Insulin Signaling in the Peripheral and Central Nervous System Regulates Female Sexual Receptivity during Starvation in Drosophila

Metabolic pathways that mediate the effects of food deprivation on reproductive behavior in female Drosophila melanogaster

In most species studied, energy deficits inhibit female reproductive behavior, but the location and nature of energy sensors and how they affect behavior are unknown. Progress has been facilitated by using Drosophila melanogaster, a species in which reproduction and food availability are closely linked. Adult males and females were either fed or food deprived (FD) and then tested in an arena with a fed, opposite-sex conspecific with no food in the testing arena. Only FD females (not FD males) significantly decreased their copulation rate and increased their copulation latency, and the effects of FD were prevented in females fed either yeast alone or glucose alone, but not sucralose alone, cholesterol alone, or amino acids alone. It is well-known that high-fat diets inhibit copulation rate in this species, and the effects of FD on copulation rate were mimicked by treatment with an inhibitor of glucose but not free fatty acid oxidation. The availability of oxidizable glucose was a necessary condition for copulation rate in females fed either yeast alone or fed a nutritive fly medium, which suggests that the critical component of yeast for female copulation rate is oxidizable glucose. Thus, female copulation rate in D. melanogaster is sensitive to the availability of oxidizable metabolic fuels, particularly the availability of oxidizable glucose or substrates/byproducts of glycolysis.NEW & NOTEWORTHY Copulation rate was decreased in food-deprived female but not in male adults when tested without food in the testing arena. Copulation rate was 1) maintained by feeding glucose alone, yeast alone, nutritive medium lacking yeast, but not sucralose, amino acids, or cholesterol alone; 2) decreased by inhibition of glycolysis in females fed either nutritive medium or yeast alone; and 3) not affected by inhibition of fatty acid oxidation. Thus, female copulation rate was linked to glycolytic status.

An integrative sensor of body states: how the mushroom body modulates behavior depending on physiological context

The brain constantly compares past and present experiences to predict the future, thereby enabling instantaneous and future behavioral adjustments. Integration of external information with the animal's current internal needs and behavioral state represents a key challenge of the nervous system. Recent advancements in dissecting the function of the Drosophila mushroom body (MB) at the single-cell level have uncovered its three-layered logic and parallel systems conveying positive and negative values during associative learning. This review explores a lesser-known role of the MB in detecting and integrating body states such as hunger, thirst, and sleep, ultimately modulating motivation and sensory-driven decisions based on the physiological state of the fly. State-dependent signals predominantly affect the activity of modulatory MB input neurons (dopaminergic, serotoninergic, and octopaminergic), but also induce plastic changes directly at the level of the MB intrinsic and output neurons. Thus, the MB emerges as a tightly regulated relay station in the insect brain, orchestrating neuroadaptations due to current internal and behavioral states leading to short- but also long-lasting changes in behavior. While these adaptations are crucial to ensure fitness and survival, recent findings also underscore how circuit motifs in the MB may reflect fundamental design principles that contribute to maladaptive behaviors such as addiction or depression-like symptoms.

The diverse roles of insulin signaling in insect behavior

In insects and other animals, nutrition-mediated behaviors are modulated by communication between the brain and peripheral systems, a process that relies heavily on the insulin/insulin-like growth factor signaling pathway (IIS). Previous studies have focused on the mechanistic and physiological functions of insulin-like peptides (ILPs) in critical developmental and adult milestones like pupation or vitellogenesis. Less work has detailed the mechanisms connecting ILPs to adult nutrient-mediated behaviors related to survival and reproductive success. Here we briefly review the range of behaviors linked to IIS in insects, from conserved regulation of feeding behavior to evolutionarily derived polyphenisms. Where possible, we incorporate information from Drosophila melanogaster and other model species to describe molecular and neural mechanisms that connect nutritional status to behavioral expression via IIS. We identify knowledge gaps which include the diverse functional roles of peripheral ILPs, how ILPs modulate neural function and behavior across the lifespan, and the lack of detailed mechanistic research in a broad range of taxa. Addressing these gaps would enable a better understanding of the evolution of this conserved and widely deployed tool kit pathway.

A neural pathway underlying hunger modulation of sexual receptivity in Drosophila females

Accepting or rejecting a mate is one of the most crucial decisions a female will make, especially when faced with food shortage. Previous studies have identified the core neural circuity from sensing male courtship or mating status to decision-making for sexual receptivity in Drosophila females, but how hunger and satiety states modulate female receptivity is poorly understood. Here, we identify the neural circuit and its neuromodulation underlying the hunger modulation of female receptivity. We find that adipokinetic hormone receptor (AkhR)-expressing neurons inhibit sexual receptivity in a starvation-dependent manner. AkhR neurons are octopaminergic and act on a subset of Octβ1R-expressing LH421 neurons. Knocking down Octβ1R expression in LH421 neurons eliminates starvation-induced suppression of female receptivity. We further find that LH421 neurons inhibit the sex-promoting pC1 neurons via GABA-resistant to dieldrin (Rdl) signaling. pC1 neurons also integrate courtship stimulation and mating status and thus serve as a common integrator of multiple internal and external cues for decision-making.

Road to sexual maturity: Behavioral event schedule from eclosion to first mating in each sex of Drosophila melanogaster

Animals achieve their first mating through the process of sexual maturation. This study examined the precise and detailed timing of a series of behavioral events, including wing expansion, first feeding, first excretion, and courtship, during sexual maturation from eclosion to first mating in D. melanogaster. We found that the time of first mating is genetically invariant and is not affected by light/dark cycle or food intake after eclosion. We also found sexual dimorphism in locomotor activity after eclosion, with females increasing locomotor activity earlier than males. In addition, we found a time rapidly changing from extremely low to high sexual activity in males post eclosion (named "drastic male courtship arousal" or DMCA). These behavioral traits leading up to the first mating could serve as clear indicators of sexual maturation and establish precisely timed developmental landmarks to explore further the mechanisms underlying the integration of behavioral and physiological sexual maturation.

Seven Questions on the Chemical Ecology and Neurogenetics of Resource-Mediated Speciation

Adaptation to different environments can result in reproductive isolation between populations and the formation of new species. Food resources are among the most important environmental factors shaping local adaptation. The chemosensory system, the most ubiquitous sensory channel in the animal kingdom, not only detects food resources and their chemical composition, but also mediates sexual communication and reproductive isolation in many taxa. Chemosensory divergence may thus play a crucial role in resource-mediated adaptation and speciation. Understanding how the chemosensory system can facilitate resource-mediated ecological speciation requires integrating mechanistic studies of the chemosensory system with ecological studies, to link the genetics and physiology of chemosensory properties to divergent adaptation. In this review, we use examples of insect research to present seven key questions that can be used to understand how the chemosensory system can facilitate resource-mediated ecological speciation in consumer populations.

Insulin and Leptin/Upd2 Exert Opposing Influences on Synapse Number in Fat-Sensing Neurons

Energy-sensing neural circuits decide to expend or conserve resources based, in part, on the tonic, steady-state, energy-store information they receive. Tonic signals, in the form of adipose tissue-derived adipokines, set the baseline level of activity in the energy-sensing neurons, thereby providing context for interpretation of additional inputs. However, the mechanism by which tonic adipokine information establishes steady-state neuronal function has heretofore been unclear. We show here that under conditions of nutrient surplus, Upd2, a Drosophila leptin ortholog, regulates actin-based synapse reorganization to reduce bouton number in an inhibitory circuit, thus establishing a neural tone that is permissive for insulin release. Unexpectedly, we found that insulin feeds back on these same inhibitory neurons to conversely increase bouton number, resulting in maintenance of negative tone. Our results point to a mechanism by which two surplus-sensing hormonal systems, Upd2/leptin and insulin, converge on a neuronal circuit with opposing outcomes to establish energy-store-dependent neuron activity.

Protein Stores Regulate When Reproductive Displays Begin in the Male Caribbean Fruit Fly

Many animals exhibit reproductive behavior that requires expenditure of valuable nutrients. In males of many species, competitive energetically demanding displays and the development of sexual ornaments require prior accumulation of nutrient stores. Males must coordinate nutrient stores with ornament development and reproductive displays or they risk depleting their resources mid-development or mid-display, reducing their chance of mating. Males may use nutrient stores to regulate their reproductive behavior. Amino acid reserves may be important for reproduction, but the roles of amino acid stores in initiating maturation and reproductive behavior are less studied than fat stores. Insects store amino acids as hexamerin storage proteins. Many fly species use a specific hexamerin, larval serum protein 2 (LSP-2), as both a juvenile storage medium and to store protein consumed after adult eclosion. Protein stored as LSP-2 has previously been suggested to regulate reproduction in females, but no role has been proposed for LSP-2 in regulating male maturation. We use males of the Caribbean fruit fly, Anastrepha suspensa, a species with nutrient-intensive male sexual displays to test whether LSP-2 stores regulate male reproductive displays. We fed adult A. suspensa males a diet with or without protein, then assayed these males for lsp-2 transcript abundance via qRT-PCR, LSP-2 protein abundance via Western blot, and reproductive display behavior via observation. We found that adult males with ad libitum dietary protein had greater lsp-2 transcript and protein abundance, earlier sexual display behavior, and were more likely to exhibit sexual display behavior than protein-deprived adult males. We show that lsp-2 knockdown via RNAi decreases the proportion of males exhibiting reproductive displays, particularly early in the onset of reproductive behavior. Our results suggest circulating LSP-2 protein stores regulate reproductive behavior in A. suspensa males, consistent with protein stores modulating reproduction in males with expensive reproductive strategies. Our results are consistent with hexamerin storage proteins performing dual roles of protein storage and protein signaling. Our work also has substantial practical applications because tephritid flies are a pest group and the timing and expression of male reproductive displays in this group are important for control efforts using the sterile insect technique.

Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

Insect Behavior and Physiological Adaptation Mechanisms Under Starvation Stress

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

Antennae sense heat stress to inhibit mating and promote escaping in Drosophila females

Environmental stress is a major factor that affects courtship behavior and evolutionary fitness. Although mature virgin females of Drosophila melanogaster usually accept a courting male to mate, they may not mate under stressful conditions. Above the temperature optimal for mating (20-25 °C), copulation success of D. melanogaster declines with increasing temperature although we observed vigorous courtship attempts by males, and no copulation takes place at temperatures over 36 °C. We attempted to identify the sensory pathway for detecting heat threat that drives a female to escape rather than to engage in mating that detects hot temperature and suppresses courtship behavior. We found that the artificial activation of warmth-sensitive neurons ('hot cells') in the antennal arista of females completely abrogates female copulation success even at permissive temperatures below 32 °C. Moreover, mutational loss of the GR28b.d thermoreceptor protein caused females to copulate even at 36 °C. These results indicate that antennal hot cells provide the input channel for detecting the high ambient temperature in the control of virgin female mating under stressful conditions.