Place learning overrides innate behaviors in Drosophila

Goal-directed behavior in Tenebrio molitor larvae

Can signs of intentional behavior be traced in an insect larva, traditionally thought to be driven only by mere reflexes? We trained Tenebrio molitor coleoptera larvae in a uniform Y-maze to prefer one target branch to get access to food, observing their ability to learn and retain access to the reward-associated side for up to 24 h. During reward devaluation, the reward food (experimental group) and a different food (control group) were paired with an aversive stimulus in a new environment. When tested again in the Y-maze, mealworms of the experimental group significantly reduced their visits to the target branch, whereas mealworms of the control group did not. Importantly, we found that the larvae did not have to experience the unpleasant consequences directly in the target branch to halt their behavior, as the exposure to the aversive taste occurred in a separate unfamiliar context. This is evidence that the mealworms formed a mental representation of action-consequence relationships, demonstrating flexible control of their actions to achieve desired outcomes at an early stage of their development.

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.

Inbreeding-Driven Innate Behavioral Changes in Drosophila melanogaster

Drosophila melanogaster has long been used to demonstrate the effect of inbreeding, particularly in relation to reproductive fitness and stress tolerance. In comparison, less attention has been given to exploring the influence of inbreeding on the innate behavior of D. melanogaster. In this study, multiple replicates of six different types of crosses were set in pair conformation of the laboratory-maintained wild-type D. melanogaster. This resulted in progeny with six different levels of inbreeding coefficients. Larvae and adult flies of varied inbreeding coefficients were subjected to different behavioral assays. In addition to the expected inbreeding depression in the-egg to-adult viability, noticeable aberrations were observed in the crawling and phototaxis behaviors of larvae. Negative geotactic behavior as well as positive phototactic behavior of the flies were also found to be adversely affected with increasing levels of inbreeding. Interestingly, positively phototactic inbred flies demonstrated improved learning compared to outbred flies, potentially the consequence of purging. Flies with higher levels of inbreeding exhibited a delay in the manifestation of aggression and courtship. In summary, our findings demonstrate that inbreeding influences the innate behaviors in D. melanogaster, which in turn may affect the overall biological fitness of the flies.

Excessive energy expenditure due to acute physical restraint disrupts Drosophila motivational feeding response

To study the behavior of Drosophila, it is often necessary to restrain and mount individual flies. This requires removal from food, additional handling, anesthesia, and physical restraint. We find a strong positive correlation between the length of time flies are mounted and their subsequent reflexive feeding response, where one hour of mounting is the approximate motivational equivalent to ten hours of fasting. In an attempt to explain this correlation, we rule out anesthesia side-effects, handling, additional fasting, and desiccation. We use respirometric and metabolic techniques coupled with behavioral video scoring to assess energy expenditure in mounted and free flies. We isolate a specific behavior capable of exerting large amounts of energy in mounted flies and identify it as an attempt to escape from restraint. We present a model where physical restraint leads to elevated activity and subsequent faster nutrient storage depletion among mounted flies. This ultimately further accelerates starvation and thus increases reflexive feeding response. In addition, we show that the consequences of the physical restraint profoundly alter aerobic activity, energy depletion, taste, and feeding behavior, and suggest that careful consideration is given to the time-sensitive nature of these highly significant effects when conducting behavioral, physiological or imaging experiments that require immobilization.

Rest Is Required to Learn an Appetitively-Reinforced Operant Task in Drosophila

Maladaptive operant conditioning contributes to development of neuropsychiatric disorders. Candidate genes have been identified that contribute to this maladaptive plasticity, but the neural basis of operant conditioning in genetic model organisms remains poorly understood. The fruit fly Drosophila melanogaster is a versatile genetic model organism that readily forms operant associations with punishment stimuli. However, operant conditioning with a food reward has not been demonstrated in flies, limiting the types of neural circuits that can be studied. Here we present the first sucrose-reinforced operant conditioning paradigm for flies. In the paradigm, flies walk along a Y-shaped track with reward locations at the terminus of each hallway. When flies turn in the reinforced direction at the center of the track, they receive a sucrose reward at the end of the hallway. Only flies that rest early in training learn the reward contingency normally. Flies rewarded independently of their behavior do not form a learned association but have the same amount of rest as trained flies, showing that rest is not driven by learning. Optogenetically-induced sleep does not promote learning, indicating that sleep itself is not sufficient for learning the operant task. We validated the sensitivity of this assay to detect the effect of genetic manipulations by testing the classic learning mutant dunce. Dunce flies are learning-impaired in the Y-Track task, indicating a likely role for cAMP in the operant coincidence detector. This novel training paradigm will provide valuable insight into the molecular mechanisms of disease and the link between sleep and learning.

Searching for relief: Drosophila melanogaster navigation in a virtual bitter maze

Animals use relief-based place learning to pinpoint a specific location where noxious stimuli are diminished or abolished. Here we show how the optogenetically-induced activation of bitter-sensing neurons in Drosophila melanogaster elicits pain-like behavioural responses and stimulates the search for a place where this activation is relieved. Under this "virtual" stimulation paradigm it would be feasible to test relief learning several times throughout an animal's lifespan, without the potentially damaging effects which may result from the repeated administration of "real" heat or electrical shock. Furthermore, virtual bitter taste could be used in place of virtual pain stimulation to guide conditioned place preference and study learning processes. We also propose that spatially-specific reduction of locomotor velocity may provide immediate evidence of relief-based place learning and spatial memory.

Anti-instinctive Learning Behavior Revealed by Locomotion-Triggered Mild Heat Stress in Drosophila

Anti-instinctive learning, an ability to modify an animal's innate behaviors in ways that go against one's innate tendency, can confer great evolutionary advantages to animals and enable them to better adapt to the changing environment. Yet, our understanding of anti-instinctive learning and its underlying mechanisms is still limited. In this work, we describe a new anti-instinctive learning behavior of fruit flies. This learning paradigm requires the fruit fly to respond to a recurring, aversive, mild heat stress by modifying its innate locomotion behavior. We found that experiencing movement-triggered mild heat stress repeatedly significantly reduced walking activity in wild type fruit flies, indicating that fruit flies are capable of anti-instinctive learning. We also report that such learning ability is reduced in dopamine 1-like receptor 1 (Dop1R1) null mutant and dopamine 2-like receptor (Dop2R) null mutant flies, suggesting that these two dopamine receptors are involved in mediating anti-instinctive learning in flies.

Experimental psychology meets behavioral ecology: what laboratory studies of learning polymorphisms mean for learning under natural conditions, and vice versa

Behavior genetics, and specifically the study of learning and memory, has benefitted immensely from the development of powerful forward- and reverse-genetic methods for investigating the relationships between genes and behavior. Application of these methods in controlled laboratory settings has led to insights into gene-behavior relationships. In this perspective article, we argue that the field is now poised to make significant inroads into understanding the adaptive value of heritable variation in behavior in natural populations. Studies of natural variation with several species, in particular, are now in a position to complement laboratory studies of mechanisms, and sometimes this work can lead to counterintuitive insights into the mechanism of gene action on behavior. We make this case using a recent example from work with the honey bee, Apis mellifera.

Finding a place and leaving a mark in memory formation

Preference for spatial locations to maximize favorable outcomes and minimize aversive experiences helps animals survive and adapt to the changing environment. Both visual and non-visual cues play a critical role in spatial navigation and memory of a place supports and guides these strategies. Here we present the neural, genetic and behavioral processes involved in place memory formation using Drosophila melanogaster with a focus on non-visual cue based spatial memories. The work presented here highlights the work done by Dr. Troy Zars and his colleagues with an emphasis on role of biogenic amines in learning, cell biological mechanisms of neural systems and behavioral plasticity of place conditioning.