Natural variation in plasticity of glucose homeostasis and food intake

The foraging gene affects alcohol sensitivity, metabolism and memory in Drosophila

The genetic basis of alcohol use disorder (AUD) is complex. Understanding how natural genetic variation contributes to alcohol phenotypes can help us identify and understand the genetic basis of AUD. Recently, a single nucleotide polymorphism in the human foraging (for) gene ortholog, Protein Kinase cGMP-Dependent 1 (PRKG1), was found to be associated with stress-induced risk for alcohol abuse. However, the mechanistic role that PRKG1 plays in AUD is not well understood. We use natural variation in the Drosophila for gene to describe how variation of cGMP-dependent protein kinase (PKG) activity modifies ethanol-induced phenotypes. We found that variation in for affects ethanol-induced increases in locomotion and memory of the appetitive properties of ethanol intoxication. Further, these differences may stem from the ability to metabolize ethanol. Together, this data suggests that natural variation in PKG modulates cue reactivity for alcohol, and thus could influence alcohol cravings by differentially modulating metabolic and behavioral sensitivities to alcohol.

The foraging gene as a modulator of division of labour in social insects

The social ants, bees, wasps, and termites include some of the most ecologically-successful groups of animal species. Their dominance in most terrestrial environments is attributed to their social lifestyle, which enable their colonies to exploit environmental resources with remarkable efficiency. One key attribute of social insect colonies is the division of labour that emerges among the sterile workers, which represent the majority of colony members. Studies of the mechanisms that drive division of labour systems across diverse social species have provided fundamental insights into the developmental, physiological, molecular, and genomic processes that regulate sociality, and the possible genetic routes that may have led to its evolution from a solitary ancestor. Here we specifically discuss the conserved role of the foraging gene, which encodes a cGMP-dependent protein kinase (PKG). Originally identified as a behaviourally polymorphic gene that drives alternative foraging strategies in the fruit fly Drosophila melanogaster, changes in foraging expression and kinase activity were later shown to play a key role in the division of labour in diverse social insect species as well. In particular, foraging appears to regulate worker transitions between behavioural tasks and specific behavioural traits associated with morphological castes. Although the specific neuroethological role of foraging in the insect brain remains mostly unknown, studies in genetically tractable insect species indicate that PKG signalling plays a conserved role in the neuronal plasticity of sensory, cognitive and motor functions, which underlie behaviours relevant to division of labour, including appetitive learning, aggression, stress response, phototaxis, and the response to pheromones.

The Drosophila melanogaster foraging gene affects social networks

Drosophila melanogaster displays social behaviors including courtship, mating, aggression, and group foraging. Recent studies employed social network analyses (SNAs) to show that D. melanogaster strains differ in their group behavior, suggesting that genes influence social network phenotypes. Aside from genes associated with sensory function, few studies address the genetic underpinnings of these networks. The foraging gene (for) is a well-established example of a pleiotropic gene that regulates multiple behavioral phenotypes and their plasticity. In D. melanogaster, there are two naturally occurring alleles of for called rover and sitter that differ in their larval and adult food-search behavior as well as other behavioral phenotypes. Here, we hypothesize that for affects behavioral elements required to form social networks and the social networks themselves. These effects are evident when we manipulate gene dosage. We found that flies of the rover and sitter strains exhibit differences in duration, frequency, and reciprocity of pairwise interactions, and they form social networks with differences in assortativity and global efficiency. Consistent with other adult phenotypes influenced by for, rover-sitter heterozygotes show intermediate patterns of dominance in many of these characteristics. Multiple generations of backcrossing a rover allele into a sitter strain showed that many but not all of these rover-sitter differences may be attributed to allelic variation at for. Our findings reveal the significant role that for plays in affecting social network properties and their behavioral elements in Drosophila melanogaster.

Drosophila as a useful model for understanding the evolutionary physiology of obesity resistance and metabolic thrift

Evolved metabolic thriftiness in humans is a proposed contributor to the obesity epidemic. Insect models have been shown to evolve both 'metabolic thrift' in response to rearing on high-protein diets that promote leanness, and 'obesity resistance' when reared on fattening high-carbohydrate, low-protein foods. Despite the hypothesis that human obesity is caused by evolved metabolic thrift, genetic contributions to this physiological trait remain elusive. Here we conducted a pilot study to determine whether thrift and obesity resistance can arise under laboratory based 'quasi-natural selection' in the genetic model organism Drosophila melanogaster. We found that both these traits can evolve within 16 generations. Contrary to predictions from the 'thrifty genotype/phenotype' hypothesis, we found that when animals from a metabolic thrift inducing high-protein environment are mismatched to fattening high-carbohydrate foods, they did not become 'obese'. Rather, they accumulate less triglyceride than control animals, not more. We speculate that this may arise through as yet un-quantified parental effects - potentially epigenetic. This study establishes that D. melanogaster could be a useful model for elucidating the role of the trans- and inter-generational effects of diet on the genetics of metabolic traits in higher animals.

Drosophila melanogaster foraging regulates a nociceptive-like escape behavior through a developmentally plastic sensory circuit

Painful or threatening experiences trigger escape responses that are guided by nociceptive neuronal circuitry. Although some components of this circuitry are known and conserved across animals, how this circuitry is regulated at the genetic and developmental levels is mostly unknown. To escape noxious stimuli, such as parasitoid wasp attacks, Drosophila melanogaster larvae generate a curling and rolling response. Rover and sitter allelic variants of the Drosophila foraging (for) gene differ in parasitoid wasp susceptibility, suggesting a link between for and nociception. By optogenetically activating cells associated with each of for's promoters (pr1-pr4), we show that pr1 cells regulate larval escape behavior. In accordance with rover and sitter differences in parasitoid wasp susceptibility, we found that rovers have higher pr1 expression and increased sensitivity to nociception relative to sitters. The for null mutants display impaired responses to thermal nociception, which are rescued by restoring for expression in pr1 cells. Conversely, knockdown of for in pr1 cells phenocopies the for null mutant. To gain insight into the circuitry underlying this response, we used an intersectional approach and activity-dependent GFP reconstitution across synaptic partners (GRASP) to show that pr1 cells in the ventral nerve cord (VNC) are required for the nociceptive response, and that multidendritic sensory nociceptive neurons synapse onto pr1 neurons in the VNC. Finally, we show that activation of the pr1 circuit during development suppresses the escape response. Our data demonstrate a role of for in larval nociceptive behavior. This function is specific to for pr1 neurons in the VNC, guiding a developmentally plastic escape response circuit.

Epigallocatechin gallate (EGCG) alters body fat and lean mass through sex-dependent metabolic mechanisms in Drosophila melanogaster

There is increasing interest in the potential role of epigallocatechin gallate (EGCG) in changing body composition to lower body fat with increased lean mass. In this study, we examined the sex-dependent effect of EGCG on body composition, locomotion, feeding behaviour, sugar levels, and transcription levels of key regulators in lipid, carbohydrate, and energy metabolisms in Drosophila melanogaster. EGCG had no effects on body weights in both females and males, but decreased fat accumulation in females compared to the control, accompanied by a reduction in food intake. EGCG treatments increased lean mass and locomotor activity, and downregulated transcription levels of brummer (bmm), adipokinetic hormone (akh), and Drosophila insulin-like peptide 2 (dilp2), and upregulated spargel (srl) in males. In addition, EGCG decreased sugar levels in both females and males. In conclusion, EGCG promotes lean phenotype in D. melanogaster via sex-specific metabolic regulations.

Levels and activity of cyclic guanosine monophosphate-dependent protein kinase in nurse and forager honeybees

Age-dependent division of labour in honeybees was shown to be connected to sensory response thresholds. Foragers show a higher gustatory responsiveness than nurse bees. It is generally assumed that nutrition-related signalling pathways underlie this behavioural plasticity. Here, one important candidate gene is the foraging gene, which encodes a cyclic guanosine monophosphate-dependent protein kinase (PKG). Several roles of members of this enzyme family were analysed in vertebrates. They own functions in important processes such as growth, secretion and neuronal adaptation. Honeybee foraging messenger RNA expression is upregulated in the brain of foragers. In vivo activation of PKG can modulate gustatory responsiveness. We present for the first time PKG protein level and activity data in the context of social behaviour and feeding. Protein level was significantly higher in brains of foragers than in those of nurse bees, substantiating the role of PKG in behavioural plasticity. However, enzyme activity did not differ between behavioural roles. The mediation of feeding status appears independent of PKG signalling. Neither PKG content nor enzyme activity differed between starved and satiated individuals. We suggest that even though nutrition-related pathways are surely involved in controlling behavioural plasticity, which involves changes in PKG signalling, mediation of satiety itself is independent of PKG.

Pleiotropy of the Drosophila melanogaster foraging gene on larval feeding-related traits

Little is known about the molecular underpinning of behavioral pleiotropy. The Drosophila melanogaster foraging gene is highly pleiotropic, affecting many independent larval and adult phenotypes. Included in foraging's multiple phenotypes are larval foraging path length, triglyceride levels, and food intake. foraging has a complex structure with four promoters and 21 transcripts that encode nine protein isoforms of a cGMP dependent protein kinase (PKG). We examined if foraging's complex molecular structure underlies the behavioral pleiotropy associated with this gene. Using a promotor analysis strategy, we cloned DNA fragments upstream of each of foraging's transcription start sites and generated four separate forpr-Gal4s. Supporting our hypothesis of modular function, they had discrete, restricted expression patterns throughout the larva. In the CNS, forpr1-Gal4 and forpr4-Gal4 were expressed in neurons while forpr2-Gal4 and forpr3-Gal4 were expressed in glia cells. In the gastric system, forpr1-Gal4 and forpr3-Gal4 were expressed in enteroendocrine cells of the midgut while forpr2-Gal4 was expressed in the stem cells of the midgut. forpr3-Gal4 was expressed in the midgut enterocytes, and midgut and hindgut visceral muscle. forpr4-Gal4's gastric system expression was restricted to the hindgut. We also found promoter specific expression in the larval fat body, salivary glands, and body muscle. The modularity of foraging's molecular structure was also apparent in the phenotypic rescues. We rescued larval path length, triglyceride levels (bordered on significance), and food intake of for0 null larvae using different forpr-Gal4s to drive UAS-forcDNA. In a foraging null genetic background, forpr1-Gal4 was the only promoter driven Gal4 to rescue larval path length, forpr3-Gal4 altered triglyceride levels, and forpr4-Gal4 rescued food intake. Our results refine the spatial expression responsible for foraging's associated phenotypes, as well as the sub-regions of the locus responsible for their expression. foraging's pleiotropy arises at least in part from the individual contributions of its four promoters.

Evolutionary responses of Drosophila melanogaster under chronic malnutrition

Drosophila species have successfully spread and adapted to diverse climates across the globe. For D. melanogaster, rotting vegetative matter provides the primary substrate for mating and oviposition, and also acts as a nutritional resource for developing larvae and adult flies. The transitory nature of decaying vegetation exposes D. melanogaster to rapidly changing nutrient availability. As evidenced by their successful global spread, flies are capable of dealing with fluctuating nutritional reserves within their respective ecological niches. Therefore, D. melanogaster populations might contain standing genetic variation to support survival during periods of nutrient scarcity. The natural history and genetic tractability of D. melanogaster make the fly an ideal model for studies on the genetic basis of resistance to nutritional stress. We review artificial selection studies on nutritionally-deprived D. melanogaster and summarize the phenotypic outcomes of selected animals. Many of the reported evolved traits phenocopy mutants of the nutrient-sensing PI3K/Akt pathway. Given that the PI3K/Akt pathway is also responsive to acute nutritional stress, the PI3K/Akt pathway might underlie traits evolved under chronic nutritional deprivation. Future studies that directly test for the genetic mechanisms driving evolutionary responses to nutritional stress will take advantage of the ease in manipulating fly nutrient availability in the laboratory.

Adipokinetic hormone signaling determines dietary fatty acid preference through maintenance of hemolymph fatty acid composition in the cricket Gryllus bimaculatus

Adipokinetic hormone (AKH), an analog of mammalian glucagon, functions in supplying the required energy by releasing lipids and carbohydrates from the fat body into the hemolymph. Our previous study showed that AKH receptor (AKHR) knockdown in the two-spotted cricket Gryllus bimaculatus decreased hemolymph lipid levels and increased its feeding frequency. To reveal underlying mechanisms by which AKH signaling modulates lipid homeostasis, we analyzed the fatty acid composition as the lipid structure in the crickets. AKH administration significantly increased the proportion of unsaturated fatty acids (USFAs) to total fatty acids with decrease of the saturated fatty acids (SFAs) in hemolymph, while these proportions were inversely changed in RNA interference-mediated AKHR-knockdowned (AKHR^RNAi) crickets. Interestingly, knockdown of hormone-sensitive lipase (Hsl) by RNAi (Hsl^RNAi) affected the proportion of USFAs and SFAs in a similar manner to that observed in AKHR^RNAi crickets. AKH administration in Hsl^RNAi crickets did not change hemolymph fatty acid composition, indicating that AKH signaling critically altered fatty acid composition in the hemolymph through Hsl. In addition, a choice assay revealed that AKHR^RNAi significantly increases the preference of USFAs. These data indicate that hemolymph lipid level and composition were modulated by AKH signaling with a complementary feeding behavior toward USFAs.

Heat and humidity induced plastic changes in body lipids and starvation resistance in the tropical Zaprionus indianus of wet - dry seasons

Insects from tropical wet or dry seasons are likely to cope starvation stress through plastic changes (developmental as well as adult acclimation) in energy metabolites. Control and experimental groups of flies of Zaprionus indianus were reared under wet or dry conditions but adults were acclimated at different thermal or humidity conditions. Adult flies of control group were acclimated at 27°C and low (50% RH) or high (60% RH) humidity. For experimental groups, adult flies were acclimated at 32℃ for 1 to 6 days and under low (40% RH) or high (70% RH). For humidity acclimation, adult flies were acclimated at 27°C but under low (40% RH) or high (70% RH) for 1 to 6 days. Plastic changes in experimental groups as compared to control group (developmental as well as adult acclimation) revealed significant accumulation of body lipids due to thermal or humidity acclimation of wet season flies but low humidity acclimation did not change the level of body lipids in dry season flies. Starvation resistance and body lipids were higher in the males of dry season but in the females of wet season. Adult acclimation under thermal or humidity conditions exhibited changes in the rate of utilization of body lipids, carbohydrates and proteins. Adult acclimation of wet or dry season flies revealed plastic changes in mean daily fecundity; and a reduction in fecundity under starvation. Thus, thermal or humidity acclimation of adults revealed plastic changes in energy metabolites to support starvation resistance of wet or dry seasons flies.

Aggressive behaviours, food deprivation and the foraging gene

A pleiotropic gene governs multiple traits, which might constrain the evolution of complexity due to conflicting selection on these traits. However, if the pleiotropic effect is modular, then this can facilitate synergistic responses to selection on functionally related traits, thereby leveraging the evolution of complexity. To understand the evolutionary consequence of pleiotropy, the relation among functionally different traits governed by the same gene is key. We examined a pleiotropic function of the foraging (for) gene with its rover and sitter allelic variants in fruit fly, Drosophila melanogaster. We measured for's effect on adult male aggressive behaviours and whether this effect was shaped by for's known role in food-related traits. Rover exhibited higher levels of offensive behaviour than sitters and s2, a sitter-like mutant on rover genetic background. With a Markov chain model, we estimated the rate of aggression escalation, and found that the rover pattern of aggressive escalation more rapidly intensified fights. Subsequent analysis revealed that this was not caused by for's effect on food-related traits, suggesting that for might directly regulate aggressive behaviours. Food deprivation did not elevate aggression, but reduced intermediate-level aggressive behaviours. Aggression and other foraging-related behaviour might comprise a synergistic trait module underlaid by this pleiotropic gene.

The Drosophila foraging gene human orthologue PRKG1 predicts individual differences in the effects of early adversity on maternal sensitivity

There is variation in the extent to which childhood adverse experience affects adult individual differences in maternal behavior. Genetic variation in the animal foraging gene, which encodes a cGMP-dependent protein kinase, contributes to variation in the responses of adult fruit flies, Drosophila melanogaster, to early life adversity and is also known to play a role in maternal behavior in social insects. Here we investigate genetic variation in the human foraging gene (PRKG1) as a predictor of individual differences in the effects of early adversity on maternal behavior in two cohorts. We show that the PRKG1 genetic polymorphism rs2043556 associates with maternal sensitivity towards their infants. We also show that rs2043556 moderates the association between self-reported childhood adversity of the mother and her later maternal sensitivity. Mothers with the TT allele of rs2043556 appeared buffered from the effects of early adversity, whereas mothers with the presence of a C allele were not. Our study used the Toronto Longitudinal Cohort (N=288 mother-16 month old infant pairs) and the Maternal Adversity and Vulnerability and Neurodevelopment Cohort (N=281 mother-18 month old infant pairs). Our findings expand the literature on the contributions of both genetics and gene-environment interactions to maternal sensitivity, a salient feature of the early environment relevant for child neurodevelopment.

Feeding-Related Traits Are Affected by Dosage of the foraging Gene in Drosophila melanogaster

Nutrient acquisition and energy storage are critical parts of achieving metabolic homeostasis. The foraging gene in Drosophila melanogaster has previously been implicated in multiple feeding-related and metabolic traits. Before foraging's functions can be further dissected, we need a precise genetic null mutant to definitively map its amorphic phenotypes. We used homologous recombination to precisely delete foraging, generating the for⁰ null allele, and used recombineering to reintegrate a full copy of the gene, generating the {for^BAC} rescue allele. We show that a total loss of foraging expression in larvae results in reduced larval path length and food intake behavior, while conversely showing an increase in triglyceride levels. Furthermore, varying foraging gene dosage demonstrates a linear dose-response on these phenotypes in relation to foraging gene expression levels. These experiments have unequivocally proven a causal, dose-dependent relationship between the foraging gene and its pleiotropic influence on these feeding-related traits. Our analysis of foraging's transcription start sites, termination sites, and splicing patterns using rapid amplification of cDNA ends (RACE) and full-length cDNA sequencing, revealed four independent promoters, pr1-4, that produce 21 transcripts with nine distinct open reading frames (ORFs). The use of alternative promoters and alternative splicing at the foraging locus creates diversity and flexibility in the regulation of gene expression, and ultimately function. Future studies will exploit these genetic tools to precisely dissect the isoform- and tissue-specific requirements of foraging's functions and shed light on the genetic control of feeding-related traits involved in energy homeostasis.

Energy Homeostasis Control in Drosophila Adipokinetic Hormone Mutants

Maintenance of biological functions under negative energy balance depends on mobilization of storage lipids and carbohydrates in animals. In mammals, glucagon and glucocorticoid signaling mobilizes energy reserves, whereas adipokinetic hormones (AKHs) play a homologous role in insects. Numerous studies based on AKH injections and correlative studies in a broad range of insect species established the view that AKH acts as master regulator of energy mobilization during development, reproduction, and stress. In contrast to AKH, the second peptide, which is processed from the Akh encoded prohormone [termed "adipokinetic hormone precursor-related peptide" (APRP)] is functionally orphan. APRP is discussed as ecdysiotropic hormone or as scaffold peptide during AKH prohormone processing. However, as in the case of AKH, final evidence for APRP functions requires genetic mutant analysis. Here we employed CRISPR/Cas9-mediated genome engineering to create AKH and AKH plus APRP-specific mutants in the model insect Drosophila melanogaster. Lack of APRP did not affect any of the tested steroid-dependent processes. Similarly, Drosophila AKH signaling is dispensable for ontogenesis, locomotion, oogenesis, and homeostasis of lipid or carbohydrate storage until up to the end of metamorphosis. During adulthood, however, AKH regulates body fat content and the hemolymph sugar level as well as nutritional and oxidative stress responses. Finally, we provide evidence for a negative autoregulatory loop in Akh gene regulation.

PKG in honey bees: spatial expression, Amfor gene expression, sucrose responsiveness, and division of labor

Division of labor is a hallmark of social insects. In honey bees, division of labor involves transition of female workers from one task to the next. The most distinct tasks are nursing (providing food for the brood) and foraging (collecting pollen and nectar). The brain mechanisms regulating this form of behavioral plasticity have largely remained elusive. Recently, it was suggested that division of labor is based on nutrition-associated signaling pathways. One highly conserved gene associated with food-related behavior across species is the foraging gene, which encodes a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG). Our analysis of this gene reveals the presence of alternative splicing in the honey bee. One isoform is expressed in the brain. Expression of this isoform is most pronounced in the mushroom bodies, the subesophageal ganglion, and the corpora allata. Division of labor and sucrose responsiveness in honey bees correlate significantly with foraging gene expression in distinct brain regions. Activating PKG selectively increases sucrose responsiveness in nurse bees to the level of foragers, whereas the same treatment does not affect responsiveness to light. These findings demonstrate a direct link between PKG signaling in distinct brain areas and division of labor. Furthermore, they demonstrate that the difference in sensory responsiveness between nurse bees and foragers can be compensated for by activating PKG. Our findings on the function of PKG in regulating specific sensory responsiveness and social organization offer valuable indications for the function of the cGMP/PKG pathway in many other insects and vertebrates.

Role of Phote-HrTH (Phormia terraenovae hypertrehalosemic hormone) in modulating the supercontractile muscles of the crop of adult Phormia regina Meigen

Phote-HrTH (Phormia terraenovae hypertrehalosemic hormone) has been demonstrated in the Diptera to be involved in flight metabolism, reproduction, and diapause. Each of these events needs the hormone's action and requirement for carbohydrates is the common denominator. In Diptera, carbohydrates are taken up during feeding by action of the cibarial pump and are then stored in the crop. Using adult Phormia regina, both a bioassay and electrophysiological recordings show that Phote-HrTH slows down or inhibits the crop lobe muscles (P5) and, at the same time, stimulates the muscles of the pump 4 (P4) involved in pushing fluids out of the crop and up into the midgut for digestion. The EC50 for P4 was in the nanomolar range while the IC50 for P5 was 1.4-75.1 pM. The effect of Phote-HrTH on P4/5 suggests that the peptide is important in coordinating the two pumps, which are involved in moving carbohydrates up into the midgut for digestion. The adult crop organ is an essential storage organ for carbohydrates and now should be considered an important structure capable of delivering nutrients to the midgut for digestion.

Tissue-specific direct microtransfer of nanomaterials into Drosophila embryos as a versatile in vivo test bed for nanomaterial toxicity assessment

Nanomaterials are the subject of intense research, focused on their synthesis, modification, and biomedical applications. Increased nanomaterial production and their wide range of applications imply a higher risk of human and environmental exposure. Unfortunately, neither environmental effects nor toxicity of nanomaterials to organisms are fully understood. Cost-effective, rapid toxicity assays requiring minimal amounts of materials are needed to establish both their biomedical potential and environmental safety standards. Drosophila exemplifies an efficient and cost-effective model organism with a vast repertoire of in vivo tools and techniques, all with high-throughput scalability and screening feasibility throughout its life cycle. Here we report tissue specific nanomaterial assessment through direct microtransfer into target tissues. We tested several nanomaterials with potential biomedical applications such as single-wall carbon nanotubes, multiwall carbon nanotubes, silver, gold, titanium dioxide, and iron oxide nanoparticles. Assessment of nanomaterial toxicity was conducted by evaluating progression through developmental morphological milestones in Drosophila. This cost-effective assessment method is amenable to high-throughput screening.

Developmental- and food-dependent foraging transcript levels in the desert locust

Drastic changes in the environment during a lifetime require developmental and physiological flexibility to ensure animal survival. Desert locusts, Schistocerca gregaria, live in an extremely changeable environment, which alternates between periods of rainfall and abundant food and periods of drought and starvation. In order to survive, locusts display an extreme form of phenotypic plasticity that allows them to rapidly cope with these changing conditions by converting from a cryptic solitarious phase to a swarming, voracious gregarious phase. To accomplish this, locusts possess different conserved mediators of phenotypic plasticity. Recently, attention has been drawn to the possible roles of protein kinases in this process. In addition to cyclic AMP-dependent protein kinase (PKA), also cyclic GMP-dependent protein kinase (PKG), which was shown to be involved in changes of food-related behavior in a variety of insects, has been associated with locust phenotypic plasticity. In this article, we study the transcript levels of the S. gregaria orthologue of the foraging gene that encodes a PKG in different food-related, developmental and crowding conditions. Transcript levels of the S. gregaria foraging orthologue are highest in different parts of the gut and differ between isolated and crowd-reared locusts. They change when the availability of food is altered, display a distinct pattern with higher levels after a moult and decrease with age during postembryonic development.

Rapid effects of humidity acclimation on stress resistance in Drosophila melanogaster

We tested the hypothesis whether developmental acclimation at ecologically relevant humidity regimes (40% and 75% RH) affects desiccation resistance of pre-adults (3rd instar larvae) and adults of Drosophila melanogaster Meigen (Diptera: Drosophilidae). Additionally, we untangled whether drought (40% RH) acclimation affects cold-tolerance in the adults of D. melanogaster. We observed that low humidity (40% RH) acclimated individuals survived significantly longer (1.6-fold) under lethal levels of desiccation stress (0-5% RH) than their counter-replicates acclimated at 75% RH. In contrast to a faster duration of development of 1st and 2nd instar larvae, 3rd instar larvae showed a delayed development at 40% RH as compared to their counterparts grown at 75% RH. Rearing to low humidity conferred an increase in bulk water, hemolymph content and dehydration tolerance, consistent with increase in desiccation resistance for replicates grown at 40% as compared to their counterparts at 75% RH. Further, we found a trade-off between the levels of carbohydrates and body lipid reserves at 40% and 75% RH. Higher levels of carbohydrates sustained longer survival under desiccation stress for individuals developed at 40% RH than their congeners at 75% RH. However, the rate of carbohydrate utilization did not differ between the individuals reared at these contrasting humidity regimes. Interestingly, our results of accelerated failure time (AFT) models showed substantial decreased death rates at a series of low temperatures (0, -2, or -4°C) for replicates acclimated at 40% RH as compared to their counter-parts at 75% RH. Therefore, our findings indicate that development to low humidity conditions constrained on multiple physiological mechanisms of water-balance, and conferred cross-tolerance towards desiccation and cold stress in D. melanogaster. Finally, we suggest that the ability of generalist Drosophila species to tolerate fluctuations in humidity might aid in their existence and abundance under expected changes in moisture level in course of global climate change.

Alleviating brain stress: what alternative animal models have revealed about therapeutic targets for hypoxia and anoxia

While the mammalian brain is highly dependent on oxygen, and can withstand only a few minutes without air, there are both vertebrate and invertebrate examples of anoxia tolerance. One example is the freshwater turtle, which can withstand days without oxygen, thus providing a vertebrate model with which to examine the physiology of anoxia tolerance without the pathology seen in mammalian ischemia/reperfusion studies. Insect models such as Drosophila melanogaster have additional advantages, such as short lifespans, low cost and well-described genetics. These models of anoxia tolerance share two common themes that enable survival without oxygen: entrance into a state of deep hypometabolism, and the suppression of cellular injury during anoxia and upon restoration of oxygen. The study of such models of anoxia tolerance, adapted through millions of years of evolution, may thus suggest protective pathways that could serve as therapeutic targets for diseases characterized by oxygen deprivation and ischemic/reperfusion injuries.

Gene-environment interplay in Drosophila melanogaster: chronic food deprivation in early life affects adult exploratory and fitness traits

Early life adversity has known impacts on adult health and behavior, yet little is known about the gene-environment interactions (GEIs) that underlie these consequences. We used the fruit fly Drosophila melanogaster to show that chronic early nutritional adversity interacts with rover and sitter allelic variants of foraging (for) to affect adult exploratory behavior, a phenotype that is critical for foraging, and reproductive fitness. Chronic nutritional adversity during adulthood did not affect rover or sitter adult exploratory behavior; however, early nutritional adversity in the larval period increased sitter but not rover adult exploratory behavior. Increasing for gene expression in the mushroom bodies, an important center of integration in the fly brain, changed the amount of exploratory behavior exhibited by sitter adults when they did not experience early nutritional adversity but had no effect in sitters that experienced early nutritional adversity. Manipulation of the larval nutritional environment also affected adult reproductive output of sitters but not rovers, indicating GEIs on fitness itself. The natural for variants are an excellent model to examine how GEIs underlie the biological embedding of early experience.

Gustatory perception and fat body energy metabolism are jointly affected by vitellogenin and juvenile hormone in honey bees

Honey bees (Apis mellifera) provide a system for studying social and food-related behavior. A caste of workers performs age-related tasks: young bees (nurses) usually feed the brood and other adult bees inside the nest, while older bees (foragers) forage outside for pollen, a protein/lipid source, or nectar, a carbohydrate source. The workers' transition from nursing to foraging and their foraging preferences correlate with differences in gustatory perception, metabolic gene expression, and endocrine physiology including the endocrine factors vitellogenin (Vg) and juvenile hormone (JH). However, the understanding of connections among social behavior, energy metabolism, and endocrine factors is incomplete. We used RNA interference (RNAi) to perturb the gene network of Vg and JH to learn more about these connections through effects on gustation, gene transcripts, and physiology. The RNAi perturbation was achieved by single and double knockdown of the genes ultraspiracle (usp) and vg, which encode a putative JH receptor and Vg, respectively. The double knockdown enhanced gustatory perception and elevated hemolymph glucose, trehalose, and JH. We also observed transcriptional responses in insulin like peptide 1 (ilp1), the adipokinetic hormone receptor (AKHR), and cGMP-dependent protein kinase (PKG, or "foraging gene" Amfor). Our study demonstrates that the Vg-JH regulatory module controls changes in carbohydrate metabolism, but not lipid metabolism, when worker bees shift from nursing to foraging. The module is also placed upstream of ilp1, AKHR, and PKG for the first time. As insulin, adipokinetic hormone (AKH), and PKG pathways influence metabolism and gustation in many animals, we propose that honey bees have conserved pathways in carbohydrate metabolism and conserved connections between energy metabolism and gustatory perception. Thus, perhaps the bee can make general contributions to the understanding of food-related behavior and metabolic disorders.

Evolution of foraging behaviour in response to chronic malnutrition in Drosophila melanogaster

Chronic exposure to food of low quality may exert conflicting selection pressures on foraging behaviour. On the one hand, more active search behaviour may allow the animal to find patches with slightly better, or more, food; on the other hand, such active foraging is energetically costly, and thus may be opposed by selection for energetic efficiency. Here, we test these alternative hypotheses in Drosophila larvae. We show that populations which experimentally evolved improved tolerance to larval chronic malnutrition have shorter foraging path length than unselected control populations. A behavioural polymorphism in foraging path length (the rover-sitter polymorphism) exists in nature and is attributed to the foraging locus (for). We show that a sitter strain (for(s2)) survives better on the poor food than the rover strain (for(R)), confirming that the sitter foraging strategy is advantageous under malnutrition. Larvae of the selected and control populations did not differ in global for expression. However, a quantitative complementation test suggests that the for locus may have contributed to the adaptation to poor food in one of the selected populations, either through a change in for allele frequencies, or by interacting epistatically with alleles at other loci. Irrespective of its genetic basis, our results provide two independent lines of evidence that sitter-like foraging behaviour is favoured under chronic larval malnutrition.

Analysis of feeding behavior of Drosophila larvae on liquid food

The food responses of Drosophila larvae offer an excellent opportunity to study the genetic and neural regulation of feeding behavior. Compared with fed larvae, hungry larvae are more likely to display aggressive foraging, rapid food intake, compensatory feeding, and stress-resistant food procurement. Behavioral assays have been developed to quantitatively assess particular aspects of the hunger-driven food response. In combination, these assays help define the specific role of signaling molecules or neurons in the regulation of feeding behavior in foraging larvae. This protocol describes the analysis of larvae feeding on liquid food. The test is designed for quantitative assessment of the food ingestion rate of individual larvae under different energy states. It provides a simple and reliable way to measure the graded modification of the baseline feeding rate of larvae as food deprivation is prolonged. The test is applicable to routine functional testing and larger-scale screening of genetic mutations and biologics that might affect food consumption.

Analysis of feeding behavior of Drosophila larvae on solid food

The food responses of Drosophila larvae offer an excellent opportunity to study the genetic and neural regulation of feeding behavior. Compared with fed larvae, hungry larvae are more likely to display aggressive foraging, rapid food intake, compensatory feeding, and stress-resistant food procurement. Behavioral assays have been developed to quantitatively assess particular aspects of the hunger-driven food response. In combination, these assays help define the specific role of signaling molecules or neurons in the regulation of feeding behavior in foraging larvae. This protocol is designed for quantitative assessment of the willingness of individual larvae to procure solid food under different energy states. It provides a simple and reliable way to measure the graded modification of the baseline feeding rate of larvae as the period of food deprivation is increased. The test is applicable to routine functional testing and larger-scale screening of genetic mutations and biologics that might affect food consumption.

Conservation of gene function in behaviour

Behaviour genetic research has shown that a given gene or gene pathway can influence categorically similar behaviours in different species. Questions about the conservation of gene function in behaviour are increasingly tractable. This is owing to the surge of DNA and 'omics data, bioinformatic tools, as well as advances in technologies for behavioural phenotyping. Here, we discuss how gene function, as a hierarchical biological phenomenon, can be used to examine behavioural homology across species. The question can be addressed independently using different levels of investigation including the DNA sequence, the gene's position in a genetic pathway, spatial-temporal tissue expression and neural circuitry. Selected examples from the literature are used to illustrate this point. We will also discuss how qualitative and quantitative comparisons of the behavioural phenotype, its function and the importance of environmental and social context should be used in cross-species comparisons. We conclude that (i) there are homologous behaviours, (ii) they are hard to define and (iii) neurogenetics and genomics investigations should help in this endeavour.

Nutritional regulation of division of labor in honey bees: toward a systems biology perspective

Organisms adapt their behavior and physiology to environmental conditions through processes of phenotypic plasticity. In one well-studied example, the division of labor among worker honey bees involves a stereotyped yet plastic pattern of behavioral and physiological maturation. Early in life, workers perform brood care and other in-hive tasks and have large internal nutrient stores; later in life, they forage for nectar and pollen outside the hive and have small nutrient stores. The pace of maturation depends on colony conditions, and the environmental, physiological, and genomic mechanisms by which this occurs are being actively investigated. Here we review current knowledge of the mechanisms by which a key environmental variable, nutritional status, influences worker honey bee division of labor. These studies demonstrate that changes in individual nutritional status and conserved food-related molecular and hormonal pathways regulate the age at which individual bees begin to forage. We then outline ways in which systems biology approaches, enabled by the sequencing of the honey bee genome, will allow researchers to gain deeper insight into nutritional regulation of honey bee behavior, and phenotypic plasticity in general.

Age- and task-dependent foraging gene expression in the bumblebee Bombus terrestris

In eusocial insects, the division of labor within a colony, based on either age or size, is correlated with a differential foraging (for) gene expression and PKG activity. This article presents in the first part a study on the for gene, encoding a cGMP-dependent protein kinase (PKG) in the bumblebee Bombus terrestris. Cloning of the open reading frame allowed phylogenetic tracing, which showed conservation of PKGs among social insects. Our results confirm the proposed role for PKGs in division of labor. Btfor gene expression is significantly higher in the larger foragers compared with the smaller sized nurses. More importantly, we discovered an age-related decrease in Btfor expression in both nursing and foraging bumblebees. We therefore speculate that the presence of BtFOR is required for correct adaptation to new external stimuli and rapid learning for foraging. In a second series of experiments, worker bumblebees of B. terrestris were treated with two insecticides imidacloprid and kinoprene, which have shown to cause impaired foraging behavior. Compared with controls, only the latter treatment resulted in a decreased Btfor expression, which concurs with a stimulation of ovarian growth and a shift in labor toward nest-related tasks. The data are discussed in relation to Btfor expression in the complex physiological event of foraging and side-effects by pesticides.

Energetics of insect diapause

Managing metabolic resources is critical for insects during diapause when food sources are limited or unavailable. Insects accumulate reserves prior to diapause, and metabolic depression during diapause promotes reserve conservation. Sufficient reserves must be sequestered to both survive the diapause period and enable postdiapause development that may involve metabolically expensive functions such as metamorphosis or long-distance flight. Nutrient utilization during diapause is a dynamic process, and insects appear capable of sensing their energy reserves and using this information to regulate whether to enter diapause and how long to remain in diapause. Overwintering insects on a tight energy budget are likely to be especially vulnerable to increased temperatures associated with climate change. Molecular mechanisms involved in diapause nutrient regulation remain poorly known, but insulin signaling is likely a major player. We also discuss other possible candidates for diapause-associated nutrient regulation including adipokinetic hormone, neuropeptide F, the cGMP-kinase For, and AMPK.

Dietary protein content affects evolution for body size, body fat and viability in Drosophila melanogaster

The ability to use different food sources is likely to be under strong selection if organisms are faced with natural variation in macro-nutrient (protein, carbohydrate and lipid) availabilities. Here, we use experimental evolution to study how variable dietary protein content affects adult body composition and developmental success in Drosophila melanogaster. We reared flies on either a standard diet or a protein-enriched diet for 17 generations before testing them on both diet types. Flies from lines selected on protein-rich diet produced phenotypes with higher total body mass and relative lipid content when compared with those selected on a standard diet, irrespective of which of the two diets they were tested on. However, selection on protein-rich diet incurred a cost as flies reared on this diet had markedly lower developmental success in terms of egg-to-adult viability on both medium types, suggesting a possible trade-off between the traits investigated.

Effects of inbreeding on aversive learning in Drosophila

Inbreeding adversely affects life history traits as well as various other fitness-related traits, but its effect on cognitive traits remains largely unexplored, despite their importance to fitness of many animals under natural conditions. We studied the effects of inbreeding on aversive learning (avoidance of an odour previously associated with mechanical shock) in multiple inbred lines of Drosophila melanogaster derived from a natural population through up to 12 generations of sib mating. Whereas the strongly inbred lines after 12 generations of inbreeding (0.75<F<0.93) consistently showed reduced egg-to-adult viability (on average by 28%), the reduction in learning performance varied among assays (average=18% reduction), being most pronounced for intermediate conditioning intensity. Furthermore, moderately inbred lines (F=0.38) showed no detectable decline in learning performance, but still had reduced egg-to-adult viability, which indicates that overall inbreeding effects on learning are mild. Learning performance varied among strongly inbred lines, indicating the presence of segregating variance for learning in the base population. However, the learning performance of some inbred lines matched that of outbred flies, supporting the dominance rather than the overdominance model of inbreeding depression for this trait. Across the inbred lines, learning performance was positively correlated with the egg-to-adult viability. This positive genetic correlation contradicts a trade-off observed in previous selection experiments and suggests that much of the genetic variation for learning is owing to pleiotropic effects of genes affecting functions related to survival. These results suggest that genetic variation that affects learning specifically (rather than pleiotropically through general physiological condition) is either low or mostly due to alleles with additive (semi-dominant) effects.

The locust foraging gene

Our knowledge of how genes act on the nervous system in response to the environment to generate behavioral plasticity is limited. A number of recent advancements in this area concern food-related behaviors and a specific gene family called foraging (for), which encodes a cGMP-dependent protein kinase (PKG). The desert locust (Schistocerca gregaria) is notorious for its destructive feeding and long-term migratory behavior. Locust phase polyphenism is an extreme example of environmentally induced behavioral plasticity. In response to changes in population density, locusts dramatically alter their behavior, from solitary and relatively sedentary behavior to active aggregation and swarming. Very little is known about the molecular and genetic basis of this striking behavioral phenomenon. Here we initiated studies into the locust for gene by identifying, cloning, and studying expression of the gene in the locust brain. We determined the phylogenetic relationships between the locust PKG and other known PKG proteins in insects. FOR expression was found to be confined to neurons of the anterior midline of the brain, the pars intercerebralis. Our results suggest that differences in PKG enzyme activity are correlated to well-established phase-related behavioral differences. These results lay the groundwork for functional studies of the locust for gene and its possible relations to locust phase polyphenism.

Social interactions in "simple" model systems

Deciphering the genetic and neurobiological underpinnings of social behavior is a difficult task. Simple model organisms such as C. elegans, Drosophila, and social insects display a wealth of social behaviors similar to those in more complex animals, including social dominance, group decision making, learning from experienced individuals, and foraging in groups. Although the study of social interactions is still in its infancy, the ability to assess the contributions of gene expression, neural circuitry, and the environment in response to social context in these simple model organisms is unsurpassed. Here, I take a comparative approach, discussing selected examples of social behavior across species and highlighting the common themes that emerge.

Genotype-by-diet interactions drive metabolic phenotype variation in Drosophila melanogaster

The rising prevalence of complex disease suggests that alterations to the human environment are increasing the proportion of individuals who exceed a threshold of liability. This might be due either to a global shift in the population mean of underlying contributing traits, or to increased variance of such underlying endophenotypes (such as body weight). To contrast these quantitative genetic mechanisms with respect to weight gain, we have quantified the effect of dietary perturbation on metabolic traits in 146 inbred lines of Drosophila melanogaster and show that genotype-by-diet interactions are pervasive. For several metabolic traits, genotype-by-diet interactions account for far more variance (between 12 and 17%) than diet alone (1-2%), and in some cases have as large an effect as genetics alone (11-23%). Substantial dew point effects were also observed. Larval foraging behavior was found to be a quantitative trait exhibiting significant genetic variation for path length (P = 0.0004). Metabolic and fitness traits exhibited a complex correlation structure, and there was evidence of selection minimizing weight under laboratory conditions. In addition, a high fat diet significantly increases population variance in metabolic phenotypes, suggesting decreased robustness in the face of dietary perturbation. Changes in metabolic trait mean and variance in response to diet indicates that shifts in both population mean and variance in underlying traits could contribute to increases in complex disease.

The adipokinetic hormone system in Culicinae (Diptera: Culicidae): molecular identification and characterization of two adipokinetic hormone (AKH) precursors from Aedes aegypti and Culex pipiens and two putative AKH receptor variants from A. aegypti

Insect neuropeptides of the adipokinetic hormone (AKH) family induce the mobilization of energy stores to fuel flight, but also affect the nutritional balance during diapause and oogenesis. They are therefore important regulators for flight, hibernation, and reproduction in mosquitoes including those that transmit human pathogens. In this study, we identified and analyzed the genes encoding two AKH preprohormones in the Yellow fever mosquito, Aedes aegypti: Aedae-AKH-I encodes the octapeptide pELFTPSWa and Aedae-AKH-II the decapeptide pEVTFSRDWNAa. Identical AKHs were identified in the West Nile virus vector, Culex pipiens, whose genes were characterized in this study as Culpi-AKH-I and Culpi-AKH-II. Using Northern blot, transcript expression was shown in A. aegypti, for Aedae-AKH-I in the head/thorax tissues of pupae and females, as well as in the abdomen of adult males; Aedae-AKH-II was only expressed in adults. In an immunocytological study using an AKH-antibody, the corpus cardiacum (CC), the intrinsic CC-cells (X-cells), the nervi corporis cardiaci, cells in the brain and thoracic ganglia were stained. In addition, two splice variants of the AKH-receptor gene were characterized in A. aegypti, (Aedae-AKHR-I and -II). RT-PCR revealed that both variants of these typical G-protein-coupled receptors were expressed in all life stages. Aedae-AKHR-I expression was also detected in the ovaries, indicating once more the influence of the AKH/AKHR system during the insect's oogenesis. Based on phylogenetic data, we postulate two closely related types of AKH-receptors that could bind selectively the two AKH peptides found in A. aegypti.

The Drosophila foraging gene mediates adult plasticity and gene-environment interactions in behaviour, metabolites, and gene expression in response to food deprivation

Nutrition is known to interact with genotype in human metabolic syndromes, obesity, and diabetes, and also in Drosophila metabolism. Plasticity in metabolic responses, such as changes in body fat or blood sugar in response to changes in dietary alterations, may also be affected by genotype. Here we show that variants of the foraging (for) gene in Drosophila melanogaster affect the response to food deprivation in a large suite of adult phenotypes by measuring gene by environment interactions (GEI) in a suite of food-related traits. for affects body fat, carbohydrates, food-leaving behavior, metabolite, and gene expression levels in response to food deprivation. This results in broad patterns of metabolic, genomic, and behavioral gene by environment interactions (GEI), in part by interaction with the insulin signaling pathway. Our results show that a single gene that varies in nature can have far reaching effects on behavior and metabolism by acting through multiple other genes and pathways.

Age, but not experience, affects courtship gene expression in male Drosophila melanogaster

Mutation screens in model organisms have helped identify the foundation of many fundamental organismal phenotypes. An emerging question in evolutionary and behavioral biology is the extent to which these “developmental” genes contribute to the subtle individual variation that characterizes natural populations. A related question is whether individual differences arise from static differences in gene expression that arose during previous life stages, or whether they are due to dynamic regulation of expression during the life stage under investigation. Here, we address these questions using genes that have been discovered to control the development of normal courtship behavior in male Drosophila melanogaster. We examined whether these genes have static or dynamic expression in the heads of adult male flies of different ages and with different levels of social experience. We found that 16 genes of the 25 genes examined were statically expressed, and 9 genes were dynamically expressed with changes related to adult age. No genes exhibited rapid dynamic expression changes due to social experience or age*experience interaction. We therefore conclude that a majority of fly “courtship” genes are statically expressed, while a minority are regulated in adults with respect to age, but not with respect to relevant social experience. These results are consistent with those from a recent microarray analysis that found none of the canonical courtship genes changed expression in male flies after brief exposure to females.

cGMP-dependent protein kinase: linking foraging to energy homeostasis

Successful foraging is necessary for procurement of nutritional resources essential for an animal’s survival. Maintenance of foraging and food acquisition is dependent on the ability to balance food intake and energy expenditure. This review examines the role of cGMP-dependent protein kinase (PKG) as a regulator of foraging behaviour, food acquisition, and energy balance. The role of PKG in food-related behaviours is highly conserved among worms, flies, bees, ants, and mammals. A growing body of literature suggests that PKG plays an integral role in the component behaviours and physiologies underlying foraging behaviour. These include energy acquisition, nutrient absorption, nutrient allocation, nutrient storage, and energy use. New evidence suggests that PKG mediates both neural and physiological mechanisms underlying these processes. This review illustrates how investigating the role of PKG in energy homeostasis in a diversity of organisms can offer a broad perspective on the mechanisms mediating energy balance.