How Social Experience and Environment Impacts Behavioural Plasticity in Drosophila

Ageing effects of social environments in 'non-social' insects

It is increasingly clear that social environments have profound impacts on the life histories of 'non-social' animals. However, it is not yet well known how species with varying degrees of sociality respond to different social contexts and whether such effects are sex-specific. To survey the extent to which social environments specifically affect lifespan and ageing in non-social species, we performed a systematic literature review, focusing on invertebrates but excluding eusocial insects. We found 80 studies in which lifespan or ageing parameters were measured in relation to changes in same-sex or opposite-sex exposure, group size or cues thereof. Most of the studies focused on manipulations of adults, often reporting sex differences in lifespan following exposure to the opposite sex. Some studies highlighted the impacts of developmental environments or social partner age on lifespan. Several studies explored potential underlying mechanisms, emphasizing that studies on insects could provide excellent opportunities to interrogate the basis of social effects on ageing. We discuss what these studies can tell us about the social environment as a stressor, or trade-offs in resources prompted by different social contexts. We suggest fruitful avenues for further research of social effects across a wider and more diverse range of taxa.This article is part of the discussion meeting issue 'Understanding age and society using natural populations'.

Socialization causes long-lasting behavioral changes

In modern human societies, social isolation acts as a negative factor for health and life quality. On the other hand, social interaction also has profound effects on animal and human, impacting aggressiveness, feeding and sleep, among many other behaviors. Here, we observe that in the fly Drosophila melanogaster these behavioral changes long-last even after social interaction has ceased, suggesting that the socialization experience triggers behavioral plasticity. These modified behaviors maintain similar levels for 24 h and persist up to 72 h, although showing a progressive decay. We also find that impairing long-term memory mechanisms either genetically or by anesthesia abolishes the expected behavioral changes in response to social interaction. Furthermore, we show that socialization increases CREB-dependent neuronal activity and synaptic plasticity in the mushroom body, the main insect memory center analogous to mammalian hippocampus. We propose that social interaction triggers socialization awareness, understood as long-lasting changes in behavior caused by experience with mechanistic similarities to long-term memory formation.

Peer-induced quiescence of male Drosophila melanogaster following copulation

Mating experience impacts the physiology and behavior of animals. Although mating effects of female Drosophila melanogaster have been studied extensively, the behavioral changes of males following copulation have not been fully understood. In this study, we characterized the mating-dependent behavioral changes of male flies, especially focusing on fly-to-fly interaction, and their dependence on rearing conditions. Our data demonstrate that male flies quiesce their courtship toward both females and males, as well as their locomotor activity. This post-copulatory quiescence appears to be contingent upon the presence of a peer, as minimal variation is noted in locomotion when the male is measured in isolation. Interestingly, copulated males influence a paired male without successful copulation to reduce his locomotion. Our findings point to a conditional behavioral quiescence following copulation, influenced by the presence of other flies.

Recovery from social isolation requires dopamine in males, but not the autism-related gene nlg3 in either sex

Social isolation causes profound changes in social behaviour in a variety of species. However, the genetic and molecular mechanisms modulating behavioural responses to social isolation and social recovery remain to be elucidated. Here, we quantified the behavioural response of vinegar flies to social isolation using two distinct protocols (social space preference and sociability, the spontaneous tendencies to form groups). We found that social isolation increased social space and reduced sociability. These effects of social isolation were reversible and could be reduced after 3 days of group housing. Flies with a loss of function of neuroligin3 (orthologue of autism-related neuroligin genes) with known increased social space in a socially enriched environment were still able to recover from social isolation. We also show that dopamine (DA) is needed for a response to social isolation and recovery in males but not in females. Furthermore, only in males, DA levels are reduced after isolation and are not recovered after group housing. Finally, in socially enriched flies mutant for neuroligin3, DA levels are reduced in males, but not in females. We propose a model to explain how DA and neuroligin3 are involved in the behavioural response to social isolation and its recovery in a dynamic and sex-specific manner.

Temporal genomics in Hawaiian crickets reveals compensatory intragenomic coadaptation during adaptive evolution

Theory predicts that compensatory genetic changes reduce negative indirect effects of selected variants during adaptive evolution, but evidence is scarce. Here, we test this in a wild population of Hawaiian crickets using temporal genomics and a high-quality chromosome-level cricket genome. In this population, a mutation, flatwing, silences males and rapidly spread due to an acoustically-orienting parasitoid. Our sampling spanned a social transition during which flatwing fixed and the population went silent. We find long-range linkage disequilibrium around the putative flatwing locus was maintained over time, and hitchhiking genes had functions related to negative flatwing-associated effects. We develop a combinatorial enrichment approach using transcriptome data to test for compensatory, intragenomic coevolution. Temporal changes in genomic selection were distributed genome-wide and functionally associated with the population's transition to silence, particularly behavioural responses to silent environments. Our results demonstrate how 'adaptation begets adaptation'; changes to the sociogenetic environment accompanying rapid trait evolution can generate selection provoking further, compensatory adaptation.

The cluster digging behavior of larvae confers trophic benefits to fitness in insects

Collective behaviors efficiently impart benefits to a diversity of species ranging from bacteria to humans. Fly larvae tend to cluster and form coordinated digging groups under crowded conditions, yet understanding the rules governing this behavior is in its infancy. We primarily took advantage of the Drosophila model to investigate cooperative foraging behavior. Here, we report that Drosophila-related species and the black soldier fly have evolved a conserved strategy of cluster digging in food foraging. Subsequently, we investigated relative factors, including larval stage, population density, and food stiffness and quality, that affect the cluster digging behavior. Remarkably, oxygen supply through the posterior breathing spiracles is necessary for the organization of digging clusters. More importantly, we theoretically devise a mathematical model to accurately calculate how the cluster digging behavior expands food resources by diving depth, cross-section area, and food volume. We found that cluster digging behavior approximately increases 2.2 fold depth, 1.7-fold cross-section area, and 1.9 fold volume than control groups, respectively. Amplification of food sources significantly facilitates survival, larval development, and reproductive success of Drosophila challenged with competition for limited food resources, thereby conferring trophic benefits to fitness in insects. Overall, our findings highlight that the cluster digging behavior is a pivotal behavior for their adaptation to food scarcity, advancing a better understanding of how this cooperative behavior confers fitness benefits in the animal kingdom.

Sensory perception of rivals has trait-dependent effects on plasticity in Drosophila melanogaster

The social environment has myriad effects on individuals, altering reproduction, immune function, cognition, and aging. Phenotypic plasticity enables animals to respond to heterogeneous environments such as the social environment but requires that they assess those environments accurately. It has been suggested that combinations of sensory cues allow animals to respond rapidly and accurately to changeable environments, but it is unclear whether the same sensory inputs are required in all traits that respond to a particular environmental cue. Drosophila melanogaster males, in the presence of rival males, exhibit a consistent behavioral response by extending mating duration. However, exposure to a rival also results in a reduction in their lifespan, a phenomenon interpreted as a trade-off associated with sperm competition strategies. D. melanogaster perceive their rivals by using multiple sensory cues; interfering with at least two olfactory, auditory, or tactile cues eliminates the extension of mating duration. Here, we assessed whether these same cues were implicated in the lifespan reduction. Removal of combinations of auditory and olfactory cues removed the extended mating duration response to a rival, as previously found. However, we found that these manipulations did not alter the reduction in lifespan of males exposed to rivals or induce any changes in activity patterns, grooming, or male-male aggression. Therefore, our analysis suggests that lifespan reduction is not a cost associated with the behavioral responses to sperm competition. Moreover, this highlights the trait-specific nature of the mechanisms underlying plasticity in response to the same environmental conditions.

Drosophila Contributions towards Understanding Neurofibromatosis 1

Neurofibromatosis 1 (NF1) is a multisymptomatic disorder with highly variable presentations, which include short stature, susceptibility to formation of the characteristic benign tumors known as neurofibromas, intense freckling and skin discoloration, and cognitive deficits, which characterize most children with the condition. Attention deficits and Autism Spectrum manifestations augment the compromised learning presented by most patients, leading to behavioral problems and school failure, while fragmented sleep contributes to chronic fatigue and poor quality of life. Neurofibromin (Nf1) is present ubiquitously during human development and postnatally in most neuronal, oligodendrocyte, and Schwann cells. Evidence largely from animal models including Drosophila suggests that the symptomatic variability may reflect distinct cell-type-specific functions of the protein, which emerge upon its loss, or mutations affecting the different functional domains of the protein. This review summarizes the contributions of Drosophila in modeling multiple NF1 manifestations, addressing hypotheses regarding the cell-type-specific functions of the protein and exploring the molecular pathways affected upon loss of the highly conserved fly homolog dNf1. Collectively, work in this model not only has efficiently and expediently modelled multiple aspects of the condition and increased understanding of its behavioral manifestations, but also has led to pharmaceutical strategies towards their amelioration.

Neuroregulation of foraging behavior mediated by the olfactory co-receptor Orco in termites

Olfaction is critical for survival because it allows animals to look for food and detect pheromonal cues. Neuropeptides modulate olfaction and behaviors in insects. While how the neuroregulation of olfactory recognition affects foraging behavior in termites is still unclear. Here, we analyzed the change after silencing the olfactory co-receptor gene (Orco) and the neuropeptide Y gene (NPY), and then investigated the impact of olfactory recognition on foraging behavior in Odontotermes formosanus under different predation pressures. The knockdown of Orco resulted in the reduced Orco protein expression in antennae and the decreased EAG response to trail pheromones. In addition, NPY silencing led to the damaged ability of olfactory response through downregulating Orco expression. Both dsOrco- and dsNPY-injected worker termites showed significantly reduced walking activity and foraging success. Additionally, we found that 0.1 pg/cm trail pheromone and nestmate soldiers could provide social buffering to relieve the adverse effect of predator ants on foraging behavior in worker termites with the normal ability of olfactory recognition. Our orthogonal experiments further verified that Orco/NPY genes are essential in manipulating termite olfactory recognition during foraging under different predation pressures, suggesting that the neuroregulation of olfactory recognition plays a crucial role in regulating termite foraging behavior.

A neural pathway for social modulation of spontaneous locomotor activity (SoMo-SLA) in Drosophila

Social enrichment or social isolation affects a range of innate behaviors, such as sex, aggression, and sleep, but whether there is a shared mechanism is not clear. Here, we report a neural mechanism underlying social modulation of spontaneous locomotor activity (SoMo-SLA), an internal-driven behavior indicative of internal states. We find that social enrichment specifically reduces spontaneous locomotor activity in male flies. We identify neuropeptides Diuretic hormone 44 (DH44) and Tachykinin (TK) to be up- and down-regulated by social enrichment and necessary for SoMo-SLA. We further demonstrate a sexually dimorphic neural circuit, in which the male-specific P1 neurons encoding internal states form positive feedback with interneurons coexpressing doublesex (dsx) and Tk to promote locomotion, while P1 neurons also form negative feedback with interneurons coexpressing dsx and DH44 to inhibit locomotion. These two opposing neuromodulatory recurrent circuits represent a potentially common mechanism that underlies the social regulation of multiple innate behaviors.

Transmission of beneficial yeasts accompanies offspring production in Drosophila-An initial evolutionary stage of insect maternal care through manipulation of microbial load?

Parent-to-offspring transmission of beneficial microorganisms is intimately interwoven with the evolution of social behaviors. Ancestral stages of complex sociality-microbe vectoring interrelationships may be characterized by high costs of intensive parental care and hence only a weak link between the transmission of microbial symbionts and offspring production. We investigate the relationship between yeast symbiont transmission and egg-laying, as well as some general factors thought to drive the "farming" of microscopic fungi by the fruit fly Drosophila melanogaster, an insect with no obvious parental care but which is highly dependent on dietary microbes during offspring development. The process of transmitting microbes involves flies ingesting microbes from their previous environment, storing and vectoring them, and finally depositing them to a new environment. This study revealed that fecal materials of adult flies play a significant role in this process, as they contain viable yeast cells that support larval development. During single patch visits, egg-laying female flies transmitted more yeast cells than non-egg-laying females, suggesting that dietary symbiont transmission is not random, but linked to offspring production. The crop, an extension of the foregut, was identified as an organ capable of storing viable yeast cells during travel between egg-laying sites. However, the amount of yeast in the crop reduced rapidly during periods of starvation. Although females starved for 24 h deposited a smaller amount of yeast than those starved for 6 h, the yeast inoculum produced still promoted the development of larval offspring. The results of these experiments suggest that female Drosophila fruit flies have the ability to store and regulate the transfer of microorganisms beneficial to their offspring via the shedding of fecal material. We argue that our observation may represent an initial evolutionary stage of maternal care through the manipulation of microbial load, from which more specialized feedbacks of sociality and microbe management may evolve.

Expression of Transposable Elements in the Brain of the Drosophila melanogaster Model for Fragile X Syndrome

Fragile X syndrome is a neuro-developmental disease affecting intellectual abilities and social interactions. Drosophila melanogaster represents a consolidated model to study neuronal pathways underlying this syndrome, especially because the model recapitulates complex behavioural phenotypes. Drosophila Fragile X protein, or FMRP, is required for a normal neuronal structure and for correct synaptic differentiation in both the peripheral and central nervous systems, as well as for synaptic connectivity during development of the neuronal circuits. At the molecular level, FMRP has a crucial role in RNA homeostasis, including a role in transposon RNA regulation in the gonads of D. m. Transposons are repetitive sequences regulated at both the transcriptional and post-transcriptional levels to avoid genomic instability. De-regulation of transposons in the brain in response to chromatin relaxation has previously been related to neurodegenerative events in Drosophila models. Here, we demonstrate for the first time that FMRP is required for transposon silencing in larval and adult brains of Drosophila "loss of function" dFmr1 mutants. This study highlights that flies kept in isolation, defined as asocial conditions, experience activation of transposable elements. In all, these results suggest a role for transposons in the pathogenesis of certain neurological alterations in Fragile X as well as in abnormal social behaviors.

Aging and memory are altered by genetically manipulating lactate dehydrogenase in the neurons or glia of flies

The astrocyte-neuron lactate shuttle hypothesis posits that glial-generated lactate is transported to neurons to fuel metabolic processes required for long-term memory. Although studies in vertebrates have revealed that lactate shuttling is important for cognitive function, it is uncertain if this form of metabolic coupling is conserved in invertebrates or is influenced by age. Lactate dehydrogenase (Ldh) is a rate limiting enzyme that interconverts lactate and pyruvate. Here we genetically manipulated expression of Drosophila melanogaster lactate dehydrogenase (dLdh) in neurons or glia to assess the impact of altered lactate metabolism on invertebrate aging and long-term courtship memory at different ages. We also assessed survival, negative geotaxis, brain neutral lipids (the core component of lipid droplets) and brain metabolites. Both upregulation and downregulation of dLdh in neurons resulted in decreased survival and memory impairment with age. Glial downregulation of dLdh expression caused age-related memory impairment without altering survival, while upregulated glial dLdh expression lowered survival without disrupting memory. Both neuronal and glial dLdh upregulation increased neutral lipid accumulation. We provide evidence that altered lactate metabolism with age affects the tricarboxylic acid (TCA) cycle, 2-hydroxyglutarate (2HG), and neutral lipid accumulation. Collectively, our findings indicate that the direct alteration of lactate metabolism in either glia or neurons affects memory and survival but only in an age-dependent manner.

Flexibility of neural circuits regulating mating behaviors in mice and flies

Mating is essential for the reproduction of animal species. As mating behaviors are high-risk and energy-consuming processes, it is critical for animals to make adaptive mating decisions. This includes not only finding a suitable mate, but also adapting mating behaviors to the animal's needs and environmental conditions. Internal needs include physical states (e.g., hunger) and emotional states (e.g., fear), while external conditions include both social cues (e.g., the existence of predators or rivals) and non-social factors (e.g., food availability). With recent advances in behavioral neuroscience, we are now beginning to understand the neural basis of mating behaviors, particularly in genetic model organisms such as mice and flies. However, how internal and external factors are integrated by the nervous system to enable adaptive mating-related decision-making in a state- and context-dependent manner is less well understood. In this article, we review recent knowledge regarding the neural basis of flexible mating behaviors from studies of flies and mice. By contrasting the knowledge derived from these two evolutionarily distant model organisms, we discuss potential conserved and divergent neural mechanisms involved in the control of flexible mating behaviors in invertebrate and vertebrate brains.