Laboratory Assay of Brood Care for Quantitative Analyses of Individual Differences in Honey Bee (Apis mellifera) Affiliative Behavior

The effect of Israeli acute paralysis virus infection on honey bee brood care behavior

To protect themselves from communicable diseases, social insects utilize social immunity-behavioral, physiological, and organizational means to combat disease transmission and severity. Within a honey bee colony, larvae are visited thousands of times by nurse bees, representing a prime environment for pathogen transmission. We investigated a potential social immune response to Israeli acute paralysis virus (IAPV) infection in brood care, testing the hypotheses that bees will respond with behaviors that result in reduced brood care, or that infection results in elevated brood care as a virus-driven mechanism to increase transmission. We tested for group-level effects by comparing three different social environments in which 0%, 50%, or 100% of nurse bees were experimentally infected with IAPV. We investigated individual-level effects by comparing exposed bees to unexposed bees within the mixed-exposure treatment group. We found no evidence for a social immune response at the group level; however, individually, exposed bees interacted with the larva more frequently than their unexposed nestmates. While this could increase virus transmission from adults to larvae, it could also represent a hygienic response to increase grooming when an infection is detected. Together, our findings underline the complexity of disease dynamics in complex social animal systems.

Gut microbiota contribute to variations in honey bee foraging intensity

Gut microbiomes are increasingly recognized for mediating diverse biological aspects of their hosts, including complex behavioral phenotypes. Although many studies have reported that experimental disruptions to the gut microbial community result in atypical host behavior, studies that address how gut microbes contribute to adaptive behavioral trait variation are rare. Eusocial insects represent a powerful model to test this, because of their simple gut microbiota and complex division of labor characterized by colony-level variation in behavioral phenotypes. Although previous studies report correlational differences in gut microbial community associated with division of labor, here, we provide evidence that gut microbes play a causal role in defining differences in foraging behavior between European honey bees (Apis mellifera). We found that gut microbial community structure differed between hive-based nurse bees and bees that leave the hive to forage for floral resources. These differences were associated with variation in the abundance of individual microbes, including Bifidobacterium asteroides, Bombilactobacillus mellis, and Lactobacillus melliventris. Manipulations of colony demography and individual foraging experience suggested that differences in gut microbial community composition were associated with task experience. Moreover, single-microbe inoculations with B. asteroides, B. mellis, and L. melliventris caused effects on foraging intensity. These results demonstrate that gut microbes contribute to division of labor in a social insect, and support a role of gut microbes in modulating host behavioral trait variation.

Beyond conflict: Kinship theory of intragenomic conflict predicts individual variation in altruistic behaviour

Behavioural variation is essential for animals to adapt to different social and environmental conditions. The Kinship Theory of Intragenomic Conflict (KTIC) predicts that parent-specific alleles can support different behavioural strategies to maximize allele fitness. Previous studies, including in honey bees (Apis mellifera), supported predictions of the KTIC for parent-specific alleles to promote selfish behaviour. Here, we test the KTIC prediction that for altruism-promoting genes (i.e. those that promote behaviours that support the reproductive fitness of kin), the allele with the higher altruism optimum should be selected to be expressed while the other is silenced. In honey bee colonies, workers act altruistically when tending to the queen by performing a 'retinue' behaviour, distributing the queen's mandibular pheromone (QMP) throughout the hive. Workers exposed to QMP do not activate their ovaries, ensuring they care for the queen's brood instead of competing to lay unfertilized eggs. Due to the haplodiploid genetics of honey bees, the KTIC predicts that response to QMP is favoured by the maternal genome. We report evidence for parent-of-origin effects on the retinue response behaviour, ovarian development and gene expression in brains of worker honey bees exposed to QMP, consistent with the KTIC. Additionally, we show enrichment for genes with parent-of-origin expression bias within gene regulatory networks associated with variation in bees' response to QMP. Our study demonstrates that intragenomic conflict can shape diverse social behaviours and influence expression patterns of single genes as well as gene networks.

Effects of pesticide-adjuvant combinations used in almond orchards on olfactory responses to social signals in honey bees (Apis mellifera)

Exposure to agrochemical sprays containing pesticides and tank-mix adjuvants has been implicated in post-bloom mortality, particularly of brood, in honey bee colonies brought into California almond orchards for pollination. Although adjuvants are generally considered to be biologically inert, some adjuvants have exhibited toxicity and sublethal effects, including decreasing survival rates of next-generation queens. Honey bees have a highly developed olfactory system to detect and discriminate among social signals. To investigate the impact of pesticide-adjuvant combinations on honey bee signal perception, we performed electroantennography assays to assess alterations in their olfactory responsiveness to the brood ester pheromone (BEP), the volatile larval pheromone β-ocimene, and the alarm pheromone 2-heptanone. These assays aimed to uncover potential mechanisms underlying changes in social behaviors and reduced brood survival after pesticide exposure. We found that combining the adjuvant Dyne-Amic with the fungicide Tilt (propiconazole) and the insecticide Altacor (chlorantraniliprole) synergistically enhanced olfactory responses to three concentrations of BEP and as well exerted dampening and compensatory effects on responses to 2-heptanone and β-ocimene, respectively. In contrast, exposure to adjuvant alone or the combination of fungicide and insecticide had no effect on olfactory responses to BEP at most concentrations but altered responses to β-ocimene and 2-heptanone. Exposure to Dyne-Amic, Altacor, and Tilt increased BEP signal amplitude, indicating potential changes in olfactory receptor sensitivity or sensilla permeability to odorants. Given that, in a previous study, next-generation queens raised by nurses exposed to the same treated pollen experienced reduced survival, these new findings highlight the potential disruption of social signaling in honey bees and its implications for colony reproductive success.

Context-dependent influence of threat on honey bee social network dynamics and brain gene expression

Adverse social experience affects social structure by modifying the behavior of individuals, but the relationship between an individual's behavioral state and its response to adversity is poorly understood. We leveraged naturally occurring division of labor in honey bees and studied the biological embedding of environmental threat using laboratory assays and automated behavioral tracking of whole colonies. Guard bees showed low intrinsic levels of sociability compared with foragers and nurse bees, but large increases in sociability following exposure to a threat. Threat experience also modified the expression of caregiving-related genes in a brain region called the mushroom bodies. These results demonstrate that the biological embedding of environmental experience depends on an individual's societal role and, in turn, affects its future sociability.

The Behavioral Toxicity of Insect Growth Disruptors on Apis mellifera Queen Care

As social insects, honey bees (Apis mellifera) rely on the coordinated performance of various behaviors to ensure that the needs of the colony are met. One of the most critical of these behaviors is the feeding and care of egg laying honey bee queens by non-fecund female worker attendants. These behaviors are crucial to honey bee reproduction and are known to be elicited by the queen’s pheromone blend. The degree to which workers respond to this blend can vary depending on their physiological status, but little is known regarding the impacts of developmental exposure to agrochemicals on this behavior. This work investigated how exposing workers during larval development to chronic sublethal doses of insect growth disruptors affected their development time, weight, longevity, and queen pheromone responsiveness as adult worker honey bees. Exposure to the juvenile hormone analog pyriproxyfen consistently shortened the duration of pupation, and pyriproxyfen and diflubenzuron inconsistently reduced the survivorship of adult bees. Finally, pyriproxyfen and methoxyfenozide treated bees were found to be less responsive to queen pheromone relative to other treatment groups. Here, we describe these results and discuss their possible physiological underpinnings as well as their potential impacts on honey bee reproduction and colony performance.

Neural and Molecular Mechanisms of Biological Embedding of Social Interactions

Animals operate in complex environments, and salient social information is encoded in the nervous system and then processed to initiate adaptive behavior. This encoding involves biological embedding, the process by which social experience affects the brain to influence future behavior. Biological embedding is an important conceptual framework for understanding social decision-making in the brain, as it encompasses multiple levels of organization that regulate how information is encoded and used to modify behavior. The framework we emphasize here is that social stimuli provoke short-term changes in neural activity that lead to changes in gene expression on longer timescales. This process, simplified-neurons are for today and genes are for tomorrow-enables the assessment of the valence of a social interaction, an appropriate and rapid response, and subsequent modification of neural circuitry to change future behavioral inclinations in anticipation of environmental changes. We review recent research on the neural and molecular basis of biological embedding in the context of social interactions, with a special focus on the honeybee.

Chronic within-hive video recordings detect altered nursing behaviour and retarded larval development of neonicotinoid treated honey bees

Risk evaluations for agricultural chemicals are necessary to preserve healthy populations of honey bee colonies. Field studies on whole colonies are limited in behavioural research, while results from lab studies allow only restricted conclusions on whole colony impacts. Methods for automated long-term investigations of behaviours within comb cells, such as brood care, were hitherto missing. In the present study, we demonstrate an innovative video method that enables within-cell analysis in honey bee (Apis mellifera) observation hives to detect chronic sublethal neonicotinoid effects of clothianidin (1 and 10 ppb) and thiacloprid (200 ppb) on worker behaviour and development. In May and June, colonies which were fed 10 ppb clothianidin and 200 ppb thiacloprid in syrup over three weeks showed reduced feeding visits and duration throughout various larval development days (LDDs). On LDD 6 (capping day) total feeding duration did not differ between treatments. Behavioural adaptation was exhibited by nurses in the treatment groups in response to retarded larval development by increasing the overall feeding timespan. Using our machine learning algorithm, we demonstrate a novel method for detecting behaviours in an intact hive that can be applied in a versatile manner to conduct impact analyses of chemicals, pests and other stressors.

Honey bee virus causes context-dependent changes in host social behavior

Anthropogenic changes create evolutionarily novel environments that present opportunities for emerging diseases, potentially changing the balance between host and pathogen. Honey bees provide essential pollination services, but intensification and globalization of honey bee management has coincided with increased pathogen pressure, primarily due to a parasitic mite/virus complex. Here, we investigated how honey bee individual and group phenotypes are altered by a virus of concern, Israeli acute paralysis virus (IAPV). Using automated and manual behavioral monitoring of IAPV-inoculated individuals, we find evidence for pathogen manipulation of worker behavior by IAPV, and reveal that this effect depends on social context; that is, within versus between colony interactions. Experimental inoculation reduced social contacts between honey bee colony members, suggesting an adaptive host social immune response to diminish transmission. Parallel analyses with double-stranded RNA (dsRNA)-immunostimulated bees revealed these behaviors are part of a generalized social immune defensive response. Conversely, inoculated bees presented to groups of bees from other colonies experienced reduced aggression compared with dsRNA-immunostimulated bees, facilitating entry into susceptible colonies. This reduction was associated with a shift in cuticular hydrocarbons, the chemical signatures used by bees to discriminate colony members from intruders. These responses were specific to IAPV infection, suggestive of pathogen manipulation of the host. Emerging bee pathogens may thus shape host phenotypes to increase transmission, a strategy especially well-suited to the unnaturally high colony densities of modern apiculture. These findings demonstrate how anthropogenic changes could affect arms races between human-managed hosts and their pathogens to potentially affect global food security.

Valence of social information is encoded in different subpopulations of mushroom body Kenyon cells in the honeybee brain

Over 600 Myr of evolutionary divergence between vertebrates and invertebrates is associated with considerable neuroanatomical variation both across and within these lineages. By contrast, valence encoding is an important behavioural trait that is evolutionarily conserved across vertebrates and invertebrates, and enables individuals to distinguish between positive (potentially beneficial) and negative (potentially harmful) situations. We tested the hypothesis that social interactions of positive and negative valence are modularly encoded in the honeybee brain (i.e. encoded in different cellular subpopulations) as in vertebrate brains. In vertebrates, neural activation patterns are distributed across distinct parts of the brain, suggesting that discrete circuits encode positive or negative stimuli. Evidence for this hypothesis would suggest a deep homology of neural organization between insects and vertebrates for valence encoding, despite vastly different brain sizes. Alternatively, overlapping localization of valenced social information in the brain would imply a 're-use' of circuitry in response to positive and negative social contexts, potentially to overcome the energetic constraints of a tiny brain. We used immediate early gene expression to map positively and negatively valenced social interactions in the brain of the western honeybee Apis mellifera. We found that the valence of a social signal is represented by distinct anatomical subregions of the mushroom bodies, an invertebrate sensory neuropil associated with social behaviour, multimodal sensory integration, learning and memory. Our results suggest that the modularization of valenced social information in the brain is a fundamental property of neuroanatomical organization.

Division of labor in honey bees is associated with transcriptional regulatory plasticity in the brain

Studies in evolutionary and developmental biology show that relationships between transcription factors (TFs) and their target genes can be altered to result in novel regulatory relationships that generate phenotypic plasticity. We hypothesized that context-dependent shifts in the nervous system associated with behavior may also be linked to changes in TF-target relationships over physiological time scales. We tested this hypothesis using honey bee (Apis mellifera) division of labor as a model system by performing bioinformatic analyses of previously published brain transcriptomic profiles together with new RNAi and behavioral experiments. The bioinformatic analyses identified five TFs that exhibited strong signatures of regulatory plasticity as a function of division of labor. RNAi targeting of one of these TFs (broad complex) and a related TF that did not exhibit plasticity (fushi tarazu transcription factor 1) was administered in conjunction with automated analyses of foraging behavior in the field, laboratory assays of aggression and brood care behavior, and endocrine treatments. The results showed that changes in the regulatory relationships of these TFs were associated with behavioral state, social context and endocrine state. These findings provide the first empirical evidence that TF-target relationships in the brain are altered in conjunction with behavior and social context. They also suggest that one mechanism for this plasticity involves pleiotropic TFs high up in regulatory hierarchies producing behavior-specific transcriptional responses by activating different downstream TFs to induce discrete context-dependent transcriptional cascades. These findings provide new insights into the dynamic nature of the transcriptional regulatory architecture underlying behavior in the brain.

Genetic accommodation and the role of ancestral plasticity in the evolution of insect eusociality

For over a century, biologists have proposed a role for phenotypic plasticity in evolution, providing an avenue for adaptation in addition to 'mutation-first' models of evolutionary change. According to the various versions of this idea, the ability of organisms to respond adaptively to their environment through phenotypic plasticity may lead to novel phenotypes that can be screened by natural selection. If these initially environmentally induced phenotypes increase fitness, then genetic accommodation can lead to allele frequency change, influencing the expression of those phenotypes. Despite the long history of 'plasticity-first' models, the importance of genetic accommodation in shaping evolutionary change has remained controversial - it is neither fully embraced nor completely discarded by most evolutionary biologists. We suggest that the lack of acceptance of genetic accommodation in some cases is related to a lack of information on its molecular mechanisms. However, recent reports of epigenetic transgenerational inheritance now provide a plausible mechanism through which genetic accommodation may act, and we review this research here. We also discuss current evidence supporting a role for genetic accommodation in the evolution of eusociality in social insects, which have long been models for studying the influence of the environment on phenotypic variation, and may be particularly good models for testing hypotheses related to genetic accommodation. Finally, we introduce 'eusocial engineering', a method by which novel social phenotypes are first induced by environmental modification and then studied mechanistically to understand how environmentally induced plasticity may lead to heritable changes in social behavior. We believe the time is right to incorporate genetic accommodation into models of the evolution of complex traits, armed with new molecular tools and a better understanding of non-genetic heritable elements.

Honey bee neurogenomic responses to affiliative and agonistic social interactions

Social interactions can be divided into two categories, affiliative and agonistic. How neurogenomic responses reflect these opposing valences is a central question in the biological embedding of experience. To address this question, we exposed honey bees to a queen larva, which evokes nursing, an affiliative alloparenting interaction, and measured the transcriptomic response of the mushroom body brain region at different times after exposure. Hundreds of genes were differentially expressed at distinct time points, revealing a dynamic temporal patterning of the response. Comparing these results to our previously published research on agonistic aggressive interactions, we found both shared and unique transcriptomic responses to each interaction. The commonly responding gene set was enriched for nuclear receptor signaling, the set specific to nursing was enriched for olfaction and neuron differentiation, and the set enriched for aggression was enriched for cytoskeleton, metabolism, and chromosome organization. Whole brain histone profiling after the affiliative interaction revealed few changes in chromatin accessibility, suggesting that the transcriptomic changes derive from already accessible areas of the genome. Although only one stimulus of each type was studied, we suggest that elements of the observed transcriptomic responses reflect molecular encoding of stimulus valence, thus priming individuals for future encounters. This hypothesis is supported by behavioral analyses showing that bees responding to either the affiliative or agonistic stimulus exhibited a higher probability of repeating the same behavior but a lower probability of performing the opposite behavior. These findings add to our understanding of the biological embedding at the molecular level.

Deep evolutionary conservation of autism-related genes

E. O. Wilson proposed in Sociobiology that similarities between human and animal societies reflect common mechanistic and evolutionary roots. When introduced in 1975, this controversial hypothesis was beyond science's ability to test. We used genomic analyses to determine whether superficial behavioral similarities in humans and the highly social honey bee reflect common molecular mechanisms. Here, we report that gene expression signatures for individual bees unresponsive to various salient social stimuli are significantly enriched for autism spectrum disorder-related genes. These signatures occur in the mushroom bodies, a high-level integration center of the insect brain. Furthermore, our finding of enrichment was unique to autism spectrum disorders; brain gene expression signatures from other honey bee behaviors do not show this enrichment, nor do datasets from other human behavioral and health conditions. These results demonstrate deep conservation for genes associated with a human social pathology and individual differences in insect social behavior, thus providing an example of how comparative genomics can be used to test sociobiological theory.