Physiological Adaptations to Sugar Intake: New Paradigms from Drosophila melanogaster

Dietary Variation Effect on Life History Traits and Energy Storage in Neotropical Species of Drosophila (Diptera; Drosophilidae)

The ability of an organism to respond to nutritional stress can be a plastic character under the action of natural selection, affecting several characteristics, including life history and energy storage. The genus Drosophila (Diptera; Drosophilidae) presents high variability regarding natural resource exploration. However, most works on this theme have studied the model species D. melanogaster Meigen, 1830 and little is known about Neotropical drosophilids. Here we evaluate the effects of three diets, with different carbohydrate-to-protein ratios, on life history (viability and development time) and metabolic pools (triglycerides, glycogen, and total soluble protein contents) of three Neotropical species of Drosophila: D. maculifrons Duda, 1927; D. ornatifrons Duda, 1927, both of the subgenus Drosophila Sturtevant, 1939, and D. willistoni Sturtevant, 1916 of the subgenus Sophophora Sturtevant, 1939. Our results showed that only D. willistoni was viable on all diets, D. maculifrons was not viable on the sugary diet, while D. ornatifrons was barely viable on this diet. The sugary diet increased the development time of D. willistoni and D. ornatifrons, and D. willistoni glycogen content. Thus, the viability of D. maculifrons and D. ornatifrons seems to depend on a certain amount of protein and/or a low concentration of carbohydrate in the diet. A more evident effect of the diets on triglyceride and protein pools was detected in D. ornatifrons, which could be related to the adult attraction to dung and carrion baited pitfall as food resource tested in nature. Our results demonstrated that the evolutionary history and differential adaptations to natural macronutrient resources are important to define the amplitude of response that a species can present when faced with dietary variation.

Immunometabolic regulation during the presence of microorganisms and parasitoids in insects

Multicellular organisms live in environments containing diverse nutrients and a wide variety of microbial communities. On the one hand, the immune response of organisms can protect from the intrusion of exogenous microorganisms. On the other hand, the dynamic coordination of anabolism and catabolism of organisms is a necessary factor for growth and reproduction. Since the production of an immune response is an energy-intensive process, the activation of immune cells is accompanied by metabolic transformations that enable the rapid production of ATP and new biomolecules. In insects, the coordination of immunity and metabolism is the basis for insects to cope with environmental challenges and ensure normal growth, development and reproduction. During the activation of insect immune tissues by pathogenic microorganisms, not only the utilization of organic resources can be enhanced, but also the activated immune cells can usurp the nutrients of non-immune tissues by generating signals. At the same time, insects also have symbiotic bacteria in their body, which can affect insect physiology through immune-metabolic regulation. This paper reviews the research progress of insect immune-metabolism regulation from the perspective of insect tissues, such as fat body, gut and hemocytes. The effects of microorganisms (pathogenic bacteria/non-pathogenic bacteria) and parasitoids on immune-metabolism were elaborated here, which provide guidance to uncover immunometabolism mechanisms in insects and mammals. This work also provides insights to utilize immune-metabolism for the formulation of pest control strategies.

Exposure to high-sugar diet induces transgenerational changes in sweet sensitivity and feeding behavior via H3K27me3 reprogramming

Human health is facing a host of new threats linked to unbalanced diets, including high-sugar diet (HSD), which contributes to the development of both metabolic and behavioral disorders. Studies have shown that diet-induced metabolic dysfunctions can be transmitted to multiple generations of offspring and exert long-lasting health burden. Meanwhile, whether and how diet-induced behavioral abnormalities can be transmitted to the offspring remains largely unclear. Here, we showed that ancestral HSD exposure suppressed sweet sensitivity and feeding behavior in the offspring in Drosophila. These behavioral deficits were transmitted through the maternal germline and companied by the enhancement of H3K27me3 modifications. PCL-PRC2 complex, a major driver of H3K27 trimethylation, was upregulated by ancestral HSD exposure, and disrupting its activity eliminated the transgenerational inheritance of sweet sensitivity and feeding behavior deficits. Elevated H3K27me3 inhibited the expression of a transcriptional factor Cad and suppressed sweet sensitivity of the sweet-sensing gustatory neurons, reshaping the sweet perception and feeding behavior of the offspring. Taken together, we uncovered a novel molecular mechanism underlying behavioral abnormalities spanning multiple generations of offspring upon ancestral HSD exposure, which would contribute to the further understanding of long-term health risk of unbalanced diet.

Effects of Fructose and Palmitic Acid on Gene Expression in Drosophila melanogaster Larvae: Implications for Neurodegenerative Diseases

One of the largest health problems worldwide is the development of chronic noncommunicable diseases due to the consumption of hypercaloric diets. Among the most common alterations are cardiovascular diseases, and a high correlation between overnutrition and neurodegenerative diseases has also been found. The urgency in the study of specific damage to tissues such as the brain and intestine led us to use Drosophila melanogaster to study the metabolic effects caused by the consumption of fructose and palmitic acid in specific tissues. Thus, third instar larvae (96 ± 4 h) of the wild Canton-S strain of D. melanogaster were used to perform transcriptomic profiling in brain and midgut tissues to test for the potential metabolic effects of a diet supplemented with fructose and palmitic acid. Our data infer that this diet can alter the biosynthesis of proteins at the mRNA level that participate in the synthesis of amino acids, as well as fundamental enzymes for the dopaminergic and GABAergic systems in the midgut and brain. These also demonstrated alterations in the tissues of flies that may help explain the development of various reported human diseases associated with the consumption of fructose and palmitic acid in humans. These studies will not only help to better understand the mechanisms by which the consumption of these alimentary products is related to the development of neuronal diseases but may also contribute to the prevention of these conditions.

Crucial roles of UCH-L1 on insulin-producing cells and carbohydrate metabolism in Drosophila melanogaster model

Ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1) is a highly expressed protein in β cells and has been implicated in β cells' viability and function, however, the role of UCH-L1 in β cells remains unclear. Herein, we examined the functions of UCH-L1 in β cells by utilizing the Drosophila melanogaster model. Our results showed that specific knockdown of dUCH (D.melanogaster homolog of UCH-L1) in Drosophila Insulin-producing cells (D.melanogaster homolog of β cells) induced mitochondria fusion, IPCs death/degeneration, interfered with DILP2 secretion, and triggered the rise of glycogen storage and body weight. Strikingly, the impairment in IPCs cellular activities can be rescued by vitamin C- a strong antioxidant compound, which suggested the relationship between knockdown dUCH and oxidative stress in IPCs; and the potential of this model in screening compounds for β cells function moderation. Since carbohydrate metabolism is an important function of beta cells, we continued to examine the ability to regulate carbohydrate metabolism of knockdown dUCH flies. Our results showed that knockdown dUCH caused the decline of IPCs number under a high-sucrose diet, which finally led to metabolic and physiological disturbances, including total lipid rise, glycogen storage reduction, circulating carbohydrate increase, and weight loss. These symptoms could be early indications of metabolic disorders, particularly β cell dysfunction-related diseases. Taken together, our results indicate that dUCH is essential in the viability and functions of IPCs through the regulation of carbohydrate metabolism in the Drosophila model.

The proximate sources of genetic variation in body size plasticity: The relative contributions of feeding behaviour and development in Drosophila melanogaster

Body size is a key life-history trait that influences many aspects of an animal's biology and is shaped by a variety of factors, both genetic and environmental. While we know that locally-adapted populations differ in the extent to which body size responds plastically to environmental conditions like diet, we have a limited understanding of what causes these differences. We hypothesized that populations could differ in the way body size responds to nutrition either by modulating growth rate, development time, feeding rate, or a combination of the above. Using three locally-adapted populations of Drosophila melanogaster from along the east coast of Australia, we investigated body size plasticity across five different diets. We then assessed how these populations differed in feeding behaviour and developmental timing on each of the diets. We observed population-specific plastic responses to nutrition for body size and feeding rate, but not development time. However, differences in feeding rate did not fully explain the differences in the way body size responded to diet. Thus, we conclude that body size variation in locally-adapted populations is shaped by a combination of growth rate and feeding behaviour. This paves the way for further studies that explore how differences in the regulation of the genetic pathways that control feeding behaviour and growth rate contribute to population-specific responses of body size to diet.

Genome-wide analysis in Drosophila reveals diet-by-gene interactions and uncovers diet-responsive genes

Genetic and environmental factors play a major role in metabolic health. However, they do not act in isolation, as a change in an environmental factor such as diet may exert different effects based on an individual’s genotype. Here, we sought to understand how such gene–diet interactions influenced nutrient storage and utilization, a major determinant of metabolic disease. We subjected 178 inbred strains from the Drosophila genetic reference panel (DGRP) to diets varying in sugar, fat, and protein. We assessed starvation resistance, a holistic phenotype of nutrient storage and utilization that can be robustly measured. Diet influenced the starvation resistance of most strains, but the effect varied markedly between strains such that some displayed better survival on a high carbohydrate diet (HCD) compared to a high-fat diet while others had opposing responses, illustrating a considerable gene × diet interaction. This demonstrates that genetics plays a major role in diet responses. Furthermore, heritability analysis revealed that the greatest genetic variability arose from diets either high in sugar or high in protein. To uncover the genetic variants that contribute to the heterogeneity in starvation resistance, we mapped 566 diet-responsive SNPs in 293 genes, 174 of which have human orthologs. Using whole-body knockdown, we identified two genes that were required for glucose tolerance, storage, and utilization. Strikingly, flies in which the expression of one of these genes, CG4607 a putative homolog of a mammalian glucose transporter, was reduced at the whole-body level, displayed lethality on a HCD. This study provides evidence that there is a strong interplay between diet and genetics in governing survival in response to starvation, a surrogate measure of nutrient storage efficiency and obesity. It is likely that a similar principle applies to higher organisms thus supporting the case for nutrigenomics as an important health strategy.

Adipokine and fat body in flies: Connecting organs

Under conditions of nutritional and environmental stress, organismal homeostasis is preserved through inter-communication between multiple organs. To do so, higher organisms have developed a system of interorgan communication through which one tissue can affect the metabolism, activity or fate of remote organs, tissues or cells. In this review, we discuss the latest findings emphasizing Drosophila melanogaster as a powerful model organism to study these interactions and may constitute one of the best documented examples depicting the long-distance communication between organs. In flies, the adipose tissue appears to be one of the main organizing centers for the regulation of insect development and behavior: it senses nutritional and hormonal signals and in turn, orchestrates the release of appropriate adipokines. We discuss the nature and the role of recently uncovered adipokines, their regulations by external cues, their secretory routes and their modes of action to adjust developmental growth and timing accordingly. These findings have the potential for identification of candidate factors and signaling pathways that mediate conserved interorgan crosstalk.

Metabolic reprogramming in cancer: mechanistic insights from Drosophila

Cancer cells constantly reprogram their metabolism as the disease progresses. However, our understanding of the metabolic complexity of cancer remains incomplete. Extensive research in the fruit fly Drosophila has established numerous tumor models ranging from hyperplasia to neoplasia. These fly tumor models exhibit a broad range of metabolic profiles and varying nutrient sensitivity. Genetic studies show that fly tumors can use various alternative strategies, such as feedback circuits and nutrient-sensing machinery, to acquire and consolidate distinct metabolic profiles. These studies not only provide fresh insights into the causes and functional relevance of metabolic reprogramming but also identify metabolic vulnerabilities as potential targets for cancer therapy. Here, we review the conceptual advances in cancer metabolism derived from comparing and contrasting the metabolic profiles of fly tumor models, with a particular focus on the Warburg effect, mitochondrial metabolism, and the links between diet and cancer.

Summary: Recent research in fruit flies has demonstrated that tumors rewire their metabolism by using diverse strategies that involve feedback regulation, nutrient sensing, intercellular or even inter-organ interactions, yielding new molecules as potential cancer markers or drug targets.

'Hangry' Drosophila: food deprivation increases male aggression

Aggressive interactions are costly, such that individuals should display modified aggression in response to environmental stress. Many organisms experience frequent periods of food deprivation, which can influence an individual's capacity and motivation to engage in aggression. However, because food deprivation can simultaneously decrease an individual's resource-holding potential and increase its valuation of food resources, its net impact on aggression is unclear. Here, we tested the influence of increasingly prolonged periods of adult food deprivation on intermale aggression in pairs of fruit flies, Drosophila melanogaster. We found that males displayed increased aggression following periods of food deprivation longer than a day. Increased aggression in food-deprived flies occurred despite their reduced mass. This result is probably explained by an increased attraction to food resources, as food deprivation increased male occupancy of central food patches, and food patch occupancy was positively associated with aggression. Our findings demonstrate that aggressive strategies in male D. melanogaster are influenced by nutritional experience, highlighting the need to consider past nutritional stresses to understand variation in aggression.

On the Origin of Carnivory: Molecular Physiology and Evolution of Plants on an Animal Diet

Charles Darwin recognized that carnivorous plants thrive in nutrient-poor soil by capturing animals. Although the concept of botanical carnivory has been known for nearly 150 years, its molecular mechanisms and evolutionary origins have not been well understood until recently. In the last decade, technical advances have fueled the genome and transcriptome sequencings of active and passive hunters, leading to a better understanding of the traits associated with the carnivorous syndrome, from trap leaf development and prey digestion to nutrient absorption, exemplified, for example, by the Venus flytrap (Dionaea muscipula), pitcher plant (Cephalotus follicularis), and bladderwort (Utricularia gibba). The repurposing of defense-related genes is an important trend in the evolution of plant carnivory. In this review, using the Venus flytrap as a representative of the carnivorous plants, we summarize the molecular mechanisms underlying their ability to attract, trap, and digest prey and discuss the origins of plant carnivory in relation to their genomic evolution.

Characterization of Glycoside Hydrolase Families 13 and 31 Reveals Expansion and Diversification of α-Amylase Genes in the Phlebotomine Lutzomyia longipalpis and Modulation of Sandfly Glycosidase Activities by Leishmania Infection

Sugar-rich food sources are essential for sandflies to meet their energy demands, achieving more prolonged survival. The digestion of carbohydrates from food is mainly realized by glycoside hydrolases (GH). To identify genes coding for α-glycosidases and α-amylases belonging to Glycoside Hydrolase Family 13 (GH13) and Glycoside Hydrolase Family 31 (GH31) in Lutzomyia longipalpis, we performed an HMMER search against its genome using known sequences from other dipteran species. The sequences retrieved were classified based on BLASTP best hit, analysis of conserved regions by alignment with sequences of proteins with known structure, and phylogenetic analysis comparing with orthologous proteins from other dipteran species. Using RT-PCR analysis, we evaluated the expression of GH13 and GH31 genes, in the gut and rest of the body of females, in four different conditions: non-fed, sugar-fed, blood-fed, and Leishmania mexicana infected females. L. longipalpis has GH13/31 genes that code for enzymes involved in various aspects of sugar metabolism, as carbohydrate digestion, storage, and mobilization of glycogen reserves, proteins involved in transport, control of N-glycosylation quality, as well as others with a putative function in the regulation of myogenesis. These proteins are representatives of GH13 and GH31 families, and their roles seem to be conserved. Most of the enzymes seem to be active with conserved consense sequences, including the expected catalytic residues. α-amylases also demonstrated the presence of calcium and chloride binding sites. L. longipalpis genome shows an expansion in the α-amylase gene family, with two clusters. In contrast, a retraction in the number of α-glucosidases occurred. The expansion of α-amylases is probably related to the specialization of these proteins for different substrates or inhibitors, which might correlate with the higher diversity of plant foods available in the natural habitat of L. longipalpis. The expression of α-glucosidase genes is higher in blood-fed females, suggesting their role in blood digestion. Besides that, in blood-fed females infected with the parasite Leishmania mexicana, these genes were also modulated. Glycoside Hydrolases from families 13 and 31 are essential for the metabolism of L. longipalpis, and GH13 enzymes seem to be involved in the interaction between sandflies and Leishmania.

Experimental evolution of post-ingestive nutritional compensation in response to a nutrient-poor diet

The geometric framework of nutrition predicts that populations restricted to a single imbalanced diet should evolve post-ingestive nutritional compensation mechanisms bringing the blend of assimilated nutrients closer to physiological optimum. The evolution of such nutritional compensation is thought to be mainly driven by the ratios of major nutrients rather than overall nutritional content of the diet. We report experimental evolution of divergence in post-ingestive nutritional compensation in populations of Drosophila melanogaster adapted to diets that contained identical imbalanced nutrient ratios but differed in total nutrient concentration. Larvae from 'Selected' populations maintained for over 200 generations on a nutrient-poor diet with a 1 : 13.5 protein : carbohydrate ratio showed enhanced assimilation of nitrogen from yeasts and reduced assimilation of carbon from sucrose than 'Control' populations evolved on a diet with the same nutrient ratio but fourfold greater nutrient concentration. Compared to the Controls, the Selected larvae also accumulated less triglycerides relative to protein. This implies that the Selected populations evolved a higher assimilation rate of amino acids from the poor imbalanced diet and a lower assimilation of carbohydrates than Controls. Thus, the evolution of nutritional compensation may be driven by changes in total nutrient abundance, even if the ratios of different nutrients remain unchanged.

A developmental checkpoint directs metabolic remodelling as a strategy against starvation in Drosophila

Steroid hormones are crucial regulators of life-stage transitions during development in animals. However, the molecular mechanisms by which developmental transition through these stages is coupled with optimal metabolic homeostasis remains poorly understood. Here, we demonstrate through mathematical modelling and experimental validation that ecdysteroid-induced metabolic remodelling from resource consumption to conservation can be a successful life-history strategy to maximize fitness in Drosophila larvae in a fluctuating environment. Specifically, the ecdysteroid-inducible protein ImpL2 protects against hydrolysis of circulating trehalose following pupal commitment in larvae. Stored glycogen and triglycerides in the fat body are also conserved, even under fasting conditions. Moreover, pupal commitment dictates reduced energy expenditure upon starvation to maintain available resources, thus negotiating trade-offs in resource allocation at the physiological and behavioural levels. The optimal stage-specific metabolic shift elucidated by our predictive and empirical approaches reveals that Drosophila has developed a highly controlled system for ensuring robust development that may be conserved among higher-order organisms in response to intrinsic and extrinsic cues.

Drosophila as a model system for deciphering the 'host physiology-nutrition-microbiome' axis

For metazoans, nutritional stressors, such as undernutrition during growth and development, results in serious outcomes, including growth impairments and organ wasting. When undernutrition is accompanied by other complications, including chronic inflammation, a more complex pathophysiology may emerge, such as environmental enteropathy. Although nutrition is one of the most important environmental factors that influences host physiology, the mechanism by which undernutrition induces host pathophysiology is not fully understood. Recently, gut microbiome was found to alleviate undernutrition-induced pathophysiology in an insect model, revealing the importance of nutrition-microbiome interactions. Here, we discussed how nutrition-microbiome interactions influence host physiology, including growth, tissue homeostasis, immunity, and behavior, by regulating the central metabolic signaling pathways with an emphasis on findings made through Drosophila, an insect model.

The Effect of Different Dietary Sugars on the Development and Fecundity of Harmonia axyridis

The aim of this study was to screen synergistic substances included in existing artificial feeds in order to improve the fertility and survival rate of Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), an efficient pest predator. To this end, we analyzed the potential effects of glucose and trehalose on the growth, development, and reproduction of H. axyridis and evaluated the effect of three different artificial feeds on the energy stress of H. axyridis. The artificial diets contained fresh pork liver, honey, sucrose, vitamin C, and royal jelly, which was marked it as Diet1. The glucose was added to diet1, which was marked it as diet2, while adding trehalose to diet1 was marked as diet3. The pre-oviposition period of H. axyridis on Diet 1 was slower than that of Diet 2 and Diet 3. Additionally, the spawning quantity and incubation rate of insects on Diet 2 and Diet 3 were significantly higher than that of those on Diet 1. Finally, the larval developmental time on Diet 1 was significantly slower than that of Diet 2 and Diet 3. These results indicate that the addition of an appropriate amount of glucose or trehalose may affect positively the growth, development, and reproduction of H. axyridis. In addition, further studies showed that ATP, amino acids and fatty acids content in the H. axyridis also increased after the addition of the synergistic substance. All these results show that proper adjustment of stored energy anabolic and catabolism is important to maintain the metabolic balance of the insect’s entire life cycle and the addition of glucose or trehalose has a certain effect on the life indicators of H. axyridis.

Trehalose metabolism confers developmental robustness and stability in Drosophila by regulating glucose homeostasis

Organisms have evolved molecular mechanisms to ensure consistent and invariant phenotypes in the face of environmental fluctuations. Developmental homeostasis is determined by two factors: robustness, which buffers against environmental variations; and developmental stability, which buffers against intrinsic random variations. However, our understanding of these noise-buffering mechanisms remains incomplete. Here, we showed that appropriate glycemic control confers developmental homeostasis in the fruit fly Drosophila. We found that circulating glucose levels are buffered by trehalose metabolism, which acts as a glucose sink in circulation. Furthermore, mutations in trehalose synthesis enzyme (Tps1) increased the among-individual and within-individual variations in wing size. Whereas wild-type flies were largely resistant to changes in dietary carbohydrate and protein levels, Tps1 mutants experienced significant disruptions in developmental homeostasis in response to dietary stress. These results demonstrate that glucose homeostasis against dietary stress is crucial for developmental homeostasis.

Identification and Expression Analysis of Chemosensory Receptor Genes in Bradysia odoriphaga (Diptera: Sciaridae)

The chive midge, Bradysia odoriphaga, is a major insect pest affecting Chinese chive production in China. Its adult life stage is nonfeeding and has a short life span. Hence, the perception of chemical stimuli is important for its adult behavior and reproductive success. To better understand its chemosensory process at the molecular level, chemosensory receptor genes were identified based on transcriptomes of B. odoriphaga. In total, 101 chemosensory genes were identified from the antenna and body transcriptomes, including 71 odorant receptors (ORs), 18 ionotropic receptors (IRs), 5 gustatory receptors (GRs), and 7 sensory neuron membrane proteins (SNMPs). Phylogenetic analysis indicated that most of these genes have homologs among other Dipteran insects. A transcript abundance comparison based on FPKM values was conducted to analyze the sex- and tissue-specific expression profiles of these chemosensory genes. Moreover, quantitative real-time PCR of OR transcripts was performed on different tissues (female antennae, male antennae, heads, and legs) to verify the transcriptional expression levels of ORs in the transcriptomes. This analysis suggested that 44 ORs showed significantly higher expression in the female antennae, while 16 OR transcripts were most highly expressed in the male antennae and may play significant roles in sex pheromone detection. In addition, some IRs and GRs might be involved in CO2 and sugar detection and temperature sensing. In the present study, 101 chemosensory genes were identified, and their putative functions were predicted. This work could provide a basis to facilitate functional clarification of these chemosensory genes at the molecular level.

Fenoxycarb and methoxyfenozide (RH-2485) affected development and chitin synthesis through disturbing glycometabolism in Lymantria dispar larvae

Fenoxycarb as a juvenile hormone analogue and methoxyfenozide (RH-2485) as a 20-hydroxyecdysone (20E) agonist are two main insect growth regulators (IGRs) used for pest control, whose insecticidal mechanisms had been widely reported in past decades. However, there were few studies focused on their effects on the carbohydrate metabolism of insects. Here, we reported that two IGRs (fenoxycarb and RH-2485) significantly affected growth and development of L. dispar larvae and caused larval lethality. Furthermore, both contens of three sugars (glycogen, threhalose, glucose) in four tissues (fat body, midgut, hemolymph and epidermis) and trehalase activity in three tissues (fat body, midgut and hemolymph) of L. dispar larvae were markedly affected by these two IGRs. Moreover, we found that mRNA expression levels of LdTPS, LdTre1 and LdTre2 in L. dispar larvae were dramatically suppressed by two IGRs. Additionally, chitin content in both midgut and epidermis decreased significantly after L. dispar larvae treated with fenoxycarb or RH-2485. Summarily, these results indicated that these two IGRs disturbed glycometabolism in L. dispar larvae, resulting in impeding chitin synthesis, generating new epidermis failure, disrupting molting and larval lethality in the end.

Suppression of glycogen synthase expression reduces glycogen and lipid storage during mosquito overwintering diapause

During diapause in mosquitoes, efficient storage and utilization of energy are crucial for surviving prolonged periods of developmental arrest and for maximizing reproductive success once diapause is terminated and development recommences. In Culex pipiens, glycogen rapidly accumulates during early diapause (7-10 days after adult eclosion) and it is used to maintain energy homeostasis during the first month of diapause. In this study, a gene encoding glycogen synthase, which converts glucose residues into a polymeric chain for storage as glycogen, was characterized. After dsi-RNA directed against glycogen synthase was injected into mosquitoes programmed for diapause (reared under short day lengths), Cx. pipiens were fed 1% d-[¹³C₆]glucose, and the knockdown effects after 7-days were monitored by measuring ¹³C-labeled carbohydrate accumulation using solid-state NMR. The use of ¹³C cross-polarization magic-angle spinning spectrum showed a 46% reduction of ¹³C-labeled glycogen and a 6% reduction in lipid accumulation in glycogen synthase knockdown adult females. In addition, the suppression of glycogen synthase dramatically increased the mortality rate of diapausing Cx. pipiens by 88% at 30-days post injection. These findings indicated that glycogen synthase plays a critical role in regulating glycogen and lipid storages during overwintering diapause, and its function is essential for successful overwintering and survival of Cx. pipiens.

Effects of starvation on respiratory metabolism and energy metabolism in the cotton bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae)

Intermittent food shortages are commonly encountered in the wild. To cope with the threat of starvation, insects initiate a suite of behavioral activities and physiological countermeasures. The cotton bollworm, Helicoverpa armigera, is a major agricultural pest worldwide, but how H. armigera modulates its metabolism under starvation remains ambiguous. In the present study, the respiratory rates (V̇_O2; ml g^-1 h^-1) from the third-larval instar to the pupal stage were first determined. Our results highlighted a transient rise during the larval-larval molt and larval-pupal transition, followed by a sharp decline in the pupal stage and, finally, an upward trend before eclosion. When subjected to food deprivation, the starved larvae experienced a significant decline in the rates of O₂ consumed and CO₂ produced, as well as in respiratory quotient (RQ) values, indicative of severe metabolic depression during starvation and a shift of metabolic substrates with prolonged starvation. For metabolic substrate analysis, an apparent decline in triglyceride and glycogen contents was observed in the starved larvae, and the hemolymph trehalose content was significantly reduced throughout starvation. In addition, comparative transcriptome analysis showed that 48 h of larval starvation caused substantial transcriptional regulations in several energetically costly processes, wherein the marked up-regulations were detected in the pathways of glycolysis and fatty acid metabolism. Overall, our work makes a comprehensive study on the respiratory rate and energy metabolism in the starved H. armigera larvae, and provides a deep insight into the physiological adaptive strategies to alleviate nutritional stress.

Female bed bugs (Cimex lectularius L) anticipate the immunological consequences of traumatic insemination via feeding cues

We show that female insects can subtly change the management of their immune system contingent on infradian feeding cycles that act as cues to immune insult during mating. We experimentally reject the possibility that this is learned behavior, and show instead that it is dependent on the predictability of feeding which in turn is a cue for mating-induced infection. Although evidence exists for insect immune anticipation over life-time scales, this study links the temporal features of feeding to the insect’s mating behavior in the context of a system with infection caused by traumatic insemination. We predict similar mating ecology in other animals is likely to select for similar reproductive immune anticipation (RIA).

Not all encounters with pathogens are stochastic and insects can adjust their immune management in relation to cues associated with the likelihood of infection within a life cycle as well as across generations. In this study we show that female insects (bed bugs) up-regulate immune function in their copulatory organ in anticipation of mating by using feeding cues. Male bed bugs only mate with recently fed females and do so by traumatic insemination (TI). Consequently, there is a tight temporal correlation between female feeding and the likelihood of her being infected via TI. Females that received predictable access to food (and therefore predictable insemination and infection cycles) up-regulated induced immunity (generic antibacterial activity) in anticipation of feeding and mating. Females that received unpredictable (but the same mean periodicity) access to food did not. Females that anticipated mating-associated immune insult received measurable fitness benefits (survival and lifetime reproductive success) despite laying eggs at the same rate as females that were not able to predict these cycles. Given that mating is a time of increased likelihood of infection in many organisms, and is often associated with temporal cues such as courtship and/or feeding, we propose that anticipation of mating-associated infection in females may be more widespread than is currently evidenced.

The role of glycogen in development and adult fitness in Drosophila

The polysaccharide glycogen is an evolutionarily conserved storage form of glucose. However, the physiological significance of glycogen metabolism on homeostatic control throughout the animal life cycle remains incomplete. Here, we describe Drosophila mutants that have defective glycogen metabolism. Null mutants of glycogen synthase (GlyS) and glycogen phosphorylase (GlyP) displayed growth defects and larval lethality, indicating that glycogen plays a crucial role in larval development. Unexpectedly, however, a certain population of larvae developed into adults with normal morphology. Semi-lethality in glycogen mutants during the larval period can be attributed to the presence of circulating sugar trehalose. Homozygous glycogen mutants produced offspring, indicating that glycogen stored in oocytes is dispensable for embryogenesis. GlyS and GlyP mutants showed distinct metabolic defects in the levels of circulating sugars and triglycerides in a life stage-specific manner. In adults, glycogen as an energy reserve is not crucial for physical fitness and lifespan under nourished conditions, but glycogen becomes important under energy stress conditions. This study provides a fundamental understanding of the stage-specific requirements for glycogen metabolism in the fruit fly.

Insect Behavior and Physiological Adaptation Mechanisms Under Starvation Stress

Intermittent food shortages are commonly encountered in the wild. During winter or starvation stress, mammals often choose to hibernate while insects-in the form of eggs, mature larvae, pupae, or adults opt to enter diapause. In response to food shortages, insects may try to find sufficient food to maintain normal growth and metabolism through distribution of populations or even migration. In the face of hunger or starvation, insect responses can include changes in behavior and/or maintenance of a low metabolic rate through physiological adaptations or regulation. For instance, in order to maintain homeostasis of the blood sugar, trehalose under starvation stress, other sugars can be transformed to sustain basic energy metabolism. Furthermore, as the severity of starvation increases, lipids (especially triglycerides) are broken down to improve hunger resistance. Starvation stress simultaneously initiates a series of neural signals and hormone regulation processes in insects. These processes involve neurons or neuropeptides, immunity-related genes, levels of autophagy, heat shock proteins and juvenile hormone levels which maintain lower levels of physiological metabolic activity. This work focuses on hunger stress in insects and reviews its effects on behavior, energy reserve utilization, and physiological regulation. In summary, we highlight the diversity in adaptive strategies of insects to hunger stress and provides potential ideas to improve hunger resistance and cold storage development of natural enemy insects. This gist of literature on insects also broadens our understanding of the factors that dictate phenotypic plasticity in adjusting development and life histories around nutritionally optimal environmental conditions.

Interplay between diet-induced obesity and oxidative stress: Comparison between Drosophila and mammals

Obesity caused by excessive fat accumulation in adipocytes is a growing global problem and is a major contributing risk factor for many chronic metabolic diseases. There is increasing evidence that oxidative stress plays a crucial role in both obesity progression and obesity-related complications. In recent years, Drosophila models of diet-induced obesity and associated pathologies have been successfully developed through manipulation of carbohydrate or fat concentrations in the food. Obese flies accumulate triacylglycerols in the fat body, an organ homologous to mammalian adipose tissue and exhibit metabolic and physiological complications including hyperglycemia, redox imbalance and shortened longevity; these are all similar to those observed in obese humans. In this review, we summarize current data on the mechanisms of oxidative stress induction in obesity, with emphasis on metabolic switches and the involvement of redox-responsive signaling pathways such as NF-κB and Nfr2. The recent achievements with D. melanogaster model suggest a complicated relationship between obesity, oxidative stress, and longevity but the Drosophila model offers probably the best opportunities to delve further into unraveling these interactions, particularly the roles of antioxidants and of Nrf2-regulated responses, in order to increase our understanding of the obese metabolic phenotype and test and develop anti-obesity pharmaceuticals.

Obesity and Aging in the Drosophila Model

Being overweight increases the risk of many metabolic disorders, but how it affects lifespan is not completely clear. Not all obese people become ill, and the exact mechanism that turns excessive fat storage into a health-threatening state remains unknown. Drosophila melanogaster has served as an excellent model for many diseases, including obesity, diabetes, and hyperglycemia-associated disorders, such as cardiomyopathy or nephropathy. Here, we review the connections between fat storage and aging in different types of fly obesity. Whereas obesity induced by high-fat or high-sugar diet is associated with hyperglycemia, cardiomyopathy, and in some cases, shortening of lifespan, there are also examples in which obesity correlates with longevity. Transgenic lines with downregulations of the insulin/insulin-like growth factor (IIS) and target of rapamycin (TOR) signaling pathways, flies reared under dietary restriction, and even certain longevity selection lines are obese, yet long-lived. The mechanisms that underlie the differential lifespans in distinct types of obesity remain to be elucidated, but fat turnover, inflammatory pathways, and dysregulations of glucose metabolism may play key roles. Altogether, Drosophila is an excellent model to study the physiology of adiposity in both health and disease.

Fat body glycogen serves as a metabolic safeguard for the maintenance of sugar levels in Drosophila

Adapting to changes in food availability is a central challenge for survival. Glucose is an important resource for energy production, and therefore, many organisms synthesize and retain sugar storage molecules. In insects, glucose is stored in two different forms: the disaccharide trehalose and the branched polymer glycogen. Glycogen is synthesized and stored in several tissues, including in muscle and the fat body. Despite the important role of the fat body as a center for energy metabolism, the importance of its glycogen content remains unclear. Here, we show that glycogen metabolism is regulated in a tissue-specific manner under starvation conditions in the fruit fly Drosophila. The mobilization of fat body glycogen in larvae is independent of adipokinetic hormone (Akh, the glucagon homolog) but is regulated by sugar availability in a tissue-autonomous manner. Fat body glycogen plays a critical role in the maintenance of circulating sugars, including trehalose, under fasting conditions. These results demonstrate the importance of fat body glycogen as a metabolic safeguard in Drosophila.

Transforming Growth Factor β/Activin signaling in neurons increases susceptibility to starvation

Animals rely on complex signaling network to mobilize its energy stores during starvation. We have previously shown that the sugar-responsive TGFβ/Activin pathway, activated through the TGFβ ligand Dawdle, plays a central role in shaping the post-prandial digestive competence in the Drosophila midgut. Nevertheless, little is known about the TGFβ/Activin signaling in sugar metabolism beyond the midgut. Here, we address the importance of Dawdle (Daw) after carbohydrate ingestion. We found that Daw expression is coupled to dietary glucose through the evolutionarily conserved Mio-Mlx transcriptional complex. In addition, Daw activates the TGFβ/Activin signaling in neuronal populations to regulate triglyceride and glycogen catabolism and energy homeostasis. Loss of those neurons depleted metabolic reserves and rendered flies susceptible to starvation.