Evolution of water balance in the genusDrosophila

Variability and General Trends in the Geographic Distribution and Climatic Niche of Endemic and Cosmopolitan Drosophilidae Species in Subtropical Regions of the Neotropics

The Neotropical region is a vast and heterogeneous ecozone harboring diverse Drosophilidae Fallén 1823 species. However, these species' distribution patterns and climatic requirements are poorly understood. In this study, we aimed to estimate differences in the climatic niche and distribution patterns among species to test the hypothesis that endemic and cosmopolitan species occurring in the Neotropics present different climatic niches, such that distribution range and niche breadth are highly correlated among species. For this task, we evaluated the geographic distributions and the climatic niches of 47 endemic and cosmopolitan drosophilids occurring in subtropical regions of the Neotropics using raw climatic data from collection records and environmental niche models (ENMs). We showed that the studied species varied in two highly correlated properties: the distribution ranges and the environmental niche breadth. Moreover, significant differences were observed between endemic and cosmopolitan drosophilids in terms of variable importance and climatic niches. Most of the studied species are distributed in regions under mild climatic conditions, but there are many species inhabiting harsher environments. Generally, the results suggest that several Neotropical drosophilid species may be highly vulnerable to global warming, potentially serving as bioindicator species for assessing the impact of climate change.

Differential water deprivation tolerances of adult Rhagoletis indifferens and Rhagoletis pomonella (Diptera: Tephritidae) as a possible factor affecting their distributional abundances in Washington State, USA

Insects that evolved in mesic regions may have difficulty establishing in xeric regions. Rhagoletis pomonella (Walsh) (Diptera: Tephritidae) was introduced into drier western North America from mesic eastern North America while Rhagoletis indifferens Curran is native to western North America. Here, we predicted that R. indifferens survives water deprivation longer than R. pomonella, as R. indifferens is more abundant than R. pomonella in dry central Washington (WA) State, USA. Sweet and bitter cherry-origin R. indifferens and apple- and hawthorn-origin R. pomonella from xeric central or mesic western WA were provided water throughout or were water-deprived at 2-4 and 14-18 d old and held at 20°C or 30/31°C and daily survival recorded. At 20°C and 30°C, western WA apple-origin R. pomonella provided water survived longer than sweet cherry-origin R. indifferens. When water-deprived, however, 2-4 d old R. indifferens, although smaller, survived significantly longer than western WA apple-origin R. pomonella of the same age. This was also generally true for 14-18 d old flies, although differences were less often significant. Central WA large-thorn hawthorn-origin R. pomonella survived water deprivation significantly longer than western WA apple-origin R. pomonella, and as long as R. indifferens. Water-deprived flies of both species survived longer at 20°C than 30/31°C. Survival analyses suggest that low water availability rather than high temperature contributes to lower R. pomonella than R. indifferens abundances in central WA, with R. pomonella populations in that region differing from western WA R. pomonella with respect to tolerance of xeric climates.

Cuticular hydrocarbons promote desiccation resistance by preventing transpiration in Drosophila melanogaster

Desiccation is a fundamental challenge confronted by all terrestrial organisms, particularly insects. With a relatively small body size and large surface-to-volume ratio, insects are susceptible to rapid evaporative water loss and dehydration. To counter these physical constraints, insects have acquired specialized adaptations, including a hydrophobic cuticle that acts as a physical barrier to transpiration. We previously reported that genetic ablation of the oenocytes - specialized cells required to produce cuticular hydrocarbons (HCs) - significantly reduced survivorship under desiccative conditions in the fruit fly, Drosophila melanogaster. Although increased transpiration - resulting from the loss of the oenocytes and HCs - was hypothesized to be responsible for the decrease in desiccation survival, this possibility was not directly tested. Here, we investigated the underlying physiological mechanisms contributing to the reduced survival of oenocyte-less (oe-) flies. Using flow-through respirometry, we show that oe- flies, regardless of sex, exhibited an increased rate of transpiration relative to wild-type controls, and that coating oe- flies with fly-derived HC extract restored the rate to near-wild-type levels. Importantly, total body water stores, including metabolic water reserves, as well as dehydration tolerance, measured as the percentage of total body water lost at the time of death, were largely unchanged in oe- flies. Together, our results directly demonstrate the critically important role played by the oenocytes and cuticular HCs to promote desiccation resistance.

Humidity and temperature preference in two Neotropical species of sand flies

BACKGROUND

Arthropods vector a multitude of human disease-causing organisms, and their geographic ranges are shifting rapidly in response to changing climatic conditions. This is, in turn, altering the landscape of disease risk for human populations that are brought into novel contact with the vectors and the diseases they carry. Sand flies in the genera Lutzomyia and Pintomyia are vectors of serious disease-causing agents such as Leishmania (the etiological agent of leishmaniasis) and may be expanding their range in the face of climate change. Understanding the climatic conditions that vector species both tolerate physiologically and prefer behaviorally is critical to predicting the direction and magnitude of range expansions and the resulting impacts on human health. Temperature and humidity are key factors that determine the geographic extent of many arthropods, including vector species.

METHODS

We characterized the habitat of two species of sand flies, Lutzomyia longipalpis and Pintomyia evansi. Additionally, we studied two behavioral factors of thermal fitness-thermal and humidity preference in two species of sand flies alongside a key aspect of physiological tolerance-desiccation resistance.

RESULTS

We found that Lu. longipalpis is found at cooler and drier conditions than Pi. evansi. Our results also show significant interspecific differences in both behavioral traits, with Pi. evansi preferring warmer, more humid conditions than Lu. longipalpis. Finally, we found that Lu. longipalpis shows greater tolerance to extreme low humidity, and that this is especially pronounced in males of the species.

CONCLUSIONS

Taken together, our results suggest that temperature and humidity conditions are key aspects of the climatic niche of Lutzomyia and Pintomyia sand flies and underscore the value of integrative studies of climatic tolerance and preference in vector biology.

Genetic basis and repeatability for desiccation resistance in Drosophila melanogaster (Diptera: Drosophilidae)

Dehydration is a stress factor for organisms inhabiting natural habitats where water is scarce. Thus, it may be expected that species facing arid environments will develop mechanisms that maximize resistance to desiccation. Insects are excellent models for studying the effects of dehydration as well as the mechanisms and processes that prevent water loss since the effect of desiccation is greater due to the higher area/volume ratio than larger animals. Even though physiological and behavioral mechanisms to cope with desiccation are being understood, the genetic basis underlying the mechanisms related to variation in desiccation resistance and the context-dependent effect remain unsolved. Here we analyze the genetic bases of desiccation resistance in Drosophila melanogaster and identify candidate genes that underlie trait variation. Our quantitative genetic analysis of desiccation resistance revealed sexual dimorphism and extensive genetic variation. The phenotype-genotype association analyses (GWAS) identified 71 candidate genes responsible for total phenotypic variation in desiccation resistance. Half of these candidate genes were sex-specific suggesting that the genetic architecture underlying this adaptive trait differs between males and females. Moreover, the public availability of desiccation data analyzed on the same lines but in a different lab allows us to investigate the reliability and repeatability of results obtained in independent screens. Our survey indicates a pervasive micro-environment lab-dependent effect since we did not detect overlap in the sets of genes affecting desiccation resistance identified between labs.

Raspberry ketone feeding makes Queensland fruit fly, Bactrocera tryoni (Froggatt), more vulnerable to desiccation but not starvation

BACKGROUND

Queensland fruit fly (Qfly) males exhibit accelerated sexual maturation when their diet is supplemented with raspberry ketone (RK) for 48 h following emergence, which is beneficial for sterile insect technique operation. The present study tests whether RK supplementation makes Qfly more vulnerable to starvation or desiccation.

RESULTS

Flies were fed for 48 h with a yeast hydrolysate and sugar diet (1:3) that contained 0% RK (control), 1.25% RK (low dose) or 5% RK (high dose) to test subsequent vulnerability to starvation and desiccation. RK feeding decreased body weight and water content in males and increased lipid levels in both sexes before exposure to any stress treatment. Under nutritional stress, flies fed the low RK dose, but not the high RK dose, had higher survival than controls. Under desiccation stress, flies fed both the low and high RK doses had lower survival than the controls. Body weight, water content and lipid reserves at death were all affected by RK dose when under nutritional stress, but not when under desiccation stress. In the absence of stress, body weight at death was higher than controls in flies provided with the high RK dose and lipids were lower than controls in flies provided with the low RK dose.

CONCLUSION

Feeding with RK makes Qflies more vulnerable to desiccation but not starvation. In most conditions, it is expected that the disadvantage of increased desiccation vulnerability would be outweighed by the benefits of accelerated sexual maturation in RK-fed young adult Qflies. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Evolution of a fatty acyl-CoA elongase underlies desert adaptation in Drosophila

Traits that allow species to survive in extreme environments such as hot-arid deserts have independently evolved in multiple taxa. However, the genetic and evolutionary mechanisms underlying these traits have thus far not been elucidated. Here, we show that Drosophila mojavensis, a desert-adapted fruit fly species, has evolved high desiccation resistance by producing long-chain methyl-branched cuticular hydrocarbons (mbCHCs) that contribute to a cuticular lipid layer reducing water loss. We show that the ability to synthesize these longer mbCHCs is due to evolutionary changes in a fatty acyl-CoA elongase (mElo). mElo knockout in D. mojavensis led to loss of longer mbCHCs and reduction of desiccation resistance at high temperatures but did not affect mortality at either high temperatures or desiccating conditions individually. Phylogenetic analysis showed that mElo is a Drosophila-specific gene, suggesting that while the physiological mechanisms underlying desert adaptation may be similar between species, the genes involved in these mechanisms may be species or lineage specific.

Gene expression differences consistent with water loss reduction underlie desiccation tolerance of natural Drosophila populations

BACKGROUND

Climate change is one of the main factors shaping the distribution and biodiversity of organisms, among others by greatly altering water availability, thus exposing species and ecosystems to harsh desiccation conditions. However, most of the studies so far have focused on the effects of increased temperature. Integrating transcriptomics and physiology is key to advancing our knowledge on how species cope with desiccation stress, and these studies are still best accomplished in model organisms.

RESULTS

Here, we characterized the natural variation of European D. melanogaster populations across climate zones and found that strains from arid regions were similar or more tolerant to desiccation compared with strains from temperate regions. Tolerant and sensitive strains differed not only in their transcriptomic response to stress but also in their basal expression levels. We further showed that gene expression changes in tolerant strains correlated with their physiological response to desiccation stress and with their cuticular hydrocarbon composition, and functionally validated three of the candidate genes identified. Transposable elements, which are known to influence stress response across organisms, were not found to be enriched nearby differentially expressed genes. Finally, we identified several tRNA-derived small RNA fragments that differentially targeted genes in response to desiccation stress.

CONCLUSIONS

Overall, our results showed that basal gene expression differences across individuals should be analyzed if we are to understand the genetic basis of differential stress survival. Moreover, tRNA-derived small RNA fragments appear to be relevant across stress responses and allow for the identification of stress-response genes not detected at the transcriptional level.

Desiccation resistance differences in Drosophila species can be largely explained by variations in cuticular hydrocarbons

Maintaining water balance is a universal challenge for organisms living in terrestrial environments, especially for insects, which have essential roles in our ecosystem. Although the high surface area to volume ratio in insects makes them vulnerable to water loss, insects have evolved different levels of desiccation resistance to adapt to diverse environments. To withstand desiccation, insects use a lipid layer called cuticular hydrocarbons (CHCs) to reduce water evaporation from the body surface. It has long been hypothesized that the water-proofing capability of this CHC layer, which can confer different levels of desiccation resistance, depends on its chemical composition. However, it is unknown which CHC components are important contributors to desiccation resistance and how these components can determine differences in desiccation resistance. In this study, we used machine-learning algorithms, correlation analyses, and synthetic CHCs to investigate how different CHC components affect desiccation resistance in 50 Drosophila and related species. We showed that desiccation resistance differences across these species can be largely explained by variation in CHC composition. In particular, length variation in a subset of CHCs, the methyl-branched CHCs (mbCHCs), is a key determinant of desiccation resistance. There is also a significant correlation between the evolution of longer mbCHCs and higher desiccation resistance in these species. Given that CHCs are almost ubiquitous in insects, we suggest that evolutionary changes in insect CHC components can be a general mechanism for the evolution of desiccation resistance and adaptation to diverse and changing environments.

Physiological and life history changes associated with seasonal adaptation in the cactophilic Drosophila mojavensis

Many insects inhabiting temperate climates are faced with changing environmental conditions throughout the year. Depending on the species, these environmental fluctuations can be experienced within a single generation or across multiple generations. Strategies for dealing with these seasonal changes vary across populations. Drosophila mojavensis is a cactophilic Drosophila species endemic to the Sonoran Desert. The Sonoran Desert regularly reaches temperatures of 50°C in the summer months. As individuals of this population are rare to collect in the summer months, we simulated the cycling temperatures experienced by D. mojavensis in the Sonoran Desert from April to July (four generations) in a temperature- and light-controlled chamber, to understand the physiological and life history changes that allow this population to withstand these conditions. In contrast to our hypothesis of a summer aestivation, we found that D. mojavensis continue to reproduce during the summer months, albeit with lower viability, but the adult survivorship of the population is highly reduced during this period. As expected, stress resistance increased during the summer months in both the adult and the larval stages. This study examines several strategies for withstanding the Sonoran Desert summer conditions which may be informative in the study of other desert endemic species.

Two putative fatty acid synthetic genes of BgFas3 and BgElo1 are responsible for respiratory waterproofing in Blattella germanica

Water retention is critical for physiological homeostasis and survival in terrestrial insects. While deposition of hydrocarbons on insect cuticles as a key measure for water conservation has been extensively investigated, we know little about other mechanisms for preventing water loss in insects. Here, we report two fatty acid synthetic genes that are independent of hydrocarbon production but crucial for water retention in the German cockroach Blattella germanica (L.). First, an integument enriched fatty acid elongase gene (BgElo1) was identified as a critical gene for desiccation resistance in B. germanica; however, knockdown of BgElo1 surprisingly failed to cause a decline in cuticular lipids. In addition, RNA interference (RNAi)-knockdown of an upstream fatty acid synthase gene (BgFas3) showed a similar phenotype, and transmission electron microscopy analysis revealed that BgFas3- or BgElo1-RNAi did not affect cuticle architecture. Bodyweight loss test showed that repression of BgFas3 and BgElo1 significantly increased the weight loss rate, but the difference disappeared when the respiration was closed by freeze killing the cockroaches. A water immersion test was performed, and we found that BgFas3- and BgElo1-RNAi made it difficult for cockroaches to recover from drowning, which was supported by the upregulation of hypoxia-related genes after a 10-h recovery from drowning. Moreover, a dyeing assay with water-soluble Eosin Y showed that this was caused by the entry of water into the respiratory system. Our research suggests that BgFas3 and BgElo1 are required for both inward and outward waterproofing of the respiratory system. This study benefits the understanding of water retention mechanisms in insects.

Water Balance and Desiccation Tolerance of the Invasive South American Tomato Pinworm

Temperature and dehydration stress are two major co-occurring environmental stressors threatening the physiology, biochemistry, and ecology of insects. As such, understanding adaptive responses to desiccation stress is critical for predicting climate change impacts, particularly its influence on insect invasions. Here, we assessed water balance and desiccation resistance of the invasive Tuta absoluta (Meyrick, 1917) (Lepidoptera: Gelechiidae), and infer how eco-physiology shapes its niche. We measured basal body water and lipid content, water loss rates (WLRs), and desiccation resistance in larvae (second to fourth instars) and adults. Body -water, -lipid, and WLRs significantly varied across life stages. Second instars recorded the lowest while fourth instars exhibited the highest body water and lipid content. Adult body water and lipid content were higher than second and third instars and lower than fourth instars while proportion of body water and lipid contents were highest in adults and second larval instars respectively. Water loss rates were significantly highest in fourth-instar larvae compared to other life stages, but differences among stages were less apparent at longer exposure durations (48 h). Desiccation resistance assays showed that second instars had greatest mortality while fourth-instar larvae and adults were the most desiccation tolerant. Our results show that T. absoluta fourth-instar larvae and adults are the most resilient developmental stages and potentially contribute most to the invasion success of the pest in arid environments. Incorporation of these species-specific eco-physiological traits in predictive models can help refine invasive species potential spread under changing climates.

Heat tolerance may determine activity time in coprophagic beetle species (Coleoptera: Scarabaeidae)

Although reports have documented loss of species diversity and ecological services caused by stressful temperature changes that result from climate change, some species cope through behavioral compensation. As temperatures and magnitudes of temperature extremes increase, animals should compensate to maintain fitness (such as through temporary behavioral shifts in activity times). Appropriate timing of activity helps avoid competition across species. Although coprophagic dung beetles exhibit species-specific temporal activity times, it is unknown whether temperature drives evolution of these species-specific temporal activity times. Using nine dung beetle species (three each of diurnal, crepuscular, and nocturnal species), we explored differences in heat stress tolerance measured as critical thermal maxima (CT_max ; the highest temperature allowing activity) and heat knockdown time (HKDT; survival time under acute heat stress) across these species, and examined the results using a phylogenetically informed approach. Our results showed that day-active species had significantly higher CT_max (diurnal > crepuscular = nocturnal species), whereas crepuscular species had higher HKDT (crepuscular > nocturnal > diurnal species). There was no correlation between heat tolerance and body size across species with distinct temporal activity, and no significant phylogenetic constraint for activity. Species with higher CT_max did not necessarily have higher HKDT, which indicates that species may respond differently to diverse heat tolerance metrics. Acute heat tolerance for diurnal beetles indicates that this trait may constrain activity time and, under high acute temperatures with climate change, species may shift activity times in more benign environments. These results contribute to elucidate the evolution of foraging behavior and management of coprophagic beetle ecosystem services under changing environments.

Plastic changes in cold and drought tolerance of Drosophila nepalensis correlate with sex-specific differences in body melanization, cuticular lipid mass, proline accumulation, and seasonal abundance

Autumn-collected flies of Himalayan Drosophila nepalensis differ in body color phenotypes (males more melanized relative to females) and in their behavior (males abundant in the open sites vs. shelters-seeking females). In contrast, winter-collected flies of both sexes are equally melanized and abundant in the open sites. We tested developmental and adult plasticity changes in cold or drought tolerance in D. nepalensis flies reared under winter or autumn simulated conditions. In D. nepalensis flies reared at 21 °C, male flies were more cold tolerant (as shown by shorter chill-coma recovery time and lower cold-shock mortality). Further, male flies also exhibited greater drought tolerance (increased levels of desiccation resistance, cuticular lipid mass, melanization, hydration level, and dehydration tolerance) as compared to females. We observed sex-specific differences in the adult plasticity responses due to rapid cold or drought hardening (RCH or RDH); and for the persistence of cold acclimation effects. RCH or RDH-induced changes in the level of proline accumulations are negatively correlated with a decrease in the chill-coma recovery time. Therefore, cold or drought hardening treatments are likely to influence cold tolerance through proline accumulation. Developmental acclimation and adult hardening responses revealed significant interaction effects between sexes and thermal treatments. Thus, sex-specific differences in morphological traits (body melanization and cuticular lipid mass) and physiological traits (adult plasticity changes in cold tolerance and proline accumulation) correlate with behavioral divergence (habitat usage) across sexes.

Gut Microbiota of Drosophila subobscura Contributes to Its Heat Tolerance and Is Sensitive to Transient Thermal Stress

The gut microbiota can contribute to host physiology leading to an increase of resistance to abiotic stress conditions. For instance, temperature has profound effects on ectotherms, and the role of the gut microbiota on the thermal tolerance of ectotherms is a matter of recent research. However, most of these studies have been focused on single static temperatures instead of evaluating thermal tolerance in a wide range of stressful temperatures. Additionally, there is evidence supporting that the gut microbiota is sensitive to environmental temperature, which induces changes in its composition and diversity. These studies have evaluated the effects of thermal acclimation (>2 weeks) on the gut microbiota, but we know little about the impact of transient thermal stress on the composition and diversity of the gut microbiota. Thus, we investigated the role of the gut microbiota on the heat tolerance of Drosophila subobscura by measuring the heat tolerance of conventional and axenic flies exposed to different heat stressful temperatures (35, 36, 37, and 38°C) and estimating the heat tolerance landscape for both microbiota treatments. Conventional flies exposed to mild heat conditions exhibited higher thermal tolerance than axenic flies, whereas at higher stressful temperatures there were no differences between axenic and conventional flies. We also assessed the impact of transient heat stress on the taxonomical abundance, diversity, and community structure of the gut microbiota, comparing non-stressed flies (exposed to 21°C) and heat-stressed flies (exposed to 34°C) from both sexes. Bacterial diversity indices, bacterial abundances, and community structure changed between non-stressed and heat-stressed flies, and this response was sex-dependent. In general, our findings provide evidence that the gut microbiota influences heat tolerance and that heat stress modifies the gut microbiota at the taxonomical and structural levels. These results demonstrate that the gut microbiota contributes to heat tolerance and is also highly sensitive to transient heat stress, which could have important consequences on host fitness, population risk extinction, and the vulnerability of ectotherms to current and future climatic conditions.

Effect of body lipid content is linked to nutritional adaptation in the acclimation responses of mesic-adapted Paederus to seasonal variations in desiccation stress

Desiccation stress causes mesic-adapted arthropods to lose their body water content. However, mesic-adapted Paederus beetles can survive over prolonged periods under dry field conditions, suggesting that these beetles adopt an array of water conservation mechanisms. We investigated the water balance mechanisms of field-collected Paederus adults over a 14-month sampling period. We also assessed their nutritional adaptations by performing a stable isotope analysis to examine their diet. The water loss rate (WLR) of the beetles was significantly associated with the rice crop cycle and saturation deficit. The cuticular permeability (CP) of adult beetles was maintained at < 30 µg cm^-2h^-1 mmHg^-1; however, CP increased significantly with the WLR. This result indicates that CP might play a minor role in reducing excessive water loss in beetles. The beetles' body water content and percentage total body water content increased when the WLR was high. Trehalose, glucose, and glycogen did not appear to play a central role in enhancing the water reserves in the insects. The body lipid content ranged from 0.22 ± 0.06 to 0.87 ± 0.07 mg and was negatively associated with the WLR. This association indicates that the increase in internal metabolic water was mediated by lipid catabolism. Stable isotope analysis results revealed that the Paederus beetles shifted their diet to carbohydrate-rich plants when the saturation deficit increased and the associated WLR reached its peak; otherwise, they consumed a high amount of staple carbohydrate-poor herbivore prey. The accumulation of energy reserves in the form of lipids through seasonal dietary shifts may exert major effects on the survival and population success of mesic-adapted Paederus beetles.

A unique Malpighian tubule architecture in Tribolium castaneum informs the evolutionary origins of systemic osmoregulation in beetles

Maintaining internal salt and water balance in response to fluctuating external conditions is essential for animal survival. This is particularly true for insects as their high surface-to-volume ratio makes them highly susceptible to osmotic stress. However, the cellular and hormonal mechanisms that mediate the systemic control of osmotic homeostasis in beetles (Coleoptera), the largest group of insects, remain largely unidentified. Here, we demonstrate that eight neurons in the brain of the red flour beetle Tribolium castaneum respond to internal changes in osmolality by releasing diuretic hormone (DH) 37 and DH47-homologs of vertebrate corticotropin-releasing factor (CRF) hormones-to control systemic water balance. Knockdown of the gene encoding the two hormones (Urinate, Urn8) reduces Malpighian tubule secretion and restricts organismal fluid loss, whereas injection of DH37 or DH47 reverses these phenotypes. We further identify a CRF-like receptor, Urinate receptor (Urn8R), which is exclusively expressed in a functionally unique secondary cell in the beetle tubules, as underlying this response. Activation of Urn8R increases K⁺ secretion, creating a lumen-positive transepithelial potential that drives fluid secretion. Together, these data show that beetle Malpighian tubules operate by a fundamentally different mechanism than those of other insects. Finally, we adopt a fluorescent labeling strategy to identify the evolutionary origin of this unusual tubule architecture, revealing that it evolved in the last common ancestor of the higher beetle families. Our work thus uncovers an important homeostatic program that is key to maintaining osmotic control in beetles, which evolved parallel to the radiation of the "advanced" beetle lineages.

When it's hot and dry: life-history strategy influences the effects of heat waves and water limitation

The frequency, duration and co-occurrence of several environmental stressors, such as heat waves and droughts, are increasing globally. Such multiple stressors may have compounding or interactive effects on animals, resulting in either additive or non-additive costs, but animals may mitigate these costs through various strategies of resource conservation or shifts in resource allocation. Through a factorial experiment, we investigated the independent and interactive effects of a simulated heat wave and water limitation on life-history, physiological and behavioral traits. We used the variable field cricket, Gryllus lineaticeps, which exhibits a wing dimorphism that mediates two distinct life-history strategies during early adulthood. Long-winged individuals invest in flight musculature and are typically flight capable, whereas short-winged individuals lack flight musculature and capacity. A comprehensive and integrative approach with G. lineaticeps allowed us to examine whether life-history strategy influenced the costs of multiple stressors as well as the resulting cost-limiting strategies. Concurrent heat wave and water limitation resulted in largely non-additive and single-stressor costs to important traits (e.g. survival and water balance), extensive shifts in resource allocation priorities (e.g. reduced prioritization of body mass) and a limited capacity to conserve resources (e.g. heat wave reduced energy use only when water was available). Life-history strategy influenced the emergency life-history stage because wing morphology and stressor(s) interacted to influence body mass, boldness behavior and immunocompetence. Our results demonstrate that water availability and life-history strategy should be incorporated into future studies integrating important conceptual frameworks of stress across a suite of traits - from survival and life history to behavior and physiology.

Eco-genetics of desiccation resistance in Drosophila

Climate change globally perturbs water circulation thereby influencing ecosystems including cultivated land. Both harmful and beneficial species of insects are likely to be vulnerable to such changes in climate. As small animals with a disadvantageous surface area to body mass ratio, they face a risk of desiccation. A number of behavioural, physiological and genetic strategies are deployed to solve these problems during adaptation in various Drosophila species. Over 100 desiccation-related genes have been identified in laboratory and wild populations of the cosmopolitan fruit fly Drosophila melanogaster and its sister species in large-scale and single-gene approaches. These genes are involved in water sensing and homeostasis, and barrier formation and function via the production and composition of surface lipids and via pigmentation. Interestingly, the genetic strategy implemented in a given population appears to be unpredictable. In part, this may be due to different experimental approaches in different studies. The observed variability may also reflect a rich standing genetic variation in Drosophila allowing a quasi-random choice of response strategies through soft-sweep events, although further studies are needed to unravel any underlying principles. These findings underline that D. melanogaster is a robust species well adapted to resist climate change-related desiccation. The rich data obtained in Drosophila research provide a framework to address and understand desiccation resistance in other insects. Through the application of powerful genetic tools in the model organism D. melanogaster, the functions of desiccation-related genes revealed by correlative studies can be tested and the underlying molecular mechanisms of desiccation tolerance understood. The combination of the wealth of available data and its genetic accessibility makes Drosophila an ideal bioindicator. Accumulation of data on desiccation resistance in Drosophila may allow us to create a world map of genetic evolution in response to climate change in an insect genome. Ultimately these efforts may provide guidelines for dealing with the effects of climate-related perturbations on insect population dynamics in the future.

Post-eclosion temperature effects on insect cuticular hydrocarbon profiles

The insect cuticle is the interface between internal homeostasis and the often harsh external environment. Cuticular hydrocarbons (CHCs) are key constituents of this hard cuticle and are associated with a variety of functions including stress response and communication. CHC production and deposition on the insect cuticle vary among natural populations and are affected by developmental temperature; however, little is known about CHC plasticity in response to the environment experienced following eclosion, during which time the insect cuticle undergoes several crucial changes. We targeted this crucial to important phase and studied post-eclosion temperature effects on CHC profiles in two natural populations of Drosophila melanogaster. A forty-eight hour post-eclosion exposure to three different temperatures (18, 25, and 30°C) significantly affected CHCs in both ancestral African and more recently derived North American populations of D. melanogaster. A clear shift from shorter to longer CHCs chain length was observed with increasing temperature, and the effects of post-eclosion temperature varied across populations and between sexes. The quantitative differences in CHCs were associated with variation in desiccation tolerance among populations. Surprisingly, we did not detect any significant differences in water loss rate between African and North American populations. Overall, our results demonstrate strong genetic and plasticity effects in CHC profiles in response to environmental temperatures experienced at the adult stage as well as associations with desiccation tolerance, which is crucial in understanding holometabolan responses to stress.

Cold tolerance, water balance, energetics, gas exchange, and diapause in overwintering brown marmorated stink bugs

Halyomorpha halys (Hemiptera: Pentatomidae) is an emerging pest which established in Ontario, Canada, in 2012. Halyomporpha halys overwinters in anthropogenic structures as an adult. We investigated seasonal variation in the cold tolerance, water balance, and energetics of H. halys in southwestern Ontario. We also induced diapause in laboratory-reared animals with short daylength at permissive temperatures and compared cold tolerance, water balance, energetics, and metabolism and gas exchange between diapausing and non-diapausing individuals. Halyomorpha halys that overwintered outside in Ontario all died, but most of those that overwintered in sheltered habitats survived. We confirm that overwintering H. halys are chill-susceptible. Over winter, Ontario H. halys depressed their supercooling point to c. -15.4 °C, and 50% survived a 1 h exposure to -17.5 °C. They reduce water loss rates over winter, and do not appear to significantly consume lipid or carbohydrate reserves to a level that might cause starvation. Overall, it appears that H. halys is dependent on built structures and other buffered microhabitats to successfully overwinter in Ontario. Laboratory-reared diapausing H. halys have lower supercooling points than their non-diapausing counterparts, but LT₅₀ is not enhanced by diapause induction. Diapausing H. halys survive desiccating conditions for 3-4 times longer than those not in diapause, through decreases in both respiratory and cuticular water loss. Diapausing H. halys do not appear to accumulate any more lipid or carbohydrate than those not in diapause, but do have lower metabolic rates, and are slightly more likely to exhibit discontinuous gas exchange.

Divergence of Desiccation-Related Traits in Sitobion avenae from Northwestern China

The impact of drought on insects has become increasingly evident in the context of global climate change, but the physiological mechanisms of aphids' responses to desiccating environments are still not well understood. We sampled the wheat aphid Sitobion avenae (Fabricius) (Hemiptera: Aphididae) from arid areas of northwestern China. Both desiccation-resistant and -nonresistant genotypes were identified, providing direct evidence of genetic divergence in desiccation resistance of S. avenae. Resistant genotypes of wingless S. avenae showed longer survival time and LT50 under the desiccation stress (i.e., 10% relative humidity) than nonresistant genotypes, and wingless individuals tended to have higher desiccation resistance than winged ones. Both absolute and relative water contents did not differ between the two kinds of genotypes. Resistant genotypes had lower water loss rates than nonresistant genotypes for both winged and wingless individuals, suggesting that modulation of water loss rates could be the primary strategy in resistance of this aphid against desiccation stress. Contents of cuticular hydrocarbons (CHC) (especially methyl-branched alkanes) showed significant increase for both resistant and nonresistant genotypes after exposure to the desiccation stress for 24 h. Under desiccation stress, survival time was positively correlated with contents of methyl-branched alkanes for resistant genotypes. Thus, the content of methyl-branched alkanes and their high plasticity could be closely linked to water loss rate and desiccation resistance in S. avenae. Our results provide insights into fundamental aspects and underlying mechanisms of desiccation resistance in aphids, and have significant implications for the evolution of aphid populations in the context of global warming.

Adaptation of gene loci to heterochromatin in the course of Drosophila evolution is associated with insulator proteins

Pericentromeric heterochromatin is generally composed of repetitive DNA forming a transcriptionally repressive environment. Dozens of genes were embedded into pericentromeric heterochromatin during evolution of Drosophilidae lineage while retaining activity. However, factors that contribute to insusceptibility of gene loci to transcriptional silencing remain unknown. Here, we find that the promoter region of genes that can be embedded in both euchromatin and heterochromatin exhibits a conserved structure throughout the Drosophila phylogeny and carries motifs for binding of certain chromatin remodeling factors, including insulator proteins. Using ChIP-seq data, we demonstrate that evolutionary gene relocation between euchromatin and pericentric heterochromatin occurred with preservation of sites of insulation of BEAF-32 in evolutionarily distant species, i.e. D. melanogaster and D. virilis. Moreover, promoters of virtually all protein-coding genes located in heterochromatin in D. melanogaster are enriched with insulator proteins BEAF-32, GAF and dCTCF. Applying RNA-seq of a BEAF-32 mutant, we show that the impairment of BEAF-32 function has a complex effect on gene expression in D. melanogaster, affecting even those genes that lack BEAF-32 association in their promoters. We propose that conserved intrinsic properties of genes, such as sites of insulation near the promoter regions, may contribute to adaptation of genes to the heterochromatic environment and, hence, facilitate the evolutionary relocation of genes loci between euchromatin and heterochromatin.

Climate change and the genetics of insecticide resistance

Changes in global temperature and humidity as a result of climate change are producing rapid evolutionary changes in many animal species, including agricultural pests and disease vectors, leading to changes in allele frequencies of genes involved in thermotolerance and desiccation resistance. As some of these genes have pleiotropic effects on insecticide resistance, climate change is likely to affect insecticide resistance in the field. In this review, we discuss how the interactions between adaptation to climate change and resistance to insecticides can affect insecticide resistance in the field using examples in phytophagous and hematophagous pest insects, focusing on the effects of increased temperature and increased aridity. We then use detailed genetic and mechanistic studies in the model insect, Drosophila melanogaster, to explain the mechanisms underlying this phenomenon. We suggest that tradeoffs or facilitation between adaptation to climate change and resistance to insecticides can alter insecticide resistance allele frequencies in the field. The dynamics of these interactions will need to be considered when managing agricultural pests and disease vectors in a changing climate. © 2019 Society of Chemical Industry.

Desiccation resistance is an adaptive life-history trait dependent upon cuticular hydrocarbons, and influenced by mating status and temperature in D. melanogaster

Terrestrial insects are susceptible to desiccation and conserve internal water stores by preventing the loss of water due to transpiration across the cuticle. The epicuticle, a thin waxy layer on the outer surface of the insect cuticle is comprised primarily of a complex blend of cuticular hydrocarbons (CHCs) and is integral to preventing cuticular water loss. How the composition of epicuticular lipids (quantity and quality of the specific hydrocarbons) relates to desiccation resistance, however, has been difficult to determine. Here, we establish a model system to test the capacity of CHCs to protect against desiccation in the vinegar fly, Drosophila melanogaster. Using this system, we demonstrate that the oenocytes and CHCs produced by these cells are critically important for desiccation resistance, as measured by survival under desiccative conditions. Additionally, we show that both mating status and developmental temperature influence desiccation resistance. Prior mating increased desiccation survival through the direct transfer of CHCs between sexual partners, as well as through a female-specific response to a male-derived factor transferred during copulation. Together, our results demonstrate that desiccation resistance is an adaptive life-history trait dependent upon CHCs and influenced by prior social interactions and environmental conditions.

Proximate mechanisms of drought resistance in Phytoseiulus persimilis eggs

Under drought stress, Phytoseiulus persimilis females are able to lay drought-resistant eggs through an adaptive maternal effect. The mechanisms making these eggs drought resistant still remain to be investigated. For this purpose, we studied the physiological differences between drought-resistant and drought-sensitive eggs. We compared the volume and the surface-area-to-volume ratio (SA:V) of the eggs, their sex ratio, their chemical composition (by gas chromatography-mass spectrometry), their internal and external structure [by scanning electron microscope (SEM) and transmission electron microscope (TEM) images], and their developmental time. Our results show that drought-resistant and drought-sensitive eggs have a different chemical composition: drought-resistant eggs contain more compatible solutes (free amino acids and sugar alcohols) and saturated hydrocarbons than drought-sensitive eggs. This difference may contribute to reducing water loss in drought-resistant eggs. Moreover, drought-resistant eggs are on average 8.4% larger in volume, and have a 2.4% smaller SA:V than drought-sensitive eggs. This larger volume and smaller SA:V, probably the result of a higher water content, may make drought-resistant eggs less vulnerable to water loss. We did not find any difference in sex ratio, internal or external structure nor developmental time between drought-resistant and drought-sensitive eggs. These results mark the first step in the understanding of the strategies and the energetic costs involved in the production of drought-resistant eggs in P. persimilis females.

Behavioral and Physiological Resistance to Desiccation in Spotted Wing Drosophila (Diptera: Drosophilidae)

With a high surface to volume ratio, small organisms must carefully regulate their internal water status. Spotted-wing drosophila, Drosophila suzukii (Matsumura), is an invasive frugivorous insect distributed across a wide range of geographical regions that can have periods of dry and hot weather, suggesting that this species has strategies to avoid stressful environments and reduce water loss. It also survives winter as an adult fly, indicating that it has adaptations to the low air humidity of this season. To determine the importance of water stress to D. suzukii, we studied their survival in environments of low humidity, which was manipulated using Drierite, and their survival and water loss in response to desiccation. Survival of both sexes was lower in drier conditions, and while female winter morph D. suzukii had higher mortality early on, remaining flies were able to survive longer in the drier conditions than the summer morphs. A bioassay method was adapted from Enjin et al. (2016) using 48-well plates to videotape the location of flies and quantify their behavioral responses to humidity. Male and female D. suzukii avoided dry conditions within the bioassay system, but only when there was at least 25% differential between humidity extremes. This response was observed for both summer and winter morphs of D. suzukii and our results provide guidance for attempts to manipulate crop environments to reduce the economic impact of this pest.

Gut yeasts do not improve desiccation survival in Drosophila melanogaster

A healthy gut microbiota generally improves the performance of its insect host. Although the effects can be specific to the species composition of the microbial community, the role of gut microbiota in determining water balance has not been well explored. We used axenic and gnotobiotic (reared with a known microbiota) Drosophila melanogaster to test three hypotheses about the effects of gut yeasts on the water balance of adult flies: 1) that gut yeasts would improve desiccation survival in adult flies; 2) that larval yeasts would improve adult desiccation survival; 3) that the effects would be species-specific, such that yeasts closely associated with D. melanogaster in nature are more likely to be beneficial than those rarely found in association with D. melanogaster. We used Saccharomyces cerevisiae (often used in Drosophila cultures, but rarely associated with D. melanogaster in nature), Lachancea kluyveri (associated with some species of Drosophila, but not D. melanogaster), and Pichia kluyveri (associated with D. melanogaster in nature). Adult inoculation with yeasts had no effect on survival of desiccating conditions. Inoculation with P. kluyveri as larvae did not change desiccation survival in adults; however, rearing with L. kluyveri or S. cerevisiae reduced adult desiccation survival. We conclude that adult inoculation with gut yeasts has no impact on desiccation survival, but that rearing with yeasts can have either no or detrimental effect. The effects appear to be species-specific: P. kluyveri did not have a negative impact on desiccation tolerance, suggesting some level of co-adaptation with D. melanogaster. We note that S. cerevisiae may not be an appropriate species for studying the effects of gut yeasts on D. melanogaster.

The physiology of forager hydration and variation among harvester ant (Pogonomyrmex barbatus) colonies in collective foraging behavior

Ants are abundant in desiccating environments despite their high surface area to volume ratios and exposure to harsh conditions outside the nest. Red harvester ant (Pogonomyrmex barbatus) colonies must spend water to obtain water: colonies lose water as workers forage outside the nest, and gain water metabolically through seeds collected in foraging trips. Here we present field experiments showing that hydrated P. barbatus foragers made more foraging trips than unhydrated nestmates. The positive effect of hydration on foraging activity is stronger as the risk of desiccation increases. Desiccation tests showed that foragers of colonies that reduce foraging in dry conditions are more sensitive to water loss, losing water and motor coordination more rapidly in desiccating conditions, than foragers of colonies that do not reduce foraging in dry conditions. Desiccation tolerance is also associated with colony reproductive success. Surprisingly, foragers that are more sensitive to water loss are from colonies more likely to produce offspring colonies. This could be because the foragers of these colonies conserve water with a more cautious response to desiccation risk. An ant's hydration status may influence its response to the olfactory interactions that regulate its decision to leave the nest to forage. Thus variation among ant colonies in worker physiology and response to ambient conditions may contribute to ecologically significant differences among colonies in collective behavior.

Plasticity for desiccation tolerance across Drosophila species is affected by phylogeny and climate in complex ways

Comparative analyses of ectotherm susceptibility to climate change often focus on thermal extremes, yet responses to aridity may be equally important. Here we focus on plasticity in desiccation resistance, a key trait shaping distributions of Drosophila species and other small ectotherms. We examined the extent to which 32 Drosophila species, varying in their distribution, could increase their desiccation resistance via phenotypic plasticity involving hardening, linking these responses to environment, phylogeny and basal resistance. We found no evidence to support the seasonality hypothesis; species with higher hardening plasticity did not occupy environments with higher and more seasonal precipitation. As basal resistance increased, the capacity of species to respond via phenotypic plasticity decreased, suggesting plastic responses involving hardening may be constrained by basal resistance. Trade-offs between basal desiccation resistance and plasticity were not universal across the phylogeny and tended to occur within specific clades. Phylogeny, environment and trade-offs all helped to explain variation in plasticity for desiccation resistance but in complex ways. These findings suggest some species have the ability to counter dry periods through plastic responses, whereas others do not; and this ability will depend to some extent on a species' placement within a phylogeny, along with its basal level of resistance.

Genomic changes associated with adaptation to arid environments in cactophilic Drosophila species

BACKGROUND

Insights into the genetic capacities of species to adapt to future climate change can be gained by using comparative genomic and transcriptomic data to reconstruct the genetic changes associated with such adaptations in the past. Here we investigate the genetic changes associated with adaptation to arid environments, specifically climatic extremes and new cactus hosts, through such an analysis of five repleta group Drosophila species.

RESULTS

We find disproportionately high rates of gene gains in internal branches in the species' phylogeny where cactus use and subsequently cactus specialisation and high heat and desiccation tolerance evolved. The terminal branch leading to the most heat and desiccation resistant species, Drosophila aldrichi, also shows disproportionately high rates of both gene gains and positive selection. Several Gene Ontology terms related to metabolism were enriched in gene gain events in lineages where cactus use was evolving, while some regulatory and developmental genes were strongly selected in the Drosophila aldrichi branch. Transcriptomic analysis of flies subjected to sublethal heat shocks showed many more downregulation responses to the stress in a heat sensitive versus heat resistant species, confirming the existence of widespread regulatory as well as structural changes in the species' differing adaptations. Gene Ontology terms related to metabolism were enriched in the differentially expressed genes in the resistant species while terms related to stress response were over-represented in the sensitive one.

CONCLUSION

Adaptations to new cactus hosts and hot desiccating environments were associated with periods of accelerated evolutionary change in diverse biochemistries. The hundreds of genes involved suggest adaptations of this sort would be difficult to achieve in the timeframes projected for anthropogenic climate change.

Insects With Survival Kits for Desiccation Tolerance Under Extreme Water Deficits

The year 2002 marked the tercentenary of Antonie van Leeuwenhoek's discovery of desiccation tolerance in animals. This remarkable phenomenon to sustain 'life' in the absence of water can be revived upon return of hydrating conditions. Today, coping with climate change-related factors, especially temperature-humidity imbalance, is a global challenge. Under such adverse circumstances, desiccation tolerance remains a prime mechanism of several plants and a few animals to escape the hostile consequences of fluctuating hydroperiodicity patterns in their habitats. Among small animals, insects have demonstrated impressive resilience to dehydration and thrive under physiological water deficits without compromising on revival and survival upon rehydration. The focus of this review is to compile research insights on insect desiccation tolerance, gathered over the past several decades from numerous laboratories worldwide working on different insect groups. We provide a comparative overview of species-specific behavioral changes, adjustments in physiological biochemistry and cellular and molecular mechanisms as few of the noteworthy desiccation-responsive survival kits in insects. Finally, we highlight the role of insects as potential mechanistic models in tracking global warming which will form the basis for translational research to mitigate periods of climatic uncertainty predicted for the future.

A phylogenetic examination of host use evolution in the quinaria and testacea groups of Drosophila

Adaptive radiations provide an opportunity to examine complex evolutionary processes such as ecological specialization and speciation. While a well-resolved phylogenetic hypothesis is critical to completing such studies, the rapid rates of evolution in these groups can impede phylogenetic studies. Here we study the quinaria and testacea species groups of the immigrans-tripunctata radiation of Drosophila, which represent a recent adaptive radiation and are a developing model system for ecological genetics. We were especially interested in understanding host use evolution in these species. In order to infer a phylogenetic hypothesis for this group we sampled loci from both the nuclear genome and the mitochondrial DNA to develop a dataset of 43 protein-coding loci for these two groups along with their close relatives in the immigrans-tripunctata radiation. We used this dataset to examine their evolutionary relationships along with the evolution of feeding behavior. Our analysis recovers strong support for the monophyly of the testacea but not the quinaria group. Results from our ancestral state reconstruction analysis suggests that the ancestor of the testacea and quinaria groups exhibited mushroom-feeding. Within the quinaria group, we infer that transition to vegetative feeding occurred twice, and that this transition did not coincide with a genome-wide change in the rate of protein evolution.

Spatiotemporal dynamics and genome-wide association genome-wide association analysis of desiccation tolerance in Drosophila melanogaster

Water availability is a major environmental challenge to a variety of terrestrial organisms. In insects, desiccation tolerance varies predictably over spatial and temporal scales and is an important physiological determinant of fitness in natural populations. Here, we examine the dynamics of desiccation tolerance in North American populations of Drosophila melanogaster using: (a) natural populations sampled across latitudes and seasons; (b) experimental evolution in field mesocosms over seasonal time; (c) genome-wide associations to identify SNPs/genes associated with variation for desiccation tolerance; and (d) subsequent analysis of patterns of clinal/seasonal enrichment in existing pooled sequencing data of populations sampled in both North America and Australia. A cline in desiccation tolerance was observed, for which tolerance exhibited a positive association with latitude; tolerance also varied predictably with culture temperature, demonstrating a significant degree of thermal plasticity. Desiccation tolerance evolved rapidly in field mesocosms, although only males showed differences in desiccation tolerance between spring and autumn collections from natural populations. Water loss rates did not vary significantly among latitudinal or seasonal populations; however, changes in metabolic rates during prolonged exposure to dry conditions are consistent with increased tolerance in higher latitude populations. Genome-wide associations in a panel of inbred lines identified twenty-five SNPs in twenty-one loci associated with sex-averaged desiccation tolerance, but there is no robust signal of spatially varying selection on genes associated with desiccation tolerance. Together, our results suggest that desiccation tolerance is a complex and important fitness component that evolves rapidly and predictably in natural populations.

Renal neuroendocrine control of desiccation and cold tolerance by Drosophila suzukii

BACKGROUND

Neuropeptides are central to the regulation of physiological and behavioural processes in insects, directly impacting cold and desiccation survival. However, little is known about the control mechanisms governing these responses in Drosophila suzukii. The close phylogenetic relationship of D. suzukii with Drosophila melanogaster allows, through genomic and functional studies, an insight into the mechanisms directing stress tolerance in D. suzukii.

RESULTS

Capability (Capa), leucokinin (LK), diuretic hormone 44 (DH44 ) and DH31 neuropeptides demonstrated a high level of conservation between D. suzukii and D. melanogaster with respect to peptide sequences, neuronal expression, receptor localisation, and diuretic function in the Malpighian tubules. Despite D. suzukii's ability to populate cold environments, it proved sensitive to both cold and desiccation. Furthermore, in D. suzukii, Capa acts as a desiccation- and cold stress-responsive gene, while DH44 gene expression is increased only after desiccation exposure, and the LK gene after nonlethal cold stress recovery.

CONCLUSION

This study provides a comparative investigation into stress tolerance mediation by neuroendocrine signalling in two Drosophila species, providing evidence that similar signalling pathways control fluid secretion in the Malpighian tubules. Identifying processes governing specific environmental stresses affecting D. suzukii could lead to the development of targeted integrated management strategies to control insect pest populations. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Functional characterization of adaptive variation within a cis-regulatory element influencing Drosophila melanogaster growth

Gene expression variation is a major contributor to phenotypic diversity within species and is thought to play an important role in adaptation. However, examples of adaptive regulatory polymorphism are rare, especially those that have been characterized at both the molecular genetic level and the organismal level. In this study, we perform a functional analysis of the Drosophila melanogaster CG9509 enhancer, a cis-regulatory element that shows evidence of adaptive evolution in populations outside the species’ ancestral range in sub-Saharan Africa. Using site-directed mutagenesis and transgenic reporter gene assays, we determined that 3 single nucleotide polymorphisms are responsible for the difference in CG9509 expression that is observed between sub-Saharan African and cosmopolitan populations. Interestingly, while 2 of these variants appear to have been the targets of a selective sweep outside of sub-Saharan Africa, the variant with the largest effect on expression remains polymorphic in cosmopolitan populations, suggesting it may be subject to a different mode of selection. To elucidate the function of CG9509, we performed a series of functional and tolerance assays on flies in which CG9509 expression was disrupted. We found that CG9509 plays a role in larval growth and influences adult body and wing size, as well as wing loading. Furthermore, variation in several of these traits was associated with variation within the CG9509 enhancer. The effect on growth appears to result from a modulation of active ecdysone levels and expression of growth factors. Taken together, our findings suggest that selection acted on 3 sites within the CG9509 enhancer to increase CG9509 expression and, as a result, reduce wing loading as D. melanogaster expanded out of sub-Saharan Africa.

Much of the phenotypic variation that is observed within species is thought to be caused by variation in gene expression. Variants within cis-regulatory elements, which affect the expression of nearby genes within the same DNA strand, are thought to be an abundant resource upon which natural selection can act. Understanding the functional consequences of adaptive cis-regulatory changes is important, as it can help elucidate the mechanisms underlying phenotypic evolution in general and provide insight into the development and maintenance of biodiversity. However, functional analyses of these types of changes remain rare. Here we present a functional analysis of an adaptively evolving enhancer element of a D. melanogaster gene called CG9509, of previously unknown function. We show that 3 single nucleotide polymorphisms located within the enhancer of this gene are responsible for an increase in CG9509 expression in cosmopolitan populations (outside of south and central Africa) relative to sub-Saharan populations, which include ancestral populations. We further show that CG9509 is involved in the regulation of growth rate and body size determination and propose that the CG9509 enhancer underwent positive selection to reduce wing loading as the species expanded out of sub-Saharan Africa.

Transcriptional profiles of plasticity for desiccation stress in Drosophila

We examined the transcriptional responses of desiccation resistance candidate genes in populations of Drosophila melanogaster divergent for desiccation resistance and in capacity to improve resistance via phenotypic plasticity. Adult females from temperate and tropical eastern Australian populations were exposed to a rapid desiccation hardening (RDH) treatment, and groups without RDH to acute desiccation stress, and the transcript expression of 12 candidate desiccation genes were temporally profiled during, and in recovery from stress. We found that desiccation exposure resulted in largely transitory, stress-specific transcriptional changes in all but one gene. However linking the expression profiles to the population-level phenotypic divergence was difficult given subtle, and time-point specific population expression variation. Nonetheless, rapid desiccation hardening had the largest effect on gene expression, resulting in distinct molecular profiles. We report a hitherto uncharacterised desiccation molecular hardening response where prior exposure essentially 'primes' genes to respond to subsequent stress without discernible transcript changes prior to stress. This, taken together with some population gene expression variation of several bona fide desiccation candidates associated with different water balance strategies speaks of the complexity of natural desiccation resistance and plasticity and provides new avenues for understanding the molecular basis of a trait of ecological significance.

Phylogeny, environment and sexual communication across the Drosophila genus

Social behaviour emerges from the local environment but is constrained by the animal's life history and its evolutionary lineage. In this perspective, we consider the genus Drosophila and provide an overview of how these constraints can shape how individuals interact. Our focus is restricted to visual and chemical signals and how their use varies across species during courtship - currently the only social behaviour well-studied across many Drosophila species. We broadly categorize species into four climatic groups - cosmopolitan, tropical, temperate and arid - which serve as discussion points as we review comparative behavioural and physiological studies and relate them to the abiotic conditions of a species environment. We discuss how the physiological and behavioural differences among many fly species may reflect life history differences as much as, or even more than, differences in phylogeny. This perspective serves not only to summarize what has been studied across drosophilids, but also to identify questions and outline gaps in the literature worth pursuing for progressing the understanding of behavioural evolution in Drosophila.

Correlation between desiccation stress response and epigenetic modifications of genes in Drosophila melanogaster: An example of environment-epigenome interaction

Animals from different phyla including arthropods tolerate water stress to different extent. This tolerance is accompanied by biochemical changes which in turn are due to transcriptional alteration. The changes in transcription can be an indirect effect on some of the genes, ensuing from the effect of stress on the regulators of transcription including epigenetic regulators. Within this paradigm, we investigated the correlation between stress response and epigenetic modification underlying gene expression modulation during desiccation stress in Canton-S. We report altered resistance of flies in desiccation stress for heterozygote mutants of PcG and TrxG members. Pc/+ mutant shows lower survival, while ash1/+ mutants show higher survival under desiccation stress as compared to Canton-S. We detect expression alteration in stress related genes as well the genes of the Polycomb and trithorax complex in Canton-S subjected to desiccation stress. Concomitant with this, there is an altered enrichment of H3K27me3 and H3K4me3 at the upstream regions of the stress responsive genes. The enrichment of activating mark, H3K4me3, is higher in non-stress condition. H3K27me3, the repressive mark, is more pronounced under stress condition, which in turn, can be correlated with the binding of Pc. Our results show that desiccation stress induces dynamic switching in expression and enrichment of PcG and TrxG in the upstream region of genes, which correlates with histone modifications. We provide evidence that epigenetic modulation could be one of the mechanisms to adapt to the desiccation stress in Drosophila. Thus, our study proposes the interaction of epigenome and environmental factors.

Ultra-low activities of a common radioisotope for permission-free tracking of a drosophilid fly in its natural habitat

Knowledge of a species’ ecology, including its movement in time and space, is key for many questions in biology and conservation. While numerous tools for tracking larger animals are available, millimetre-sized insects are averse to standard tracking and labelling procedures. Here, we evaluated the applicability of ultra-low, permission-exempt activities of the metastable isomer of the radionuclide Technetium-99 for labelling and field detection of the mountain fly Drosophila nigrosparsa. We demonstrate that an activity of less than 10 MBq is sufficient to label dozens of flies and detect single individuals using standard radiation protection monitors. The methodology presented here is applicable to many small-sized, low-mobility animals as well as independent from light and weather conditions and visual contact with the target organism.

An Experimental Evolution Test of the Relationship between Melanism and Desiccation Survival in Insects

We used experimental evolution to test the ‘melanism-desiccation’ hypothesis, which proposes that dark cuticle in several Drosophila species is an adaptation for increased desiccation tolerance. We selected for dark and light body pigmentation in replicated populations of D. melanogaster and assayed several traits related to water balance. We also scored pigmentation and desiccation tolerance in populations selected for desiccation survival. Populations in both selection regimes showed large differences in the traits directly under selection. However, after over 40 generations of pigmentation selection, dark-selected populations were not more desiccation-tolerant than light-selected and control populations, nor did we find significant changes in mass or carbohydrate amounts that could affect desiccation resistance. Body pigmentation of desiccation-selected populations did not differ from control populations after over 140 generations of selection, although selected populations lost water less rapidly. Our results do not support an important role for melanization in Drosophila water balance.

Effects of saturation deficit on desiccation resistance and water balance in seasonal populations of the tropical drosophilid Zaprionus indianus

Seasonally varying populations of ectothermic insect taxa from a given locality are expected to cope with simultaneous changes in temperature and humidity through phenotypic plasticity. Accordingly, we investigated the effect of saturation deficit on resistance to desiccation in wild-caught flies from four seasons (spring, summer, rainy and autumn) and corresponding flies reared in the laboratory under season-specific simulated temperature and humidity growth conditions. Flies raised under summer conditions showed approximately three times higher desiccation resistance and increased levels of cuticular lipids compared with flies raised in rainy season conditions. In contrast, intermediate trends were observed for water balance-related traits in flies reared under spring or autumn conditions but trait values overlapped across these two seasons. Furthermore, a threefold difference in saturation deficit (an index of evaporative water loss due to a combined thermal and humidity effect) between summer (27.5 mB) and rainy (8.5 mB) seasons was associated with twofold differences in the rate of water loss. Higher dehydration stress due to a high saturation deficit in summer is compensated by storage of higher levels of energy metabolite (trehalose) and cuticular lipids, and these traits correlated positively with desiccation resistance. In Z. indianus, the observed changes in desiccation-related traits due to plastic effects of simulated growth conditions correspond to similar changes exhibited by seasonal wild-caught flies. Our results show that developmental plastic effects under ecologically relevant thermal and humidity conditions can explain seasonal adaptations for water balance-related traits in Z. indianus and are likely to be associated with its invasive potential.

Desiccation resistance in tropical insects: causes and mechanisms underlying variability in a Panama ant community

Desiccation resistance, the ability of an organism to reduce water loss, is an essential trait in arid habitats. Drought frequency in tropical regions is predicted to increase with climate change, and small ectotherms are often under a strong desiccation risk. We tested hypotheses regarding the underexplored desiccation potential of tropical insects. We measured desiccation resistance in 82 ant species from a Panama rainforest by recording the time ants can survive desiccation stress. Species' desiccation resistance ranged from 0.7 h to 97.9 h. We tested the desiccation adaptation hypothesis, which predicts higher desiccation resistance in habitats with higher vapor pressure deficit (VPD) - the drying power of the air. In a Panama rainforest, canopy microclimates averaged a VPD of 0.43 kPa, compared to a VPD of 0.05 kPa in the understory. Canopy ants averaged desiccation resistances 2.8 times higher than the understory ants. We tested a number of mechanisms to account for desiccation resistance. Smaller insects should desiccate faster given their higher surface area to volume ratio. Desiccation resistance increased with ant mass, and canopy ants averaged 16% heavier than the understory ants. A second way to increase desiccation resistance is to carry more water. Water content was on average 2.5% higher in canopy ants, but total water content was not a good predictor of ant desiccation resistance or critical thermal maximum (CT max), a measure of an ant's thermal tolerance. In canopy ants, desiccation resistance and CT max were inversely related, suggesting a tradeoff, while the two were positively correlated in understory ants. This is the first community level test of desiccation adaptation hypothesis in tropical insects. Tropical forests do contain desiccation-resistant species, and while we cannot predict those simply based on their body size, high levels of desiccation resistance are always associated with the tropical canopy.

Aquatic insects dealing with dehydration: do desiccation resistance traits differ in species with contrasting habitat preferences?

Background

Desiccation resistance shapes the distribution of terrestrial insects at multiple spatial scales. However, responses to drying stress have been poorly studied in aquatic groups, despite their potential role in constraining their distribution and diversification, particularly in arid and semi-arid regions.

Methods

We examined desiccation resistance in adults of four congeneric water beetle species (Enochrus, family Hydrophilidae) with contrasting habitat specificity (lentic vs. lotic systems and different salinity optima from fresh- to hypersaline waters). We measured survival, recovery capacity and key traits related to desiccation resistance (fresh mass, % water content, % cuticle content and water loss rate) under controlled exposure to desiccation, and explored their variability within and between species.

Results

Meso- and hypersaline species were more resistant to desiccation than freshwater and hyposaline ones, showing significantly lower water loss rates and higher water content. No clear patterns in desiccation resistance traits were observed between lotic and lentic species. Intraspecifically, water loss rate was positively related to specimens’ initial % water content, but not to fresh mass or % cuticle content, suggesting that the dynamic mechanism controlling water loss is mainly regulated by the amount of body water available.

Discussion

Our results support previous hypotheses suggesting that the evolution of desiccation resistance is associated with the colonization of saline habitats by aquatic beetles. The interespecific patterns observed in Enochrus also suggest that freshwater species may be more vulnerable than saline ones to drought intensification expected under climate change in semi-arid regions such as the Mediterranean Basin.

Molecular characterization of Tps1 and Treh genes in Drosophila and their role in body water homeostasis

In insects, trehalose serves as the main sugar component of haemolymph. Trehalose is also recognized as a mediator of desiccation survival due to its proposed ability to stabilize membranes and proteins. Although the physiological role of trehalose in insects has been documented for decades, genetic evidence to support the importance of trehalose metabolism remains incomplete. We here show on the basis of genetic and biochemical evidence that the trehalose synthesis enzyme Tps1 is solely responsible for the de novo synthesis of trehalose in Drosophila. Conversely, a lack of the gene for the trehalose hydrolyzing enzyme Treh causes an accumulation of trehalose that is lethal during the pupal period, as is observed with Tps1 mutants. Lack of either Tps1 or Treh results in a significant reduction in circulating glucose, suggesting that the maintenance of glucose levels requires a continuous turnover of trehalose. Furthermore, changes in trehalose levels are positively correlated with the haemolymph water volume. In addition, both Tps1 and Treh mutant larvae exhibit a high lethality after desiccation stress. These results demonstrate that the regulation of trehalose metabolism is essential for normal development, body water homeostasis, and desiccation tolerance in Drosophila.

Water loss in tree weta (Hemideina): adaptation to the montane environment and a test of the melanisation-desiccation resistance hypothesis

Montane insects are at a higher risk of desiccation than their lowland counterparts and are expected to have evolved reduced water loss. Hemideina spp. (tree weta; Orthoptera: Anostostomatidae) have both lowland (Hemideina femorata, Hemideina crassidens and Hemideina thoracica) and montane (Hemideina maori and Hemideina ricta) species. H. maori has both melanic and yellow morphs. We use these weta to test two hypotheses: that montane insects lose water more slowly than lowland species, and that cuticular water loss rates are lower in darker insects than lighter morphs, because of incorporation of melanin in the cuticle. We used flow-through respirometry to compare water loss rates among Hemideina species and found that montane weta have reduced cuticular water loss by 45%, reduced respiratory water loss by 55% and reduced the molar ratio of V̇H2 O:V̇CO2  by 64% compared with lowland species. Within H. maori, cuticular water loss was reduced by 46% when compared with yellow morphs. Removal of cuticular hydrocarbons significantly increased total water loss in both melanic and yellow morphs, highlighting the role that cuticular hydrocarbons play in limiting water loss; however, the dark morph still lost water more slowly after removal of cuticular hydrocarbons (57% less), supporting the melanisation-desiccation resistance hypothesis.

Reproductive arrest and stress resistance in winter-acclimated Drosophila suzukii

Overwintering insects must survive the multiple-stress environment of winter, which includes low temperatures, reduced food and water availability, and cold-active pathogens. Many insects overwinter in diapause, a developmental arrest associated with high stress tolerance. Drosophila suzukii (Diptera: Drosophilidae), spotted wing drosophila, is an invasive agricultural pest worldwide. Its ability to overwinter and therefore establish in temperate regions could have severe implications for fruit crop industries. We demonstrate here that laboratory populations of Canadian D. suzukii larvae reared under short-day, low temperature, conditions develop into dark 'winter morph' adults similar to those reported globally from field captures, and observed by us in southern Ontario, Canada. These winter-acclimated adults have delayed reproductive maturity, enhanced cold tolerance, and can remain active at low temperatures, although they do not have the increased desiccation tolerance or survival of fungal pathogen challenges that might be expected from a more heavily melanised cuticle. Winter-acclimated female D. suzukii have underdeveloped ovaries and altered transcript levels of several genes associated with reproduction and stress. While superficially indicative of reproductive diapause, the delayed reproductive maturity of winter-acclimated D. suzukii appears to be temperature-dependent, not regulated by photoperiod, and is thus unlikely to be 'true' diapause. The traits of this 'winter morph', however, likely facilitate overwintering in southern Canada, and have probably contributed to the global success of this fly as an invasive species.