A novel paradigm for nonassociative long-term memory in Drosophila: predator-induced changes in oviposition behavior

Diverse memory paradigms in Drosophila reveal diverse neural mechanisms

In this review, we aggregated the different types of learning and memory paradigms developed in adult Drosophila and attempted to assess the similarities and differences in the neural mechanisms supporting diverse types of memory. The simplest association memory assays are conditioning paradigms (olfactory, visual, and gustatory). A great deal of work has been done on these memories, revealing hundreds of genes and neural circuits supporting this memory. Variations of conditioning assays (reversal learning, trace conditioning, latent inhibition, and extinction) also reveal interesting memory mechanisms, whereas mechanisms supporting spatial memory (thermal maze, orientation memory, and heat box) and the conditioned suppression of innate behaviors (phototaxis, negative geotaxis, anemotaxis, and locomotion) remain largely unexplored. In recent years, there has been an increased interest in multisensory and multicomponent memories (context-dependent and cross-modal memory) and higher-order memory (sensory preconditioning and second-order conditioning). Some of this work has revealed how the intricate mushroom body (MB) neural circuitry can support more complex memories. Finally, the most complex memories are arguably those involving social memory: courtship conditioning and social learning (mate-copying and egg-laying behaviors). Currently, very little is known about the mechanisms supporting social memories. Overall, the MBs are important for association memories of multiple sensory modalities and multisensory integration, whereas the central complex is important for place, orientation, and navigation memories. Interestingly, several different types of memory appear to use similar or variants of the olfactory conditioning neural circuitry, which are repurposed in different ways.

Future avenues in Drosophila mushroom body research

How does the brain translate sensory information into complex behaviors? With relatively small neuronal numbers, readable behavioral outputs, and an unparalleled genetic toolkit, the Drosophila mushroom body (MB) offers an excellent model to address this question in the context of associative learning and memory. Recent technological breakthroughs, such as the freshly completed full-brain connectome, multiomics approaches, CRISPR-mediated gene editing, and machine learning techniques, led to major advancements in our understanding of the MB circuit at the molecular, structural, physiological, and functional levels. Despite significant progress in individual MB areas, the field still faces the fundamental challenge of resolving how these different levels combine and interact to ultimately control the behavior of an individual fly. In this review, we discuss various aspects of MB research, with a focus on the current knowledge gaps, and an outlook on the future methodological developments required to reach an overall view of the neurobiological basis of learning and memory.

Entomopathogenic fungal infection following immune gene silencing decreased behavioral and physiological fitness in Aedes aegypti mosquitoes

Entomopathogenic fungi are a promising category of biocontrol agents with mosquitocidal properties. Prior studies have proved their potential to reduce fecundity, human biting and vector competence, all of them together determine vectorial capacity of the mosquitoes. Unfortunately, conventional vector control strategies are inadequate with growing problem of insecticide resistance and environmental deterioration. Therefore, alternate vector control measures are immediately needed and to accomplish that, an improved understanding of behavioral and physiological defense mechanisms of the mosquitoes against fungal infection is essential. In this study, fitness was considered with respect to different behavioral (self-grooming and flight), physiological (antifungal activity and antimicrobial peptides) parameters and survival rates as compared to the control group. We found a significant upregulation in CLSP2, TEP22, Rel1 and Rel2 genes at multiple time periods of fungal infection, which indicates the successful fungal infection and activation of Toll and IMD pathways in mosquitoes. RNAi-mediated silencing of Rel1 and Rel2 genes (transcription factors of Toll and IMD pathways, respectively) significantly reduced the survival, self-grooming frequencies and durations, and flight locomotor activity among adult Ae. aegypti female mosquitoes. Moreover, Rel1 and Rel2 knockdown significantly decreased antifungal activity and antimicrobial peptides expression levels in target mosquitoes. These results indicate an overall decrease in fitness of the mosquitoes after fungal challenge following Rel1 and Rel2 silencing. These findings provide an improved understanding of behavioral and physiological responses in mosquitoes with altered immunity against entomopathogenic fungal infections which can guide us towards the development of novel biocontrol strategies against mosquitoes.

Evaluating Learning and Memory in Drosophila melanogaster to Study the Neurodevelopmental Impacts of Toxicants

Neurodevelopmental disorders are a heterogeneous group of behaviorally defined disorders with both genetic and environmental risk factors. Given that many neurodevelopmental disorders are characterized by impaired learning and/or intellectual abilities, behavioral paradigms that assess cognition in animal models have been effective tools in delineating underlying genetic variants that impact disease pathophysiology. For example, learning and memory paradigms in the common fruit fly Drosophila melanogaster have been successfully used to study risk genes and biological pathways associated with several neurodevelopmental disorders, including fragile X syndrome, autism spectrum disorder, and CHARGE syndrome. While these established Drosophila behavioral paradigms have historically been used to investigate genetic risk factors, they also have many other applications, including use in developmental neurotoxicology studies to determine environmental risk factors for neurodevelopmental disorders. There is, however, a deficit of step-by-step protocols that describe how to apply learning and memory assays in fruit flies to developmental neurotoxicology studies. Here, we describe two quantitative behavioral paradigms for Drosophila-predator-induced oviposition and courtship conditioning-that can be used to measure different forms of learning and memory in the context of a developmental neurotoxicology study. Non-associative learning and memory are measured here by examining female Drosophila oviposition behavior in response to endoparasitoid wasps, while associative learning and memory are measured in males using courtship conditioning. Our protocols outline procedures for oral toxicant exposure of developing fruit flies, culturing of endoparasitoid wasps, measuring Drosophila oviposition activity, and assessing conditioned courtship in order to identify the impact of toxicants on learning and memory in both females and males. As an example, we present the protocols using bisphenol A, a chemical utilized in the synthesis of polycarbonate plastics, to determine its impacts on learning and memory. These protocols are inexpensive and relatively simple to perform, and can be used by labs that are new to Drosophila behavioral research to evaluate how toxicant exposure influences learning and memory in male and female flies. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Preparation of toxicant-containing food and developmental exposure Basic Protocol 2: Predator-induced oviposition assay Support Protocol: Culture of Leptopilina heterotoma Basic Protocol 3: Conditioned courtship assay.

Bisphenol A affects neurodevelopmental gene expression, cognitive function, and neuromuscular synaptic morphology in Drosophila melanogaster

Bisphenol A (BPA) is an environmentally prevalent endocrine disrupting chemical that can impact human health and may be an environmental risk factor for neurodevelopmental disorders. BPA has been associated with behavioral impairment in children and a variety of neurodevelopmental phenotypes in model organisms. We used Drosophila melanogaster to explore the consequences of developmental BPA exposure on gene expression, cognitive function, and synapse development. Our transcriptome analysis indicated neurodevelopmentally relevant genes were predominantly downregulated by BPA. Among the misregulated genes were those with roles in learning, memory, and synapse development, as well as orthologs of human genes associated with neurodevelopmental and neuropsychiatric disorders. To examine how gene expression data corresponded to behavioral and cellular phenotypes, we first used a predator-response behavioral paradigm and found that BPA disrupts visual perception. Further analysis using conditioned courtship suppression showed that BPA impairs associative learning. Finally, we examined synapse morphology within the larval neuromuscular junction and found that BPA significantly increased the number of axonal branches. Given that our findings align with studies of BPA in mammalian model organisms, this data indicates that BPA impairs neurodevelopmental pathways that are functionally conserved from invertebrates to mammals. Further, because Drosophila do not possess classic estrogen receptors or estrogen, this research suggests that BPA can impact neurodevelopment by molecular mechanisms distinct from its role as an estrogen mimic.

Impact of Microorganisms and Parasites on Neuronally Controlled Drosophila Behaviours

Like all invertebrates, flies such as Drosophila lack an adaptive immune system and depend on their innate immune system to protect them against pathogenic microorganisms and parasites. In recent years, it appears that the nervous systems of eucaryotes not only control animal behavior but also cooperate and synergize very strongly with the animals’ immune systems to detect and fight potential pathogenic threats, and allow them to adapt their behavior to the presence of microorganisms and parasites that coexist with them. This review puts into perspective the latest progress made using the Drosophila model system, in this field of research, which remains in its infancy.

Neuropeptide F signaling regulates parasitoid-specific germline development and egg-laying in Drosophila

Drosophila larvae and pupae are at high risk of parasitoid infection in nature. To circumvent parasitic stress, fruit flies have developed various survival strategies, including cellular and behavioral defenses. We show that adult Drosophila females exposed to the parasitic wasps, Leptopilina boulardi, decrease their total egg-lay by deploying at least two strategies: Retention of fully developed follicles reduces the number of eggs laid, while induction of caspase-mediated apoptosis eliminates the vitellogenic follicles. These reproductive defense strategies require both visual and olfactory cues, but not the MB247-positive mushroom body neuronal function, suggesting a novel mode of sensory integration mediates reduced egg-laying in the presence of a parasitoid. We further show that neuropeptide F (NPF) signaling is necessary for both retaining matured follicles and activating apoptosis in vitellogenic follicles. Whereas previous studies have found that gut-derived NPF controls germ stem cell proliferation, we show that sensory-induced changes in germ cell development specifically require brain-derived NPF signaling, which recruits a subset of NPFR-expressing cell-types that control follicle development and retention. Importantly, we found that reduced egg-lay behavior is specific to parasitic wasps that infect the developing Drosophila larvae, but not the pupae. Our findings demonstrate that female fruit flies use multimodal sensory integration and neuroendocrine signaling via NPF to engage in parasite-specific cellular and behavioral survival strategies.

The Panopticon-Assessing the Effect of Starvation on Prolonged Fly Activity and Place Preference

Animal behaviours are demonstrably governed by sensory stimulation, previous experience and internal states like hunger. With increasing hunger, priorities shift towards foraging and feeding. During foraging, flies are known to employ efficient path integration strategies. However, general long-term activity patterns for both hungry and satiated flies in conditions of foraging remain to be better understood. Similarly, little is known about how permanent contact chemosensory stimulation affects locomotion. To address these questions, we have developed a novel, simplistic fly activity tracking setup—the Panopticon. Using a 3D-printed Petri dish inset, our assay allows recording of walking behaviour, of several flies in parallel, with all arena surfaces covered by a uniform substrate layer. We tested two constellations of providing food: (i) in single patches and (ii) omnipresent within the substrate layer. Fly tracking is done with FIJI, further assessment, analysis and presentation is done with a custom-built MATLAB analysis framework. We find that starvation history leads to a long-lasting reduction in locomotion, as well as a delayed place preference for food patches which seems to be not driven by immediate hunger motivation.

Social competition stimulates cognitive performance in a sex-specific manner

Social interactions are thought to be a critical driver in the evolution of cognitive ability. Cooperative interactions, such as pair bonding, rather than competitive interactions have been largely implicated in the evolution of increased cognition. This is despite competition traditionally being a very strong driver of trait evolution. Males of many species track changes in their social environment and alter their reproductive strategies in response to anticipated levels of competition. We predict this to be cognitively challenging. Using a Drosophila melanogaster model, we are able to distinguish between the effects of a competitive environment versus generic social contact by exposing flies to same-sex same-species competition versus different species partners, shown to present non-competitive contacts. Males increase olfactory learning/memory and visual memory after exposure to conspecific males only, a pattern echoed by increased expression of synaptic genes and an increased need for sleep. For females, largely not affected by mating competition, the opposite pattern was seen. The results indicate that specific social contacts dependent on sex, not simply generic social stimulation, may be an important evolutionary driver for cognitive ability in fruit flies.

Behavioral and environmental contributions to drosophilid social networks

Animals interact with each other in species-specific reproducible patterns. These patterns of organization are captured by social network analysis, and social interaction networks (SINs) have been described for a wide variety of species including fish, insects, birds, and mammals. The aim of this study is to understand the evolution of social organization in Drosophila Using a comparative ecological, phylogenetic, and behavioral approach, the different properties of SINs formed by 20 drosophilids were compared. We investigate whether drosophilid network structures arise from common ancestry, a response to the species' past climate, other social behaviors, or a combination of these factors. This study shows that differences in past climate predicted the species' current SIN properties. The drosophilid phylogeny offered no value to predicting species' differences in SINs through phylogenetic signal tests. This suggests that group-level social behaviors in drosophilid species are shaped by divergent climates. However, we find that the social distance at which flies interact correlated with the drosophilid phylogeny, indicating that behavioral elements of SINs have remained largely unchanged in their evolutionary history. We find a significant correlation of leg length to social distance, outlining the interdependence of anatomy and complex social structures. Although SINs display a complex evolutionary relationship across drosophilids, this study suggests that the ecology, and not common ancestry, contributes to diversity in social structure in Drosophila.

What can a non-eusocial insect tell us about the neural basis of group behaviour?

Group behaviour has been extensively studied in canonically social swarming, shoaling and flocking vertebrates and invertebrates, providing great insight into the behavioural and ecological aspects of group living. However, the search for its neuronal basis is lagging behind. In the natural environment, Drosophila melanogaster, increasingly used as a model to study neuronal circuits and behaviour, spend their lives surrounded by several conspecifics of different stages, as well as heterospecifics. Despite their dynamic multi-organism natural environment, the neuronal basis of social behaviours has been typically studied in dyadic interactions, such as mating or aggression. This review will focus on recent studies regarding how the behaviour of fruit flies can be shaped by the nature of the surrounding group. We argue that the rich social environment of Drosophila melanogaster, its arsenal of neurogenetic tools and the ability to use large sample sizes for detailed quantitative behavioural analysis makes this species ideal for mechanistic studies of group behaviour.

Peptidoglycan-dependent NF-κB activation in a small subset of brain octopaminergic neurons controls female oviposition

When facing microbes, animals engage in behaviors that lower the impact of the infection. We previously demonstrated that internal sensing of bacterial peptidoglycan reduces Drosophila female oviposition via NF-κB pathway activation in some neurons (Kurz et al., 2017). Although we showed that the neuromodulator octopamine is implicated, the identity of the involved neurons, as well as the physiological mechanism blocking egg-laying, remained unknown. In this study, we identified few ventral nerve cord and brain octopaminergic neurons expressing an NF-κB pathway component. We functionally demonstrated that NF-κB pathway activation in the brain, but not in the ventral nerve cord octopaminergic neurons, triggers an egg-laying drop in response to infection. Furthermore, we demonstrated via calcium imaging that the activity of these neurons can be directly modulated by peptidoglycan and that these cells do not control other octopamine-dependent behaviors such as female receptivity. This study shows that by sensing peptidoglycan and hence activating NF-κB cascade, a couple of brain neurons modulate a specific octopamine-dependent behavior to adapt female physiology status to their infectious state.

Neural circuitry of social learning in Drosophila requires multiple inputs to facilitate inter-species communication

Drosophila species communicate the threat of parasitoid wasps to naïve individuals. Communication of the threat between closely related species is efficient, while more distantly related species exhibit a dampened, partial communication. Partial communication between D. melanogaster and D. ananassae about wasp presence is enhanced following a period of cohabitation, suggesting that species-specific natural variations in communication 'dialects' can be learned through socialization. In this study, we identify six regions of the Drosophila brain essential for dialect training. We pinpoint subgroups of neurons in these regions, including motion detecting neurons in the optic lobe, layer 5 of the fan-shaped body, the D glomerulus in the antennal lobe, and the odorant receptor Or69a, where activation of each component is necessary for dialect learning. These results reveal functional neural circuits that underlie complex Drosophila social behaviors, and these circuits are required for integration several cue inputs involving multiple regions of the Drosophila brain.

Wolbachia infection may improve learning and memory capacity of Drosophila by altering host gene expression through microRNA

Wolbachia are endosymbiotic bacteria present in a wide range of invertebrates. Although their dramatic effects on host reproductive biology have been well studied, little is known about the effects of Wolbachia on the learning and memory capacity (LMC) of hosts, despite their distribution in the host nervous system, including brain. In this study, we found that Wolbachia infection significantly enhanced LMC in both Drosophila melanogaster and D. simulans. Expression of LMC-related genes was significantly increased in the head of D. melanogaster infected with the wMel strain, and among these genes, crebA was up-regulated the most. Knockdown of crebA in Wolbachia-infected flies significantly decreased LMC, while overexpression of crebA in Wolbachia-free flies significantly enhanced the LMC of flies. More importantly, a microRNA (miRNA), dme-miR-92b, was identified to be complementary to the 3'UTR of crebA. Wolbachia infection was correlated with reduced expression of dme-miR-92b in D. melanogaster, and dme-miR-92b negatively regulated crebA through binding to its 3'UTR region. Overexpression of dme-miR-92b in Wolbachia-infected flies by microinjection of agomirs caused a significant decrease in crebA expression and LMC, while inhibition of dme-miR-92b in Wolbachia-free flies by microinjection of antagomirs resulted in a significant increase in crebA expression and LMC. These results suggest that Wolbachia may improve LMC in Drosophila by altering host gene expression through a miRNA-target pathway. Our findings help better understand the host-endosymbiont interactions and, in particular, the impact of Wolbachia on cognitive processes in invertebrate hosts.

New Drosophila Long-Term Memory Genes Revealed by Assessing Computational Function Prediction Methods

A major bottleneck to our understanding of the genetic and molecular foundation of life lies in the ability to assign function to a gene and, subsequently, a protein. Traditional molecular and genetic experiments can provide the most reliable forms of identification, but are generally low-throughput, making such discovery and assignment a daunting task. The bottleneck has led to an increasing role for computational approaches. The Critical Assessment of Functional Annotation (CAFA) effort seeks to measure the performance of computational methods. In CAFA3, we performed selected screens, including an effort focused on long-term memory. We used homology and previous CAFA predictions to identify 29 key Drosophila genes, which we tested via a long-term memory screen. We identify 11 novel genes that are involved in long-term memory formation and show a high level of connectivity with previously identified learning and memory genes. Our study provides first higher-order behavioral assay and organism screen used for CAFA assessments and revealed previously uncharacterized roles of multiple genes as possible regulators of neuronal plasticity at the boundary of information acquisition and memory formation.

Drosophila species learn dialects through communal living

Many species are able to share information about their environment by communicating through auditory, visual, and olfactory cues. In Drosophila melanogaster, exposure to parasitoid wasps leads to a decline in egg laying, and exposed females communicate this threat to naïve flies, which also depress egg laying. We find that species across the genus Drosophila respond to wasps by egg laying reduction, activate cleaved caspase in oocytes, and communicate the presence of wasps to naïve individuals. Communication within a species and between closely related species is efficient, while more distantly related species exhibit partial communication. Remarkably, partial communication between some species is enhanced after a cohabitation period that requires exchange of visual and olfactory signals. This interspecies “dialect learning” requires neuronal cAMP signaling in the mushroom body, suggesting neuronal plasticity facilitates dialect learning and memory. These observations establish Drosophila as genetic models for interspecies social communication and evolution of dialects.

In this study, we find that many different Drosophila species never having been exposed to parasitoid wasps can trigger caspase activation in the ovary and depress egg-laying when placed next to flies that had visual experience with wasps. Interestingly, when teacher flies of one species are placed with a student of a different species, communication exists, to varying degrees, which seems dependent on evolutionary relatedness. Cohabitation of two species that can partially communicate can learn each other’s “dialect”, yielding effective interspecies communication. There are various inputs involved in dialect learning, including the presence of visual and olfactory cues and memory functions, including genes implicated in social learning defects in murine models, such as PTEN. The neuroplasticity of adult Drosophila allows for learning of dialects, but the specific dialect learned is dependent on social interactions exclusive to a communal environmental context, which provides both visual and olfactory inputs. We find flies can communicate with one another about an anticipated danger, which is suggestive of a fly “language.” The presence of a neurologically plastic system, allowing for social learning, can subsequently lead to a dramatic physiological response, requiring active learning and memory formation through integration of multiple inputs.

Predator experience changes spider mites' habitat choice even without current threat

As recent studies have revealed, previous exposure to a predator can change prey behavior even in the absence of current threat. We hypothesized that experiencing a predator increases prey avoidance of lower-quality resources even in the absence of a predator, which in turn influences the prey's spatial distribution. We examined these hypotheses using the herbivorous spider mite Tetranychus kanzawai and the specialist predatory mite Neoseiulus womersleyi. We used Phaseolus vulgaris as a high-quality host plant and Hydrangea macrophylla as a low-quality host plant. First we examined whether T. kanzawai females that were previously exposed to predators preferred P. vulgaris to H. macrophylla under no current threat more than those without predator experience. Second, we tested the effect of predator experience on dispersal by T. kanzawai females on P. vulgaris or on H. macrophylla. Our results show that: (1) predator-experienced T. kanzawai females expressed stronger avoidance of the low-quality plant H. macrophylla than those without predator experiences; and (2) T. kanzawai females transferred to H. macrophylla traveled farther than those on P. vulgaris, especially females with previous predator experience. These findings reveal neglected aspects of the evolutionary interaction between predators and the habitat choices of their prey.

Modeling Fragile X Syndrome in Drosophila

Intellectual disability (ID) and autism are hallmarks of Fragile X Syndrome (FXS), a hereditary neurodevelopmental disorder. The gene responsible for FXS is Fragile X Mental Retardation gene 1 (FMR1) encoding the Fragile X Mental Retardation Protein (FMRP), an RNA-binding protein involved in RNA metabolism and modulating the expression level of many targets. Most cases of FXS are caused by silencing of FMR1 due to CGG expansions in the 5'-UTR of the gene. Humans also carry the FXR1 and FXR2 paralogs of FMR1 while flies have only one FMR1 gene, here called dFMR1, sharing the same level of sequence homology with all three human genes, but functionally most similar to FMR1. This enables a much easier approach for FMR1 genetic studies. Drosophila has been widely used to investigate FMR1 functions at genetic, cellular, and molecular levels since dFMR1 mutants have many phenotypes in common with the wide spectrum of FMR1 functions that underlay the disease. In this review, we present very recent Drosophila studies investigating FMRP functions at genetic, cellular, molecular, and electrophysiological levels in addition to research on pharmacological treatments in the fly model. These studies have the potential to aid the discovery of pharmacological therapies for FXS.

Machine Learning Analysis Identifies Drosophila Grunge/Atrophin as an Important Learning and Memory Gene Required for Memory Retention and Social Learning

High-throughput experiments are becoming increasingly common, and scientists must balance hypothesis-driven experiments with genome-wide data acquisition. We sought to predict novel genes involved in Drosophila learning and long-term memory from existing public high-throughput data. We performed an analysis using PILGRM, which analyzes public gene expression compendia using machine learning. We evaluated the top prediction alongside genes involved in learning and memory in IMP, an interface for functional relationship networks. We identified Grunge/Atrophin (Gug/Atro), a transcriptional repressor, histone deacetylase, as our top candidate. We find, through multiple, distinct assays, that Gug has an active role as a modulator of memory retention in the fly and its function is required in the adult mushroom body. Depletion of Gug specifically in neurons of the adult mushroom body, after cell division and neuronal development is complete, suggests that Gug function is important for memory retention through regulation of neuronal activity, and not by altering neurodevelopment. Our study provides a previously uncharacterized role for Gug as a possible regulator of neuronal plasticity at the interface of memory retention and memory extinction.

A systems level approach to temporal expression dynamics in Drosophila reveals clusters of long term memory genes

The ability to integrate experiential information and recall it in the form of memory is observed in a wide range of taxa, and is a hallmark of highly derived nervous systems. Storage of past experiences is critical for adaptive behaviors that anticipate both adverse and positive environmental factors. The process of memory formation and consolidation involve many synchronized biological events including gene transcription, protein modification, and intracellular trafficking: However, many of these molecular mechanisms remain illusive. With Drosophila as a model system we use a nonassociative memory paradigm and a systems level approach to uncover novel transcriptional patterns. RNA sequencing of Drosophila heads during and after memory formation identified a number of novel memory genes. Tracking the dynamic expression of these genes over time revealed complex gene networks involved in long term memory. In particular, this study focuses on two functional gene clusters of signal peptides and proteases. Bioinformatics network analysis and prediction in combination with high-throughput RNA sequencing identified previously unknown memory genes, which when genetically knocked down resulted in behaviorally validated memory defects.

Drosophila increase exploration after visually detecting predators

Novel stimuli elicit behaviors that are collectively known as specific exploration. These behaviors allow the animal to become more familiar with the novel objects within its environment. Specific exploration is frequently suppressed by defensive reactions to predator cues. Herein, we examine if this suppression occurs in Drosophila melanogaster by measuring the response of these flies to wild harvested predators. The flies used in our experiments have been cultured and had not lived under predator threat for multiple decades. In a circular arena with centrally-caged predators, wild type Drosophila actively avoided the pantropical jumping spider, Plexippus paykulli, and the Texas unicorn mantis, Phyllovates chlorophaena, indicating an innate defensive reaction to these predators. Interestingly, wild type Drosophila males also avoided a centrally-caged mock spider, and the avoidance of the mock spider became exaggerated when it was made to move within the cage. Visually impaired Drosophila failed to detect and avoid the Plexippus paykulli and the moving mock spider, while the broadly anosmic orco² mutants were fully capable of detecting and avoiding Plexippus paykulli, indicating that these flies principally relied upon vison to perceive the predator stimuli. During early exploration of the arena, exploratory activity increased in the presence of Plexippus paykulli and the moving mock spider. The elevated activity induced by Plexippus paykulli disappeared after the fly had finished exploring, suggesting the flies were capable of habituating the predator cues. Taken together, these results indicate that despite being isolated from predators for decades Drosophila will visually detect these predators, retain innate defensive behaviors, respond by increasing exploratory activity in the arena rather than suppressing activity, and may habituate to normal predator cues.

Ethanol confers differential protection against generalist and specialist parasitoids of Drosophila melanogaster

As parasites coevolve with their hosts, they can evolve counter-defenses that render host immune responses ineffective. These counter-defenses are more likely to evolve in specialist parasites than generalist parasites; the latter face variable selection pressures between the different hosts they infect. Natural populations of the fruit fly Drosophila melanogaster are commonly threatened by endoparasitoid wasps in the genus Leptopilina, including the specialist L. boulardi and the generalist L. heterotoma, and both wasp species can incapacitate the cellular immune response of D. melanogaster larvae. Given that ethanol tolerance is high in D. melanogaster and stronger in the specialist wasp than the generalist, we tested whether fly larvae could use ethanol as an anti-parasite defense and whether its effectiveness would differ against the two wasp species. We found that fly larvae benefited from eating ethanol-containing food during exposure to L. heterotoma; we observed a two-fold decrease in parasitization intensity and a 24-fold increase in fly survival to adulthood. Although host ethanol consumption did not affect L. boulardi parasitization rates or intensities, it led to a modest increase in fly survival. Thus, ethanol conferred stronger protection against the generalist wasp than the specialist. We tested whether fly larvae can self-medicate by seeking ethanol-containing food after being attacked by wasps, but found no support for this hypothesis. We also allowed female flies to choose between control and ethanol-containing oviposition sites in the presence vs. absence of wasps and generally found significant preferences for ethanol regardless of wasp presence. Overall, our results suggest that D. melanogaster larvae obtain protection from certain parasitoid wasp species through their mothers’ innate oviposition preferences for ethanol-containing food sources.

Regulated Intron Removal Integrates Motivational State and Experience

Myriad experiences produce transient memory, yet, contingent on the internal state of the organism and the saliency of the experience, only some memories persist over time. How experience and internal state influence the duration of memory at the molecular level remains unknown. A self-assembled aggregated state of Drosophila Orb2A protein is required specifically for long-lasting memory. We report that in the adult fly brain the mRNA encoding Orb2A protein exists in an unspliced non-protein-coding form. The convergence of experience and internal drive transiently increases the spliced protein-coding Orb2A mRNA. A screen identified pasilla, the fly ortholog of mammalian Nova-1/2, as a mediator of Orb2A mRNA processing. A single-nucleotide substitution in the intronic region that reduces Pasilla binding and intron removal selectively impairs long-term memory. We posit that pasilla-mediated processing of unspliced Orb2A mRNA integrates experience and internal state to control Orb2A protein abundance and long-term memory formation.

Behavioural flexibility in spider mites: oviposition site shifts based on past and present stimuli from conspecifics and predators

Predator-experienced individuals often change their predation avoidance response when they re-encounter the same predators or their cues. Recent reports show that behavioural change sometimes occurs even before the re-encounter. To function as an adaptive strategy in the wild, such prospective experience-induced behaviour should change flexibly in response to changing situations. We assessed flexibility of experience-induced oviposition site shift in two closely related species of spider mites, Tetranychus kanzawai and T. urticae, from the viewpoint of reducing future predation risk on their eggs. We found that: (i) individuals of T. kanzawai shifted oviposition site depending on the presence of conspecific eggs; (ii) after experiencing predation threat T. kanzawai females shifted oviposition site even in the absence of any current predation threat; (iii) this experience-induced shift of oviposition site was weakened in the presence of conspecific males; and (iv) experience-induced behaviour was retained for a shorter period in T. urticae than in T. kanzawai, possibly because the demand for learning may differ with regard to biological conditions encountered in the wild.

Peptidoglycan sensing by octopaminergic neurons modulates Drosophila oviposition

As infectious diseases pose a threat to host integrity, eukaryotes have evolved mechanisms to eliminate pathogens. In addition to develop strategies reducing infection, animals can engage in behaviors that lower the impact of the infection. The molecular mechanisms by which microbes impact host behavior are not well understood. We demonstrate that bacterial infection of Drosophila females reduces oviposition and that peptidoglycan, the component that activates Drosophila antibacterial response, is also the elicitor of this behavioral change. We show that peptidoglycan regulates egg-laying rate by activating NF-κB signaling pathway in octopaminergic neurons and that, a dedicated peptidoglycan degrading enzyme acts in these neurons to buffer this behavioral response. This study shows that a unique ligand and signaling cascade are used in immune cells to mount an immune response and in neurons to control fly behavior following infection. This may represent a case of behavioral immunity.

DOI:http://dx.doi.org/10.7554/eLife.21937.001

Bacteria are all around us: they are on our skin, in the food that we eat and inside our bodies, particularly in the gut. While many of these bacteria are harmless and some even help us digest our food, others can make us ill. Upon detecting harmful bacteria, our bodies therefore trigger an immune response intended to destroy them.

Some insects – including butterflies, moths and grasshoppers – have an additional way of defending themselves against bacteria besides their immune response. Whenever they detect harmful microorganisms, the insects change their behavior so as to reduce their chances of becoming infected and limit the damage an infection would cause. The insects move away from areas containing harmful bacteria, for example, and temporarily stop eating. But whereas the insects’ immune response to bacteria is well documented, little was known about the mechanisms that underlie these changes in behavior.

Kurz, Charroux et al. set out to rectify this using another insect species, the fruit fly Drosophila. Flies that are infected with bacteria lay fewer eggs than healthy flies: a change in behavior that helps protect the offspring from infection. Kurz, Charroux et al. show that fruit flies are able to detect a component of the cell wall that surrounds all bacteria. This substance, known as peptidoglycan, activates a set of neurons in the fly that produce a chemical called octopamine. These neurons in turn activate a signaling pathway featuring a molecule known as NF-κB, and this causes the flies to lay fewer eggs.

Notably, peptidoglycan and NF-κB are also the molecules that trigger the anti-bacterial immune response. Fruit flies thus use the same pathway in immune cells and in neurons to trigger immune responses and behavioral changes, respectively. The challenge now is to identify precisely which neurons respond to bacterial peptidoglycan, and to work out how peptidoglycan changes the activity of these cells. Furthermore, studies have recently shown that bacterial peptidoglycan can influence the development of the mouse brain, as well as mouse behavior. This suggests that mechanisms for detecting harmful bacteria may be conserved across evolution, a possibility that requires further investigation.

DOI:http://dx.doi.org/10.7554/eLife.21937.002

Neuropeptides as Regulators of Behavior in Insects

Neuropeptides are by far the largest and most diverse group of signaling molecules in multicellular organisms. They are ancient molecules important in regulating a multitude of processes. Their small proteinaceous character allowed them to evolve and radiate quickly into numerous different molecules. On average, hundreds of distinct neuropeptides are present in animals, sometimes with unique classes that do not occur in distantly related species. Acting as neurotransmitters, neuromodulators, hormones, or growth factors, they are extremely diverse and are involved in controlling growth, development, ecdysis, digestion, diuresis, and many more physiological processes. Neuropeptides are also crucial in regulating myriad behavioral actions associated with feeding, courtship, sleep, learning and memory, stress, addiction, and social interactions. In general, behavior ensures that an organism can survive in its environment and is defined as any action that can change an organism's relationship to its surroundings. Even though the mode of action of neuropeptides in insects has been vigorously studied, relatively little is known about most neuropeptides and only a few model insects have been investigated. Here, we provide an overview of the roles neuropeptides play in insect behavior. We conclude that multiple neuropeptides need to work in concert to coordinate certain behaviors. Additionally, most neuropeptides studied to date have more than a single function.

A Putative Biochemical Engram of Long-Term Memory

How a transient experience creates an enduring yet dynamic memory remains an unresolved issue in studies of memory. Experience-dependent aggregation of the RNA-binding protein CPEB/Orb2 is one of the candidate mechanisms of memory maintenance. Here, using tools that allow rapid and reversible inactivation of Orb2 protein in neurons, we find that Orb2 activity is required for encoding and recall of memory. From a screen, we have identified a DNA-J family chaperone, JJJ2, which facilitates Orb2 aggregation, and ectopic expression of JJJ2 enhances the animal's capacity to form long-term memory. Finally, we have developed tools to visualize training-dependent aggregation of Orb2. We find that aggregated Orb2 in a subset of mushroom body neurons can serve as a "molecular signature" of memory and predict memory strength. Our data indicate that self-sustaining aggregates of Orb2 may serve as a physical substrate of memory and provide a molecular basis for the perduring yet malleable nature of memory.

Amyloidogenic Oligomerization Transforms Drosophila Orb2 from a Translation Repressor to an Activator

Memories are thought to be formed in response to transient experiences, in part through changes in local protein synthesis at synapses. In Drosophila, the amyloidogenic (prion-like) state of the RNA binding protein Orb2 has been implicated in long-term memory, but how conformational conversion of Orb2 promotes memory formation is unclear. Combining in vitro and in vivo studies, we find that the monomeric form of Orb2 represses translation and removes mRNA poly(A) tails, while the oligomeric form enhances translation and elongates the poly(A) tails and imparts its translational state to the monomer. The CG13928 protein, which binds only to monomeric Orb2, promotes deadenylation, whereas the putative poly(A) binding protein CG4612 promotes oligomeric Orb2-dependent translation. Our data support a model in which monomeric Orb2 keeps target mRNA in a translationally dormant state and experience-dependent conversion to the amyloidogenic state activates translation, resulting in persistent alteration of synaptic activity and stabilization of memory.

Social communication of predator-induced changes in Drosophila behavior and germ line physiology

Behavioral adaptation to environmental threats and subsequent social transmission of adaptive behavior has evolutionary implications. In Drosophila, exposure to parasitoid wasps leads to a sharp decline in oviposition. We show that exposure to predator elicits both an acute and learned oviposition depression, mediated through the visual system. However, long-term persistence of oviposition depression after predator removal requires neuronal signaling functions, a functional mushroom body, and neurally driven apoptosis of oocytes through effector caspases. Strikingly, wasp-exposed flies (teachers) can transmit egg-retention behavior and trigger ovarian apoptosis in naive, unexposed flies (students). Acquisition and behavioral execution of this socially learned behavior by naive flies requires all of the factors needed for primary learning. The ability to teach does not require ovarian apoptosis. This work provides new insight into genetic and physiological mechanisms that underlie an ecologically relevant form of learning and mechanisms for its social transmission.

DOI:http://dx.doi.org/10.7554/eLife.07423.001

Every animal must be able to adapt to threats and changes to their environment that could affect their survival. Some ‘social’ animals, such as honeybees and ants, go further than this, and also transmit information about a threat—and how to survive it—to other members of their species. This helpful behavior is now known to occur to some extent even in animals that have not been considered to be social, like the Drosophila species of fruit fly.

Parasitoid wasps lay their eggs in the larvae and pupae of certain insect species. When the wasp eggs hatch, they feed on the host insect, eventually killing it. Drosophila fruit flies have evolved various behaviors to protect their offspring from these wasps. For example, female fruit flies reduce the number of eggs they lay when they are in the presence of a wasp.

Kacsoh, Bozler et al. exposed female flies to wasps for a day. These flies produced fewer eggs than flies that were not exposed to wasps and continued to lay fewer eggs for 24 hours after the wasps were removed. Introducing these flies to ‘naive’ flies that had not encountered a wasp caused the naive flies to produce fewer eggs as well.

After ruling out several possible ways that the wasp-exposed flies might ‘teach’ the naive flies to produce and lay fewer eggs, Kacsoh, Bozler et al. found that naive flies cannot learn this behavior when they are blind. In addition, exposed flies cannot instruct other flies of the threat if their wings are absent or deformed. These and other findings, therefore, suggest that information about the wasp threat is transmitted through visual cues that involve the wings.

Kacsoh, Bozler et al. found that the flies must have certain brain circuits associated with memory and learning to be able to teach others and to reduce the numbers of eggs they lay after the wasp has been removed. This suggests that signals from this brain region must be continually sent out to alter the physiology of the developing eggs in order to maintain the lower rate of egg laying; understanding how flies use visual cues for communication and how the brain signals to the ovary remain key challenges for future work.

DOI:http://dx.doi.org/10.7554/eLife.07423.002