Structural plasticity of identified glomeruli in the antennal lobes of the adult worker honey bee

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.

Natural variability in bee brain size and symmetry revealed by micro-CT imaging and deep learning

Analysing large numbers of brain samples can reveal minor, but statistically and biologically relevant variations in brain morphology that provide critical insights into animal behaviour, ecology and evolution. So far, however, such analyses have required extensive manual effort, which considerably limits the scope for comparative research. Here we used micro-CT imaging and deep learning to perform automated analyses of 3D image data from 187 honey bee and bumblebee brains. We revealed strong inter-individual variations in total brain size that are consistent across colonies and species, and may underpin behavioural variability central to complex social organisations. In addition, the bumblebee dataset showed a significant level of lateralization in optic and antennal lobes, providing a potential explanation for reported variations in visual and olfactory learning. Our fast, robust and user-friendly approach holds considerable promises for carrying out large-scale quantitative neuroanatomical comparisons across a wider range of animals. Ultimately, this will help address fundamental unresolved questions related to the evolution of animal brains and cognition.

Effects of thiamethoxam on brain structure of Bombus terrestris (Hymenoptera: Apidae) workers

Neonicotinoids are the most widely used pesticide compared to other major insecticide classes known worldwide and have the fastest growing market share. Many studies showed that neonicotinoid pesticides harm honeybee learning and farming activities, negatively affect colony adaptation and reduce pollination abilities. Bumblebees are heavily preferred species all over the world in order to ensure pollination in plant production. In this study, sublethal effects of the neonicotinoid insecticide thiamethoxam on the brain of Bombus terrestris workers were analyzed. Suspensions (1/1000, 1/100, 1/10) of the maximum recommended dose of thiamethoxam were applied to the workers. 48 h after spraying, morphological effects on the brains of workers were studied. According to area measurements of ICC's of Kenyon cells, there was a significant difference between 1/10 dose and all groups. On the other hand, areas of INC's of Kenyon cells showed a significant difference between the control group and all dose groups. Neuropil disorganization in the calyces increased gradually and differed significantly between the groups and was mostly detected at the highest dose (1/10). Apart from optic lobes, pycnotic nuclei were also observed in the middle region of calyces of mushroom bodies in the high dose group. Also, the width of the lamina, medulla and lobula parts of the optic lobes of each group and the areas of the antennal lobes were measured and significant differences were determined between the groups. The results of the study revealed that sublethal doses of thiamethoxam caused some negative impacts on brain morphology of B. terrestris workers.

Repeated short-term exposure to diesel exhaust reduces honey bee colony fitness

Production of insect-pollinated crops is often reliant on honey bee (Apis mellifera) pollination services. Colonies can be managed and moved to meet the demands of modern intensified monoculture farming systems. Increased colony mortalities have been observed, which are thought be caused by interacting factors including exposure to pesticides, parasites, viruses, agricultural intensification, and changes in global and regional climate. However, whilst common tropospheric air pollutants (e.g. NO_x, particulate matter etc) are known to cause a range of negative effects on human health, there is little evidence of their impact on the health of A. mellifera. This study investigates the effects of exposure to diesel exhaust on A. mellifera, both at the level of individual foragers and on the whole colony. We exposed a series of colonies to diesel exhaust fumes for 2 h a day over the course of three weeks and contrasted their performance to a series of paired control colonies located at the same field site. We investigated markers of neuronal health in the brains of individual foragers and measured the prevalence of common viruses. Electronic counters monitored daily colony activity patterns and pollen samples from returning foragers were analysed to investigate plant species richness and diversity. The amounts of honey, brood and pollen in each colony were measured regularly. We demonstrated an upregulation of the synapse protein Neurexin 1 in forager brains repeatedly exposed to diesel exhaust. Furthermore, we found that colonies exposed to diesel exhaust lost colony weight after the exposure period until the end of the summer season, whereas control colonies gained weight towards the end of the season. Further investigations are required, but we hypothesise that such effects on both individual foragers and whole colony fitness parameters could ultimately contribute to winter losses of honey bee colonies, particularly in the presence of additional stressors.

Age and social experience induced plasticity across brain regions of the paper wasp Polistes fuscatus

Developmental studies of brain volumes can reveal which portions of neural circuits are sensitive to environmental inputs. In social insects, differences in relative investment across brain regions emerge as behavioural repertoires change during ontogeny or as a result of experience. Here, we test the effects of maturation and social experience on morphological brain development in Polistes fuscatus paper wasps, focusing on brain regions involved in visual and olfactory processing. We find that mature wasps regardless of social experience have relatively larger brains than newly emerged wasps and this difference is driven by changes to mushroom body calyx and visual regions but not olfactory processing neuropils. Notably, social wasps invest more in the anterior optic tubercle (AOT), a visual glomerulus involved in colour and object processing in other taxa, relative to other visual integration centres the mushroom body calyces compared with aged socially naive wasps. Differences in developmental plasticity between visual and olfactory neuropil volumes are discussed in light of behavioural maturation in paper wasps, especially as it relates to social recognition. Previous research has shown that P. fuscatus need social experience to develop specialized visual processing of faces, which is used to individually recognize conspecifics. The present study suggests that the AOT is a candidate brain region that could mediate facial processing in this species.

Learning-dependent plasticity in the antennal lobe improves discrimination and recognition of odors in the honeybee

Honeybees are extensively used to study olfactory learning and memory processes thanks to their ability to discriminate and remember odors and because of their advantages for optophysiological recordings of the circuits involved in memory and odor perception. There are evidences that the encoding of odors in areas of primary sensory processing is not rigid, but undergoes changes caused by olfactory experience. The biological meaning of these changes is focus of intense discussions. Along this review, we present evidences of plasticity related to different forms of learning and discuss its function in the context of olfactory challenges that honeybees have to solve. So far, results in honeybees are consistent with a model in which changes in early olfactory processing contributes to the ability of an animal to recognize the presence of relevant odors and facilitates the discrimination of odors in a way adjusted to its own experience.

Plasticity and modulation of olfactory circuits in insects

Olfactory circuits change structurally and physiologically during development and adult life. This allows insects to respond to olfactory cues in an appropriate and adaptive way according to their physiological and behavioral state, and to adapt to their specific abiotic and biotic natural environment. We highlight here findings on olfactory plasticity and modulation in various model and non-model insects with an emphasis on moths and social Hymenoptera. Different categories of plasticity occur in the olfactory systems of insects. One type relates to the reproductive or feeding state, as well as to adult age. Another type of plasticity is context-dependent and includes influences of the immediate sensory and abiotic environment, but also environmental conditions during postembryonic development, periods of adult behavioral maturation, and short- and long-term sensory experience. Finally, plasticity in olfactory circuits is linked to associative learning and memory formation. The vast majority of the available literature summarized here deals with plasticity in primary and secondary olfactory brain centers, but also peripheral modulation is treated. The described molecular, physiological, and structural neuronal changes occur under the influence of neuromodulators such as biogenic amines, neuropeptides, and hormones, but the mechanisms through which they act are only beginning to be analyzed.

Structural plasticity of olfactory neuropils in relation to insect diapause

Many insects that live in temperate zones spend the cold season in a state of dormancy, referred to as diapause. As the insect must rely on resources that were gathered before entering diapause, keeping a low metabolic rate is of utmost importance. Organs that are metabolically expensive to maintain, such as the brain, can therefore become a liability to survival if they are too large.Insects that go through diapause as adults generally do so before entering the season of reproduction. This order of events introduces a conflict between maintaining low metabolism during dormancy and emerging afterward with highly developed sensory systems that improve fitness during the mating season.We investigated the timing of when investments into the olfactory system are made by measuring the volumes of primary and secondary olfactory neuropils in the brain as they fluctuate in size throughout the extended diapause life-period of adult Polygonia c-album butterflies.Relative volumes of both olfactory neuropils increase significantly during early adult development, indicating the importance of olfaction to this species, but still remain considerably smaller than those of nondiapausing conspecifics. However, despite butterflies being kept under the same conditions as before the dormancy, their olfactory neuropil volumes decreased significantly during the postdormancy period.The opposing directions of change in relative neuropil volumes before and after diapause dormancy indicate that the investment strategies governing structural plasticity during the two life stages could be functionally distinct. As butterflies were kept in stimulus-poor conditions, we find it likely that investments into these brain regions rely on experience-expectant processes before diapause and experience-dependent processes after diapause conditions are broken.As the shift in investment strategies coincides with a hard shift from premating season to mating season, we argue that these developmental characteristics could be adaptations that mitigate the trade-off between dormancy survival and reproductive fitness.

Early olfactory, but not gustatory processing, is affected by the selection of heritable cognitive phenotypes in honey bee

Associative learning enables animals to predict rewards or punishments by their associations with predictive stimuli, while non-associative learning occurs without reinforcement. The latter includes latent inhibition (LI), whereby animals learn to ignore an inconsequential 'familiar' stimulus. Individual honey bees display heritable differences in expression of LI. We examined the behavioral and neuronal responses between honey bee genetic lines exhibiting high and low LI. We observed, as in previous studies, that high LI lines learned a familiar odor more slowly than low LI bees. By measuring gustatory responses to sucrose, we determined that perception of sucrose reward was similar between both lines, thereby not contributing to the LI phenotype. We then used extracellular electrophysiology to determine differences in neural responses of the antennal lobe (AL) to familiar and novel odors between the lines. Low LI bees responded significantly more strongly to both familiar and novel odors than the high LI bees, but the lines showed equivalent differences in response to the novel and familiar odors. This work suggests that some effects of genotype are present in early olfactory processing, and those effects could complement how LI is manifested at later stages of processing in brains of bees in the different lines.

The Neuroproteomic Basis of Enhanced Perception and Processing of Brood Signals That Trigger Increased Reproductive Investment in Honeybee (Apis mellifera) Workers

The neuronal basis of complex social behavior is still poorly understood. In honeybees, reproductive investment decisions are made at the colony-level. Queens develop from female-destined larvae that receive alloparental care from nurse bees in the form of ad-libitum royal jelly (RJ) secretions. Typically, the number of raised new queens is limited but genetic breeding of "royal jelly bees" (RJBs) for enhanced RJ production over decades has led to a dramatic increase of reproductive investment in queens. Here, we compare RJBs to unselected Italian bees (ITBs) to investigate how their cognitive processing of larval signals in the mushroom bodies (MBs) and antennal lobes (ALs) may contribute to their behavioral differences. A cross-fostering experiment confirms that the RJB syndrome is mainly due to a shift in nurse bee alloparental care behavior. Using olfactory conditioning of the proboscis extension reflex, we show that the RJB nurses spontaneously respond more often to larval odors compared with ITB nurses but their subsequent learning occurs at similar rates. These phenotypic findings are corroborated by our demonstration that the proteome of the brain, particularly of the ALs differs between RJBs and ITBs. Notably, in the ALs of RJB newly emerged bees and nurses compared with ITBs, processes of energy and nutrient metabolism, signal transduction are up-regulated, priming the ALs for receiving and processing the brood signals from the antennae. Moreover, highly abundant major royal jelly proteins and hexamerins in RJBs compared with ITBs during early life when the nervous system still develops suggest crucial new neurobiological roles for these well-characterized proteins. Altogether, our findings reveal that RJBs have evolved a strong olfactory response to larvae, enabled by numerous neurophysiological adaptations that increase the nurse bees' alloparental care behavior.

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.

Neuroanatomical correlates of mobility: Sensory brain centres are bigger in winged than in wingless parthenogenetic pea aphid females

Many aphid species reproduce parthenogenetically throughout most of the year, with individuals having identical genomes. Nevertheless, aphid clones display a marked polyphenism with associated behavioural differences. Pea aphids (Acyrthosiphon pisum), when crowded, produce winged individuals, which have a larger dispersal range than wingless individuals. We examined here if brain structures linked to primary sensory processing and high-order motor control change in size as a function of wing polyphenism. Using micro-computing tomography (micro-CT) scans and immunocytochemical staining with anti-synapsin antibody, we reconstructed primary visual (optic lobes) and olfactory (antennal lobes) neuropils, together with the central body of winged and wingless parthenogenetic females of A. pisum for volume measurements. Absolute neuropil volumes were generally bigger in anti-synapsin labelled brains compared to micro-CT scans. This is potentially due to differences in rearing conditions of the used aphids. Independent of the method used, however, winged females consistently had larger antennal lobes and optic lobes than wingless females in spite of a larger overall body size of wingless compared to winged females. The volume of the central body, on the other hand was not significantly different between the two morphs. The larger primary sensory centres in winged aphids might thus provide the neuronal substrate for processing different environmental information due to the increased mobility during flight.

Adaptations during Maturation in an Identified Honeybee Interneuron Responsive to Waggle Dance Vibration Signals

Honeybees are social insects, and individual bees take on different social roles as they mature, performing a multitude of tasks that involve multi-modal sensory integration. Several activities vital for foraging, like flight and waggle dance communication, involve sensing air vibrations through their antennae. We investigated changes in the identified vibration-sensitive interneuron DL-Int-1 in the honeybee Apis mellifera during maturation by comparing properties of neurons from newly emerged adult and forager honeybees. Although comparison of morphological reconstructions of the neurons revealed no significant changes in gross dendritic features, consistent and region-dependent changes were found in dendritic density. Comparison of electrophysiological properties showed an increase in the firing rate differences between stimulus and nonstimulus periods in foragers compared with newly emerged adult bees. The observed differences in neurons of foragers compared with newly emerged adult honeybees suggest refined connectivity, improved signal propagation, and enhancement of response features possibly important for the network processing of air vibration signals relevant for the waggle dance communication of honeybees.

Insect brain plasticity: effects of olfactory input on neuropil size

Insect brains are known to express a high degree of experience-dependent structural plasticity. One brain structure in particular, the mushroom body (MB), has been attended to in numerous studies as it is implicated in complex cognitive processes such as olfactory learning and memory. It is, however, poorly understood to what extent sensory input per se affects the plasticity of the mushroom bodies. By performing unilateral blocking of olfactory input on immobilized butterflies, we were able to measure the effect of passive sensory input on the volumes of antennal lobes (ALs) and MB calyces. We showed that the primary and secondary olfactory neuropils respond in different ways to olfactory input. ALs show absolute experience-dependency and increase in volume only if receiving direct olfactory input from ipsilateral antennae, while MB calyx volumes were unaffected by the treatment and instead show absolute age-dependency in this regard. We therefore propose that cognitive processes related to behavioural expressions are needed in order for the calyx to show experience-dependent volumetric expansions. Our results indicate that such experience-dependent volumetric expansions of calyces observed in other studies may have been caused by cognitive processes rather than by sensory input, bringing some causative clarity to a complex neural phenomenon.

An Early Sensitive Period Induces Long-Lasting Plasticity in the Honeybee Nervous System

The effect of early experiences on the brain during a sensitive period exerts a long-lasting influence on the mature individual. Despite behavioral and neural plasticity caused by early experiences having been reported in the honeybee Apis mellifera, the presence of a sensitive period in which associative experiences lead to pronounced modifications in the adult nervous system is still unclear. Laboratory-reared bees were fed with scented food within specific temporal windows and were assessed for memory retention, in the regulation of gene expression related to the synaptic formation and in the olfactory perception of their antennae at 17 days of age. Bees were able to retain a food-odor association acquired 5–8 days after emergence, but not before, and showed better retention than those exposed to an odor at 9–12 days. In the brain, the odor-rewarded experiences that occurred at 5–8 days of age boosted the expression levels of the cell adhesion proteins neurexin 1 (Nrx1) and neuroligin 2 (Nlg2) involved in synaptic strength. At the antennae, the experiences increased the electrical response to a novel odor but not to the one experienced. Therefore, a sensitive period that induces long-lasting behavioral, functional and structural changes is found in adult honeybees.

Proteomics Reveals the Molecular Underpinnings of Stronger Learning and Memory in Eastern Compared to Western Bees

The eastern (Apis cerana cerana, Acc) and western (Apis mellifera ligustica, Aml) honeybee are two major honeybee species. Surprisingly, little is known about the fundamental molecular neurobiology of brain suborgans of Acc and Aml. We characterized and compared the proteomes of mushroom bodies (MBs), antennal lobes (ALs) and optical lobes (OLs) in the brain of both species, and biologically validated the functions related to learning and memory. Acc and Aml have evolved similar proteome signatures in MBs and OLs to drive the domain-specific neural activities. In MBs of both species, commonly enriched and enhanced functional groups related to protein metabolism and Ca²⁺ transport relative to ALs and OLs, suggests that proteins and Ca²⁺ are vital for consolidating learning and memory via modulation of synaptic structure and signal transduction. Furthermore, in OLs of both species, the mainly enriched ribonucleoside metabolism suggests its vital role as second messenger in promoting phototransduction. Notably, in ALs of both species, distinct proteome settings have shaped to prime olfactory learning and memory. In ALs of Acc, this is supported by the enriched cytoskeleton organization to sustain olfactory signaling through modulation of plasticity in glomeruli and intracellular transport. In ALs of Aml, however, the enriched functional groups implicated in hydrogen ion transport are indicative of their importance in supporting olfactory processes by regulation of synaptic transmission. The biological confirmation of enhanced activities of protein metabolism and signal transduction in ALs and MBs of Acc relative to in Aml demonstrates that a stronger sense of olfactory learning and memory has evolved in Acc. The reported first in-depth proteome data of honeybee brain suborgans provide a novel insight into the molecular basis of neurobiology, and is potentially useful for further neurological studies in honeybees and other insects.

Morphofunctional experience-dependent plasticity in the honeybee brain

Repeated or prolonged exposure to an odorant without any positive or negative reinforcement produces experience-dependent plasticity, which results in habituation and latent inhibition. In the honeybee (Apis mellifera), it has been demonstrated that, even if the absolute neural representation of an odor in the primary olfactory center, the antennal lobe (AL), is not changed by repeated presentations, its relative representation with respect to unfamiliar stimuli is modified. In particular, the representation of a stimulus composed of a 50:50 mixture of a familiar and a novel odorant becomes more similar to that of the novel stimulus after repeated stimulus preexposure. In a calcium-imaging study, we found that the same functional effect develops following prolonged odor exposure. By analyzing the brains of the animals subjected to this procedure, we found that such functional changes are accompanied by morphological changes in the AL (i.e., a decrease in volume in specific glomeruli). The AL glomeruli that exhibited structural plasticity also modified their functional responses to the three stimuli (familiar odor, novel odor, binary mixture). We suggest a model in which rebalancing inhibition within the AL glomeruli may be sufficient to elicit structural and functional correlates of experience-dependent plasticity.

Morphogenesis of honeybee hypopharyngeal gland during pupal development

Background

The hypopharyngeal gland of worker bees contributes to the production of the royal jelly fed to queens and larvae. The gland consists of thousands of two-cell units that are composed of a secretory cell and a duct cell and that are arranged in sets of about 12 around a long collecting duct.

Results

By fluorescent staining, we have examined the morphogenesis of the hypopharyngeal gland during pupal life, from a saccule lined by a pseudostratified epithelium to the elaborate organ of adult worker bees. The hypopharyngeal gland develops as follows. (1) Cell proliferation occurs during the first day of pupal life in the hypopharyngeal gland primordium. (2) Subsequently, the epithelium becomes organized into rosette-like units of three cells. Two of these will become the secretory cell and the duct cell of the adult secretory units; the third cell contributes only temporarily to the development of the secretory units and is eliminated by apoptosis in the second half of pupal life. (3) The three-cell units of flask-shaped cells undergo complex changes in cell morphology. Thus, by mid-pupal stage, the gland is structurally similar to the adult hypopharyngeal gland. (4) Concomitantly, the prospective secretory cell attains its characteristic subcellular organization by the invagination of a small patch of apical membrane domain, its extension to a tube of about 100 μm in length (termed a canaliculus), and the expansion of the tube to a diameter of about 3 μm. (6) Finally, the canaliculus-associated F-actin system becomes reorganized into rings of bundled actin filaments that are positioned at regular distances along the membrane tube.

Conclusions

The morphogenesis of the secretory units in the hypopharyngeal gland of the worker bee seems to be based on a developmental program that is conserved, with slight modification, among insects for the production of dermal glands. Elaboration of the secretory cell as a unicellular seamless epithelial tube occurs by invagination of the apical membrane, its extension likely by targeted exocytosis and its expansion, and finally the reorganisation of the membrane-associated F-actin system. Our work is fundamental for future studies of environmental effects on hypopharyngeal gland morphology and development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-017-0207-z) contains supplementary material, which is available to authorized users.

Odor Experiences during Preimaginal Stages Cause Behavioral and Neural Plasticity in Adult Honeybees

In eusocial insects, experiences acquired during the development have long-term consequences on mature behavior. In the honeybee that suffers profound changes associated with metamorphosis, the effect of odor experiences at larval instars on the subsequent physiological and behavioral response is still unclear. To address the impact of preimaginal experiences on the adult honeybee, colonies containing larvae were fed scented food. The effect of the preimaginal experiences with the food odor was assessed in learning performance, memory retention and generalization in 3–5- and 17–19 day-old bees, in the regulation of their expression of synaptic-related genes and in the perception and morphology of their antennae. Three-five day old bees that experienced 1-hexanol (1-HEX) as food scent responded more to the presentation of the odor during the 1-HEX conditioning than control bees (i.e., bees reared in colonies fed unscented food). Higher levels of proboscis extension response (PER) to 1-HEX in this group also extended to HEXA, the most perceptually similar odor to the experienced one that we tested. These results were not observed for the group tested at older ages. In the brain of young adults, larval experiences triggered similar levels of neurexins (NRXs) and neuroligins (Nlgs) expression, two proteins that have been involved in synaptic formation after associative learning. At the sensory periphery, the experience did not alter the number of the olfactory sensilla placoidea, but did reduce the electrical response of the antennae to the experienced and novel odor. Our study provides a new insight into the effects of preimaginal experiences in the honeybee and the mechanisms underlying olfactory plasticity at larval stage of holometabolous insects.

Neuroethology of Olfactory-Guided Behavior and Its Potential Application in the Control of Harmful Insects

Harmful insects include pests of crops and storage goods, and vectors of human and animal diseases. Throughout their history, humans have been fighting them using diverse methods. The fairly recent development of synthetic chemical insecticides promised efficient crop and health protection at a relatively low cost. However, the negative effects of those insecticides on human health and the environment, as well as the development of insect resistance, have been fueling the search for alternative control tools. New and promising alternative methods to fight harmful insects include the manipulation of their behavior using synthetic versions of "semiochemicals", which are natural volatile and non-volatile substances involved in the intra- and/or inter-specific communication between organisms. Synthetic semiochemicals can be used as trap baits to monitor the presence of insects, so that insecticide spraying can be planned rationally (i.e., only when and where insects are actually present). Other methods that use semiochemicals include insect annihilation by mass trapping, attract-and- kill techniques, behavioral disruption, and the use of repellents. In the last decades many investigations focused on the neural bases of insect's responses to semiochemicals. Those studies help understand how the olfactory system detects and processes information about odors, which could lead to the design of efficient control tools, including odor baits, repellents or ways to confound insects. Here we review our current knowledge about the neural mechanisms controlling olfactory responses to semiochemicals in harmful insects. We also discuss how this neuroethology approach can be used to design or improve pest/vector management strategies.

Plasticity in Insect Olfaction: To Smell or Not to Smell?

In insects, olfaction plays a crucial role in many behavioral contexts, such as locating food, sexual partners, and oviposition sites. To successfully perform such behaviors, insects must respond to chemical stimuli at the right moment. Insects modulate their olfactory system according to their physiological state upon interaction with their environment. Here, we review the plasticity of behavioral responses to different odor types according to age, feeding state, circadian rhythm, and mating status. We also summarize what is known about the underlying neural and endocrinological mechanisms, from peripheral detection to central nervous integration, and cover neuromodulation from the molecular to the behavioral level. We describe forms of olfactory plasticity that have contributed to the evolutionary success of insects and have provided them with remarkable tools to adapt to their ever-changing environment.

The Bee as a Model to Investigate Brain and Behavioural Asymmetries

The honeybee Apis mellifera, with a brain of only 960,000 neurons and the ability to perform sophisticated cognitive tasks, has become an excellent model in life sciences and in particular in cognitive neurosciences. It has been used in our laboratories to investigate brain and behavioural asymmetries, i.e., the different functional specializations of the right and the left sides of the brain. It is well known that bees can learn to associate an odour stimulus with a sugar reward, as demonstrated by extension of the proboscis when presented with the trained odour in the so-called Proboscis Extension Reflex (PER) paradigm. Bees recall this association better when trained using their right antenna than they do when using their left antenna. They also retrieve short-term memory of this task better when using the right antenna. On the other hand, when tested for long-term memory recall, bees respond better when using their left antenna. Here we review a series of behavioural studies investigating bees’ lateralization, integrated with electrophysiological measurements to study asymmetries of olfactory sensitivity, and discuss the possible evolutionary origins of these asymmetries. We also present morphological data obtained by scanning electron microscopy and two-photon microscopy. Finally, a behavioural study conducted in a social context is summarised, showing that honeybees control context-appropriate social interactions using their right antenna, rather than the left, thus suggesting that lateral biases in behaviour might be associated with requirements of social life.

Plasticity of the worker bumblebee brain in relation to age and rearing environment

The environment experienced during development can dramatically affect the brain, with possible implications for sensory processing, learning, and memory. Although the effects of single sensory modalities on brain development have been repeatedly explored, the additive or interactive effects of multiple modalities have been less thoroughly investigated. We asked how experience with multisensory stimuli affected brain development in the bumblebee Bombus impatiens. First, to establish the timeline of brain development during early adulthood, we estimated regional brain volumes across a range of ages. We discovered significant age-related volume changes in nearly every region of the brain. Next, to determine whether these changes were dependent upon certain environmental stimuli, we manipulated the visual and olfactory stimuli available to newly emerged bumblebee workers in a factorial manner. Newly emerged bumblebees were maintained in the presence or absence of supplemental visual and/or olfactory stimuli for 7 days, after which the volumes of several brain regions were estimated. We found that the volumes of the mushroom body lobes and calyces were larger in the absence of visual stimuli. Additionally, visual deprivation was associated with the expression of larger antennal lobes, the primary olfactory processing regions of the brain. In contrast, exposure to plant-derived olfactory stimuli did not have a significant effect on brain region volumes. This study is the first to explore the separate and interactive effects of visual and olfactory stimuli on bee brain development. Assessing the timing and sensitivity of brain development is a first step toward understanding how different rearing environments differentially affect regional brain volumes in this species. Our findings suggest that environmental factors experienced during the first week of adulthood can modify bumblebee brain development in many subtle ways.

Behavioral and neural plasticity caused by early social experiences: the case of the honeybee

Cognitive experiences during the early stages of life play an important role in shaping future behavior. Behavioral and neural long-term changes after early sensory and associative experiences have been recently reported in the honeybee. This invertebrate is an excellent model for assessing the role of precocious experiences on later behavior due to its extraordinarily tuned division of labor based on age polyethism. These studies are mainly focused on the role and importance of experiences occurred during the first days of the adult lifespan, their impact on foraging decisions, and their contribution to coordinate food gathering. Odor-rewarded experiences during the first days of honeybee adulthood alter the responsiveness to sucrose, making young hive bees more sensitive to assess gustatory features about the nectar brought back to the hive and affecting the dynamic of the food transfers and the propagation of food-related information within the colony. Early olfactory experiences lead to stable and long-term associative memories that can be successfully recalled after many days, even at foraging ages. Also they improve memorizing of new associative learning events later in life. The establishment of early memories promotes stable reorganization of the olfactory circuits inducing structural and functional changes in the antennal lobe (AL). Early rewarded experiences have relevant consequences at the social level too, biasing dance and trophallaxis partner choice and affecting recruitment. Here, we revised recent results in bees' physiology, behavior, and sociobiology to depict how the early experiences affect their cognition abilities and neural-related circuits.

Nonassociative plasticity alters competitive interactions among mixture components in early olfactory processing

Experience-related plasticity is an essential component of networks involved in early olfactory processing. However, the mechanisms and functions of plasticity in these neural networks are not well understood. We studied nonassociative plasticity by evaluating responses to two pure odors (A and X) and their binary mixture using calcium imaging of odor-elicited activity in output neurons of the honey bee antennal lobe. Unreinforced exposure to A or X produced no change in the neural response elicited by the pure odors. However, exposure to one odor (e.g. A) caused the response to the mixture to become more similar to that of the other component (X). We also show in behavioral analyses that unreinforced exposure to A caused the mixture to become perceptually more similar to X. These results suggest that nonassociative plasticity modifies neural networks in such a way that it affects local competitive interactions among mixture components. We used a computational model to evaluate the most likely targets for modification. Hebbian modification of synapses from inhibitory local interneurons to projection neurons most reliably produced the observed shift in response to the mixture. These results are consistent with a model in which the antennal lobe acts to filter olfactory information according to its relevance for performing a particular task.

The involvement of the antennae in mediating the brood influence on circadian rhythms in "nurse" honey bee (Apis mellifera) workers

Age-related division of labor in honey bees is associated with plasticity in circadian rhythms. Forager bees that are typically older than 3 weeks of age show strong behavioral and molecular circadian rhythms with higher activity during the day. Younger bees that typically care for ("nurse") the brood are active around the clock with similar brain clock gene levels throughout the day. However, nurses that are caged on brood-less combs inside or outside the hive show robust circadian rhythms with higher activity during the day, suggesting that direct contact with the brood mediates the plasticity in the circadian system. The nature of the brood signals affecting the workers' circadian system and the modalities by which they are detected are unknown. Given that the antennae are pivotal sensory organs in bees, we hypothesized that they are involved in mediating the brood influence on the plasticity in circadian rhythms. The flagella of the antennae are densely covered with diverse sensory structures able to detect a wide range of signals. To test our hypothesis, we removed the flagella of nurses and observed their behavior in isolation and in free-foraging colonies. We found that individually-isolated flagella-less bees under constant laboratory conditions show robust circadian rhythms in locomotor activity. In observation hives, flagella-less bees cared for the brood, but were more active during the day. By contrast, sham-treated bees were active around the clock as typical of nurses. Detailed video recordings showed that the brood-tending behavior of flagella-less and sham-treated bees is similar. These observations suggest that the difference in the patterns of brood care activity is not because the flagella-less bees did not contact the brood. Our results suggest that nurses are able to find the brood in the dark environment of the hive without their flagella, perhaps by using other sensory organs. The higher activity of flagella-less bees during the day further suggests that the flagella are involved in mediating the brood signals modulating plasticity in the circadian system.

Heterogeneity in dendritic morphology of moth antennal lobe projection neurons

A population of projection neurons (PNs) in the antennal lobe (AL) integrates sensory information from the antenna and is essential for processing odor information in the insect brain. We examined the anatomy of this neuronal population in the brain of the silkmoth Bombyx mori. Using intracellular dye injection, we labeled a total of 246 PNs and systematically analyzed their morphological features, including the soma position, antennocerebral tract, and number of innervating glomeruli. For example, we analyzed PNs that had somata in the different cell clusters, innervated overlapping but different groups of glomeruli, and ran through different pathways. We also identified glomeruli innervated by PNs using a previously established procedure that first classifies glomeruli into regional groups and then identifies individual glomeruli. We analyzed uniglomerular PNs (75.6% of the total) and found heterogeneity in the dendritic morphology of the PNs that was dependent on the regions and/or the innervating glomeruli. For example, most PNs innervating the macroglomerular complex did not have extraglomerular processes, whereas most PNs innervating ordinary glomeruli did. Moreover, PNs innervating the toroid glomerulus showed heterogeneity in their dendritic morphology. These PNs had dendritic arborization in different areas within the glomerulus. We found that, in some cases, the innervation pattern of the PN dendrite correlated with individual variation in the glomerular organization. These results indicate that PNs are not homogeneous populations, and in some cases morphological heterogeneity in PNs correlated with change in glomerular organization in the silkmoth AL.

Transcriptional response to foraging experience in the honey bee mushroom bodies

Enriched environmental conditions induce neuroanatomical plasticity in a variety of vertebrate and invertebrate species. We explored the molecular processes associated with experience-induced plasticity, using naturally occurring foraging behavior in adult worker honey bees (Apis mellifera). In honey bees, the mushroom bodies exhibit neuroanatomical plasticity that is dependent on accumulated foraging experience. To investigate molecular processes associated with foraging experience, we performed a time-course microarray study to examine gene expression changes in the mushroom bodies as a function of days foraged. We found almost 500 genes that were regulated by duration of foraging experience. Bioinformatic analyses of these genes suggest that foraging experience is associated with multiple molecular processes in the mushroom bodies, including some that may contribute directly to neuropil growth, and others that could potentially protect the brain from the effects of aging and physiological stress.

Division of labor in the hyperdiverse ant genus Pheidole is associated with distinct subcaste- and age-related patterns of worker brain organization

The evolutionary success of ants and other social insects is considered to be intrinsically linked to division of labor among workers. The role of the brains of individual ants in generating division of labor, however, is poorly understood, as is the degree to which interspecific variation in worker social phenotypes is underscored by functional neurobiological differentiation. Here we demonstrate that dimorphic minor and major workers of different ages from three ecotypical species of the hyperdiverse ant genus Pheidole have distinct patterns of neuropil size variation. Brain subregions involved in sensory input (optic and antennal lobes), sensory integration, learning and memory (mushroom bodies), and motor functions (central body and subesophageal ganglion) vary significantly in relative size, reflecting differential investment in neuropils that likely regulate subcaste- and age-correlated task performance. Worker groups differ in brain size and display patterns of altered isometric and allometric subregion scaling that affect brain architecture independently of brain size variation. In particular, mushroom body size was positively correlated with task plasticity in the context of both age- and subcaste-related polyethism, providing strong, novel support that greater investment in this neuropil increases behavioral flexibility. Our findings reveal striking levels of developmental plasticity and evolutionary flexibility in Pheidole worker neuroanatomy, supporting the hypothesis that mosaic alterations of brain composition contribute to adaptive colony structure and interspecific variation in social organization.

Early olfactory experience induces structural changes in the primary olfactory center of an insect brain

The antennal lobe (AL) is the first olfactory center of the insect brain and is constituted of different functional units, the glomeruli. In the AL, odors are coded as spatiotemporal patterns of glomerular activity. In honeybees, olfactory learning during early adulthood modifies neural activity in the AL on a long-term scale and also enhances later memory retention. By means of behavioral experiments, we first verified that olfactory learning between the fifth and eighth day of adulthood induces better retention performances at a late adult stage than the same experience acquired before or after this period. We checked that the specificity of memory for the odorants used was improved. We then studied whether such early olfactory learning also induces long-term structural changes in the AL consistent with the formation of long-term olfactory memories. We also measured the volume of 15 identified glomeruli in the ALs of 17-day-old honeybees that either experienced an odor associated with sucrose solution between the fifth and eighth day of adulthood or were left untreated. We found that early olfactory experience induces glomerulus-selective increases in volume that were specific to the learned odor. By comparing our volumetric measures with calcium-imaging recordings from a previous study, performed in 17-day-old bees subjected to the same treatment and experimental conditions, we found that glomeruli that showed structural changes after early learning were those that exhibited a significant increase in neural activity. Our results make evident a correlation between structural and functional changes in the AL following early olfactory learning.

A multimodal approach for tracing lateralisation along the olfactory pathway in the honeybee through electrophysiological recordings, morpho-functional imaging, and behavioural studies

Recent studies have revealed asymmetries between the left and right sides of the brain in invertebrate species. Here we present a review of a series of recent studies from our laboratories, aimed at tracing asymmetries at different stages along the honeybee's (Apis mellifera) olfactory pathway. These include estimates of the number of sensilla present on the two antennae, obtained by scanning electron microscopy, as well as electroantennography recordings of the left and right antennal responses to odorants. We describe investigative studies of the antennal lobes, where multi-photon microscopy was used to search for possible morphological asymmetries between the two brain sides. Moreover, we report on recently published results obtained by two-photon calcium imaging for functional mapping of the antennal lobe aimed at comparing patterns of activity evoked by different odours. Finally, possible links to the results of behavioural tests, measuring asymmetries in single-sided olfactory memory recall, are discussed.

Searching for anatomical correlates of olfactory lateralization in the honeybee antennal lobes: a morphological and behavioural study

The honeybee, Apis mellifera L. (Hymenoptera: Apidae), has recently become a model for studying brain asymmetry among invertebrates. A strong lateralization favouring the right antenna was discovered in odour learning and short-term memory recall experiments, and a lateral shift favouring the left antenna for long-term memory recall. Corresponding morphological asymmetries have been found in the distribution of olfactory sensilla between the antennae and confirmed by electrophysiological odour response measurements in isolated right and left antennae. The aim of this study was to investigate whether a morphological asymmetry can be observed in the volume of the primary olfactory centres of the central nervous system, the antennal lobes (ALs). Precise volume measurements of a subset of their functional units, the glomeruli, were performed in both sides of the brain, exploiting the advantages of two-photon microscopy. This novel method allowed minimal invasive acquisition of volume images of the ALs, avoiding artefacts from brain extraction and dehydration. The study was completed by a series of behavioural experiments in which response asymmetry in odour recall following proboscis extension reflex conditioning was assessed for odours, chosen to stimulate strong activity in the same glomeruli as in the morphological study. The volumetric measurements found no evidence of lateralization in the investigated glomeruli within the experimental limits. Instead, in the behavioural experiments, a striking odour dependence of the lateralization was observed. The results are discussed on the basis of recent neurophysiological and ethological experiments in A. mellifera.

Aminergic control and modulation of honeybee behaviour

Biogenic amines are important messenger substances in the central nervous system and in peripheral organs of vertebrates and of invertebrates. The honeybee, Apis mellifera, is excellently suited to uncover the functions of biogenic amines in behaviour, because it has an extensive behavioural repertoire, with a number of biogenic amine receptors characterised in this insect.In the honeybee, the biogenic amines dopamine, octopamine, serotonin and tyramine modulate neuronal functions in various ways. Dopamine and serotonin are present in high concentrations in the bee brain, whereas octopamine and tyramine are less abundant. Octopamine is a key molecule for the control of honeybee behaviour. It generally has an arousing effect and leads to higher sensitivity for sensory inputs, better learning performance and increased foraging behaviour. Tyramine has been suggested to act antagonistically to octopamine, but only few experimental data are available for this amine. Dopamine and serotonin often have antagonistic or inhibitory effects as compared to octopamine.Biogenic amines bind to membrane receptors that primarily belong to the large gene-family of GTP-binding (G) protein coupled receptors. Receptor activation leads to transient changes in concentrations of intracellular second messengers such as cAMP, IP(3) and/or Ca(2+). Although several biogenic amine receptors from the honeybee have been cloned and characterised more recently, many genes still remain to be identified. The availability of the completely sequenced genome of Apis mellifera will contribute substantially to closing this gap.In this review, we will discuss the present knowledge on how biogenic amines and their receptor-mediated cellular responses modulate different behaviours of honeybees including learning processes and division of labour.

Sleeping together using social interactions to understand the role of sleep in plasticity

Social experience alters the expression of genes related to synaptic function and plasticity, induces elaborations in the morphology of neural structures throughout the brain (Volkmar and Greenough, 1972; Greenough et al., 1978; Technau, 2007), improves cognitive and behavioral performance (Pham et al., 1999a; Toscano et al., 2006) and alters subsequent sleep (Ganguly-Fitzgerald et al., 2006). In this review, we discuss the plastic mechanisms that are induced in response to social experience and how social enrichment can provide insight into the biological functions of sleep.

Brain plasticity in Diptera and Hymenoptera

To mediate different types of behaviour, nervous systems need to coordinate the proper operation of their neural circuits as well as short- and long-term alterations that occur within those circuits. The latter ultimately devolve upon specific changes in neuronal structures, membrane properties and synaptic connections that are all examples of plasticity. This reorganization of the adult nervous system is shaped by internal and external influences both during development and adult maturation. In adults, behavioural experience is a major driving force of neuronal plasticity studied particularly in sensory systems. The range of adaptation depends on features that are important to a particular species, and is therefore specific, so that learning is essential for foraging in honeybees, while regenerative capacities are important in hemimetabolous insects with long appendages. Experience is usually effective during a critical period in early adult life, when neural function becomes tuned to future conditions in an insect's life. Tuning occur at all levels, in synaptic circuits, neuropile volumes, and behaviour. There are many examples, and this review incorporates only a select few, mainly those from Diptera and Hymenoptera.

Parallel olfactory systems in insects: anatomy and function

A striking commonality across insects and vertebrates is the recurring presence of parallel olfactory subsystems, suggesting that such an organization has a highly adaptive value. Conceptually, two different categories of parallel systems must be distinguished. In one, specific sensory organs or processing streams analyze different chemical stimuli (segregate parallel systems). In the other, similar odor stimuli are processed but analyzed with respect to different features (dual parallel systems). Insects offer many examples for both categories. For example, segregate parallel systems for different chemical stimuli are realized in specialized neuronal streams for processing sex pheromones and CO(2). Dual parallel streams related to similar or overlapping odor stimuli are prominent in Hymenoptera. Here, a clear separation of sensory tracts to higher-order brain centers is present despite no apparent differences regarding the classes or categories of olfactory stimuli being processed. In this paper, we review the situation across insect species and offer hypotheses for the function and evolution of parallel olfactory systems.

Associative learning during early adulthood enhances later memory retention in honeybees

Background

Cognitive experiences during the early stages of life play an important role in shaping the future behavior in mammals but also in insects, in which precocious learning can directly modify behaviors later in life depending on both the timing and the rearing environment. However, whether olfactory associative learning acquired early in the adult stage of insects affect memorizing of new learning events has not been studied yet.

Methodology

Groups of adult honeybee workers that experienced an odor paired with a sucrose solution 5 to 8 days or 9 to 12 days after emergence were previously exposed to (i) a rewarded experience through the offering of scented food, or (ii) a non-rewarded experience with a pure volatile compound in the rearing environment.

Principal Findings

Early rewarded experiences (either at 1–4 or 5–8 days of adult age) enhanced retention performance in 9–12-day-conditioned bees when they were tested at 17 days of age. The highest retention levels at this age, which could not be improved with prior rewarded experiences, were found for memories established at 5–8 days of adult age. Associative memories acquired at 9–12 days of age showed a weak effect on retention for some pure pre-exposed volatile compounds; whereas the sole exposure of an odor at any younger age did not promote long-term effects on learning performance.

Conclusions

The associative learning events that occurred a few days after adult emergence improved memorizing in middle-aged bees. In addition, both the timing and the nature of early sensory inputs interact to enhance retention of new learning events acquired later in life, an important matter in the social life of honeybees.

Early olfactory experience modifies neural activity in the antennal lobe of a social insect at the adult stage

In the antennal lobe (AL), the first olfactory centre of the insect brain, odorants are represented as spatiotemporal patterns of glomerular activity. Whether and how such patterns are modified in the long term after precocious olfactory experiences (i.e. in the first days of adulthood) remains unknown. To address this question, we used in vivo optical imaging of calcium activity in the antennal lobe of 17-day-old honeybees which either experienced an odorant associated with sucrose solution 5-8 days after emergence or were left untreated. In both cases, we imaged neural responses to the learned odor and to three novel odors varying in functional group and carbon-chain length. Two different odor concentrations were used. We also measured behavioral responses of 17-day-old honeybees, treated and untreated, to these stimuli. We show that precocious olfactory experience increased general odor-induced activity and the number of activated glomeruli in the adult AL, but also affected qualitative odor representations, which appeared shifted in the neural space of treated animals relative to control animals. Such effects were not limited to the experienced odor, but were generalized to other perceptually similar odors. A similar trend was found in behavioral experiments, in which increased responses to the learned odor extended to perceptually similar odors in treated bees. Our results show that early olfactory experiences have long-lasting effects, reflected in behavioral responses to odorants and concomitant neural activity in the adult olfactory system.

Long-term memory shapes the primary olfactory center of an insect brain

The storage of stable memories is generally considered to rely on changes in the functional properties and/or the synaptic connectivity of neural networks. However, these changes are not easily tractable given the complexity of the learning procedures and brain circuits studied. Such a search can be narrowed down by studying memories of specific stimuli in a given sensory modality and by working on networks with a modular and relatively simple organization. We have therefore focused on associative memories of individual odors and the possible related changes in the honeybee primary olfactory center, the antennal lobe (AL). As this brain structure is organized in well-identified morpho-functional units, the glomeruli, we looked for evidence of structural and functional plasticity in these units in relation with the bees' ability to store long-term memories (LTMs) of specific odors. Restrained bees were trained to form an odor-specific LTM in an appetitive Pavlovian conditioning protocol. The stability and specificity of this memory was tested behaviorally 3 d after conditioning. At that time, we performed both a structural and a functional analysis on a subset of 17 identified glomeruli by measuring glomerular volume under confocal microscopy, and odor-evoked activity, using in vivo calcium imaging. We show that long-term olfactory memory for a given odor is associated with volume increases in a subset of glomeruli. Independent of these structural changes, odor-evoked activity was not modified. Lastly, we show that structural glomerular plasticity can be predicted based on a putative model of interglomerular connections.

Age and rearing environment interact in the retention of early olfactory memories in honeybees

Due to the changing behavioral contexts at which social insects are exposed during the adult lifespan, they are ideal models to analyze the effect of particular sensory stimuli during young adulthood on later behavior. Specifically, our goal is to understand early influences on later foraging behavior. For that, olfactory memories were established by worker honeybees to different pre-foraging ages using either (1) classical conditioning in the proboscis extension response (PER) paradigm or (2) the offering of scented-sugar solution under different rearing conditions. By testing long-term memories (LTM) through a single PER test in workers of foraging ages (17-25 days), we found that retention of the early olfactory memories in honey bees is age-dependent and not time-dependent. Independently of the environmental conditions in which they were reared (laboratory cages or hives), bees were able to retain food-odor association from 5 days after emergence, but rarely before. In most experiments we observed a bi-modal pattern of response: bees exposed to scented-food at 5-8 and 13-16 days showed better retention than those exposed at 9-12 days. These differences disappeared for bees reared in hives. Retrieval of LTMs depending on the timing and the continuous inputs of appropriate sensory stimuli are discussed.

Appetitive odor learning does not change olfactory coding in a subpopulation of honeybee antennal lobe neurons

Odors elicit spatio-temporal patterns of activity in the olfactory bulb of vertebrates and the antennal lobe of insects. There have been several reports of changes in these patterns following olfactory learning. These studies pose a conundrum: how can an animal learn to efficiently respond to a particular odor with an adequate response, if its primary representation already changes during this process? In this study, we offer a possible solution for this problem. We measured odor-evoked calcium responses in a subpopulation of uniglomerular AL output neurons in honeybees. We show that their responses to odors are remarkably resistant to plasticity following a variety of appetitive olfactory learning paradigms. There was no significant difference in the changes of odor-evoked activity between single and multiple trial forward or backward conditioning, differential conditioning, or unrewarded successive odor stimulation. In a behavioral learning experiment we show that these neurons are necessary for conditioned odor responses. We conclude that these uniglomerular projection neurons are necessary for reliable odor coding and are not modified by learning in this paradigm. The role that other projection neurons play in olfactory learning remains to be investigated.

Multiple olfactory receptor neurons and their axonal projections in the antennal lobe of the honeybee Apis mellifera

The poreplate sensilla of honeybees are equipped with multiple olfactory receptor neurons (ORNs), which innervate glomeruli of the antennal lobe (AL). We investigated the axonal projection pattern in glomeruli of the AL (glomerular pattern), formed by the multiple ORNs of individual poreplate sensilla. We used the different glomerular patterns to draw conclusions about the equipment of poreplate sensilla with different ORN types. ORNs of single poreplate sensilla were stained and analyzed by laser-scanning confocal microscopy and 3D software (AMIRA). In 13 specimens we found between 7 and 23 ORNs. This is in accordance with data found in the literature (5-35 ORNs) suggesting that all ORNs of the single poreplate sensilla were stained. The ORNs innervate the AL via all four sensory tracts (T1-T4), and glomeruli of the anterior part of the AL are more often innervated. Each ORN innervates a single glomerulus (uniglomerular), and all ORNs of one poreplate sensillum project to different glomeruli. Visual inspection and individual identification of glomeruli, based on the honeybee digital AL atlas, were used to evaluate mapping of glomeruli by a rigid transformation of the experimental ALs onto a reference AL. ORNs belonging to individual poreplate sensilla form variable glomerular patterns, and we did not find a common organization of glomerular patterns. We conclude that poreplate sensilla are equipped with different ORN types but that the same ORN types can be found in different poreplate sensilla. The equipment of poreplate sensilla with ORNs is overlapping. The mapping of glomeruli by rigid transformation is revealed to be a powerful tool for comparative neuroanatomy.

Honey bees as a model for understanding mechanisms of life history transitions

As honey bee workers switch from in-hive tasks to foraging, they undergo transition from constant exposure to the controlled homogenous physical and sensory environment of the hive to prolonged diurnal exposures to a far more heterogeneous environment outside the hive. The switch from hive work to foraging offers an opportunity for the integrative study of the physiological and genetic mechanisms that produce the behavioral plasticity required for major life history transitions. Although such transitions have been studied in a number of animals, currently there is no model system where the evolution, development, physiology, molecular biology, neurobiology and behavior of such a transition can all be studied in the same organism in its natural habitat. With a large literature covering its evolution, behavior and physiology (plus the recent sequencing of the honey bee genome), the honey bee is uniquely suited to integrative studies of the mechanisms of behavior. In this review we discuss the physiological and genetic mechanisms of this behavioral transition, which include large scale changes in hormonal activity, metabolism, flight ability, circadian rhythms, sensory perception and processing, neural architecture, learning ability, memory and gene expression.

Maturation of odor representation in the honeybee antennal lobe

The antennal lobe (AL) is the first center for processing odors in the insect brain, as is the olfactory bulb (OB) in vertebrates. Both the AL and the OB have a characteristic glomerular structure; odors sensed by olfactory receptor neurons are represented by patterns of glomerular activity. Little is known about when and how an odor begins to be perceived in a developing brain. We address this question by using calcium imaging to monitor odor-evoked neural activity in the ALs of bees of different ages. We find that odor-evoked neural activity already occurs in the ALs of bees as young as 1 or 2 days. In young bees, the responses to odors are relatively weak and restricted to a small number of glomeruli. However, different odors already evoke responses in different combinations of glomeruli. In mature bees, the responses are stronger and are evident in more glomeruli, but continue to have distinct odor-dependent signatures. Our findings indicate that the specific glomerular patterns for odors are conserved during the development, and that odor representations are fully developed in the AL during the first 2 weeks following emergence.

A Macroglomerulus in the Antennal Lobe of Leaf-cutting Ant Workers and its Possible Functional Significance

Ants have a well-developed olfactory system, and pheromone communication is essential for regulating social life within their colonies. We compared the organization of primary olfactory centers (antennal lobes, ALs) in the brain of two closely related species of leaf-cutting ants (Atta vollenweideri, Atta sexdens). Both species express a striking size polymorphism associated with polyethism. We discovered that the ALs of large workers contain a substantially enlarged glomerulus (macroglomerulus, MG) at the entrance of the antennal nerve. This is the first description of an MG in non-sexual individuals of an insect. The location of the MG is laterally reversed in the two species, and workers of different size express a disproportional allometry of glomerular volumes. While ALs of large workers contain an MG, glomeruli in small workers are all similar in size. We further compared electroantennogram (EAG) responses to two common trail pheromone components of leaf-cutting ants: 4-methylpyrrol-2-carboxylate and 2-ethyl-3,6-dimethylpyrazine. At high concentrations the ratio of the EAG signals to 2-ethyl-3,6-dimethylpyrazine versus 4-methylpyrrol-2-carboxylate was significantly smaller in A. vollenweideri compared with the ratio of EAG signals to the same two components in A. sexdens. The differences in EAG signals and the species specific MG location in large workers provide correlative evidence that the MG may be involved in the detection of the trail pheromone.