Social Immunity Research Articles

Two pathogens change cuticular hydrocarbon profiles but neither elicit a social behavioural change in infected honey bees, Apis mellifera (Apidae: Hymenoptera)

AbstractSocial immunity is the cooperation of individual group members towards the control of disease transmission arising from social living. It has been identified as an important mechanism of natural disease control in honey bees, Apis mellifera Linnaeus, which can exhibit hygienic behaviours such as evicting pathogen‐infected or parasitised nest mates. Here, we examine if these bees exhibit a change in behaviour towards nest mates infected by the microsporidian pathogens Nosema apis and N. ceranae. We observed no significant change in the frequency of interactive and non‐interactive behaviours of bees towards Nosema‐infected nest mates, for both N. apis and N. ceranae. Thus, uninfected bees interacted and were exposed to pathogen‐infected bees equally often compared with uninfected bees. This lack of any behavioural modification suggests that bees are unable to recognise Nosema‐infected nest mates. These results were surprising given our analysis indicated that Nosema infection changes the hydrocarbon profiles of the bees, enabling the statistical discrimination of uninfected, N. apis and N. ceranae infected bees. The hydrocarbons we analysed were 16 straight‐chain alkanes ranging from heptadecane (C17H36) to tritriacontane (C33H68). Our results and those of others suggest that hygienic behaviour is currently unlikely to provide resistance towards Nosema pathogens in honey bees. The lack of social immunity was unexpected given the co‐evolution of one of the pathogens and honey bees. However, the modification of cuticular hydrocarbons by these pathogens provides a specific cue that may enable future selection and rearing of bees for Nosema‐related social immunity.

Fungi That Infect Insects: Altering Host Behavior and Beyond.

Approximately 1,000 species of the fungal phyla Microsporidia, Chytridiomycota, Entomophthoromycota (order: Entomophthorales), Basidiomycota, and Ascomycota are known to infect and kill insects [1]. Of these, species such as Beauveria bassiana and Metarhizium robertsii (Ascomycota: Hypocreales) are well-studied models for exploring the mechanisms of fungus–insect physiological interactions, and they are used as biological controls for insect pests [2]. Entomopathogenic Hypocreales are phylogenetically closely related to plant pathogens and endophytes [3], and their sexual stages belong to Cordyceps sensu lato [4]. However, these species are taxonomically diverse and differ from each other considerably in their genomic features [5–8] and, consequently, their host ranges, infection cycles, and life strategies. Fungal infection normally, but not always, results in the death of an insect in situ. The parasitic fungi such as the host-specific pathogen Ophiocordyceps unilateralis sensu lato can control insect brains and manipulate their behavior to reach death locations that are optimal for spore dispersal, the so-called fungal extended phenotype [9]. On the other hand, on top of physiological immune surveillance, insects (especially social insects such as ants and termites) can smell and avoid fungal pathogens, groom each other to clear pathogenic spores, generate a fever response, or die well away from their nestmates, which is termed behavioral or social immunity [10,11]. Either side of these interactions will benefit from behavioral changes in insects that maximize their adaptive fitness. In this paper, both types of insect behavior alternations are reviewed, and the underlying mechanisms are discussed.

Acid, silk and grooming: alternative strategies in social immunity in ants?

Parasites are an important force in evolution, driving the need for costly resistance mechanisms. The threat from disease is potentially high in group-living species such as social insects, which have accordingly evolved behavioural and chemical defences that vary between species depending on their life histories. Several ant genera have lost a key exocrine antimicrobial defence, the metapleural gland, and yet are still able to thrive in environments abundant with parasites. We investigate, in species lacking the metapleural gland, how the production of antimicrobial venom, grooming behaviours and the use of potentially antimicrobial larval silk may have evolved as alternative antiparasite defences. We focus on the Australasian weaver ant Oecophylla smaragdina and compare this to Polyrhachis weaver ants. We show that the production of venom by O. smaragdina workers is important for disease resistance but that the presence of larval silk is not, and that workers use their acidic venom to maintain nest hygiene. The grooming defences of O. smaragdina differ between castes, with minor workers allogrooming more and major workers showing greater upregulation of grooming in response to parasites. Chemical and behavioural defences differ interspecifically between O. smaragdina and Polyrhachis, with O. smaragdina appearing to rely primarily on its venom while Polyrhachis use higher rates of grooming. The results show how alternative investment strategies can evolve for disease defence, notably the highly effective application of acidic venom by O. smaragdina, and highlight the need for targeted comparative studies to understand how organisms respond to the ubiquitous threat from parasites.

Social immunity and the evolution of group living in insects.

The evolution of group living requires that individuals limit the inherent risks of parasite infection. To this end, group living insects have developed a unique capability of mounting collective anti-parasite defences, such as allogrooming and corpse removal from the nest. Over the last 20 years, this phenomenon (called social immunity) was mostly studied in eusocial insects, with results emphasizing its importance in derived social systems. However, the role of social immunity in the early evolution of group living remains unclear. Here, I investigate this topic by first presenting the definitions of social immunity and discussing their applications across social systems. I then provide an up-to-date appraisal of the collective and individual mechanisms of social immunity described in eusocial insects and show that they have counterparts in non-eusocial species and even solitary species. Finally, I review evidence demonstrating that the increased risks of parasite infection in group living species may both decrease and increase the level of personal immunity, and discuss how the expression of social immunity could drive these opposite effects. By highlighting similarities and differences of social immunity across social systems, this review emphasizes the potential importance of this phenomenon in the early evolution of the multiple forms of group living in insects.

Social instability and immunity in rhesus monkeys: the role of the sympathetic nervous system

Social instability can adversely affect endocrine, immune and health outcomes, and recent evidence suggests that the sympathetic nervous system (SNS) might mediate these effects. We conducted two studies with adult male rhesus monkeys (Macaca mulatta) to understand how social conditions affect measures of SNS activity and immune function. In Experiment 1, animals were socialized in stable social conditions, then were switched to unstable (stressful) social conditions, then were returned to stable conditions. Analysis revealed quadratic effects for measures of behaviour, urinary metabolites of epinephrine and norepinephrine, and expression of immune response genes: as expected, social instability adversely impacted most measures, and the effects remediated upon re-imposition of stable conditions. Cortisol levels were unaffected. In Experiment 2, we used the sympathomimetic drug methamphetamine to challenge the SNS; animals also underwent socialization in stable or unstable groups. Surprisingly, while methamphetamine elevated plasma catecholamines, responses in lymph nodes tracked the social, and not the drug, condition: social instability upregulated the density of SNS fibres in lymph nodes and downregulated Type I interferon gene expression. Together, these results indicate that the SNS is extremely sensitive to social conditions; full understanding of the adverse effects of social instability on health should therefore incorporate measures of this health-relevant system.

Emergency measures: Adaptive response to pathogen intrusion in the ant nest

Ants have developed prophylactic and hygienic behaviours in order to limit risks of pathogenic outbreaks inside their nest, which are often called social immunity. Here, we test whether ants can adapt the “social immune response” to the level of pathogenic risk in the colony. We challenged Myrmica rubra colonies with dead nestmates that had either died from being frozen or from infection by the fungus Metarhizium anisopliae. Ant survival was compromised by the presence of the fungus-bearing corpses: workers died faster with a significantly lower survival from the 4th day compared to workers challenged with freeze-killed corpses. When faced with fungus-bearing corpses, workers responded quickly by increasing hygienic behaviours: they spent more time cleaning the nest, moving the corpses, and self-grooming. Ants in fungus-threatened colonies also decreased contact rates with other workers, and moved corpses further in the corners of the nest than in colonies in contact with non-infected corpses. These results show that ant colonies are able to assess the risk level associated with the presence of corpses in the nest, and adjust their investment in terms of hygienic behaviour.

Feces production as a form of social immunity in an insect with facultative maternal care.

BackgroundSocial animals have the unique capability of mounting social defenses against pathogens. Over the last decades, social immunity has been extensively studied in species with obligatory and permanent forms of social life. However, its occurrence in less derived social systems and thus its role in the early evolution of group-living remains unclear. Here, we investigated whether lining nests with feces is a form of social immunity against microbial growth in the European earwig Forficula auricularia, an insect with temporary family life and facultative maternal care.ResultsUsing a total of 415 inhibition zone assays, we showed that earwig feces inhibit the growth of two GRAM+ bacteria, two fungi, but not of a GRAM- bacteria. These inhibitions did not result from the consumed food or the nesting environment. We then demonstrated that the antimicrobial activity against fungus was higher in offspring than maternal feces, but that this difference was absent against bacteria. Finally, we showed that family interactions inhibited the antibacterial activity of maternal feces against one of the two GRAM+ bacteria, whereas it had no effect on the one of nymphal feces. By contrast, antifungal activities of the feces were independent of mother-offspring interactions.ConclusionThese results demonstrate that social immunity occurs in a species with simple and facultative social life, and thus shed light on the general importance of this process in the evolution of group-living. These results also emphasize that defecation can be under selection for other life-history traits than simple waste disposal.

Fungal disease dynamics in insect societies: optimal killing rates and the ambivalent effect of high social interaction rates.

Entomopathogenic fungi are potent biocontrol agents that are widely used against insect pests, many of which are social insects. Nevertheless, theoretical investigations of their particular life history are scarce. We develop a model that takes into account the main distinguishing features between traditionally studied diseases and obligate killing pathogens, like the (biocontrol-relevant) insect-pathogenic fungi Metarhizium and Beauveria. First, obligate killing entomopathogenic fungi produce new infectious particles (conidiospores) only after host death and not yet on the living host. Second, the killing rates of entomopathogenic fungi depend strongly on the initial exposure dosage, thus we explicitly consider the pathogen load of individual hosts. Further, we make the model applicable not only to solitary host species, but also to group living species by incorporating social interactions between hosts, like the collective disease defences of insect societies. Our results identify the optimal killing rate for the pathogen that minimises its invasion threshold. Furthermore, we find that the rate of contact between hosts has an ambivalent effect: dense interaction networks between individuals are considered to facilitate disease outbreaks because of increased pathogen transmission. In social insects, this is compensated by their collective disease defences, i.e., social immunity. For the type of pathogens considered here, we show that even without social immunity, high contact rates between live individuals dilute the pathogen in the host colony and hence can reduce individual pathogen loads below disease-causing levels.

A search for protein biomarkers links olfactory signal transduction to social immunity.

BackgroundThe Western honey bee (Apis mellifera L.) is a critical component of human agriculture through its pollination activities. For years, beekeepers have controlled deadly pathogens such as Paenibacillus larvae, Nosema spp. and Varroa destructor with antibiotics and pesticides but widespread chemical resistance is appearing and most beekeepers would prefer to eliminate or reduce the use of in-hive chemicals. While such treatments are likely to still be needed, an alternate management strategy is to identify and select bees with heritable traits that allow them to resist mites and diseases. Breeding such bees is difficult as the tests involved to identify disease-resistance are complicated, time-consuming, expensive and can misidentify desirable genotypes. Additionally, we do not yet fully understand the mechanisms behind social immunity. Here we have set out to discover the molecular mechanism behind hygienic behavior (HB), a trait known to confer disease resistance in bees.ResultsAfter confirming that HB could be selectively bred for, we correlated measurements of this behavior with protein expression over a period of three years, at two geographically distinct sites, using several hundred bee colonies. By correlating the expression patterns of individual proteins with HB scores, we identified seven putative biomarkers of HB that survived stringent control for multiple hypothesis testing. Intriguingly, these proteins were all involved in semiochemical sensing (odorant binding proteins), nerve signal transmission or signal decay, indicative of the series of events required to respond to an olfactory signal from dead or diseased larvae. We then used recombinant versions of two odorant-binding proteins to identify the classes of ligands that these proteins might be helping bees detect.ConclusionsOur data suggest that neurosensory detection of odors emitted by dead or diseased larvae is the likely mechanism behind a complex and important social immunity behavior that allows bees to co-exist with pathogens.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-014-1193-6) contains supplementary material, which is available to authorized users.

Social context affects immune gene expression in a subterranean termite

Social living may burden the immune system by increasing susceptibility to contagion. Animals may, however, evolve socially enabled defenses that limit their demographic vulnerability to disease. In this study we test for evidence of social immunity by exposing single versus grouped termites to a topical application of fungal spores, and measuring the transcriptional response at immune loci. We find that immune-related genes tend to be less responsive within infected groups, compared to the response of singly infected individuals. Two gram-negative binding protein genes (GNBP1, GNBP2) and a leucine-rich repeat immune gene are up-regulated upon infection of singletons, but are not disregulated upon grouped infection. A fourth locus (termicin) was, by contrast, disproportionately up-regulated in social groups, relative to singletons. These socially responsive patterns to immune gene expression in the Eastern subterranean termite suggests a real-time interplay between the individual immune response and the immediate social context.

Seasonal benefits of a natural propolis envelope to honey bee immunity and colony health.

Honey bees, as social insects, rely on collective behavioral defenses that produce a colony-level immune phenotype, or social immunity, which in turn impacts the immune response of individuals. One behavioral defense is the collection and deposition of antimicrobial plant resins, or propolis, in the nest. We tested the effect of a naturally constructed propolis envelope within standard beekeeping equipment on the pathogen and parasite load of large field colonies, and on immune system activity, virus and storage protein levels of individual bees over the course of a year. The main effect of the propolis envelope was a decreased and more uniform baseline expression of immune genes in bees during summer and autumn months each year, compared with the immune activity in bees with no propolis envelope in the colony. The most important function of the propolis envelope may be to modulate costly immune system activity. As no differences were found in levels of bacteria, pathogens and parasites between the treatment groups, the propolis envelope may act directly on the immune system, reducing the bees' need to activate the physiologically costly production of humoral immune responses. Colonies with a natural propolis envelope had increased colony strength and vitellogenin levels after surviving the winter in one of the two years of the study, despite the fact that the biological activity of the propolis diminished over the winter. A natural propolis envelope acts as an important antimicrobial layer enshrouding the colony, benefiting individual immunity and ultimately colony health.

Juvenile hormone downregulates vitellogenin production in Ectatomma tuberculatum (Hymenoptera: Formicidae) sterile workers.

In the ant Ectatomma tuberculatum (Olivier 1792), workers have active ovaries and lay trophic eggs that are eaten by the queen and larvae. Vitellogenins are the main proteins found in the eggs of insects and are the source of nutrients for the embryo in the fertilized eggs and for adults in the trophic eggs. In social insects, vitellogenin titres vary between castes and affect reproductive social status, nursing, foraging, longevity, somatic maintenance, and immunity. In most insects, vitellogenin synthesis is mainly regulated by juvenile hormone. However, in non-reproductive worker ants, this relationship is poorly characterized. This study determined the effects of juvenile hormone on vitellogenin synthesis in non-reproductive E. tuberculatum workers. Juvenile hormone was topically applied onto workers, and the effect on vitellogenin synthesis in the fat body and vitellogenin titres in the haemolymph were analysed by ELISA and qPCR. Juvenile hormone downregulated protein synthesis and reduced vitellogenin titres in the haemolymph, suggesting that in workers of E. tuberculatum, juvenile hormone loses its gonadotrophic function.

Regional variation in composition and antimicrobial activity of US propolis against Paenibacillus larvae and Ascosphaera apis

Propolis is a substance derived from antimicrobial plant resins that honey bees use in the construction of their nests. Propolis use in the hive is an important component of honey bee social immunity and confers a number of positive physiological benefits to bees. The benefits that bees derive from resins are mostly due to their antimicrobial properties, but it is unknown how the diversity of antimicrobial activities among resins might impact bee health. In our previous work, we found that resins from different North American Populus spp. differed in their ability to inhibit in vitro growth of the bee bacterial pathogen Paenibacillus larvae. The goal of our current work was to characterize the antimicrobial activity of propolis from 12 climatically diverse regions across the US against the bee pathogens P. larvae and Ascosphaeraapis and compare the metabolite profiles among those samples using LC–MS-based metabolomic methods. Samples differed greatly in their ability to inhibit both bacterial and fungal growth in vitro, but propolis from Nevada, Texas, and California displayed high activity against both pathogens. Interestingly, propolis from Georgia, New York, Louisiana, and Minnesota were active against A. apis, but not very active against P. larvae. Metabolomic analysis of regional propolis samples revealed that each sample was compositionally distinct, and LC–FTMS profiles from each sample contained a unique number of shared and exclusive peaks. Propolis from Aspen, CO, Tuscon, AZ, and Raleigh, NC, contained relatively large numbers of exclusive peaks, which may indicate that these samples originated from relatively unique botanical sources. This is the first study to characterize how the diversity of bee preferred resinous plants in the US may affect bee health, and could guide future studies on the therapeutic potential of propolis for bees.

Diversity of honey stores and their impact on pathogenic bacteria of the honeybee, Apis mellifera.

Honeybee colonies offer an excellent environment for microbial pathogen development. The highest virulent, colony killing, bacterial agents are Paenibacillus larvae causing American foulbrood (AFB), and European foulbrood (EFB) associated bacteria. Besides the innate immune defense, honeybees evolved behavioral defenses to combat infections. Foraging of antimicrobial plant compounds plays a key role for this “social immunity” behavior. Secondary plant metabolites in floral nectar are known for their antimicrobial effects. Yet, these compounds are highly plant specific, and the effects on bee health will depend on the floral origin of the honey produced. As worker bees not only feed themselves, but also the larvae and other colony members, honey is a prime candidate acting as self-medication agent in honeybee colonies to prevent or decrease infections. Here, we test eight AFB and EFB bacterial strains and the growth inhibitory activity of three honey types. Using a high-throughput cell growth assay, we show that all honeys have high growth inhibitory activity and the two monofloral honeys appeared to be strain specific. The specificity of the monofloral honeys and the strong antimicrobial potential of the polyfloral honey suggest that the diversity of honeys in the honey stores of a colony may be highly adaptive for its “social immunity” against the highly diverse suite of pathogens encountered in nature. This ecological diversity may therefore operate similar to the well-known effects of host genetic variance in the arms race between host and parasite.

No genetic tradeoffs between hygienic behaviour and individual innate immunity in the honey bee, Apis mellifera.

Many animals have individual and social mechanisms for combating pathogens. Animals may exhibit short-term physiological tradeoffs between social and individual immunity because the latter is often energetically costly. Genetic tradeoffs between these two traits can also occur if mutations that enhance social immunity diminish individual immunity, or vice versa. Physiological tradeoffs between individual and social immunity have been previously documented in insects, but there has been no study of genetic tradeoffs involving these traits. There is strong evidence that some genes influence both innate immunity and behaviour in social insects – a prerequisite for genetic tradeoffs. Quantifying genetic tradeoffs is critical for understanding the evolution of immunity in social insects and for devising effective strategies for breeding disease-resistant pollinator populations. We conducted two experiments to test the hypothesis of a genetic tradeoff between social and individual immunity in the honey bee, Apis mellifera. First, we estimated the relative contribution of genetics to individual variation in innate immunity of honey bee workers, as only heritable traits can experience genetic tradeoffs. Second, we examined if worker bees with hygienic sisters have reduced individual innate immune response. We genotyped several hundred workers from two colonies and found that patriline genotype does not significantly influence the antimicrobial activity of a worker’s hemolymph. Further, we did not find a negative correlation between hygienic behaviour and the average antimicrobial activity of a worker’s hemolymph across 30 honey bee colonies. Taken together, our work indicates no genetic tradeoffs between hygienic behaviour and innate immunity in honey bees. Our work suggests that using artificial selection to increase hygienic behaviour of honey bee colonies is not expected to concurrently compromise individual innate immunity of worker bees.

Fungus Exposed Solenopsis invicta Ants Benefit from Grooming

We investigated aspects of resistance to entomopathogenic fungi in the social insect Solenopsis invicta, the red imported fire ant (RIFA). RIFA reared individually were significantly more susceptible to the entomopathogenic fungi Metarhizium anisopliae var. anisopliae M09 than reared in groups. Fungus exposed ants performed more self-grooming behavior when isolated as individuals and received more allo-grooming when accompanied with four healthy nestmates. Using fluorescence microscopy, we counted the number of fluorescein isothiocyanate (FITC)-labeled conidia on the cuticle of fungus exposed ants reared individually or as groups. The number of conidia on the surface of grouped ants decreased more rapidly than on isolated individuals. Allo-grooming behavior appears to be important in removing the conidia on the surface of RIFA. Individuals help fungus exposed ants by performing intensive grooming behaviors, which either risk infecting themselves or get them immunized as social immunity. We show evidence that contacting with fungus exposed ants would decrease susceptibility of nestmates to the fungus. All these results indicate that RIFA benefit from grooming behavior to fight against the fungal pathogens. Future advances in biological control of RIFA with entomopathogenic fungi are also discussed.

Keep the nest clean: survival advantages of corpse removal in ants.

Sociality increases exposure to pathogens. Therefore, social insects have developed a wide range of behavioural defences, known as ‘social immunity’. However, the benefits of these behaviours in terms of colony survival have been scarcely investigated. We tested the survival advantage of prophylaxis, i.e. corpse removal, in ants. Over 50 days, we compared the survival of ants in colonies that were free to remove corpses with those that were restricted in their corpse removal. From Day 8 onwards, the survival of adult workers was significantly higher in colonies that were allowed to remove corpses normally. Overall, larvae survived better than adults, but were slightly affected by the presence of corpses in the nest. When removal was restricted, ants removed as many corpses as they could and moved the remaining corpses away from brood, typically to the nest corners. These results show the importance of nest maintenance and prophylactic behaviour in social insects.

A carbohydrate-rich diet increases social immunity in ants

Increased potential for disease transmission among nest-mates means living in groups has inherent costs. This increased potential is predicted to select for disease resistance mechanisms that are enhanced by cooperative exchanges among group members, a phenomenon known as social immunity. One potential mediator of social immunity is diet nutritional balance because traits underlying immunity can require different nutritional mixtures. Here, we show how dietary protein-carbohydrate balance affects social immunity in ants. When challenged with a parasitic fungus Metarhizium anisopliae, workers reared on a high-carbohydrate diet survived approximately 2.8× longer in worker groups than in solitary conditions, whereas workers reared on an isocaloric, high-protein diet survived only approximately 1.3× longer in worker groups versus solitary conditions. Nutrition had little effect on social grooming, a potential mechanism for social immunity. However, experimentally blocking metapleural glands, which secrete antibiotics, completely eliminated effects of social grouping and nutrition on immunity, suggesting a causal role for secretion exchange. A carbohydrate-rich diet also reduced worker mortality rates when whole colonies were challenged with Metarhizium. These results provide a novel mechanism by which carbohydrate exploitation could contribute to the ecological dominance of ants and other social groups.

Parental care influences social immunity in burying beetle larvae

In this study, evidence is provided of social immunity in the offspring of a sub‐social species, the burying beetle, Nicrophorus vespilloides. Nicrophorus vespilloides is a carrion breeder and, in a similar fashion to the adult beetles, the offspring produce exudates that exhibit lytic activity, which are used to coat the breeding resource. This strategy defends against the microbial community. The lytic activity in larval exudates declines as the brood develops, perhaps being most beneficial at the start of the breeding bout. Changing levels of parental care through widowing/orphaning affects lytic activity in the larval exudates, with levels decreasing in the absence of both parents.

Exploring the hidden honeybee (Apis mellifera) venom proteome by integrating a combinatorial peptide ligand library approach with FTMS

At present, 30 compounds have been described in the venom of the honeybee, and 16 of them were confirmed by mass spectrometry. Previous studies typically combined 2-D PAGE with MALDI-TOF/TOF MS, a technology which now appears to lack sensitivity to detect additional venom compounds. Here, we report an in-depth study of the honeybee venom proteome using a combinatorial peptide ligand library sample pretreatment to enrich for minor components followed by shotgun LC-FT-ICR MS analysis. This strategy revealed an unexpectedly rich venom composition: in total 102 proteins and peptides were found, with 83 of them never described in bee venom samples before. Based on their predicted function and subcellular location, the proteins could be divided into two groups. A group of 33 putative toxins is proposed to contribute to venom activity by exerting toxic functions or by playing a role in social immunity. The other group, considered as venom trace molecules, appears to be secreted for their functions in the extracellular space, or is unintentionally secreted by the venom gland cells due to insufficient protein recycling or co-secretion with other compounds. In conclusion, our approach allowed to explore the hidden honeybee venom proteome and extended the list of potential venom allergens. This study dug deeper into the complex honeybee venom proteome than ever before by applying a highly performing sample pretreatment and mass spectrometric technology. We present putative biological functions for all identified compounds, largely extending our knowledge of the venom toxicity. In addition, this study offers a long list of potential new venom allergens.