Species differences in brain gene expression profiles associated with adult behavioral maturation in honey bees

Background

Honey bees are known for several striking social behaviors, including a complex pattern of behavioral maturation that gives rise to an age-related colony division of labor and a symbolic dance language, by which successful foragers communicate the location of attractive food sources to their nestmates. Our understanding of honey bees is mostly based on studies of the Western honey bee, Apis mellifera, even though there are 9–10 other members of genus Apis, showing interesting variations in social behavior relative to A. mellifera. To facilitate future in-depth genomic and molecular level comparisons of behavior across the genus, we performed a microarray analysis of brain gene expression for A. mellifera and three key species found in Asia, A. cerana, A. florea and A. dorsata.

Results

For each species we compared brain gene expression patterns between foragers and adult one-day-old bees on an A. mellifera cDNA microarray and calculated within-species gene expression ratios to facilitate cross-species analysis. The number of cDNA spots showing hybridization fluorescence intensities above the experimental threshold was reduced by an average of 16% in the Asian species compared to A. mellifera, but an average of 71% of genes on the microarray were available for analysis. Brain gene expression profiles between foragers and one-day-olds showed differences that are consistent with a previous study on A. mellifera and were comparable across species. Although 1772 genes showed significant differences in expression between foragers and one-day-olds, only 218 genes showed differences in forager/one-day-old expression between species (p < 0.001). Principal Components Analysis revealed dominant patterns of expression that clearly distinguished between the four species but did not reflect known differences in behavior and ecology. There were species differences in brain expression profiles for functionally related groups of genes.

Conclusion

We conclude that the A. mellifera cDNA microarray can be used effectively for cross-species comparisons within the genus. Our results indicate that there is a widespread conservation of the molecular processes in the honey bee brain underlying behavioral maturation. Species differences in brain expression profiles for functionally related groups of genes provide possible clues to the basis of behavioral variation in the genus.

Central projections of sensory systems involved in honey bee dance language communication

Honey bee dance language is a unique and complex form of animal communication used to inform nest mates in the colony about the specific location of food sources or new nest sites. Five different sensory systems have been implicated in acquiring and communicating the information necessary for dance language communication. We present results from neuronal tracer studies identifying the central projections from four of the five. Sensory neurons of the dorsal rim area of the compound eyes, involved in acquiring sun-compass based information, project to the dorsal-most part of the medulla. Sensory neurons of the neck hair plates, required to transpose sun-compass based information to gravity-based information in the dark hive, project to the dorsal labial neuromere of the subesophageal ganglion. Sensory neurons from the antennal joint hair sensilla and the Johnston's organ, which perceive information on dance direction and distance from mechanostimuli generated by abdomen waggling and wing vibration, project to the deutocerebral dorsal lobe and the subesophageal ganglion, and the posterior protocerebrum, respectively. We found no 'dance-specific' projections relative to those previously described for drone and queen honey bees and other insect species that do not exhibit dance communication. We suggest that the evolution of dance language communication was likely based on the modification of central neural pathways associated with path integration, the capability to calculate distance, and directional information during flight.

Evo-devo and the evolution of social behavior

The integration of evolutionary biology with developmental genetics into the hybrid field of 'evo-devo' resulted in major advances in understanding multicellular development and morphological evolution. Here we show how insights from evo-devo can be applied to study the evolution of social behavior. We develop this idea by reviewing studies that suggest that molecular pathways controlling feeding behavior and reproduction in solitary insects are part of a 'genetic toolkit' underlying the evolution of a particularly complex form of social behavior, division of labor among workers in honeybee colonies. The evo-devo approach, coupled with advances in genomics for non-model genetic organisms, including the recent sequencing of the honeybee genome, promises to advance our understanding of the evolution of social behavior.

Vitellogenin, juvenile hormone, insulin signaling, and queen honey bee longevity

In most animals, longevity is achieved at the expense of fertility, but queen honey bees do not show this tradeoff. Queens are both long-lived and fertile, whereas workers, derived from the same genome, are both relatively short-lived and normally sterile. It has been suggested, on the basis of results from workers, that vitellogenin (Vg), best known as a yolk protein synthesized in the abdominal fat body, acts as an antioxidant to promote longevity in queen bees. We explored this hypothesis, as well as related roles of insulin-IGF-1 signaling and juvenile hormone. Vg was expressed in thorax and head fat body cells in an age-dependent manner, with old queens showing much higher expression than workers. In contrast, Vg expression in worker head was much lower. Queens also were more resistant to oxidative stress than workers. These results support the hypothesis that caste-specific differences in Vg expression are involved in queen longevity. Consistent with predictions from Drosophila, old queens had lower head expression of insulin-like peptide and its putative receptors than did old workers. Juvenile hormone affected the expression of Vg and insulin-IGF-1 signaling genes in opposite directions. These results suggest that conserved and species-specific mechanisms interact to regulate queen bee longevity without sacrificing fecundity.

Comparing injection, feeding and topical application methods for treatment of honeybees with octopamine

Entomologists have used a range of techniques to treat insects with neuroactive compounds, but it is not always clear whether different treatment methods are equally effective in delivering a compound to a target organ. Here, we used five different techniques to treat honeybees with 3H-octopamine (3H-OA), and analysed the distribution of the 3H radiolabelled compound within different tissues and how it changed over time. All treatment methods, including injection of the median ocellus, resulted in 3H-OA detection in all parts of the honeybee. Injection through the median ocellus was the most effective method for delivering 3H-OA to the brain. Topical application of 3H-OA dissolved in dimethylformamide (dMF) to the thorax was as effective as thoracic injections of 3H-OA in delivering 3H-OA to the brain, but topical applications to the abdomen were less so. Most of the 3H-OA applied topically remained associated with the cuticle and the tissues of the body segment to which it had been applied. For all treatment methods, 3H-OA was rapidly lost from the brain and head capsule, and accumulated in the abdomen. Our findings demonstrate the value of thoracic topical treatment with compounds dissolved in dMF as an effective non-invasive method for short-term, systemic pharmacological treatments.

Octopamine modulates honey bee dance behavior

Honey bees communicate the location and desirability of valuable forage sites to their nestmates through an elaborate, symbolic "dance language." The dance language is a uniquely complex communication system in invertebrates, and the neural mechanisms that generate dances are largely unknown. Here we show that treatments with controlled doses of the biogenic amine neuromodulator octopamine selectively increased the reporting of resource value in dances by forager bees. Oral and topical octopamine treatments modulated aspects of dances related to resource profitability in a dose-dependent manner. Dances for pollen and sucrose responded similarly to octopamine treatment, and these effects were eliminated by treatment with the octopamine antagonist mianserin. We propose that octopamine modulates the representation of floral rewards in dances by changing the processing of reward in the honey bee brain. Octopamine is known to modulate appetitive behavior in a range of solitary insects; the role of octopamine in dance provides an example of how neural substrates can be adapted for new behavioral innovations in the process of social evolution.

Endocrine modulation of a pheromone-responsive gene in the honey bee brain

Pheromones cause dramatic changes in behavior and physiology, and are critical for honey bee colony organization. Queen mandibular pheromone (QMP) regulates multiple behaviors in worker bees (Slessor et al. in J Chem Ecol 31(11):2731-2745, 2005). We also identified genes whose brain expression levels were altered by exposure to QMP (Grozinger et al. in Proc Natl Acad Sci USA 100(Suppl 2):14519-14525, 2003). Krüppel-homolog 1 (Kr-h1) RNA levels were significantly downregulated by QMP, and were higher in foragers than in nurses (Whitfield et al. in Science 302(5643):296-299, 2003). Here we report on results of behavioral and pharmacological experiments that characterize factors regulating expression of Kr-h1. Foragers have higher brain levels of Kr-h1 than in-hive bees, regardless of age and pheromone exposure. Furthermore, forager Kr-h1 levels were not affected by QMP. Since the onset of foraging is caused, in part, by increasing juvenile hormone blood titers and brain octopamine levels, we investigated the effects of octopamine and methoprene (a juvenile hormone analog) on Kr-h1 expression. Methoprene produced a marginal (not significant) increase in Kr-h1 expression, but Kr-h1 brain levels in methoprene-treated bees were no longer downregulated by QMP. Octopamine did not modulate Kr-h1 expression. Our results demonstrate that the gene expression response to QMP is not hard-wired in the brain but is instead dependent on worker behavioral state.

Functional CpG methylation system in a social insect

DNA methylation systems are well characterized in vertebrates, but methylation in Drosophila melanogaster and other invertebrates remains controversial. Using the recently sequenced honey bee genome, we present a bioinformatic, molecular, and biochemical characterization of a functional DNA methylation system in an insect. We report on catalytically active orthologs of the vertebrate DNA methyltransferases Dnmt1 and Dnmt3a and b, two isoforms that contain a methyl-DNA binding domain, genomic 5-methyl-deoxycytosine, and CpG-methylated genes. The honey bee provides an opportunity to study the roles of methylation in social contexts.

Genome scan for cis-regulatory DNA motifs associated with social behavior in honey bees

Honey bees (Apis mellifera) undergo an age-related, socially regulated transition from working in the hive to foraging, which is associated with changes in the expression of thousands of genes in the brain. To begin to study the cis-regulatory code underlying this massive social regulation of gene expression, we used the newly sequenced honey bee genome to scan the promoter regions of eight sets of behaviorally related genes differentially expressed in the brain in the context of division of labor among worker bees, for 41 cis-regulatory motifs previously characterized in Drosophila melanogaster. Binding sites for the transcription factors Hairy, GAGA, Adf1, Cf1, Snail, and Dri, known to function in nervous system development, olfactory learning, or hormone binding in Drosophila, were significantly associated with one or more gene sets. The presence of some binding sites also predicted expression patterns for as many as 71% of the genes in some gene sets. These results suggest that there is a robust relationship between cis and social regulation of brain gene expression, especially considering that we studied <15% of all known transcription factors. These results also suggest that transcriptional networks involved in the regulation of development in Drosophila are used to regulate behavioral development in adult honey bees. However, differences in gene regulation between these two processes are suggested by the finding that the promoter regions for the behaviorally related bee genes differed in both motif occurrence and G/C content relative to their Drosophila orthologs.

From the Genome to the Proteome: Uncovering Peptides in the Apis Brain

Neuropeptides, critical brain peptides that modulate animal behavior by affecting the activity of almost every neuronal circuit, are inherently difficult to predict directly from a nascent genome sequence because of extensive posttranslational processing. The combination of bioinformatics and proteomics allows unprecedented neuropeptide discovery from an unannotated genome. Within the Apis mellifera genome, we have inferred more than 200 neuropeptides and have confirmed the sequences of 100 peptides. This study lays the groundwork for future molecular studies of Apis neuropeptides with the identification of 36 genes, 33 of which were previously unreported.

Genomic dissection of behavioral maturation in the honey bee

Honey bees undergo an age-related, socially regulated transition from working in the hive to foraging that has been previously associated with changes in the expression of thousands of genes in the brain. To understand the meaning of these changes, we conducted microarray analyses to examine the following: (i) the ontogeny of gene expression preceding the onset of foraging, (ii) the effects of physiological and genetic factors that influence this behavioral transition, and (iii) the effects of foraging experience. Although >85% of approximately 5,500 genes showed brain differences, principal component analysis revealed discrete influences of age, behavior, genotype, environment, and experience. Young bees not yet competent to forage showed extensive, age-related expression changes, essentially complete by 8 days of age, coinciding with previously described structural brain changes. Subsequent changes were not age-related but were largely related to effects of juvenile hormone (JH), suggesting that the increase in JH that influences the hive bee-forager transition may cause many of these changes. Other treatments that also influence the onset age of foraging induced many changes but with little overlap, suggesting that multiple pathways affect behavioral maturation. Subspecies differences in onset age of foraging were correlated with differences in JH and JH-target gene expression, suggesting that this endocrine system mediates the genetic differences. We also used this multifactorial approach to identify candidate genes for behavioral maturation. This successful dissection of gene expression indicates that, for social behavior, gene expression in the brain can provide a robust indicator of the interaction between hereditary and environmental information.

Nuclear receptors of the honey bee: annotation and expression in the adult brain

The Drosophila genome encodes 18 canonical nuclear receptors. All of the Drosophila nuclear receptors are here shown to be present in the genome of the honey bee (Apis mellifera). Given that the time since divergence of the Drosophila and Apis lineages is measured in hundreds of millions of years, the identification of matched orthologous nuclear receptors in the two genomes reveals the fundamental set of nuclear receptors required to 'make' an endopterygote insect. The single novelty is the presence in the A. mellifera genome of a third insect gene similar to vertebrate photoreceptor-specific nuclear receptor (PNR). Phylogenetic analysis indicates that this novel gene, which we have named AmPNR-like, is a new member of the NR2 subfamily not found in the Drosophila or human genomes. This gene is expressed in the developing compound eye of the honey bee. Like their vertebrate counterparts, arthropod nuclear receptors play key roles in embryonic and postembryonic development. Studies in Drosophila have focused primarily on the role of these transcription factors in embryogenesis and metamorphosis. Examination of an expressed sequence tag library developed from the adult bee brain and analysis of transcript expression in brain using in situ hybridization and quantitative RT-PCR revealed that several members of the nuclear receptor family (AmSVP, AmUSP, AmERR, AmHr46, AmFtz-F1, and AmHnf-4) are expressed in the brain of the adult bee. Further analysis of the expression of AmUSP and AmSVP in the mushroom bodies, the major insect brain centre for learning and memory, revealed changes in transcript abundance and, in the case of AmUSP, changes in transcript localization, during the development of foraging behaviour in the adult. Study of the honey bee therefore provides a model for understanding nuclear receptor function in the adult brain.

Genes of the antioxidant system of the honey bee: annotation and phylogeny

Antioxidant enzymes perform a variety of vital functions including the reduction of life-shortening oxidative damage. We used the honey bee genome sequence to identify the major components of the honey bee antioxidant system. A comparative analysis of honey bee with Drosophila melanogaster and Anopheles gambiae shows that although the basic components of the antioxidant system are conserved, there are important species differences in the number of paralogs. These include the duplication of thioredoxin reductase and the expansion of the thioredoxin family in fly; lack of expansion of the Theta, Delta and Omega GST classes in bee and no expansion of the Sigma class in dipteran species. The differential expansion of antioxidant gene families among honey bees and dipteran species might reflect the marked differences in life history and ecological niches between social and solitary species.

Carbohydrate metabolism genes and pathways in insects: insights from the honey bee genome

Carbohydrate-metabolizing enzymes may have particularly interesting roles in the honey bee, Apis mellifera, because this social insect has an extremely carbohydrate-rich diet, and nutrition plays important roles in caste determination and socially mediated behavioural plasticity. We annotated a total of 174 genes encoding carbohydrate-metabolizing enzymes and 28 genes encoding lipid-metabolizing enzymes, based on orthology to their counterparts in the fly, Drosophila melanogaster, and the mosquito, Anopheles gambiae. We found that the number of genes for carbohydrate metabolism appears to be more evolutionarily labile than for lipid metabolism. In particular, we identified striking changes in gene number or genomic organization for genes encoding glycolytic enzymes, cellulase, glucose oxidase and glucose dehydrogenases, glucose-methanol-choline (GMC) oxidoreductases, fucosyltransferases, and lysozymes.

Semiparametric approach to characterize unique gene expression trajectories across time

Background:

A semiparametric approach was used to identify groups of cDNAs and genes with distinct expression profiles across time and overcome the limitations of clustering to identify groups. The semiparametric approach allows the generalization of mixtures of distributions while making no specific parametric assumptions about the distribution of the hidden heterogeneity of the cDNAs. The semiparametric approach was applied to study gene expression in the brains of Apis mellifera ligustica honey bees raised in two colonies (A. m. mellifera and ligustica) with consistent patterns across five maturation ages.

Results:

The semiparametric approach provided unambiguous criteria to detect groups of genes, trajectories and probability of gene membership to groups. The semiparametric results were cross-validated in both colony data sets. Gene Ontology analysis enhanced by genome annotation helped to confirm the semiparametric results and revealed that most genes with similar or related neurobiological function were assigned to the same group or groups with similar trajectories. Ten groups of genes were identified and nine groups had highly similar trajectories in both data sets. Differences in the trajectory of the reminder group were consistent with reports of accelerated maturation in ligustica colonies compared to mellifera colonies.

Conclusion:

The combination of microarray technology, genomic information and semiparametric analysis provided insights into the genomic plasticity and gene networks linked to behavioral maturation in the honey bee.

Nutritional status influences socially regulated foraging ontogeny in honey bees

In many social insects, including honey bees, worker energy reserve levels are correlated with task performance in the colony. Honey bee nest workers have abundant stored lipid and protein while foragers are depleted of these reserves; this depletion precedes the shift from nest work to foraging. The first objective of this study was to test the hypothesis that lipid depletion has a causal effect on the age at onset of foraging in honey bees (Apis mellifera L.). We found that bees treated with a fatty acid synthesis inhibitor (TOFA) were more likely to forage precociously. The second objective of this study was to determine whether there is a relationship between social interactions, nutritional state and behavioral maturation. Since older bees are known to inhibit the development of young bees into foragers, we asked whether this effect is mediated nutritionally via the passage of food from old to young bees. We found that bees reared in social isolation have low lipid stores, but social inhibition occurs in colonies in the field, whether young bees are starved or fed. These results indicate that although social interactions affect the nutritional status of young bees, social and nutritional factors act independently to influence age at onset of foraging. Our findings suggest that mechanisms linking internal nutritional physiology to foraging in solitary insects have been co-opted to regulate altruistic foraging in a social context.

Division of labor in the honey bee (Apis mellifera): the role of tyramine β-hydroxylase

The biogenic amine octopamine (OA) is involved in the regulation of honey bee behavioral development; brain levels are higher in foragers than bees working in the hive, especially in the antennal lobes, and treatment causes precocious foraging. We measured brain mRNA and protein activity of tyramineβ-hydroxylase (T βh), an enzyme vital for OA synthesis, in order to begin testing the hypothesis that this enzyme is responsible for the rising levels of OA during honey bee behavioral development. Brain OA levels were greater in forager bees than in bees engaged in brood care, as in previous studies, but T βh activity was not correlated with bee behavior. Tβh mRNA levels, however, did closely track OA levels during behavioral development, and T βh mRNA was localized to previously identified octopaminergic neurons in the bee brain. Our results show that the transcription of this neurotransmitter synthetic enzyme is associated with regulation of social behavior in honey bees, but other factors may be involved.

Stimulation of muscarinic receptors mimics experience-dependent plasticity in the honey bee brain

Honey bees begin life working in the hive. At approximately 3 weeks of age, they shift to visiting flowers to forage for pollen and nectar. Foraging is a complex task associated with enlargement of the mushroom bodies, a brain region important in insects for certain forms of learning and memory. We report here that foraging bees had a larger volume of mushroom body neuropil than did age-matched bees confined to the hive. This result indicates that direct experience of the world outside the hive causes mushroom body neuropil growth in bees. We also show that oral treatment of caged bees with pilocarpine, a muscarinic agonist, induced an increase in the volume of the neuropil similar to that seen after a week of foraging experience. Effects of pilocarpine were blocked by scopolamine, a muscarinic antagonist. Our results suggest that signaling in cholinergic pathways couples experience to structural brain plasticity.

Gene expression patterns associated with queen honey bee longevity

The oxidative stress theory of aging proposes that accumulation of oxidative damage is the main proximate cause of aging and that lifespan is determined by the rate at which this damage occurs. Two predictions from this theory are that long-lived organisms produce fewer ROS or have increased antioxidant production. Based in these predictions, molecular mechanisms to promote longevity could include either changes in the regulation of mitochondrial genes that affect ROS production or elevated expression of antioxidant genes. We explored these possibilities in the honey bee, a good model for the study of aging because it has a caste system in which the same genome produces both a long-lived queen and a short-lived worker. We measured mRNA levels for genes encoding eight of the most prominent antioxidant enzymes and five mitochondrial proteins involved in respiration. The expression of antioxidant genes generally decreased with age in queens, but not in workers. Expression of most mitochondrial genes, in particular CytC, was higher in young queens, but these genes showed a faster age-related decline relative to workers. One exception to this trend was COX-I in thorax. This resulted in higher COX-I/CytC ratios in old queens compared to old workers, which suggests caste-specific differences in mitochondrial function that might be related to the caste-specific differences in longevity. Queen honey bee longevity appears to have evolved via mechanisms other than increased antioxidant gene expression.

Selective modulation of task performance by octopamine in honey bee (Apis mellifera) division of labour

Octopamine treatment has previously been shown to increase honey bee foraging behaviour. We determined the effects of octopamine on other tasks to learn how octopamine affects division of labour in honey bee colonies. Octopamine treatment did not increase the rate of corpse removal from the hive, suggesting that elevated brain levels of octopamine do not act to increase the performance of all flight-related tasks. Octopamine treatment also did not increase attendance in the queen's retinue, suggesting that elevated brain levels of octopamine do not act to increase responsiveness to all olfactory stimuli. Consistent with these findings, octopamine treatment enhanced the foraging response to brood pheromone but not the cell capping response, a component of brood care. These results demonstrate a relatively specific form of neuromodulation by octopamine in the regulation of division of labour in honey bee colonies.

Sociogenomics: social life in molecular terms

Spectacular progress in molecular biology, genome-sequencing projects and genomics makes this an appropriate time to attempt a comprehensive understanding of the molecular basis of social life. Promising results have already been obtained in identifying genes that influence animal social behaviour and genes that are implicated in social evolution. These findings - derived from an eclectic mix of species that show varying levels of sociality - provide the foundation for the integration of molecular biology, genomics, neuroscience, behavioural biology and evolutionary biology that is necessary for this endeavour.

Behavior and the limits of genomic plasticity: power and replicability in microarray analysis of honeybee brains

Transcription is slow relative to many post-transcriptional processes in the brain. Using the rich system of division of labor in the honeybee (Apis mellifera), we found extreme differences in the extent to which behavioral occupations of different durations were associated with gene-expression differences in the brain. Nursing and foraging, occupations lasting > 1 week, were associated with significant expression differences for nearly one-quarter of the genes tested (1208 of 5563 cDNAs tested; P < 0.01, anova), consistent with previous results. In contrast, transitional occupations, performed for 1-2 days after nursing and before the onset of foraging, were associated with either no differences (guards vs. undertakers; 19 cDNAs, fewer than the expectation of 56 false-positives) or few differences (comb builders vs. guards and undertakers; 248 cDNAs), but extensive differences relative to both nursing and foraging (> 500 cDNAs, all contrasts). Statistical power analysis indicated that expression differences of two-, 1.5- and 1.25-fold should have been detected in 100, 92 and 37% of cases, respectively. Replication of previous results at these magnitudes was 95, 71 and 51%, with no genes showing differences in the opposite direction. These results indicate that behavioral plasticity over different time-scales may be associated with substantial differences in the extent of genomic plasticity in the brain.

Candidate genes for behavioural ecology

In spite of millions of years of evolutionary divergence, the conservation of gene function is common across distant lineages. As such, genes that are known to influence behaviour in one organism are likely to influence similar behaviours in other organisms. Recent studies of the evolution of behaviour and morphological adaptation support this notion. Thus, the candidate gene approach offers great potential to expand our understanding of behavioural ecology. Changes in the expression of candidate genes can reveal their contribution to behavioural variation and/or phenotypic plasticity. Knowledge of gene function also enables experimental manipulation of behaviour in the lab and in the field. The candidate gene approach provides an accessible and useful tool for generating insights about animals that are not typically associated with genetic experimentation.

Phenotypic deconstruction reveals involvement of manganese transporter malvolio in honey bee division of labor

Molecular analysis of a complex behavioral phenotype is facilitated by dissecting it into simpler behavioral components. Using this approach, we present evidence implicating increased manganese transport by the malvolio (mvl) gene into brain cells as one factor that influences age-related division of labor in honey bee colonies. We studied mvl because manganese affects sucrose responsiveness in Drosophila melanogaster, and sucrose responsiveness is related to division of labor in honey bee colonies. Honey bee foragers are more responsive to sucrose in the laboratory than are younger nurse bees, and pollen foragers are more responsive to sucrose than nectar foragers. Levels of mvl mRNA in the brain and manganese in the head were higher in pollen foragers compared with nurses, with nectar foragers intermediate. Manganese treatment increased honey bee sucrose responsiveness and caused precocious foraging. Manganese levels showed a similar pattern to mvl mRNA but manganese treatment did not increase pollen foraging. These results suggest that, while there are molecular pathways common to sucrose responsiveness and division of labor, linkages between a complex behavior and some of its simpler behavioral components are not obligatory. Together with previous findings, these results support the idea that some feeding-related genes in Drosophila have been used in social evolution to regulate division of labor.