Decoding odor quality and intensity in the Drosophila brain

The innexin 7 gap junction protein contributes to synchronized activity in the Drosophila antennal lobe and regulates olfactory function

In the mammalian olfactory bulb (OB), gap junctions coordinate synchronous activity among mitral and tufted cells to process olfactory information. In insects, gap junctions are also present in the antennal lobe (AL), a structure homologous to the mammalian OB. The invertebrate gap junction protein ShakB contributes to electrical synapses between AL projection neurons (PNs) in Drosophila. Other gap junction proteins, including innexin 7 (Inx7), are also expressed in the Drosophila AL, but little is known about their contribution to intercellular communication during olfactory information processing. In this study, we report spontaneous calcium transients in PNs grown in cell culture that are highly synchronous when these neurons are physically connected. RNAi-mediated knockdown of Inx7 in cultured PNs blocks calcium transient neuronal synchronization. In vivo, downregulation of Inx7 in the AL impairs both vinegar-induced electrophysiological calcium responses and behavioral responses to this appetitive stimulus. These results demonstrate that Inx7-encoded gap junctions functionally coordinate PN activity and modulate olfactory information processing in the adult Drosophila AL.

Olfactory projection neuron rewiring in the brain of an ecological specialist

Animal behaviors can differ greatly between closely related species. These behavioral changes are frequently linked to sensory system modifications, but central brain cell-type alterations might also be involved. Here, we develop advanced genetic tools to compare homologous central neurons in Drosophila sechellia, an ecological specialist, with the generalist Drosophila melanogaster. Through systematic morphological analysis of olfactory projection neurons (PNs), we reveal that the global anatomy of these second-order neurons is conserved. However, high-resolution, quantitative comparisons identify a striking case of convergent rewiring of PNs in two olfactory pathways critical for D. sechellia's host location. Calcium imaging and labeling of pre-synaptic sites in these evolved D. sechellia PNs indicate that species-specific connections with third-order partners are formed. This work demonstrates that peripheral sensory evolution is accompanied by selective wiring changes in the central brain to facilitate ecological specialization and paves the way to compare other cell types throughout the nervous system.

Neuroecology of alcohol risk and reward: Methanol boosts pheromones and courtship success in Drosophila melanogaster

Attraction of Drosophila melanogaster toward by-products of alcoholic fermentation, especially ethanol, has been extensively studied. Previous research has provided several interpretations of this attraction, including potential drug abuse, or a self-medicating coping strategy after mate rejection. We posit that the ecologically adaptive value of alcohol attraction has not been fully explored. Here, we assert a simple yet vital biological rationale for this alcohol preference. Flies display attraction to fruits rich in alcohol, specifically ethanol and methanol, where contact results in a rapid amplification of fatty acid-derived pheromones that enhance courtship success. We also identify olfactory sensory neurons that detect these alcohols, where we reveal roles in both attraction and aversion, and show that valence is balanced around alcohol concentration. Moreover, we demonstrate that methanol can be deadly, and adult flies must therefore accurately weigh the trade-off between benefits and costs for exposure within their naturally fermented and alcohol-rich environments.

Innate floral object identification in a solitary pollinator employs a combination of both visual and olfactory cues

Object identification is fundamental to animal behaviour and identifying nutritive objects is key for survival. Solitary insects emerging far from food sources and without access to social learning must therefore employ innate identification of food cues to locate relevant nutritive objects from a distance. Such innate preferences for food cues should be both specific enough to allow discrimination between food and non-food objects and general enough to allow for the variety of food objects relevant to the insect species. Here, we examined innate floral object identification behaviour in solitary generalist insect pollinator Eristalinus aeneus using an artificial floral object with both visual and olfactory cues previously found to be attractive to several hoverfly species across multiple environments. We used a subtractive two choice assay to present flower-naïve E. aeneus with a choice between this object and a similar object that differed in either a single visual or olfactory cue. We found that innate floral choices of the hoverfly E. aeneus are a product of broad, plant-based olfactory cues and visual cues, where a combination of radial symmetry and reflectance in the 300-400 nm and 500-700 nm wavelength range was particularly important for innate floral object preference. Our study, therefore, shows how solitary animals without prior experience can efficiently employ multimodal cues to identify multiple relevant nutritive objects.

Impact of developmental temperature on neural growth, connectivity, and function

Environmental temperature dictates the developmental pace of poikilothermic animals. In Drosophila, slower development at lower temperatures results in higher brain connectivity, but the generality of such scaling across temperatures and brain regions and its impact on function are unclear. Here, we show that brain connectivity scales continuously across temperatures, in agreement with a first-principle model that postulates different metabolic constraints for the growth of the brain and the organism. The model predicts brain wiring under temperature cycles and the nonuniform temporal scaling of neural development across temperatures. Developmental temperature has notable effects on odor-driven behavior. Dissecting the circuit architecture and function of neurons in the olfactory pathway, we demonstrate that developmental temperature does not alter odor encoding in first- and second-order neurons, but it shifts the specificity of connections onto third-order neurons that mediate innate behaviors. We conclude that while some circuit computations are robust to the effects of developmental temperature on wiring, others exhibit phenotypic plasticity with possible adaptive advantages.

Convergent olfactory circuits for courtship in Drosophila revealed by ds-Tango

Animals exhibit sex-specific behaviors that are governed by sexually dimorphic circuits. One such behavior in male Drosophila melanogaster, courtship, is regulated by various sensory modalities, including olfaction. Here, we reveal how sexually dimorphic olfactory pathways in male flies converge at the third-order, onto lateral horn output neurons, to regulate courtship. To achieve this, we developed ds-Tango, a modified version of the monosynaptic tracing and manipulation tool trans-Tango. In ds-Tango, two distinct configurations of trans-Tango are positioned in series, thus providing selective genetic access not only to the monosynaptic partners of starter neurons but also to their disynaptic connections. Using ds-Tango, we identified a node of convergence for three sexually dimorphic olfactory pathways. Silencing this node results in deficits in sex recognition of potential partners. Our results identify lateral horn output neurons required for proper courtship behavior in male flies and establish ds-Tango as a tool for disynaptic circuit tracing.

Analysis of fast calcium dynamics of honey bee olfactory coding

Odour processing exhibits multiple parallels between vertebrate and invertebrate olfactory systems. Insects, in particular, have emerged as relevant models for olfactory studies because of the tractability of their olfactory circuits. Here, we used fast calcium imaging to track the activity of projection neurons in the honey bee antennal lobe (AL) during olfactory stimulation at high temporal resolution. We observed a heterogeneity of response profiles and an abundance of inhibitory activities, resulting in various response latencies and stimulus-specific post-odour neural signatures. Recorded calcium signals were fed to a mushroom body (MB) model constructed implementing the fundamental features of connectivity between olfactory projection neurons, Kenyon cells (KC), and MB output neurons (MBON). The model accounts for the increase of odorant discrimination in the MB compared to the AL and reveals the recruitment of two distinct KC populations that represent odorants and their aftersmell as two separate but temporally coherent neural objects. Finally, we showed that the learning-induced modulation of KC-to-MBON synapses can explain both the variations in associative learning scores across different conditioning protocols used in bees and the bees' response latency. Thus, it provides a simple explanation of how the time contingency between the stimulus and the reward can be encoded without the need for time tracking. This study broadens our understanding of olfactory coding and learning in honey bees. It demonstrates that a model based on simple MB connectivity rules and fed with real physiological data can explain fundamental aspects of odour processing and associative learning.

Common principles for odour coding across vertebrates and invertebrates

The olfactory system is an ideal and tractable system for exploring how the brain transforms sensory inputs into behaviour. The basic tasks of any olfactory system include odour detection, discrimination and categorization. The challenge for the olfactory system is to transform the high-dimensional space of olfactory stimuli into the much smaller space of perceived objects and valence that endows odours with meaning. Our current understanding of how neural circuits address this challenge has come primarily from observations of the mechanisms of the brain for processing other sensory modalities, such as vision and hearing, in which optimized deep hierarchical circuits are used to extract sensory features that vary along continuous physical dimensions. The olfactory system, by contrast, contends with an ill-defined, high-dimensional stimulus space and discrete stimuli using a circuit architecture that is shallow and parallelized. Here, we present recent observations in vertebrate and invertebrate systems that relate the statistical structure and state-dependent modulation of olfactory codes to mechanisms of perception and odour-guided behaviour.

Experience-dependent tuning of the olfactory system

Insects rely on their sense of smell to navigate complex environments and make decisions regarding food and reproduction. However, in natural settings, the odors that convey this information may come mixed with environmental odors that can obscure their perception. Therefore, recognizing the presence of informative odors involves generalization and discrimination processes, which can be facilitated when there is a high contrast between stimuli, or the internal representation of the odors of interest outcompetes that of concurrent ones. The first two layers of the olfactory system, which involve the detection of odorants by olfactory receptor neurons and their encoding by the first postsynaptic partners in the antennal lobe, are critical for achieving such optimal representation. In this review, we summarize evidence indicating that experience-dependent changes adjust these two levels of the olfactory system. These changes are discussed in the context of the advantages they provide for detection of informative odors.

Evolution at multiple processing levels underlies odor-guided behavior in the genus Drosophila

Olfaction is a fundamental sense guiding animals to their food. How the olfactory system evolves and influences behavior is still poorly understood. Here, we selected five drosophilid species, including Drosophila melanogaster, inhabiting different ecological niches to compare their olfactory systems at multiple levels. We first identified ecologically relevant natural food odorants from every species and established species-specific odorant preferences. To compare odor coding in sensory neurons, we analyzed the antennal lobe (AL) structure, generated glomerular atlases, and developed GCaMP transgenic lines for all species. Although subsets of glomeruli showed distinct tuning profiles, odorants inducing species-specific preferences were coded generally similarly. Species distantly related or occupying different habitats showed more evident differences in odor coding, and further analysis revealed that changes in olfactory receptor (OR) sequences partially explain these differences. Our results demonstrate that genetic distance in phylogeny and ecological niche occupancy are key determinants in the evolution of ORs, AL structures, odor coding, and behavior. Interestingly, changes in odor coding among species could not be explained by evolutionary changes at a single olfactory processing level but rather are a complex phenomenon based on changes at multiple levels.

Internal-state-dependent modulation of olfactory responses: a tale of dopamine neurons in the adult Drosophila mushroom body

Olfaction is a vital sense that insects use to forage and interact with each other. When an insect smells an odor, its nervous system processes the odor information and transforms it into an appropriate behavioral decision. Olfactory processing and transformation are not label-lined, but instead are modulated by internal states. The vinegar fly, Drosophila melanogaster, has become a primary model organism for studying this modulation. It has been observed that internal state modulates olfactory behaviors in multiple sites of the fly brain. In this review article, I focus on the mushroom body, a computational center in the fly brain, and discuss how the dopamine system in this brain region mediates internal-state signals and shapes olfactory responses in adult flies.

Neural manifolds for odor-driven innate and acquired appetitive preferences

Sensory stimuli evoke spiking neural responses that innately or after learning drive suitable behavioral outputs. How are these spiking activities intrinsically patterned to encode for innate preferences, and could the neural response organization impose constraints on learning? We examined this issue in the locust olfactory system. Using a diverse odor panel, we found that ensemble activities both during ('ON response') and after stimulus presentations ('OFF response') could be linearly mapped onto overall appetitive preference indices. Although diverse, ON and OFF response patterns generated by innately appetitive odorants (higher palp-opening responses) were still limited to a low-dimensional subspace (a 'neural manifold'). Similarly, innately non-appetitive odorants evoked responses that were separable yet confined to another neural manifold. Notably, only odorants that evoked neural response excursions in the appetitive manifold could be associated with gustatory reward. In sum, these results provide insights into how encoding for innate preferences can also impact associative learning.

Olfactory navigation in arthropods

Using odors to find food and mates is one of the most ancient and highly conserved behaviors. Arthropods from flies to moths to crabs use broadly similar strategies to navigate toward odor sources-such as integrating flow information with odor information, comparing odor concentration across sensors, and integrating odor information over time. Because arthropods share many homologous brain structures-antennal lobes for processing olfactory information, mechanosensors for processing flow, mushroom bodies (or hemi-ellipsoid bodies) for associative learning, and central complexes for navigation, it is likely that these closely related behaviors are mediated by conserved neural circuits. However, differences in the types of odors they seek, the physics of odor dispersal, and the physics of locomotion in water, air, and on substrates mean that these circuits must have adapted to generate a wide diversity of odor-seeking behaviors. In this review, we discuss common strategies and specializations observed in olfactory navigation behavior across arthropods, and review our current knowledge about the neural circuits subserving this behavior. We propose that a comparative study of arthropod nervous systems may provide insight into how a set of basic circuit structures has diversified to generate behavior adapted to different environments.

Geosmin suppresses defensive behaviour and elicits unusual neural responses in honey bees

Geosmin is an odorant produced by bacteria in moist soil. It has been found to be extraordinarily relevant to some insects, but the reasons for this are not yet fully understood. Here we report the first tests of the effect of geosmin on honey bees. A stinging assay showed that the defensive behaviour elicited by the bee's alarm pheromone component isoamyl acetate (IAA) is strongly suppressed by geosmin. Surprisingly, the suppression is, however, only present at very low geosmin concentrations, and disappears at higher concentrations. We investigated the underlying mechanisms at the level of the olfactory receptor neurons by means of electroantennography, finding the responses to mixtures of geosmin and IAA to be lower than to pure IAA, suggesting an interaction of both compounds at the olfactory receptor level. Calcium imaging of the antennal lobe (AL) revealed that neuronal responses to geosmin decreased with increasing concentration, correlating well with the observed behaviour. Computational modelling of odour transduction and coding in the AL suggests that a broader activation of olfactory receptor types by geosmin in combination with lateral inhibition could lead to the observed non-monotonic increasing-decreasing responses to geosmin and thus underlie the specificity of the behavioural response to low geosmin concentrations.

Experience-dependent plasticity in the olfactory system of Drosophila melanogaster and other insects

It is long known that the nervous system of vertebrates can be shaped by internal and external factors. On the other hand, the nervous system of insects was long assumed to be stereotypic, although evidence for plasticity effects accumulated for several decades. To cover the topic comprehensively, this review recapitulates the establishment of the term "plasticity" in neuroscience and introduces its original meaning. We describe the basic composition of the insect olfactory system using Drosophila melanogaster as a representative example and outline experience-dependent plasticity effects observed in this part of the brain in a variety of insects, including hymenopterans, lepidopterans, locusts, and flies. In particular, we highlight recent advances in the study of experience-dependent plasticity effects in the olfactory system of D. melanogaster, as it is the most accessible olfactory system of all insect species due to the genetic tools available. The partly contradictory results demonstrate that morphological, physiological and behavioral changes in response to long-term olfactory stimulation are more complex than previously thought. Different molecular mechanisms leading to these changes were unveiled in the past and are likely responsible for this complexity. We discuss common problems in the study of experience-dependent plasticity, ways to overcome them, and future directions in this area of research. In addition, we critically examine the transferability of laboratory data to natural systems to address the topic as holistically as possible. As a mechanism that allows organisms to adapt to new environmental conditions, experience-dependent plasticity contributes to an animal's resilience and is therefore a crucial topic for future research, especially in an era of rapid environmental changes.

Multisite imaging of neural activity using a genetically encoded calcium sensor in the honey bee

Understanding of the neural bases for complex behaviors in Hymenoptera insect species has been limited by a lack of tools that allow measuring neuronal activity simultaneously in different brain regions. Here, we developed the first pan-neuronal genetic driver in a Hymenopteran model organism, the honey bee, and expressed the calcium indicator GCaMP6f under the control of the honey bee synapsin promoter. We show that GCaMP6f is widely expressed in the honey bee brain, allowing to record neural activity from multiple brain regions. To assess the power of this tool, we focused on the olfactory system, recording simultaneous responses from the antennal lobe, and from the more poorly investigated lateral horn (LH) and mushroom body (MB) calyces. Neural responses to 16 distinct odorants demonstrate that odorant quality (chemical structure) and quantity are faithfully encoded in the honey bee antennal lobe. In contrast, odor coding in the LH departs from this simple physico-chemical coding, supporting the role of this structure in coding the biological value of odorants. We further demonstrate robust neural responses to several bee pheromone odorants, key drivers of social behavior, in the LH. Combined, these brain recordings represent the first use of a neurogenetic tool for recording large-scale neural activity in a eusocial insect and will be of utility in assessing the neural underpinnings of olfactory and other sensory modalities and of social behaviors and cognitive abilities.

Behavioral algorithms and neural mechanisms underlying odor-modulated locomotion in insects

Odors released from mates and resources such as a host and food are often the first sensory signals that an animal can detect. Changes in locomotion in response to odors are an important mechanism by which animals access resources important to their survival. Odor-modulated changes in locomotion in insects constitute a whole suite of flexible behaviors that allow insects to close in on these resources from long distances and perform local searches to locate and subsequently assess them. Here, we review changes in odor-mediated locomotion across many insect species. We emphasize that changes in locomotion induced by odors are diverse. In particular, the olfactory stimulus is sporadic at long distances and becomes more continuous at short distances. This distance-dependent change in temporal profile produces a corresponding change in an insect's locomotory strategy. We also discuss the neural circuits underlying odor modulation of locomotion.

Drosophila olfaction: past, present and future

Among the many wonders of nature, the sense of smell of the fly Drosophila melanogaster might seem, at first glance, of esoteric interest. Nevertheless, for over a century, the 'nose' of this insect has been an extraordinary system to explore questions in animal behaviour, ecology and evolution, neuroscience, physiology and molecular genetics. The insights gained are relevant for our understanding of the sensory biology of vertebrates, including humans, and other insect species, encompassing those detrimental to human health. Here, I present an overview of our current knowledge of D. melanogaster olfaction, from molecules to behaviours, with an emphasis on the historical motivations of studies and illustration of how technical innovations have enabled advances. I also highlight some of the pressing and long-term questions.

A dopamine-gated learning circuit underpins reproductive state-dependent odor preference in Drosophila females

Motherhood induces a drastic, sometimes long-lasting, change in internal state and behavior in many female animals. How a change in reproductive state or the discrete event of mating modulates specific female behaviors is still incompletely understood. Using calcium imaging of the whole brain of Drosophila females, we find that mating does not induce a global change in brain activity. Instead, mating modulates the pheromone response of dopaminergic neurons innervating the fly's learning and memory center, the mushroom body (MB). Using the mating-induced increased attraction to the odor of important nutrients, polyamines, we show that disruption of the female fly's ability to smell, for instance the pheromone cVA, during mating leads to a reduction in polyamine preference for days later indicating that the odor environment at mating lastingly influences female perception and choice behavior. Moreover, dopaminergic neurons including innervation of the β'1 compartment are sufficient to induce the lasting behavioral increase in polyamine preference. We further show that MB output neurons (MBON) of the β'1 compartment are activated by pheromone odor and their activity during mating bidirectionally modulates preference behavior in mated and virgin females. Their activity is not required, however, for the expression of polyamine attraction. Instead, inhibition of another type of MBON innervating the β'2 compartment enables expression of high odor attraction. In addition, the response of a lateral horn (LH) neuron, AD1b2, which output is required for the expression of polyamine attraction, shows a modulated polyamine response after mating. Taken together, our data in the fly suggests that mating-related sensory experience regulates female odor perception and expression of choice behavior through a dopamine-gated learning circuit.

Higher-order olfactory neurons in the lateral horn support odor valence and odor identity coding in Drosophila

Understanding neuronal representations of odor-evoked activities and their progressive transformation from the sensory level to higher brain centers features one of the major aims in olfactory neuroscience. Here, we investigated how odor information is transformed and represented in higher-order neurons of the lateral horn, one of the higher olfactory centers implicated in determining innate behavior, using Drosophila melanogaster. We focused on a subset of third-order glutamatergic lateral horn neurons (LHNs) and characterized their odor coding properties in relation to their presynaptic partner neurons, the projection neurons (PNs) by two-photon functional imaging. We show that odors evoke reproducible, stereotypic, and odor-specific response patterns in LHNs. Notably, odor-evoked responses in these neurons are valence-specific in a way that their response amplitude is positively correlated with innate odor preferences. We postulate that this valence-specific activity is the result of integrating inputs from multiple olfactory channels through second-order neurons. GRASP and micro-lesioning experiments provide evidence that glutamatergic LHNs obtain their major excitatory input from uniglomerular PNs, while they receive an odor-specific inhibition through inhibitory multiglomerular PNs. In summary, our study indicates that odor representations in glutamatergic LHNs encode hedonic valence and odor identity and primarily retain the odor coding properties of second-order neurons.

Shedding Light on Inter-Individual Variability of Olfactory Circuits in Drosophila

Inter-individual differences in behavioral responses, anatomy or functional properties of neuronal populations of animals having the same genotype were for a long time disregarded. The majority of behavioral studies were conducted at a group level, and usually the mean behavior of all individuals was considered. Similarly, in neurophysiological studies, data were pooled and normalized from several individuals. This approach is mostly suited to map and characterize stereotyped neuronal properties between individuals, but lacks the ability to depict inter-individual variability regarding neuronal wiring or physiological characteristics. Recent studies have shown that behavioral biases and preferences to olfactory stimuli can vary significantly among individuals of the same genotype. The origin and the benefit of these diverse "personalities" is still unclear and needs to be further investigated. A perspective taken into account the inter-individual differences is needed to explore the cellular mechanisms underlying this phenomenon. This review focuses on olfaction in the vinegar fly Drosophila melanogaster and summarizes previous and recent studies on odor-guided behavior and the underlying olfactory circuits in the light of inter-individual variability. We address the morphological and physiological variabilities present at each layer of the olfactory circuitry and attempt to link them to individual olfactory behavior. Additionally, we discuss the factors that might influence individuality with regard to olfactory perception.

Optogenetic and thermogenetic manipulation of defined neural circuits and behaviors in Drosophila

Neural network dynamics underlying flexible animal behaviors remain elusive. The fruit fly Drosophila melanogaster is considered an excellent model in behavioral neuroscience because of its simple neuroanatomical architecture and the availability of various genetic methods. Moreover, Drosophila larvae's transparent body allows investigators to use optical methods on freely moving animals, broadening research directions. Activating or inhibiting well-defined events in excitable cells with a fine temporal resolution using optogenetics and thermogenetics led to the association of functions of defined neural populations with specific behavioral outputs such as the induction of associative memory. Furthermore, combining optogenetics and thermogenetics with state-of-the-art approaches, including connectome mapping and machine learning-based behavioral quantification, might provide a complete view of the experience- and time-dependent variations of behavioral responses. These methodologies allow further understanding of the functional connections between neural circuits and behaviors such as chemosensory, motivational, courtship, and feeding behaviors and sleep, learning, and memory.

The neural basis of interspecies interactions in insects

As insects move through the world, they continuously engage in behavioral interactions with other species. These interactions take on a spectrum of forms, from inconsequential encounters to predation, defense, and specialized symbiotic partnerships. All such interactions rely on sensorimotor pathways that carry out efficient categorization of different organisms and enact behaviors that cross species boundaries. Despite the universality of interspecies interactions, how insect brains perceive and process salient features of other species remains unexplored. Here, we present an overview of major questions concerning the neurobiology and evolution of behavioral interactions between species, providing a framework for future research on this critical role of the insect nervous system.

Multimodal Information Processing and Associative Learning in the Insect Brain

Simple Summary

Insect behaviors are a great indicator of evolution and provide useful information about the complexity of organisms. The realistic sensory scene of an environment is complex and replete with multisensory inputs, making the study of sensory integration that leads to behavior highly relevant. We summarize the recent findings on multimodal sensory integration and the behaviors that originate from them in our review.

Abstract

The study of sensory systems in insects has a long-spanning history of almost an entire century. Olfaction, vision, and gustation are thoroughly researched in several robust insect models and new discoveries are made every day on the more elusive thermo- and mechano-sensory systems. Few specialized senses such as hygro- and magneto-reception are also identified in some insects. In light of recent advancements in the scientific investigation of insect behavior, it is not only important to study sensory modalities individually, but also as a combination of multimodal inputs. This is of particular significance, as a combinatorial approach to study sensory behaviors mimics the real-time environment of an insect with a wide spectrum of information available to it. As a fascinating field that is recently gaining new insight, multimodal integration in insects serves as a fundamental basis to understand complex insect behaviors including, but not limited to navigation, foraging, learning, and memory. In this review, we have summarized various studies that investigated sensory integration across modalities, with emphasis on three insect models (honeybees, ants and flies), their behaviors, and the corresponding neuronal underpinnings.

Evolution of Olfactory Receptors Tuned to Mustard Oils in Herbivorous Drosophilidae

The diversity of herbivorous insects is attributed to their propensity to specialize on toxic plants. In an evolutionary twist, toxins betray the identity of their bearers when herbivores coopt them as cues for host-plant finding, but the evolutionary mechanisms underlying this phenomenon are poorly understood. We focused on Scaptomyza flava, an herbivorous drosophilid specialized on isothiocyanate (ITC)-producing (Brassicales) plants, and identified Or67b paralogs that were triplicated as mustard-specific herbivory evolved. Using in vivo heterologous systems for the expression of olfactory receptors, we found that S. flava Or67bs, but not the homologs from microbe-feeding relatives, responded selectively to ITCs, each paralog detecting different ITC subsets. Consistent with this, S. flava was attracted to ITCs, as was Drosophila melanogaster expressing S. flava Or67b3 in the homologous Or67b olfactory circuit. ITCs were likely coopted as olfactory attractants through gene duplication and functional specialization (neofunctionalization and subfunctionalization) in S. flava, a recently derived herbivore.

The female sex pheromone (Z)-4-undecenal mediates flight attraction and courtship in Drosophila melanogaster

Specific mate communication and recognition underlies reproduction and hence speciation. Our study provides new insights in Drosophila melanogaster premating olfactory communication. Mate communication evolves during adaptation to ecological niches and makes use of social signals and habitat cues. Female-produced, species-specific volatile pheromone (Z)-4-undecenal (Z4-11Al) and male pheromone (Z)-11-octadecenyl acetate (cVA) interact with food odour in a sex-specific manner. Furthermore, Z4-11Al, which mediates upwind flight attraction in both sexes, also elicits courtship in experienced males. Two isoforms of the olfactory receptor Or69a are co-expressed in the same olfactory sensory neurons. Z4-11Al is perceived via Or69aB, while the food odorant (R)-linalool is a main ligand for the other variant, Or69aA. However, only Z4-11Al mediates courtship in experienced males, not (R)-linalool. Behavioural discrimination is reflected by calcium imaging of the antennal lobe, showing distinct glomerular activation patterns by these two compounds. Male sex pheromone cVA is known to affect male and female courtship at close range, but does not elicit upwind flight attraction as a single compound, in contrast to Z4-11Al. A blend of the food odour vinegar and cVA attracted females, while a blend of vinegar and female pheromone Z4-11Al attracted males, instead. Sex-specific upwind flight attraction to blends of food volatiles and male and female pheromone, respectively, adds a new element to Drosophila olfactory premating communication and is an unambiguous paradigm for identifying the behaviourally active components, towards a more complete concept of food-pheromone odour objects.

Consensus driven by a minority in heterogenous groups of the cockroach Periplaneta american a

Many social species are able to perform collective decisions and reach consensus. However, how the interplay between social interactions, the diversity of preferences among the group members and the group size affects these dynamics is usually overlooked. The collective choice between odourous and odorless shelters is tested for the following three groups of social cockroaches (Periplaneta americana) which are solitary foragers: naive (individuals preferring the odorous shelter), conditioned (individuals without preference), and mixed (combining, unevenly, conditioned, and naive individuals). The robustness of the consensus is not affected by the naive individuals' proportion, but the rate and the frequency of selection of the odorous shelter are correlated to this proportion. In mixed groups, the naive individuals act as influencers. Simulations based on the mechanisms highlighted in our experiments predict that the consensus emerges only for intermediate group sizes. The universality of these mechanisms suggests that such phenomena are widely present in social systems.

Manipulations of the olfactory circuit highlight the role of sensory stimulation in regulating sleep amount

STUDY OBJECTIVES

While wake duration is a major sleep driver, an important question is if wake quality also contributes to controlling sleep. In particular, we sought to determine whether changes in sensory stimulation affect sleep in Drosophila. As Drosophila rely heavily on their sense of smell, we focused on manipulating olfactory input and the olfactory sensory pathway.

METHODS

Sensory deprivation was first performed by removing antennae or applying glue to antennae. We then measured sleep in response to neural activation, via expression of the thermally gated cation channel TRPA1, or inhibition, via expression of the inward rectifying potassium channel KIR2.1, of subpopulations of neurons in the olfactory pathway. Genetically restricting manipulations to adult animals prevented developmental effects.

RESULTS

We find that olfactory deprivation reduces sleep, largely independently of mushroom bodies that integrate olfactory signals for memory consolidation and have previously been implicated in sleep. However, specific neurons in the lateral horn, the other third-order target of olfactory input, affect sleep. Also, activation of inhibitory second-order projection neurons increases sleep. No single neuronal population in the olfactory processing pathway was found to bidirectionally regulate sleep, and reduced sleep in response to olfactory deprivation may be masked by temperature changes.

CONCLUSIONS

These findings demonstrate that Drosophila sleep is sensitive to sensory stimulation, and identify novel sleep-regulating neurons in the olfactory circuit. Scaling of signals across the circuit may explain the lack of bidirectional effects when neuronal activity is manipulated. We propose that olfactory inputs act through specific circuit components to modulate sleep in flies.

Unc13A and Unc13B contribute to the decoding of distinct sensory information in Drosophila

The physical distance between presynaptic Ca²⁺ channels and the Ca²⁺ sensors triggering the release of neurotransmitter-containing vesicles regulates short-term plasticity (STP). While STP is highly diversified across synapse types, the computational and behavioral relevance of this diversity remains unclear. In the Drosophila brain, at nanoscale level, we can distinguish distinct coupling distances between Ca²⁺ channels and the (m)unc13 family priming factors, Unc13A and Unc13B. Importantly, coupling distance defines release components with distinct STP characteristics. Here, we show that while Unc13A and Unc13B both contribute to synaptic signalling, they play distinct roles in neural decoding of olfactory information at excitatory projection neuron (ePN) output synapses. Unc13A clusters closer to Ca²⁺ channels than Unc13B, specifically promoting fast phasic signal transfer. Reduction of Unc13A in ePNs attenuates responses to both aversive and appetitive stimuli, while reduction of Unc13B provokes a general shift towards appetitive values. Collectively, we provide direct genetic evidence that release components of distinct nanoscopic coupling distances differentially control STP to play distinct roles in neural decoding of sensory information.

The physical distance between synaptic Ca²⁺ channels and sensors modulates short-term plasticity (STP). Here, the authors show that synaptic release factors Unc13A and Unc13B distinctly couple with Ca²⁺ channels and contribute to the neural decoding of distinct sensory information in Drosophila.

The neuroethology of olfactory sex communication in the honeybee Apis mellifera L

The honeybee Apis mellifera L. is a crucial pollinator as well as a prominent scientific model organism, in particular for the neurobiological study of olfactory perception, learning, and memory. A wealth of information is indeed available about how the worker bee brain detects, processes, and learns about odorants. Comparatively, olfaction in males (the drones) and queens has received less attention, although they engage in a fascinating mating behavior that strongly relies on olfaction. Here, we present our current understanding of the molecules, cells, and circuits underlying bees' sexual communication. Mating in honeybees takes place at so-called drone congregation areas and places high in the air where thousands of drones gather and mate in dozens with virgin queens. One major queen-produced olfactory signal-9-ODA, the major component of the queen pheromone-has been known for decades to attract the drones. Since then, some of the neural pathways responsible for the processing of this pheromone have been unraveled. However, olfactory receptor expression as well as brain neuroanatomical data point to the existence of three additional major pathways in the drone brain, hinting at the existence of 4 major odorant cues involved in honeybee mating. We discuss current evidence about additional not only queen- but also drone-produced pheromonal signals possibly involved in bees' sexual behavior. We also examine data revealing recent evolutionary changes in drone's olfactory system in the Apis genus. Lastly, we present promising research avenues for progressing in our understanding of the neural basis of bees mating behavior.

Dopamine modulation of sensory processing and adaptive behavior in flies

Behavioral flexibility for appropriate action selection is an advantage when animals are faced with decisions that will determine their survival or death. In order to arrive at the right decision, animals evaluate information from their external environment, internal state, and past experiences. How these different signals are integrated and modulated in the brain, and how context- and state-dependent behavioral decisions are controlled are poorly understood questions. Studying the molecules that help convey and integrate such information in neural circuits is an important way to approach these questions. Many years of work in different model organisms have shown that dopamine is a critical neuromodulator for (reward based) associative learning. However, recent findings in vertebrates and invertebrates have demonstrated the complexity and heterogeneity of dopaminergic neuron populations and their functional implications in many adaptive behaviors important for survival. For example, dopaminergic neurons can integrate external sensory information, internal and behavioral states, and learned experience in the decision making circuitry. Several recent advances in methodologies and the availability of a synaptic level connectome of the whole-brain circuitry of Drosophila melanogaster make the fly an attractive system to study the roles of dopamine in decision making and state-dependent behavior. In particular, a learning and memory center-the mushroom body-is richly innervated by dopaminergic neurons that enable it to integrate multi-modal information according to state and context, and to modulate decision-making and behavior.

Internal state configures olfactory behavior and early sensory processing in Drosophila larvae

Animals exhibit different behavioral responses to the same sensory cue depending on their internal state at a given moment. How and where in the brain are sensory inputs combined with state information to select an appropriate behavior? Here, we investigate how food deprivation affects olfactory behavior in Drosophila larvae. We find that certain odors repel well-fed animals but attract food-deprived animals and that feeding state flexibly alters neural processing in the first olfactory center, the antennal lobe. Hunger differentially modulates two output pathways required for opposing behavioral responses. Upon food deprivation, attraction-mediating uniglomerular projection neurons show elevated odor-evoked activity, whereas an aversion-mediating multiglomerular projection neuron receives odor-evoked inhibition. The switch between these two pathways is regulated by the lone serotonergic neuron in the antennal lobe, CSD. Our findings demonstrate how flexible behaviors can arise from state-dependent circuit dynamics in an early sensory processing center.

Olfactory processing in the lateral horn of Drosophila

Sensing olfactory signals in the environment represents a crucial and significant task of sensory systems in almost all organisms to facilitate survival and reproduction. Notably, the olfactory system of diverse animal phyla shares astonishingly many fundamental principles with regard to anatomical and functional properties. Binding of odor ligands by chemosensory receptors present in the olfactory peripheral organs leads to a neuronal activity that is conveyed to first and higher-order brain centers leading to a subsequent odor-guided behavioral decision. One of the key centers for integrating and processing innate olfactory behavior is the lateral horn (LH) of the protocerebrum in insects. In recent years the LH of Drosophila has garnered increasing attention and many studies have been dedicated to elucidate its circuitry. In this review we will summarize the recent advances in mapping and characterizing LH-specific cell types, their functional properties with respect to odor tuning, their neurotransmitter profiles, their connectivity to pre-synaptic and post-synaptic partner neurons as well as their impact for olfactory behavior as known so far.

Olfactory coding in honeybees

With less than a million neurons, the western honeybee Apis mellifera is capable of complex olfactory behaviors and provides an ideal model for investigating the neurophysiology of the olfactory circuit and the basis of olfactory perception and learning. Here, we review the most fundamental aspects of honeybee's olfaction: first, we discuss which odorants dominate its environment, and how bees use them to communicate and regulate colony homeostasis; then, we describe the neuroanatomy and the neurophysiology of the olfactory circuit; finally, we explore the cellular and molecular mechanisms leading to olfactory memory formation. The vastity of histological, neurophysiological, and behavioral data collected during the last century, together with new technological advancements, including genetic tools, confirm the honeybee as an attractive research model for understanding olfactory coding and learning.

Muscarinic Modulation of Antennal Lobe GABAergic Local Neurons Shapes Odor Coding and Behavior

In the antennal lobe (AL), the first olfactory relay of Drosophila, excitatory neurons are predominantly cholinergic. Ionotropic nicotinic receptors play a vital role in the effects of acetylcholine in the AL. However, the AL also has a high expression level of metabotropic muscarinic acetylcholine receptors type A (mAChRs-A). Nevertheless, the neurons expressing them and their role in the AL are unknown. Elucidating their function may reveal principles in olfactory modulation. Here, we show that mAChRs-A shape AL output and affect behavior. We localized mAChRs-A effects to a sub-population of GABAergic local neurons (iLNs), where they play a dual role: direct excitation of iLNs and stabilization of the synapse between receptor neurons and iLNs, which undergoes strong short-term depression. Our results reveal modulatory functions of the AL main excitatory neurotransmitter. Striking similarities to the mammalian olfactory system predict that mammalian glutamatergic metabotropic receptors could be associated with similar modulations.

Complete Connectomic Reconstruction of Olfactory Projection Neurons in the Fly Brain

Nervous systems contain sensory neurons, local neurons, projection neurons, and motor neurons. To understand how these building blocks form whole circuits, we must distil these broad classes into neuronal cell types and describe their network connectivity. Using an electron micrograph dataset for an entire Drosophila melanogaster brain, we reconstruct the first complete inventory of olfactory projections connecting the antennal lobe, the insect analog of the mammalian olfactory bulb, to higher-order brain regions in an adult animal brain. We then connect this inventory to extant data in the literature, providing synaptic-resolution "holotypes" both for heavily investigated and previously unknown cell types. Projection neurons are approximately twice as numerous as reported by light level studies; cell types are stereotyped, but not identical, in cell and synapse numbers between brain hemispheres. The lateral horn, the insect analog of the mammalian cortical amygdala, is the main target for this olfactory information and has been shown to guide innate behavior. Here, we find new connectivity motifs, including axo-axonic connectivity between projection neurons, feedback, and lateral inhibition of these axons by a large population of neurons, and the convergence of different inputs, including non-olfactory inputs and memory-related feedback onto third-order olfactory neurons. These features are less prominent in the mushroom body calyx, the insect analog of the mammalian piriform cortex and a center for associative memory. Our work provides a complete neuroanatomical platform for future studies of the adult Drosophila olfactory system.

The Wiring Logic of an Identified Serotonergic Neuron That Spans Sensory Networks

Serotonergic neurons project widely throughout the brain to modulate diverse physiological and behavioral processes. However, a single-cell resolution understanding of the connectivity of serotonergic neurons is currently lacking. Using a whole-brain EM dataset of a female Drosophila, we comprehensively determine the wiring logic of a broadly projecting serotonergic neuron (the CSDn) that spans several olfactory regions. Within the antennal lobe, the CSDn differentially innervates each glomerulus, yet surprisingly, this variability reflects a diverse set of presynaptic partners, rather than glomerulus-specific differences in synaptic output, which is predominately to local interneurons. Moreover, the CSDn has distinct connectivity relationships with specific local interneuron subtypes, suggesting that the CSDn influences distinct aspects of local network processing. Across olfactory regions, the CSDn has different patterns of connectivity, even having different connectivity with individual projection neurons that also span these regions. Whereas the CSDn targets inhibitory local neurons in the antennal lobe, the CSDn has more distributed connectivity in the LH, preferentially synapsing with principal neuron types based on transmitter content. Last, we identify individual novel synaptic partners associated with other sensory domains that provide strong, top-down input to the CSDn. Together, our study reveals the complex connectivity of serotonergic neurons, which combine the integration of local and extrinsic synaptic input in a nuanced, region-specific manner.SIGNIFICANCE STATEMENT All sensory systems receive serotonergic modulatory input. However, a comprehensive understanding of the synaptic connectivity of individual serotonergic neurons is lacking. In this study, we use a whole-brain EM microscopy dataset to comprehensively determine the wiring logic of a broadly projecting serotonergic neuron in the olfactory system of Drosophila Collectively, our study demonstrates, at a single-cell level, the complex connectivity of serotonergic neurons within their target networks, identifies specific cell classes heavily targeted for serotonergic modulation in the olfactory system, and reveals novel extrinsic neurons that provide strong input to this serotonergic system outside of the context of olfaction. Elucidating the connectivity logic of individual modulatory neurons provides a ground plan for the seemingly heterogeneous effects of modulatory systems.

Cellular and Molecular Targets of Waterbuck Repellent Blend Odors in Antennae of Glossina fuscipes fuscipes Newstead, 1910

Insects that transmit many of the world's deadliest animal diseases, for instance trypanosomosis, find their suitable hosts and avoid non-preferred hosts mostly through olfactory cues. The waterbuck repellent blend (WRB) comprising geranylacetone, guaiacol, pentanoic acid, and δ-octalactone derived from waterbuck skin odor is a repellent to some savannah-adapted tsetse flies and reduces trap catches of riverine species. However, the cellular and molecular mechanisms associated with detection and coding of the repellent odors remain to be elucidated. Here, we demonstrated that WRB inhibited blood feeding in both Glossina pallidipes Austen, 1903 and Glossina fuscipes fuscipes Newstead, 1910. Using the DREAM (Deorphanization of Receptors based on Expression Alterations in odorant receptor mRNA levels) technique, combined with ortholog comparison and molecular docking, we predicted the putative odorant receptors (ORs) for the WRB in G. f. fuscipes, a non-model insect. We show that exposure of G. f. fuscipes in vivo to WRB odorant resulted in up- and downregulation of mRNA transcript of several ORs. The WRB component with strong feeding inhibition altered mRNA transcript differently as compared to an attractant odor, showing these two odors of opposing valence already segregate at the cellular and molecular levels. Furthermore, molecular dynamics simulations demonstrated that the predicted ligand-OR binding pockets consisted mostly of hydrophobic residues with a few hydrogen bonds but a stable interaction. Finally, our electrophysiological response showed the olfactory sensory neurons of G. f. fuscipes tuned to the tsetse repellent components in different sensitivity and selectivity.

Valence and State-Dependent Population Coding in Dopaminergic Neurons in the Fly Mushroom Body

Neuromodulation permits flexibility of synapses, neural circuits, and ultimately behavior. One neuromodulator, dopamine, has been studied extensively in its role as a reward signal during learning and memory across animal species. Newer evidence suggests that dopaminergic neurons (DANs) can modulate sensory perception acutely, thereby allowing an animal to adapt its behavior and decision making to its internal and behavioral state. In addition, some data indicate that DANs are not homogeneous but rather convey different types of information as a heterogeneous population. We have investigated DAN population activity and how it could encode relevant information about sensory stimuli and state by taking advantage of the confined anatomy of DANs innervating the mushroom body (MB) of the fly Drosophila melanogaster. Using in vivo calcium imaging and a custom 3D image registration method, we found that the activity of the population of MB DANs encodes innate valence information of an odor or taste as well as the physiological state of the animal. Furthermore, DAN population activity is strongly correlated with movement, consistent with a role of dopamine in conveying behavioral state to the MB. Altogether, our data and analysis suggest that DAN population activities encode innate odor and taste valence, movement, and physiological state in a MB-compartment-specific manner. We propose that dopamine shapes innate perception through combinatorial population coding of sensory valence, physiological, and behavioral context.

The natverse, a versatile toolbox for combining and analysing neuroanatomical data

To analyse neuron data at scale, neuroscientists expend substantial effort reading documentation, installing dependencies and moving between analysis and visualisation environments. To facilitate this, we have developed a suite of interoperable open-source R packages called the natverse. The natverse allows users to read local and remote data, perform popular analyses including visualisation and clustering and graph-theoretic analysis of neuronal branching. Unlike most tools, the natverse enables comparison across many neurons of morphology and connectivity after imaging or co-registration within a common template space. The natverse also enables transformations between different template spaces and imaging modalities. We demonstrate tools that integrate the vast majority of Drosophila neuroanatomical light microscopy and electron microscopy connectomic datasets. The natverse is an easy-to-use environment for neuroscientists to solve complex, large-scale analysis challenges as well as an open platform to create new code and packages to share with the community.

Differential Role for a Defined Lateral Horn Neuron Subset in Naïve Odor Valence in Drosophila

Value coding of external stimuli in general, and odor valence in particular, is crucial for survival. In flies, odor valence is thought to be coded by two types of neurons: mushroom body output neurons (MBONs) and lateral horn (LH) neurons. MBONs are classified as neurons that promote either attraction or aversion, but not both, and they are dynamically activated by upstream neurons. This dynamic activation updates the valence values. In contrast, LH neurons receive scaled, but non-dynamic, input from their upstream neurons. It remains unclear how such a non-dynamic system generates differential valence values. Recently, PD2a1/b1 LH neurons were demonstrated to promote approach behavior at low odor concentration in starved flies. Here, we demonstrate that at high odor concentrations, these same neurons contribute to avoidance in satiated flies. The contribution of PD2a1/b1 LH neurons to aversion is context dependent. It is diminished in starved flies, although PD2a1/b1 neural activity remains unchanged, and at lower odor concentration. In addition, PD2a1/b1 aversive effect develops over time. Thus, our results indicate that, even though PD2a1/b1 LH neurons transmit hard-wired output, their effect on valence can change. Taken together, we suggest that the valence model described for MBONs does not hold for LH neurons.

Alpha oscillations govern interhemispheric spike timing coordination in the honey bee brain

In 1929 Hans Berger discovered the alpha oscillations: prominent, ongoing oscillations around 10 Hz in the electroencephalogram of the human brain. These alpha oscillations are among the most widely studied brain signals, related to cognitive phenomena such as attention, memory and consciousness. However, the mechanisms by which alpha oscillations affect human cognition await demonstration. Here, we suggest the honey bee brain as an experimentally more accessible model system for investigating the functional role of alpha oscillations. We found a prominent spontaneous oscillation around 18 Hz that is reduced in amplitude upon olfactory stimulation. Similar to alpha oscillations in primates, the phase of this oscillation biased both timing of neuronal spikes and amplitude of high-frequency gamma activity (40-450 Hz). These results suggest a common role of alpha oscillations across phyla and provide an unprecedented new venue for causal studies on the relationship between neuronal spikes, brain oscillations and cognition.

Stochastic and Arbitrarily Generated Input Patterns to the Mushroom Bodies Can Serve as Conditioned Stimuli in Drosophila

Single neurons in the brains of insects often have individual genetic identities and can be unambiguously identified between animals. The overall neuronal connectivity is also genetically determined and hard-wired to a large degree. Experience-dependent structural and functional plasticity is believed to be superimposed onto this more-or-less fixed connectome. However, in Drosophila melanogaster, it has been shown that the connectivity between the olfactory projection neurons (OPNs) and Kenyon cells, the intrinsic neurons of the mushroom body, is highly stochastic and idiosyncratic between individuals. Ensembles of distinctly and sparsely activated Kenyon cells represent information about the identity of the olfactory input, and behavioral relevance can be assigned to this representation in the course of associative olfactory learning. Previously, we showed that in the absence of any direct sensory input, artificially and stochastically activated groups of Kenyon cells could be trained to encode aversive cues when their activation coincided with aversive stimuli. Here, we have tested the hypothesis that the mushroom body can learn any stochastic neuronal input pattern as behaviorally relevant, independent of its exact origin. We show that fruit flies can learn thermogenetically generated, stochastic activity patterns of OPNs as conditioned stimuli, irrespective of glomerular identity, the innate valence that the projection neurons carry, or inter-hemispheric symmetry.

The brain of a nocturnal migratory insect, the Australian Bogong moth

Every year, millions of Australian Bogong moths (Agrotis infusa) complete an astonishing journey: In Spring, they migrate over 1,000 km from their breeding grounds to the alpine regions of the Snowy Mountains, where they endure the hot summer in the cool climate of alpine caves. In autumn, the moths return to their breeding grounds, where they mate, lay eggs and die. These moths can use visual cues in combination with the geomagnetic field to guide their flight, but how these cues are processed and integrated into the brain to drive migratory behavior is unknown. To generate an access point for functional studies, we provide a detailed description of the Bogong moth's brain. Based on immunohistochemical stainings against synapsin and serotonin (5HT), we describe the overall layout as well as the fine structure of all major neuropils, including the regions that have previously been implicated in compass-based navigation. The resulting average brain atlas consists of 3D reconstructions of 25 separate neuropils, comprising the most detailed account of a moth brain to date. Our results show that the Bogong moth brain follows the typical lepidopteran ground pattern, with no major specializations that can be attributed to their spectacular migratory lifestyle. These findings suggest that migratory behavior does not require widespread modifications of brain structure, but might be achievable via small adjustments of neural circuitry in key brain areas. Locating these subtle changes will be a challenging task for the future, for which our study provides an essential anatomical framework.

Experience-dependent tuning of early olfactory processing in the adult honey bee, Apis mellifera

Experience-dependent plasticity in the central nervous system allows an animal to adapt its responses to stimuli over different time scales. In this study, we explored the impacts of adult foraging experience on early olfactory processing by comparing naturally foraging honey bees, Apis mellifera, with those that experienced a chronic reduction in adult foraging experience. We placed age-matched sets of sister honey bees into two different olfactory conditions, in which animals were allowed to forage ad libitum In one condition, we restricted foraging experience by placing honey bees in a tent in which both sucrose and pollen resources were associated with a single odor. In the second condition, honey bees were allowed to forage freely and therefore encounter a diversity of naturally occurring resource-associated olfactory experiences. We found that honey bees with restricted foraging experiences had altered antennal lobe development. We measured the glomerular responses to odors using calcium imaging in the antennal lobe, and found that natural olfactory experience also enhanced the inter-individual variation in glomerular response profiles to odors. Additionally, we found that honey bees with adult restricted foraging experience did not distinguish relevant components of an odor mixture in a behavioral assay as did their freely foraging siblings. This study highlights the impacts of individual experience on early olfactory processing at multiple levels.

The role of volatiles in plant communication

Volatiles mediate the interaction of plants with pollinators, herbivores and their natural enemies, other plants and micro-organisms. With increasing knowledge about these interactions the underlying mechanisms turn out to be increasingly complex. The mechanisms of biosynthesis and perception of volatiles are slowly being uncovered. The increasing scientific knowledge can be used to design and apply volatile-based agricultural strategies.

Scaling the interactive effects of attractive and repellent odours for insect search behaviour

Insects searching for resources are exposed to a complexity of mixed odours, often involving both attractant and repellent substances. Understanding how insects respond to this complexity of cues is crucial for understanding consumer-resource interactions, but also to develop novel tools to control harmful pests. To advance our understanding of insect responses to combinations of attractive and repellent odours, we formulated three qualitative hypotheses; the response-ratio hypothesis, the repellent-threshold hypothesis and the odour-modulation hypothesis. The hypotheses were tested by exposing Drosophila melanogaster in a wind tunnel to combinations of vinegar as attractant and four known repellents; benzaldehyde, 1-octen-3-ol, geosmin and phenol. The responses to benzaldehyde, 1-octen-3-ol and geosmin provided support for the response-ratio hypothesis, which assumes that the behavioural response depends on the ratio between attractants and repellents. The response to phenol, rather supported the repellent-threshold hypothesis, where aversion only occurs above a threshold concentration of the repellent due to overshadowing of the attractant. We hypothesize that the different responses may be connected to the localization of receptors, as receptors detecting phenol are located on the maxillary palps whereas receptors detecting the other odorants are located on the antennae.

Long-term memory is formed immediately without the need for protein synthesis-dependent consolidation in Drosophila

It is believed that long-term memory (LTM) cannot be formed immediately because it must go through a protein synthesis-dependent consolidation process. However, the current study uses Drosophila aversive olfactory conditioning to show that such processes are dispensable for context-dependent LTM (cLTM). Single-trial conditioning yields cLTM that is formed immediately in a protein-synthesis independent manner and is sustained over 14 days without decay. Unlike retrieval of traditional LTM, which requires only the conditioned odour and is mediated by mushroom-body neurons, cLTM recall requires both the conditioned odour and reinstatement of the training-environmental context. It is mediated through lateral-horn neurons that connect to multiple sensory brain regions. The cLTM cannot be retrieved if synaptic transmission from any one of these centres is blocked, with effects similar to those of altered encoding context during retrieval. The present study provides strong evidence that long-term memory can be formed easily without the need for consolidation.

New protein synthesis is known to be indispensable for the consolidation of long-term memory. Here, the authors report that an olfactory memory can be successfully recalled after 14 days without protein synthesis when the training context is also provided in addition to the conditioned odor.

Species- and size-related differences in dopamine-like immunoreactive clusters in the brain of Nasonia vitripennis and N. giraulti

An extreme reduction in body size has been shown to negatively impact the memory retention level of the parasitic wasp Nasonia vitripennis. In addition, N. vitripennis and Nasonia giraulti, closely related parasitic wasps, differ markedly in the number of conditioning trials required to form long-term memory. These differences in memory dynamics may be associated with differences in the dopaminergic neurons in the Nasonia brains. Here, we used dopamine immunoreactivity to identify and count the number of cell bodies in dopaminergic clusters of normal- and small-sized N. vitripennis and normal-sized N. giraulti. We counted in total a maximum of approximately 160 dopaminergic neurons per brain. These neurons were present in 9 identifiable clusters (D1a, D1b, D2, D3, D4a, D4b, D5, D6 and D7). Our analysis revealed that N. giraulti had fewer cells in the D2 and D4a clusters but more in D4b, compared with normal-sized N. vitripennis. In addition, we found fewer cells in the D5 and D7 cluster of small-sized N. vitripennis compared to normal-sized N. vitripennis. A comparison of our findings with the literature on dopaminergic clusters in the fruit fly Drosophila melanogaster and the honey bee Apis mellifera indicates that clusters D2, D3 and D5 may play a role in memory formation in Nasonia wasps. The results from both the species comparison and the size comparison are therefore of high interest and importance for our understanding of the complex intricacies that underlie the memory dynamics of insects.