism could be involved in counteracting OA induced behavioural changes. This may serve as a fitness advantage of this respective species compared to B. saida in a future warmer, more acidified polar ocean.

Electronic supplementary material

The online version of this article (10.1186/s12983-017-0238-5) contains supplementary material, which is available to authorized users.

Metabolomic Studies in Drosophila

Metabolomic analysis provides a powerful new tool for studies of Drosophila physiology. This approach allows investigators to detect thousands of chemical compounds in a single sample, representing the combined contributions of gene expression, enzyme activity, and environmental context. Metabolomics has been used for a wide range of studies in Drosophila, often providing new insights into gene function and metabolic state that could not be obtained using any other approach. In this review, we survey the uses of metabolomic analysis since its entry into the field. We also cover the major methods used for metabolomic studies in Drosophila and highlight new directions for future research.

Comparative metabolomics analysis of Callosobruchus chinensis larvae under hypoxia, hypoxia/hypercapnia and normoxia

BACKGROUND

Insect tolerance to low oxygen (hypoxia) and high carbon dioxide (hypercapnia) is critical for insect control. On the basis of bioassay, metabolism profiles were built to investigate adaptive mechanisms in bean weevil under hypoxia (2% O₂ ), hypoxia/hypercapnia (2% O₂ + 18% CO₂ ) and normoxia (control, 20% O₂ + 80% N₂ ) using gas chromatography/time-of-flight mass spectrometry (GC/TOF-MS).

RESULTS

The growth and development of bean weevils were significantly suppressed by the two hypoxia conditions; hypercapnia enhanced the mortality, but after 24 days of exposure, the surviving insects emerged as adults earlier than those under hypoxia only. Metabolism profiles also showed striking differences in metabolites among the treatment and control groups, both quantitatively and qualitatively. Pairwise comparisons of the three groups showed that 61 metabolites changed significantly, 40 in the hypoxia group and 37 in the hypoxia/hypercapnia group relative to the control group, while only 16 were shared equally by the hypoxia and hypoxia/hypercapnia groups. Increased metabolites were mainly carbohydrates, amino acids and organic acids, while free fatty acids were decreased. Furthermore, the changes were strengthened by the addition of hypercapnia, but excluding free fatty acids.

CONCLUSION

The findings show that bean weevil has high tolerance to hypoxia or even hypoxia/hypercapnia at biologically achievable levels and provide more direct evidence for stored product insect mechanism regulation under hypoxia stress, especially free fatty acid regulation by hypercapnia but not by hypoxia. © 2016 Society of Chemical Industry.

Metabolic and functional characterization of effects of developmental temperature in Drosophila melanogaster

The ability of ectotherms to respond to changes in their thermal environment through plastic mechanisms is central to their adaptive capability. However, we still lack knowledge on the physiological and functional responses by which ectotherms acclimate to temperatures during development, and in particular, how physiological stress at extreme temperatures may counteract beneficial acclimation responses at benign temperatures. We exposed Drosophila melanogaster to 10 developmental temperatures covering their entire permissible temperature range. We obtained metabolic profiles and reaction norms for several functional traits: egg-to-adult viability, developmental time, and heat and cold tolerance. Females were more heat tolerant than males, whereas no sexual dimorphism was found in cold tolerance. A group of metabolites, mainly free amino acids, had linear reaction norms. Several energy-carrying molecules, as well as some sugars, showed distinct inverted U-shaped norms of reaction across the thermal range, resulting in a positive correlation between metabolite intensities and egg-to-adult viability. At extreme temperatures, low levels of these metabolites were interpreted as a response characteristic of costs of homeostatic perturbations. Our results provide novel insights into a range of metabolites reported to be central for the acclimation response and suggest several new candidate metabolites. Low and high temperatures result in different adaptive physiological responses, but they also have commonalities likely to be a result of the failure to compensate for the physiological stress. We suggest that the regulation of metabolites that are tightly connected to the performance curve is important for the ability of ectotherms to cope with variation in temperature.

Chronic dietary salt stress mitigates hyperkalemia and facilitates chill coma recovery in Drosophila melanogaster

Chill susceptible insects like Drosophila lose the ability to regulate water and ion homeostasis at low temperatures. This loss of hemolymph ion and water balance drives a hyperkalemic state that depolarizes cells, causing cellular injury and death. The ability to maintain ion homeostasis at low temperatures and/or recover ion homeostasis upon rewarming is closely related to insect cold tolerance. We thus hypothesized that changes to organismal ion balance, which can be achieved in Drosophila through dietary salt loading, could alter whole animal cold tolerance phenotypes. We put Drosophila melanogaster in the presence of diets highly enriched in NaCl, KCl, xylitol (an osmotic control) or sucrose (a dietary supplement known to impact cold tolerance) for 24h and confirmed that they consumed the novel food. Independently of their osmotic effects, NaCl, KCl, and sucrose supplementation all improved the ability of flies to maintain K⁺ balance in the cold, which allowed for faster recovery from chill coma after 6h at 0°C. These supplements, however, also slightly increased the CT_min and had little impact on survival rates following chronic cold stress (24h at 0°C), suggesting that the effect of diet on cold tolerance depends on the measure of cold tolerance assessed. In contrast to prolonged salt stress, brief feeding (1.5h) on diets high in salt slowed coma recovery, suggesting that the long-term effects of NaCl and KCl on chilling tolerance result from phenotypic plasticity, induced in response to a salty diet, rather than simply the presence of the diet in the gut lumen.

Inducing Cold-Sensitivity in the Frigophilic Fly Drosophila montana by RNAi

Cold acclimation is a critical physiological adaptation for coping with seasonal cold. By increasing their cold tolerance individuals can remain active for longer at the onset of winter and can recover more quickly from a cold shock. In insects, despite many physiological studies, little is known about the genetic basis of cold acclimation. Recently, transcriptomic analyses in Drosophila virilis and D. montana revealed candidate genes for cold acclimation by identifying genes upregulated during exposure to cold. Here, we test the role of myo-inositol-1-phosphate synthase (Inos), in cold tolerance in D. montana using an RNAi approach. D. montana has a circumpolar distribution and overwinters as an adult in northern latitudes with extreme cold. We assessed cold tolerance of dsRNA knock-down flies using two metrics: chill-coma recovery time (CCRT) and mortality rate after cold acclimation. Injection of dsRNAInos did not alter CCRT, either overall or in interaction with the cold treatment, however it did induced cold-specific mortality, with high levels of mortality observed in injected flies acclimated at 5°C but not at 19°C. Overall, injection with dsRNAInos induced a temperature-sensitive mortality rate of over 60% in this normally cold-tolerant species. qPCR analysis confirmed that dsRNA injection successfully reduced gene expression of Inos. Thus, our results demonstrate the involvement of Inos in increasing cold tolerance in D. montana. The potential mechanisms involved by which Inos increases cold tolerance are also discussed.

Comparison of thermal traits of Polistes dominula and Polistes gallicus, two European paper wasps with strongly differing distribution ranges

The two paper wasps, Polistes dominula and Polistes gallicus, are related species with strongly differing distribution ranges. We investigated thermal tolerance traits (critical thermal limits and metabolic response to temperature) to gain knowledge about physiological adaptations to their local climate conditions and to get evidence for the reasons of P. dominula’s successful dispersion. Body and ambient temperature measurements at the nests revealed behavioural adaptations to microclimate. The species differed clearly in critical thermal minimum (P. dominula −1.4 °C, P. gallicus −0.4 °C), but not significantly in critical thermal maximum of activity (P. dominula 47.1 °C, P. gallicus 47.6 °C). The metabolic response did not reveal clear adaptations to climate conditions. At low and high temperatures, the metabolic rate of P. dominula was higher, and at intermediate temperatures, we determined higher values in P. gallicus. However, the species exhibited remarkably differing thermoregulatory behaviour at the nest. On average, P. gallicus tolerated a thoracic temperature up to ~41 °C, whereas P. dominula already tried at ~37 °C to keep the thorax below ambient temperature. We suggest this to be an adaptation to the higher mean ambient temperature we measured at the nest during a breeding season. Although we determined for P. dominula a 0.5 °C larger thermal tolerance range, we do not presume this parameter to be solely responsible for the successful distribution of P. dominula. Additional factors, such as the thermal tolerance of the queens could limit the overwintering success of P. gallicus in a harsher climate.

Transcriptional Differences between Diapausing and Non-Diapausing D. montana Females Reared under the Same Photoperiod and Temperature

BACKGROUND

A wide range of insects living at higher latitudes enter diapause at the end of the warm season, which increases their chances of survival through harsh winter conditions. In this study we used RNA sequencing to identify genes involved in adult reproductive diapause in a northern fly species, Drosophila montana. Both diapausing and non-diapausing flies were reared under a critical day length and temperature, where about half of the emerging females enter diapause enabling us to eliminate the effects of varying environmental conditions on gene expression patterns of the two types of female flies.

RESULTS

RNA sequencing revealed large differences between gene expression patterns of diapausing and non-diapausing females, especially in genes involved with metabolism, fatty acid biosynthesis, and metal and nucleotide binding. Differently expressed genes included several gene groups, including myosin, actin and cytochromeP450 genes, which have been previously associated with diapause. This study also identified new candidate genes, including some involved in cuticular hydrocarbon synthesis or regulation (desat1 and desat2), and acyl-CoA Δ11-desaturase activity (CG9747), and few odorant-binding protein genes (e.g. Obp44A). Also, several transposable elements (TEs) showed differential expression between the two female groups motivating future research on their roles in diapause.

CONCLUSIONS

Our results demonstrate that the adult reproductive diapause in D. montana involves changes in the expression level of a variety of genes involved in key processes (e.g. metabolism and fatty acid biosynthesis) which help diapausing females to cope with overwintering. This is consistent with the view that diapause is a complex adaptive phenotype where not only sexual maturation is arrested, but also changes in adult physiology are required in order to survive over the winter.

Contribution of Drosophila TRPA1 to Metabolism

Transient receptor potential (TRP) cation channels are highly conserved in humans and insects. Some of these channels are expressed in internal organs and their functions remain incompletely understood. By direct knock-in of the GAL4 gene into the trpA1 locus in Drosophila, we identified the expression of this gene in the subesophageal ganglion (SOGs) region. In addition, the neurites present in the dorsal posterior region as well as the drosophila insulin-like peptide 2 (dILP2)-positive neurons send signals to the SOGs. The signal is sent to the crop, which is an enlarged organ of the esophagus and functions as a storage place for food in the digestive system. To systematically investigate the role of TRPA1 in metabolism, we applied non-targeted metabolite profiling analysis together with gas-chromatography/time-of-flight mass spectrometry, with an aim to identify a wide range of primary metabolites. We effectively captured distinctive metabolomic phenotypes and identified specific metabolic dysregulation triggered by TRPA1 mutation based on reconstructed metabolic network analysis. Primarily, the network analysis pinpointed the simultaneous down-regulation of intermediates in the methionine salvation pathway, in contrast to the synchronized up-regulation of a range of free fatty acids. The gene dosage-dependent dynamics of metabolite levels among wild-type, hetero- and homozygous mutants, and their coordinated metabolic modulation under multiple gene settings across five different genotypes confirmed the direct linkages of TRPA1 to metabolism.

Seasonal cues induce phenotypic plasticity of Drosophila suzukii to enhance winter survival

BACKGROUND

As global climate change and exponential human population growth intensifies pressure on agricultural systems, the need to effectively manage invasive insect pests is becoming increasingly important to global food security. Drosophila suzukii is an invasive pest that drastically expanded its global range in a very short time since 2008, spreading to most areas in North America and many countries in Europe and South America. Preliminary ecological modeling predicted a more restricted distribution and, for this reason, the invasion of D. suzukii to northern temperate regions is especially unexpected. Investigating D. suzukii phenology and seasonal adaptations can lead to a better understanding of the mechanisms through which insects express phenotypic plasticity, which likely enables invasive species to successfully colonize a wide range of environments.

RESULTS

We describe seasonal phenotypic plasticity in field populations of D. suzukii. Specifically, we observed a trend of higher proportions of flies with the winter morph phenotype, characterized by darker pigmentation and longer wing length, as summer progresses to winter. A laboratory-simulated winter photoperiod and temperature (12:12 L:D and 10 °C) were sufficient to induce the winter morph phenotype in D. suzukii. This winter morph is associated with increased survival at 1 °C when compared to the summer morph, thus explaining the ability of D. suzukii to survive cold winters. We then used RNA sequencing to identify gene expression differences underlying seasonal differences in D. suzukii physiology. Winter morph gene expression is consistent with known mechanisms of cold-hardening such as adjustments to ion transport and up-regulation of carbohydrate metabolism. In addition, transcripts involved in oogenesis and DNA replication were down-regulated in the winter morph, providing the first molecular evidence of a reproductive diapause in D. suzukii.

CONCLUSIONS

To date, D. suzukii cold resistance studies suggest that this species cannot overwinter in northern locations, e.g. Canada, even though they are established pests in these regions. Combining physiological investigations with RNA sequencing, we present potential mechanisms by which D. suzukii can overwinter in these regions. This work may contribute to more accurate population models that incorporate seasonal variation in physiological parameters, leading to development of better management strategies.

Hemolymph metabolites and osmolality are tightly linked to cold tolerance of Drosophila species: a comparative study

Drosophila, like most insects, are susceptible to low temperatures, and will succumb to temperatures above the freezing point of their hemolymph. For these insects, cold exposure causes a loss of extracellular ion and water homeostasis, leading to chill injury and eventually death. Chill tolerant species are characterized by lower hemolymph [Na+] than chill susceptible species and this lowered hemolymph [Na+] is suggested to improve ion and water homeostasis during cold exposure. It has therefore also been hypothesized that hemolymph Na+ is replaced by other “cryoprotective” osmolytes in cold tolerant species. Here, we compare the hemolymph metabolite profiles of five drosophilid species with marked difference in chill tolerance. All species were examined under “normal” thermal conditions (i.e. 20°C) and following cold exposure (4 hours at 0°C). Under benign conditions total hemolymph osmolality was similar among all species despite chill tolerant species having lower hemolymph [Na+]. Using NMR spectroscopy we found that chill tolerant species instead have higher levels of sugars and free amino acids in their hemolymph, including classical “cryoprotectants” such as trehalose and proline. In addition, we found that chill tolerant species maintain a relatively stable hemolymph osmolality and metabolite profile when exposed to cold stress while sensitive species suffer from large increases in osmolality and massive changes in their metabolic profiles during a cold stress. We suggest that the larger contribution of classical “cryoprotectants” in chill tolerant Drosophila play a non-colligative role for cold tolerance that contributes to osmotic and ion homeostasis during cold exposures and in addition we discuss how these comparative differences may represent an evolutionary pathway toward more extreme cold tolerance of insects.

Seasonal gene expression kinetics between diapause phases in Drosophila virilis group species and overwintering differences between diapausing and non-diapausing females

Most northern insect species experience a period of developmental arrest, diapause, which enables them to survive over the winter and postpone reproduction until favorable conditions. We studied the timing of reproductive diapause and its long-term effects on the cold tolerance of Drosophila montana, D. littoralis and D. ezoana females in seasonally varying environmental conditions. At the same time we traced expression levels of 219 genes in D. montana using a custom-made microarray. We show that the seasonal switch to reproductive diapause occurs over a short time period, and that overwintering in reproductive diapause has long-lasting effects on cold tolerance. Some genes, such as Hsc70, Jon25Bi and period, were upregulated throughout the diapause, while others, including regucalcin, couch potato and Thor, were upregulated only at its specific phases. Some of the expression patterns induced during the sensitive stage, when the females either enter diapause or not, remained induced regardless of the later conditions. qPCR analyses confirmed the findings of the microarray analysis in D. montana and revealed similar gene expression changes in D. littoralis and D. ezoana. The present study helps to achieve a better understanding of the genetic regulation of diapause and of the plasticity of seasonal responses in general.

Entometabolomics: applications of modern analytical techniques to insect studies

Metabolomic analyses can reveal associations between an organism's metabolome and further aspects of its phenotypic state, an attractive prospect for many life-sciences researchers. The metabolomic approach has been employed in some, but not many, insect study systems, starting in 1990 with the evaluation of the metabolic effects of parasitism on moth larvae. Metabolomics has now been applied to a variety of aspects of insect biology, including behaviour, infection, temperature stress responses, CO 2 sedation, and bacteria-insect symbiosis. From a technical and reporting standpoint, these studies have adopted a range of approaches utilising established experimental methodologies. Here, we review current literature and evaluate the metabolomic approaches typically utilised by entomologists. We suggest that improvements can be made in several areas, including sampling procedures, the reduction in sampling and equipment variation, the use of sample extracts, statistical analyses, confirmation, and metabolite identification. Overall, it is clear that metabolomics can identify correlations between phenotypic states and underlying cellular metabolism that previous, more targeted, approaches are incapable of measuring. The unique combination of untargeted global analyses with high-resolution quantitative analyses results in a tool with great potential for future entomological investigations.

Comparative ecophysiology of cold-tolerance-related traits: assessing range expansion potential for an invasive insect at high latitude

Survival at high latitude requires the capability to cope with seasonally imposed stress, such as low winter temperatures or large temperature fluctuations. The Colorado potato beetle, Leptinotarsa decemlineata, is an invasive pest of potato that has rapidly spread from low latitudes to higher latitudes. During the last 30 years, a decrease in range expansion speed is apparent in Europe. We use a comparative approach to assess whether this could be due to an inability of L. decemlineata to cope with the harsher winters encountered at high latitude, when compared to two native northern chrysomelid beetles with similar overwintering ecology. We investigated several cold-tolerance-related physiological traits at different time points during winter. Cold tolerance followed a latitudinal pattern; the northern species were more tolerant to short-term subzero temperatures than the invasive L. decemlineata. The other northern species, the knotgrass leaf beetle, Chrysolina polita, was found to tolerate internal freezing. Interestingly, the pattern for overwinter survival at 5°C was the opposite and higher in L. decemlineata than the northern species and could be related to behavioral differences between species in overwintering location selection and a potential physiological trade-off between tolerance to cold shock and to chronic cold exposure. Furthermore, while the northern species accumulated large amounts of different sugars and polyols with probable cryoprotectant functions, none were detected in L. decemlineata at high concentrations. This lack of cryoprotectant accumulation could explain the difference in cold tolerance between the species and also suggests that a lack of physiological capacity to tolerate low temperatures could slow further latitudinal range expansion of L. decemlineata.

How consistent are the transcriptome changes associated with cold acclimation in two species of the Drosophila virilis group?

For many organisms the ability to cold acclimate with the onset of seasonal cold has major implications for their fitness. In insects, where this ability is widespread, the physiological changes associated with increased cold tolerance have been well studied. Despite this, little work has been done to trace changes in gene expression during cold acclimation that lead to an increase in cold tolerance. We used an RNA-Seq approach to investigate this in two species of the Drosophila virilis group. We found that the majority of genes that are differentially expressed during cold acclimation differ between the two species. Despite this, the biological processes associated with the differentially expressed genes were broadly similar in the two species. These included: metabolism, cell membrane composition, and circadian rhythms, which are largely consistent with previous work on cold acclimation/cold tolerance. In addition, we also found evidence of the involvement of the rhodopsin pathway in cold acclimation, a pathway that has been recently linked to thermotaxis. Interestingly, we found no evidence of differential expression of stress genes implying that long-term cold acclimation and short-term stress response may have a different physiological basis.

Invariance and plasticity in the Drosophila melanogaster metabolomic network in response to temperature

Background

Metabolomic responses to extreme thermal stress have recently been investigated in Drosophila melanogaster. However, a network level understanding of metabolomic responses to longer and less drastic temperature changes, which more closely reflect variation in natural ambient temperatures experienced during development and adulthood, is currently lacking. Here we use high-resolution, non-targeted metabolomics to dissect metabolomic changes in D. melanogaster elicited by moderately cool (18°C) or warm (27°C) developmental and adult temperature exposures.

Results

We find that temperature at which larvae are reared has a dramatic effect on metabolomic network structure measured in adults. Using network analysis, we are able to identify modules that are highly differentially expressed in response to changing developmental temperature, as well as modules whose correlation structure is strongly preserved across temperature.

Conclusions

Our results suggest that the effect of temperature on the metabolome provides an easily studied and powerful model for understanding the forces that influence invariance and plasticity in biological networks.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-014-0139-6) contains supplementary material, which is available to authorized users.

Application of functional 'Omics' in environmental stress physiology: insights, limitations, and future challenges

Omic technologies have revolutionised how environmental physiologists investigate stress response pathways. To date, however, omic screens typically constitute simple presence/absence correlations, and fall short of explaining mechanism. Disentangling function necessitates hypothesis-driven manipulation of selected molecular signals, and a systems level view will only come from more detailed tissue-specific and time series sampling. The increasing accessibility of omic applications means that species can be selected based on Krogh principles, but focus also needs to be given to core models where multi-platform approaches can be combined to provide a deeper understanding. This review highlights recent technological and intellectual advances in the application of omics to understanding insect stress adaptation, and sets out how to address remaining knowledge gaps.

Molecular basis of chill resistance adaptations in poikilothermic animals

Chill and freeze represent very different components of low temperature stress. Whilst the principal mechanisms of tissue damage and of acquired protection from freeze-induced effects are reasonably well established, those for chill damage and protection are not. Non-freeze cold exposure (i.e. chill) can lead to serious disruption to normal life processes, including disruption to energy metabolism, loss of membrane perm-selectivity and collapse of ion gradients, as well as loss of neuromuscular coordination. If the primary lesions are not relieved then the progressive functional debilitation can lead to death. Thus, identifying the underpinning molecular lesions can point to the means of building resistance to subsequent chill exposures. Researchers have focused on four specific lesions: (i) failure of neuromuscular coordination, (ii) perturbation of bio-membrane structure and adaptations due to altered lipid composition, (iii) protein unfolding, which might be mitigated by the induced expression of compatible osmolytes acting as 'chemical chaperones', (iv) or the induced expression of protein chaperones along with the suppression of general protein synthesis. Progress in all these potential mechanisms has been ongoing but not substantial, due in part to an over-reliance on straightforward correlative approaches. Also, few studies have intervened by adoption of single gene ablation, which provides much more direct and compelling evidence for the role of specific genes, and thus processes, in adaptive phenotypes. Another difficulty is the existence of multiple mechanisms, which often act together, thus resulting in compensatory responses to gene manipulations, which may potentially mask disruptive effects on the chill tolerance phenotype. Consequently, there is little direct evidence of the underpinning regulatory mechanisms leading to induced resistance to chill injury. Here, we review recent advances mainly in lower vertebrates and in arthropods, but increasingly in genetic model species from a broader range of taxa.