Age- and behaviour-related changes in the expression of biogenic amine receptor genes in the antennae of honey bees (Apis mellifera)

Early life exposure to queen mandibular pheromone mediates persistent transcriptional changes in the brain of honey bee foragers

Behavioural regulation in insect societies remains a fundamental question in sociobiology. In hymenopteran societies, the queen plays a crucial role in regulating group behaviour by affecting individual behaviour and physiology through modulation of worker gene expression. Honey bee (Apis mellifera) queens signal their presence via queen mandibular pheromone (QMP). While QMP has been shown to influence behaviour and gene expression of young workers, we know little about how these changes translate in older workers. The effects of the queen pheromone could have prolonged molecular impacts on workers that depend on an early sensitive period. We demonstrate that removal of QMP impacts long-term gene expression in the brain and antennae in foragers that were treated early in life (1 day post emergence), but not when treated later in life. Genes important for division of labour, learning, chemosensory perception and ageing were among those differentially expressed in the antennae and brain tissues, suggesting that QMP influences diverse physiological and behavioural processes in workers. Surprisingly, removal of QMP did not have an impact on foraging behaviour. Overall, our study suggests a sensitive period early in the life of workers, where the presence or absence of a queen has potentially life-long effects on transcriptional activity.

MicroRNA ame-let-7 targets Amdop2 to increase sucrose sensitivity in honey bees (Apis mellifera)

BACKGROUND

As an important catecholamine neurotransmitter in invertebrates and vertebrates, dopamine plays multiple roles in the life of the honey bee. Dopamine receptors (DA), which specifically bind to dopamine to activate downstream cascades, have been reported to be involved in honey bee reproduction, division of labour, as well as learning and motor behaviour. However, how dopamine receptors regulate honey bee behavior remains uninvestigated.

RESULTS

The expression level of Amdop2 in the brain increased with the age of worker bees, which was just the opposite trend of ame-let-7. Inhibition of ame-let-7 through feeding an inhibitor upregulated Amdop2 expression; conversely, overexpression of ame-let-7 through a mimic downregulated Amdop2. Moreover, knockdown of Amdop2 in forager brain led to significantly higher sucrose responsiveness, which is similar to the phenotype of overexpression of ame-let-7. Finally, we confirmed that ame-let-7 directly targets Amdop2 in vitro by a luciferase reporter assay.

CONCLUSIONS

ame-let-7 is involved in the dopamine receptor signaling pathway to modulate the sucrose sensitivity in honey bees. Specifically, it down-regulates Amdop2, which then induces higher responses to sucrose. These results further unraveled the diverse mechanisms of the dopamine pathway in the regulation of insect behavior.

CRISPR/Cas9 mediated knockout of Amyellow-y gene results in melanization defect of the cuticle in adult Apis mellifera

Visible genetic markers are critical to gene function studies using genome editing technology in insects. However, there is no report about visible phenotypic markers in Apis mellifera, which extremely influences the application of genomic editing in honey bees. Here, we cloned and characterized the Amyellow-y gene in A. mellifera. Stage expression profiles showed that Amyellow-y gene was highly expressed in 2-, 4-day-old pupae, and newly emerged bees, and a high expression level was detected in the leg, thorax, wing and sting. To understand its functional role in pigmentation, Amyellow-y edited honeybees were created using CRISPR/Cas9, and it was found that the black pigment was decreased in the cuticle of mosaic workers and mutant drones. In particular, mutant drones manifested an overall appearance of yellowish cuticle in the body and appendages, including antennae, wings and legs, indicating that mutagenesis induced by disruption of Amyellow-y with CRISPR/Cas9 are heritable. Furthermore, the expression levels of genes associated with melanin pigmentation was investigated in mutant and wild-type drones using quantitative reverse transcription PCR. Transcription levels of Amyellow-y and aaNAT decreased markedly in mutant drones than that in wild-type ones, whereas laccase 2 was significantly up-regulated. Our results provide the first evidence, to our knowledge, that CRISPR/Cas9 edited G1 mutant drones of A. mellifera have a dramatic body pigmentation defect that can be visualized in adults, suggesting that Amyellow-y may serve as a promising visible phenotypic marker for genome editing in honey bees.

Use of waggle dance information in honey bees is linked to gene expression in the antennae, but not in the brain

Communication is essential for social animals, but deciding how to utilize information provided by conspecifics is a complex process that depends on environmental and intrinsic factors. Honey bees use a unique form of communication, the waggle dance, to inform nestmates about the location of food sources. However, as in many other animals, experienced individuals often ignore this social information and prefer to rely on prior experiences, i.e., private information. The neurosensory factors that drive the decision to use social information are not yet understood. Here we test whether the decision to use social dance information or private information is linked to gene expression differences in different parts of the nervous system. We trained bees to collect food from sugar water feeders and observed whether they utilize social or private information when exposed to dances for a new food source. We performed transcriptome analysis of four brain parts (11-16 bees per tissue type) critical for cognition: the subesophageal ganglion, the central brain, the mushroom bodies, and the antennal lobes but, unexpectedly, detected no differences between social or private information users. In contrast, we found 413 differentially expressed genes in the antennae, suggesting that variation in sensory perception mediates the decision to use social information. Social information users were characterized by the upregulation of biogenic amine genes, while private information users upregulated several genes coding for odour perception. These results highlight that decision-making in honey bees might also depend on peripheral processes of perception rather than higher-order brain centres of information integration.

Characterization of Halyomorpha halys TAR1 reveals its involvement in (E)-2-decenal pheromone perception

In insects, tyramine receptor 1 (TAR1) has been shown to control several physiological functions, including olfaction. We investigated the molecular and functional profile of the Halyomorpha halys type 1 tyramine receptor gene (HhTAR1) and its role in olfactory functions of this pest. Molecular and pharmacological analyses confirmed that the HhTAR1 gene codes for a true TAR1. RT-qPCR analysis revealed that HhTAR1 is expressed mostly in adult brain and antennae as well as in early development stages (eggs, 1st and 2nd instar nymphs). In particular, among the antennomeres that compose a typical H. halys antenna, HhTAR1 was more expressed in flagellomeres. Scanning electron microscopy investigation revealed the type and distribution of sensilla on adult H. halys antennae: both flagellomeres appear rich in trichoid and grooved sensilla, known to be associated with olfactory functions. Through an RNAi approach, topically delivered HhTAR1 dsRNA induced a 50% downregulation in gene expression after 24 h in H. halys 2nd instar nymphs. An innovative behavioural assay revealed that HhTAR1 RNAi-silenced 2nd instar nymphs were less susceptible to the alarm pheromone component (E)-2 decenal as compared with controls. These results provide critical information concerning the role of TAR1 in olfaction regulation, especially alarm pheromone reception, in H. halys. Furthermore, considering the emerging role of TAR1 as target of biopesticides, this work opens the way for further investigation on innovative methods for controlling H. halys.

The Insect Type 1 Tyramine Receptors: From Structure to Behavior

Simple Summary

This review aims to describe the type 1 tyramine receptors (TAR1s) in insects with a multidisciplinary approach and might be an important tool for a wide scientific audience, including biochemists, molecular physiologists, ethologists, and neurobiologists with a biological entomology background. In fact, in the last years, TAR1 has received much attention due to its broad general interest. The review is composed of a general introduction about the tyraminergic and octopaminergic systems and the corresponding tyramine (TA) and octopamine (OA) receptors, including the recent classification as well as their brief structural and functional information. The four chapters then describe TAR1s: (1) Molecular and structural characterization, with the purpose to provide a clear biochemical overview of the receptor that ensures a well-defined TAR1 identity; (2) pharmacology, in which a clear TAR1-mediated intracellular signaling pathway is detailed; (3) physiology and behavior, focusing on the TAR1-controlled traits in insects; (4) insecticide target, in which the knowledge on TAR1 roles in insects is associated with the growing evidence about the pest management strategies based on this receptor. The conclusions summarize TAR1 features as well as future directions on which the receptor research should move.

Abstract

Tyramine is a neuroactive compound that acts as neurotransmitter, neuromodulator, and neurohormone in insects. Three G protein-coupled receptors, TAR1-3, are responsible for mediating the intracellular pathway in the complex tyraminergic network. TAR1, the prominent player in this system, was initially classified as an octopamine receptor which can also be activated by tyramine, while it later appeared to be a true tyramine receptor. Even though TAR1 is currently considered as a well-defined tyramine receptor and several insect TAR1s have been characterized, a defined nomenclature is still inconsistent. In the last years, our knowledge on the structural, biochemical, and functional properties of TAR1 has substantially increased. This review summarizes the available information on TAR1 from different insect species in terms of basic structure, its regulation and signal transduction mechanisms, and its distribution and functions in the brain and the periphery. A special focus is given to the TAR1-mediated intracellular signaling pathways as well as to their physiological role in regulating behavioral traits. Therefore, this work aims to correlate, for the first time, the physiological relevance of TAR1 functions with the tyraminergic system in insects. In addition, pharmacological studies have shed light on compounds with insecticidal properties having TAR1 as a target and on the emerging trend in the development of novel strategies for pest control.

Regulation of the antennal transcriptome of the dengue vector, Aedes aegypti, during the first gonotrophic cycle

Background

In the light of dengue being the fastest growing transmissible disease, there is a dire need to identify the mechanisms regulating the behaviour of the main vector Aedes aegypti. Disease transmission requires the female mosquito to acquire the pathogen from a blood meal during one gonotrophic cycle, and to pass it on in the next, and the capacity of the vector to maintain the disease relies on a sustained mosquito population.

Results

Using a comprehensive transcriptomic approach, we provide insight into the regulation of the odour-mediated host- and oviposition-seeking behaviours throughout the first gonotrophic cycle. We provide clear evidence that the age and state of the female affects antennal transcription differentially. Notably, the temporal- and state-dependent patterns of differential transcript abundance of chemosensory and neuromodulatory genes extends across families, and appears to be linked to concerted differential modulation by subsets of transcription factors.

Conclusions

By identifying these regulatory pathways, we provide a substrate for future studies targeting subsets of genes across disparate families involved in generating key vector behaviours, with the goal to develop novel vector control tools.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07336-w.

Forager age and foraging state, but not cumulative foraging activity, affect biogenic amine receptor gene expression in the honeybee mushroom bodies

Foraging behavior is crucial for the development of a honeybee colony. Biogenic amines are key mediators of learning and the transition from in-hive tasks to foraging. Foragers vary considerably in their behavior, but whether and how this behavioral diversity depends on biogenic amines is not yet well understood. For example, forager age, cumulative foraging activity or foraging state may all be linked to biogenic amine signaling. Furthermore, expression levels may fluctuate depending on daytime. We tested if these intrinsic and extrinsic factors are linked to biogenic amine signaling by quantifying the expression of octopamine, dopamine and tyramine receptor genes in the mushroom bodies, important tissues for learning and memory. We found that older foragers had a significantly higher expression of Amdop1, Amdop2, AmoctαR1, and AmoctβR1 compared to younger foragers, whereas Amtar1 showed the opposite pattern. Surprisingly, our measures of cumulative foraging activity were not related to the expression of the same receptor genes in the mushroom bodies. Furthermore, we trained foragers to collect sucrose solution at a specific time of day and tested if the foraging state of time-trained foragers affected receptor gene expression. Bees engaged in foraging had a higher expression of Amdop1 and AmoctβR3/4 than inactive foragers. Finally, the expression of Amdop1, Amdop3, AmoctαR1, and Amtar1 also varied with daytime. Our results show that receptor gene expression in forager mushroom bodies is complex and depends on both intrinsic and extrinsic factors.

Correlation between octopaminergic signalling and foraging task specialisation in honeybees

Regulation of pollen and nectar foraging in honeybees is linked to differences in the sensitivity to the reward. Octopamine (OA) participates in the processing of reward-related information in the bee brain, being a candidate to mediate and modulate the division of labour among pollen and nectar foragers. Here we tested the hypothesis that OA affects the resource preferences of foragers. We first investigated whether oral administration of OA is involved in the transition from nectar to pollen foraging. We quantified the percentage of OA-treated bees that switched from a sucrose solution to a pollen feeder when the sugar concentration was decreased experimentally. We also evaluated if feeding the colonies sucrose solution containing OA increases the rate of bees collecting pollen. Finally, we quantified OA and tyramine (TYR) receptor genes expression of pollen and nectar foragers in different parts of the brain, as a putative mechanism that affects the decision-making process regarding the resource type collected. Adding OA in the food modified the probability that foragers switch from nectar to pollen collection. The proportion of pollen foragers also increased after feeding colonies with OA-containing food. Furthermore, the expression level of the AmoctαR1 was upregulated in foragers arriving at pollen sources compared with those arriving at sugar-water feeders. Using age-matched pollen and nectar foragers that returned to the hive, we detected an upregulated expression of a TYR receptor gene in the suboesophageal ganglia. These findings support our prediction that OA signalling affects the decision in honeybee foragers to collect pollen or nectar.

Reference gene selection for qRT-PCR analysis of season- and tissue-specific gene expression profiles in the honey bee Apis mellifera

Honey bees are both important pollinators and model insects due to their highly developed sociality and colony management. To better understand the molecular mechanisms underlying honey bee colony management, it is important to investigate the expression of genes putatively involved in colony physiology. Although quantitative real-time PCR (qRT-PCR) can be used to quantify the relative expression of target genes, internal reference genes (which are stably expressed across different conditions) must first be identified to ensure accurate normalisation of target genes. To identify reliable reference genes in honey bee (Apis mellifera) colonies, therefore, we evaluated seven candidate genes (ACT, EIF, EF1, RPN2, RPS5, RPS18 and GAPDH) in samples collected from three honey bee tissue types (head, thorax and abdomen) across all four seasons using three analysis programmes (NormFinder, BestKeeper and geNorm). Subsequently, we validated various normalisation methods using each of the seven reference genes and a combination of multiple genes by calculating the expression of catalase (CAT). Although the genes ranked as the most stable gene were slightly different on conditions and analysis methods, our results suggest that RPS5, RPS18 and GAPDH represent optimal honey bee reference genes for target gene normalisation in qRT-PCR analysis of various honey bee tissue samples collected across seasons.

Sex-specific molecular specialization and activity rhythm-dependent gene expression in honey bee antennae

We performed an RNA-seq-based comparison of gene expression levels in the antennae of honey bee drones and time-trained foragers (workers) collected at different times of the day and different activity states. Interestingly, olfaction-related genes [i.e. odorant receptor (Or) genes, odorant binding protein (Obp) genes, carboxyl esterase (CEst) genes, etc.] showed stable gene expression differences between drone and worker antennae. Drone antennae showed higher expression of 24 Or genes, of which 21 belong to the clade X which comprises the receptor for the major queen pheromone compound 9-ODA. This high number of drone-biased Or genes suggests that more than previously thought play a role in sex-pheromone communication. In addition, we found higher expression levels for many non-olfaction-related genes including nitric oxide synthase (NOS), and the potassium channel Shaw In contrast, workers showed higher expression of 67 Or genes, which belong to different Or clades that are involved in pheromone communication as well as the perception of cuticular hydrocarbons and floral scents. Further, drone antennae showed higher expression of genes involved in energy metabolism, whereas worker antennae showed higher expression of genes involved in neuronal communication, consistent with earlier reports on peripheral olfactory plasticity. Finally, drones that perform mating flight in the afternoon (innate) and foragers that are trained to forage in the afternoon (adapted) showed similar daily changes in the expression of two major clock genes, period and cryptochrome2 Most of the other genes showing changes with time or onset of daily flight activity were specific to drones and foragers.

Transcriptomics supports local sensory regulation in the antenna of the kissing-bug Rhodnius prolixus

Background

Rhodnius prolixus has become a model for revealing the molecular bases of insect sensory biology due to the publication of its genome and its well-characterized behavioural repertoire. Gene expression modulation underlies behaviour-triggering processes at peripheral and central levels. Still, the regulation of sensory-related gene transcription in sensory organs is poorly understood. Here we study the genetic bases of plasticity in antennal sensory function, using R. prolixus as an insect model.

Results

Antennal expression of neuromodulatory genes such as those coding for neuropeptides, neurohormones and their receptors was characterized in fifth instar larvae and female and male adults by means of RNA-Sequencing (RNA-Seq). New nuclear receptor and takeout gene sequences were identified for this species, as well as those of enzymes involved in the biosynthesis and processing of neuropeptides and biogenic amines.

Conclusions

We report a broad repertoire of neuromodulatory and neuroendocrine-related genes expressed in the antennae of R. prolixus and suggest that they may serve as the local basis for modulation of sensory neuron physiology. Diverse neuropeptide precursor genes showed consistent expression in the antennae of all stages studied. Future studies should characterize the role of these modulatory components acting over antennal sensory processes to assess the relative contribution of peripheral and central regulatory systems on the plastic expression of insect behaviour.

Inhibitory learning of phototaxis by honeybees in a passive-avoidance task

Honeybees are a standard model for the study of appetitive learning and memory. Yet, fewer attempts have been performed to characterize aversive learning and memory in this insect and uncover its molecular underpinnings. Here, we took advantage of the positive phototactic behavior of bees kept away from the hive in a dark environment and established a passive-avoidance task in which they had to suppress positive phototaxis. Bees placed in a two-compartment box learned to inhibit spontaneous attraction to a compartment illuminated with blue light by associating and entering into that chamber with shock delivery. Inhibitory learning resulted in an avoidance memory that could be retrieved 24 h after training and that was specific to the punished blue light. The memory was mainly operant but involved a Pavlovian component linking the blue light and the shock. Coupling conditioning with transcriptional analyses in key areas of the brain showed that inhibitory learning of phototaxis leads to an up-regulation of the dopaminergic receptor gene Amdop1 in the calyces of the mushroom bodies, consistently with the role of dopamine signaling in different forms of aversive learning in insects. Our results thus introduce new perspectives for uncovering further cellular and molecular underpinnings of aversive learning and memory in bees. Overall, they represent an important step toward comparative learning studies between the appetitive and the aversive frameworks.

Comparative transcriptomics of social insect queen pheromones

Queen pheromones are chemical signals that mediate reproductive division of labor in eusocial animals. Remarkably, queen pheromones are composed of identical or chemically similar compounds in some ants, wasps and bees, even though these taxa diverged >150MYA and evolved queens and workers independently. Here, we measure the transcriptomic consequences of experimental exposure to queen pheromones in workers from two ant and two bee species (genera: Lasius, Apis, Bombus), and test whether they are similar across species. Queen pheromone exposure affected transcription and splicing at many loci. Many genes responded consistently in multiple species, and the set of pheromone-sensitive genes was enriched for functions relating to lipid biosynthesis and transport, olfaction, production of cuticle, oogenesis, and histone (de)acetylation. Pheromone-sensitive genes tend to be evolutionarily ancient, positively selected, peripheral in the gene coexpression network, hypomethylated, and caste-specific in their expression. Our results reveal how queen pheromones achieve their effects, and suggest that ants and bees use similar genetic modules to achieve reproductive division of labor.

Origanum vulgare terpenoids modulate Myrmica scabrinodis brain biogenic amines and ant behaviour

Coordinated social behaviour is fundamental for ant ecological success. However, even distantly-related organisms, such as plants, have evolved the ability to manipulate ant collective performances to their own advantage. In the parasitic system encompassing Maculinea butterflies, Myrmica ants, and Origanum vulgare plants, the ant-plant interaction elicits the release of a volatile terpenoid compound (carvacrol) which is used by the gravid butterfly to locate the ideal oviposition site. Here we show that this ant-plant association is maintained by the effect of O. vulgare terpenoids on ant behaviour and that food plants might gain protection by Myrmica ants by chemically manipulating workers to forage in their surroundings. The variation in the locomotor ability of three ant species (Formica cinerea, Tetramorium caespitum, and Myrmica scabrinodis) was studied after treatment with the two major O. vulgare terpenoid volatile compounds (i.e., carvacrol and thymol). The brain levels of three biogenic amines (dopamine, tyramine and serotonin) were analysed in ants exposed to the O. vulgare terpenoids by HPLC-ESI-MS/MS. Carvacrol and thymol increased the locomotor activity of all ant species tested, but if blended reduced the movement propensity of Myrmica scabrinodis. Dopamine and tyramine production was positively correlated with the worker locomotor activity. In Myrmica ants, both brain biogenic ammines were negatively correlated with the aggressive behaviour. Blends of O. vulgare volatiles affected the locomotor ability while increased the aggressiveness of Myrmica workers by altering the aminergic regulation in the ant brains. This behavioural manipulation, might enhance partner fidelity and plant protection. Our findings provide new insights supporting a direct role of plant volatiles in driving behavioural changes in social insects through biogenic amine modulation.

Biogenic amine modulation of honey bee sociability and nestmate affiliation

Biogenic amines modulate a range of social behaviours, including sociability and mechanisms of group cohesion, in both vertebrates and invertebrates. Here, we tested if the biogenic amines modulate honey bee (Apis mellifera) sociability and nestmate affiliation. We examined the consequences of treatments with biogenic amines, agonists and antagonists on a bee’s approach to, and subsequent social interactions with, conspecifics in both naturally hive-reared bees and isolated bees. We used two different treatment methods. Bees were first treated topically with compounds dissolved in the solvent dimethylformamide (dMF) applied to the dorsal thorax, but dMF had a significant effect on the locomotion and behaviour of the bees during the behavioural test that interfered with their social responses. Our second method used microinjection to deliver biogenic amines to the head capsule via the ocellar tract. Microinjection of dopamine and a dopamine antagonist had strong effects on bee sociability, likelihood of interaction with bees, and nestmate affiliation. Octopamine treatment reduced social interaction with other bees, and serotonin increased the likelihood of social interactions. HPLC measurements showed that isolation reduced brain levels of biogenic amines compared to hive-reared bees. Our findings suggest that dopamine is an important neurochemical component of social motivation in bees. This finding advances a comparative understanding of the processes of social evolution.

Biogenic amine signaling systems in the red imported fire ant, Solenopsis invicta - Possible contributors to worker division of labor

The red imported fire ant, Solenopsis invicta Buren, is a dangerous invasive pest in the United States, China and other countries. Efficient division of labor is one of the main reasons for the success of this social insect. Biogenic amines are important regulators of worker division of labor in this eusocial insect, but the related molecular mechanisms are largely unknown. In this study, we identified 10 candidate biogenic amine synthetic enzyme genes and 17 candidate biogenic amine receptor genes in the genome of S. invicta. Quantitative real-time PCR results indicated that foragers had higher head transcripts levels of all the tested enzyme genes than nurses did. In the abdomen, only the rate-limiting enzyme genes for the biosynthesis of serotonin and dopamine were higher in foragers than in nurses. Among the tested serotonin receptors, only the expression of 5-HT_2A gene showed significant difference between foragers and nurses. In the head, more abundant 5-HT_2A transcripts were detected in foragers than in nurses. Foragers expressed higher Octβ4R than nurses in the head and abdomen. However, much lower mRNA levels of Dop3 receptor gene were detected in both body regions of foragers than nurses. Several other octopamine and tyramine receptor genes were also differentially expressed between foragers and nurses in the head and/or in the abdomen. Our results will improve the understanding of molecular mechanisms underlying biogenic amine modulation of the worker division of labor in S. invicta.

Genome-Wide Identification and Characterization of Fox Genes in the Honeybee, Apis cerana, and Comparative Analysis with Other Bee Fox Genes

The forkhead box (Fox) gene family, one of the most important families of transcription factors, participates in various biological processes. However, Fox genes in Hymenoptera are still poorly known. In this study, 14 Fox genes were identified in the genome of Apis cerana. In addition, 16 (Apis mellifera), 13 (Apis dorsata), 16 (Apis florea), 17 (Bombus terrestris), 16 (Bombus impatiens), and 18 (Megachile rotundata) Fox genes were identified in their genomes, respectively. Phylogenetic analyses suggest that FoxA is absent in the genome of A. dorsata genome. Similarly, FoxG is missing in the genomes A. cerana and A. dorsata. Temporal expression profiles obtained by quantitative real-time PCR revealed that Fox genes have distinct expression patterns in A. cerana, especially for three genes ACSNU03719T0 (AcFoxN4), ACSNU05765T0 (AcFoxB), and ACSNU07465T0 (AcFoxL2), which displayed high expression at the egg stage. Tissue expression patterns showed that FoxJ1 is significantly higher in the antennae of A. cerana and A. mellifera compared to other tissues. These results may facilitate a better understanding of the potential physiological functions of the Fox gene family in A. cerana and provide valuable information for a comprehensive functional analysis of the Fox gene family in Hymenopterans.

Biogenic Amines in Insect Antennae

Insect antenna is a multisensory organ, each modality of which can be modulated by biogenic amines. Octopamine (OA) and its metabolic precursor tyramine (TA) affect activity of antennal olfactory receptor neurons. There is some evidence that dopamine (DA) modulates gustatory neurons. Serotonin can serve as a neurotransmitter in some afferent mechanosensory neurons and both as a neurotransmitter and neurohormone in efferent fibers targeted at the antennal vessel and mechanosensory organs. As a neurohormone, serotonin affects the generation of the transepithelial potential by sensillar accessory cells. Other possible targets of biogenic amines in insect antennae are hygro- and thermosensory neurons and epithelial cells. We suggest that the insect antenna is partially autonomous in the sense that biologically active substances entering its hemolymph may exert their effects and be cleared from this compartment without affecting other body parts.

Age, worksite location, neuromodulators, and task performance in the ant Pheidole dentata

Social insect workers modify task performance according to age-related schedules of behavioral development, and/or changing colony labor requirements based on flexible responses that may be independent of age. Using known-age minor workers of the ant Pheidole dentata throughout 68% of their 140-day laboratory lifespan, we asked whether workers found inside or outside the nest differed in task performance and if behaviors were correlated with and/or causally linked to changes in brain serotonin (5HT) and dopamine (DA). Our results suggest that task performance patterns of individually assayed minors collected at these two spatially different worksites were independent of age. Outside-nest minors displayed significantly higher levels of predatory behavior and greater activity than inside-nest minors, but these groups did not differ in brood care or phototaxis. We examined the relationship of 5HT and DA to these behaviors in known-age minors by quantifying individual brain titers. Both monoamines did not increase significantly from 20 to 95 days of age. DA did not appear to directly regulate worksite location, although titers were significantly higher in outside-nest than inside-nest workers. Pharmacological depletion of 5HT did not affect nursing, predation, phototaxis or activity. Our results suggest that worker task capabilities are independent of age beyond 20 days, and only predatory behavior can be consistently predicted by spatial location. This could reflect worker flexibility or variability in the behavior of individuals collected at each location, which could be influenced by complex interactions between age, worksite location, social interactions, neuromodulators, and other environmental and internal regulators of behavior.

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.

Effects of Sublethal Doses of Imidacloprid on Young Adult Honeybee Behaviour

Imidacloprid (IMI), a neonicotinoid used for its high selective toxicity to insects, is one of the most commonly used pesticides. However, its effect on beneficial insects such as the honeybee Apis mellifera L is still controversial. As young adult workers perform in-hive duties that are crucial for colony maintenance and survival, we aimed to assess the effect of sublethal IMI doses on honeybee behaviour during this period. Also, because this insecticide acts as a cholinergic-nicotinic agonist and these pathways take part in insect learning and memory processes; we used IMI to assess their role and the changes they suffer along early adulthood. We focused on appetitive behaviours based on the proboscis extension response. Laboratory reared adults of 2 to 10 days of age were exposed to sublethal IMI doses (0.25 or 0.50ng) administered orally or topically prior to behavioural assessment. Modification of gustatory responsiveness and impairment of learning and memory were found as a result of IMI exposure. These outcomes differed depending on age of evaluation, type of exposure and IMI dose, being the youngest bees more sensitive and the highest oral dose more toxic. Altogether, these results imply that IMI administered at levels found in agroecosystems can reduce sensitivity to reward and impair associative learning in young honeybees. Therefore, once a nectar inflow with IMI traces is distributed within the hive, it could impair in-door duties with negative consequences on colony performance.

Gene expression during zombie ant biting behavior reflects the complexity underlying fungal parasitic behavioral manipulation

BACKGROUND

Adaptive manipulation of animal behavior by parasites functions to increase parasite transmission through changes in host behavior. These changes can range from slight alterations in existing behaviors of the host to the establishment of wholly novel behaviors. The biting behavior observed in Carpenter ants infected by the specialized fungus Ophiocordyceps unilateralis s.l. is an example of the latter. Though parasitic manipulation of host behavior is generally assumed to be due to the parasite's gene expression, few studies have set out to test this.

RESULTS

We experimentally infected Carpenter ants to collect tissue from both parasite and host during the time period when manipulated biting behavior is experienced. Upon observation of synchronized biting, samples were collected and subjected to mixed RNA-Seq analysis. We also sequenced and annotated the O. unilateralis s.l. genome as a reference for the fungal sequencing reads.

CONCLUSIONS

Our mixed transcriptomics approach, together with a comparative genomics study, shows that the majority of the fungal genes that are up-regulated during manipulated biting behavior are unique to the O. unilateralis s.l. genome. This study furthermore reveals that the fungal parasite might be regulating immune- and neuronal stress responses in the host during manipulated biting, as well as impairing its chemosensory communication and causing apoptosis. Moreover, we found genes up-regulated during manipulation that putatively encode for proteins with reported effects on behavioral outputs, proteins involved in various neuropathologies and proteins involved in the biosynthesis of secondary metabolites such as alkaloids.

Effect of a thymol application on olfactory memory and gene expression levels in the brain of the honeybee Apis mellifera

Essential oils are used by beekeepers to control the Varroa mites that infest honeybee colonies. So, bees can be exposed to thymol formulations in the hive. The effects of the monoterpenoid thymol were explored on olfactory memory and gene expression in the brain of the honeybee. In bees previously exposed to thymol (10 or 100 ng/bee), the specificity of the response to the conditioned stimulus (CS) was lost 24 h after learning. Besides, the octopamine receptor OA1 gene Amoa1 showed a significant decrease of expression 3 h after exposure with 10 or 100 ng/bee of thymol. With the same doses, expression of Rdl gene, coding for a GABA receptor subunit, was not significantly modified but the trpl gene was upregulated 1 and 24 h after exposure to thymol. These data indicated that the genes coding for the cellular targets of thymol could be rapidly regulated after exposure to this molecule. Memory and sensory processes should be investigated in bees after chronic exposure in the hive to thymol-based preparations.

Antennae hold a key to Varroa-sensitive hygiene behaviour in honey bees

In honey bees, Varroa sensitive hygiene (VSH) behaviour, which involves the detection and removal of brood parasitised by the mite Varroa destructor, can actively participate in the survival of colonies facing Varroa outbreaks. This study investigated the mechanisms of VSH behaviour, by comparing the antennal transcriptomes of bees that do and do not perform VSH behaviour. Results indicate that antennae likely play a key role in the expression of VSH behaviour. Comparisons with the antennal transcriptome of nurse and forager bees suggest that VSH profile is more similar to that of nurse bees than foragers. Enhanced detection of certain odorants in VSH bees may be predicted from transcriptional patterns, as well as a higher metabolism and antennal motor activity. Interestingly, Deformed wing virus/Varroa destructor virus infections were detected in the antennae, with higher level in non-VSH bees; a putative negative impact of viral infection on bees' ability to display VSH behaviour is proposed. These results bring new perspectives to the understanding of VSH behaviour and the evolution of collective defence by focusing attention on the importance of the peripheral nervous system. In addition, such data might be useful for promoting marker-assisted selection of honey bees that can survive Varroa infestations.

Neurophysiological mechanisms underlying sex- and maturation-related variation in pheromone responses in honey bees (Apis mellifera)

In the honey bee (Apis mellifera), social organization is primarily mediated by pheromones. Queen-produced 9-oxo-2-decenoic acid (9-ODA) functions as both a social and sex pheromone, eliciting attraction in both female workers and male drones, but also affecting other critical aspects of worker physiology and behavior. These effects are also maturation related, as younger workers and sexually mature drones are most receptive to 9-ODA. While changes in the peripheral nervous system drive sex-related differences in sensitivity to 9-ODA, the mechanisms driving maturation-related shifts in receptivity to 9-ODA remain unknown. Here, we investigate the hypothesis that changes at the peripheral nervous system may be mediating plastic responses to 9-ODA by characterizing expression levels of AmOR11 (the olfactory receptor tuned to 9-ODA) and electrophysiological responses to 9-ODA. We find that receptor expression correlates significantly with behavioral receptivity to 9-ODA, with nurses and sexually mature drones exhibiting higher levels of expression than foragers and immature drones, respectively. Electrophysiological responses to 9-ODA were not found to correlate with behavioral receptivity or receptor expression, however. Thus, while receptor expression at the periphery exhibits a level of plasticity that correlates with behavior, the mechanisms driving maturation-dependent responsiveness to 9-ODA appear to function primarily in the central nervous system.

Social modulation of stress reactivity and learning in young worker honey bees

Alarm pheromone and its major component isopentylacetate induce stress-like responses in forager honey bees, impairing their ability to associate odors with a food reward. We investigated whether isopentylacetate exposure decreases appetitive learning also in young worker bees. While isopentylacetate-induced learning deficits were observed in guards and foragers collected from a queen-right colony, learning impairments resulting from exposure to this pheromone could not be detected in bees cleaning cells. As cell cleaners are generally among the youngest workers in the colony, effects of isopentylacetate on learning behavior were examined further using bees of known age. Adult workers were maintained under laboratory conditions from the time of adult emergence. Fifty percent of the bees were exposed to queen mandibular pheromone during this period, whereas control bees were not exposed to this pheromone. Isopentylacetate-induced learning impairments were apparent in young (less than one week old) controls, but not in bees of the same age exposed to queen mandibular pheromone. This study reveals young worker bees can exhibit a stress-like response to alarm pheromone, but isopentylacetate-induced learning impairments in young bees are suppressed by queen mandibular pheromone. While isopentylacetate exposure reduced responses during associative learning (acquisition), it did not affect one-hour memory retrieval.

Division of labour in honey bees: age- and task-related changes in the expression of octopamine receptor genes

The honey bee (Apis mellifera L.) has developed into an important ethological model organism for social behaviour and behavioural plasticity. Bees perform a complex age-dependent division of labour with the most pronounced behavioural differences occurring between in-hive bees and foragers. Whereas nurse bees, for example, stay inside the hive and provide the larvae with food, foragers leave the hive to collect pollen and nectar for the entire colony. The biogenic amine octopamine appears to play a major role in division of labour but the molecular mechanisms involved are unknown. We here investigated the role of two characterized octopamine receptors in honey bee division of labour. AmOctαR1 codes for a Ca(2+) -linked octopamine receptor. AmOctβR3/4 codes for a cyclic adenosine monophosphate-coupled octopamine receptor. Messenger RNA expression of AmOctαR1 in different brain neuropils correlates with social task, whereas expression of AmOctβR3/4 changes with age rather than with social role per se. Our results for the first time link the regulatory role of octopamine in division of labour to specific receptors and brain regions. They are an important step forward in our understanding of complex behavioural organization in social groups.

Juvenile hormone enhances aversive learning performance in 2-day old worker honey bees while reducing their attraction to queen mandibular pheromone

Previous studies have shown that exposing young worker bees (Apis mellifera) to queen mandibular pheromone (QMP) reduces their aversive learning performance, while enhancing their attraction to QMP. As QMP has been found to reduce the rate of juvenile hormone (JH) synthesis in worker bees, we examined whether aversive learning in 2-day old workers exposed to QMP from the time of adult emergence could be improved by injecting JH (10 µg in a 2 µl volume) into the haemolymph. We examined in addition, the effects of JH treatment on worker attraction to QMP, and on the levels of expression of amine receptor genes in the antennae, as well as in the mushroom bodies of the brain. We found that memory acquisition and 1-hour memory recall were enhanced by JH. In contrast, JH treatment reduced the bees' attraction towards a synthetic strip impregnated with QMP (Bee Boost). Levels of expression of the dopamine receptor gene Amdop1 were significantly lower in the mushroom bodies of JH-treated bees than in bees treated with vehicle alone (acetone diluted with bee ringer). Expression of the octopamine receptor gene, Amoa1, in this brain region was also affected by JH treatment, and in the antennae, Amoa1 transcript levels were significantly lower in JH-treated bees compared to controls. The results of this study suggest that QMP's effects on JH synthesis may contribute to reducing aversive learning performance and enhancing attraction to QMP in young worker bees.

The GPCR membrane receptor, DopEcR, mediates the actions of both dopamine and ecdysone to control sex pheromone perception in an insect

Olfactory information mediating sexual behavior is crucial for reproduction in many animals, including insects. In male moths, the macroglomerular complex (MGC) of the primary olfactory center, the antennal lobe (AL) is specialized in the treatment of information on the female-emitted sex pheromone. Evidence is accumulating that modulation of behavioral pheromone responses occurs through neuronal plasticity via the action of hormones and/or catecholamines. We recently showed that a G-protein-coupled receptor (GPCR), AipsDopEcR, with its homologue known in Drosophila for its double affinity to the main insect steroid hormone 20-hydroxyecdysone (20E), and dopamine (DA), present in the ALs, is involved in the behavioral response to pheromone in the moth, Agrotis ipsilon. Here we tested the role of AipsDopEcR as compared to nuclear 20E receptors in central pheromone processing combining receptor inhibition with intracellular recordings of AL neurons. We show that the sensitivity of AL neurons for the pheromone in males decreases strongly after AipsDopEcR-dsRNA injection but also after inhibition of nuclear 20E receptors. Moreover we tested the involvement of 20E and DA in the receptor-mediated behavioral modulation in wind tunnel experiments, using ligand applications and receptor inhibition treatments. We show that both ligands are necessary and act on AipsDopEcR-mediated behavior. Altogether these results indicate that the GPCR membrane receptor, AipsDopEcR, controls sex pheromone perception through the action of both 20E and DA in the central nervous system, probably in concert with 20E action through nuclear receptors.

Gene co-citation networks associated with worker sterility in honey bees

BACKGROUND

The evolution of reproductive self-sacrifice is well understood from kin theory, yet our understanding of how actual genes influence the expression of reproductive altruism is only beginning to take shape. As a model in the molecular study of social behaviour, the honey bee Apis mellifera has yielded hundreds of genes associated in their expression with differences in reproductive status of females, including genes directly associated with sterility, yet there has not been an attempt to link these candidates into functional networks that explain how workers regulate sterility in the presence of queen pheromone. In this study we use available microarray data and a co-citation analysis to describe what gene interactions might regulate a worker's response to ovary suppressing queen pheromone.

RESULTS

We reconstructed a total of nine gene networks that vary in size and gene composition, but that are significantly enriched for genes of reproductive function. The networks identify, for the first time, which candidate microarray genes are of functional importance, as evidenced by their degree of connectivity to other genes within each of the inferred networks. Our study identifies single genes of interest related to oogenesis, including eggless, and further implicates pathways related to insulin, ecdysteroid, and dopamine signaling as potentially important to reproductive decision making in honey bees.

CONCLUSIONS

The networks derived here appear to be variable in gene composition, hub gene identity, and the overall interactions they describe. One interpretation is that workers use different networks to control personal reproduction via ovary activation, perhaps as a function of age or environmental circumstance. Alternatively, the multiple networks inferred here may represent segments of the larger, single network that remains unknown in its entirety. The networks generated here are provisional but do offer a new multi-gene framework for understanding how honey bees regulate personal reproduction within their highly social breeding system.

Involvement of the G-protein-coupled dopamine/ecdysteroid receptor DopEcR in the behavioral response to sex pheromone in an insect

Most animals including insects rely on olfaction to find their mating partners. In moths, males are attracted by female-produced sex pheromones inducing stereotyped sexual behavior. The behaviorally relevant olfactory information is processed in the primary olfactory centre, the antennal lobe (AL). Evidence is now accumulating that modulation of sex-linked behavioral output occurs through neuronal plasticity via the action of hormones and/or catecholamines. A G-protein-coupled receptor (GPCR) binding to 20-hydroxyecdysone, the main insect steroid hormone, and dopamine, has been identified in Drosophila (DmDopEcR), and was suggested to modulate neuronal signaling. In the male moth Agrotis ipsilon, the behavioral and central nervous responses to pheromone are age-dependent. To further unveil the mechanisms of this olfactory plasticity, we searched for DopEcR and tested its potential role in the behavioral response to sex pheromone in A. ipsilon males. Our results show that A. ipsilon DopEcR (named AipsDopEcR) is predominantly expressed in the nervous system. The corresponding protein was detected immunohistochemically in the ALs and higher brain centers including the mushroom bodies. Moreover, AipsDopEcR expression increased with age. Using a strategy of RNA interference, we also show that silencing of AipsDopEcR inhibited the behavioral response to sex pheromone in wind tunnel experiments. Altogether our results indicate that this GPCR is involved in the expression of sexual behavior in the male moth, probably by modulating the central nervous processing of sex pheromone through the action of one or both of its ligands.

Functional Development of the Octenol Response in Aedes aegypti

Attraction of female Aedes aegypti mosquitoes to 1-octen-3-ol (octenol), CO2, lactic acid, or ammonia emitted by vertebrate hosts is not only contingent on the presence of odorants in the environment, but is also influenced by the insect's physiological state. For anautogenous mosquito species, like A. aegypti, newly emerged adult females neither respond to host odors nor engage in blood-feeding; the bases for these behaviors are poorly understood. Here we investigated detection of two components of an attractant blend emitted by vertebrate hosts, octenol, and CO2, by female A. aegypti mosquitoes using electrophysiological, behavioral, and molecular approaches. An increase in sensitivity of octenol olfactory receptor neurons (ORNs) was correlated with an increase in odorant receptor gene (Or) expression and octenol-mediated attractive behavior from day 1 to day 6 post-emergence. While the sensitivity of octenol ORNs was maintained through day 10, behavioral responses to octenol decreased as did the ability of females to discriminate between octenol and octenol + CO2. Our results show differing age-related roles for the peripheral receptors for octenol and higher order neural processing in the behavior of female mosquitoes.

General Stress Responses in the Honey Bee

The biological concept of stress originated in mammals, where a “General Adaptation Syndrome” describes a set of common integrated physiological responses to diverse noxious agents. Physiological mechanisms of stress in mammals have been extensively investigated through diverse behavioral and physiological studies. One of the main elements of the stress response pathway is the endocrine hypothalamo-pituitary-adrenal (HPA) axis, which underlies the “fight-or-flight” response via a hormonal cascade of catecholamines and corticoid hormones. Physiological responses to stress have been studied more recently in insects: they involve biogenic amines (octopamine, dopamine), neuropeptides (allatostatin, corazonin) and metabolic hormones (adipokinetic hormone, diuretic hormone). Here, we review elements of the physiological stress response that are or may be specific to honey bees, given the economical and ecological impact of this species. This review proposes a hypothetical integrated honey bee stress pathway somewhat analogous to the mammalian HPA, involving the brain and, particularly, the neurohemal organ corpora cardiaca and peripheral targets, including energy storage organs (fat body and crop). We discuss how this system can organize rapid coordinated changes in metabolic activity and arousal, in response to adverse environmental stimuli. We highlight physiological elements of the general stress responses that are specific to honey bees, and the areas in which we lack information to stimulate more research into how this fascinating and vital insect responds to stress.