Individual and social immunisation in insects

Uncovering the function of insulin receptor substrate in termites' immunity through active immunization

Insulin receptor substrate (IRS) proteins are key mediators in insulin signaling pathway. In social insect lives, IRS proteins played important roles in caste differentiation and foraging, but there function in disease defenses such as active immunization has not been reported yet. To investigate the issue, we successfully suppressed the IRS gene 3 days after dsRNA injection. Suppressing IRS gene increased the contents of glucose, trehalose, glycogen, and triglyceride and decreased the content of pyruvate in termites, and led to the metabolic disorder of glucose and lipids. IRS suppressing significantly enhanced grooming behaviors of nestmates of fungus-contaminated termites and hence increased the conidial load in the guts of the nestmates. Additionally, IRS suppressing led to significant downregulation of the immune genes Gram-negative bacteria-binding protein2 (GNBP2) and termicin and upregulation of the apoptotic gene caspase8, and hence diminished antifungal activity of nestmates of fungus-contaminated termites. The above abnormal behavioral and physiological responses significantly decreased the survival rate of dsIRS-injected nestmates of the fungus-contaminated termites. These findings suggest that IRS is involved in regulation of active immunization in termites, providing a better understanding of the link between insulin signaling and the social immunity of termites.

The complex interactions between nutrition, immunity and infection in insects

Insects are the most diverse animal group on the planet. Their success is reflected by the diversity of habitats in which they live. However, these habitats have undergone great changes in recent decades; understanding how these changes affect insect health and fitness is an important challenge for insect conservation. In this Review, we focus on the research that links the nutritional environment with infection and immune status in insects. We first discuss the research from the field of nutritional immunology, and we then investigate how factors such as intracellular and extracellular symbionts, sociality and transgenerational effects may interact with the connection between nutrition and immunity. We show that the interactions between nutrition and resistance can be highly specific to insect species and/or infection type - this is almost certainly due to the diversity of insect social interactions and life cycles, and the varied environments in which insects live. Hence, these connections cannot be easily generalised across insects. We finally suggest that other environmental aspects - such as the use of agrochemicals and climatic factors - might also influence the interaction between nutrition and resistance, and highlight how research on these is essential.

Defensive behavior is linked to altered surface chemistry following infection in a termite society

The care-kill response determines whether a sick individual will be treated or eliminated from an insect society, but little is known about the physiological underpinnings of this process. We exploited the stepwise infection dynamics of an entomopathogenic fungus in a termite to explore how care-kill transitions occur, and identify the chemical cues behind these shifts. We found collective responses towards pathogen-injected individuals to vary according to severity and timing of pathogen challenge, with elimination, via cannibalism, occurring sooner in response to a severe active infection. However, injection with inactivated fungal blastospores also resulted in increased albeit delayed cannibalism, even though it did not universally cause host death. This indicates that the decision to eliminate an individual is triggered before pathogen viability or terminal disease status has been established. We then compared the surface chemistry of differently challenged individuals, finding increased amounts of long-chained methyl-branched alkanes with similar branching patterns in individuals injected with both dead and viable fungal blastospores, with the latter showing the largest increase. This coincided with the highest amounts of observed cannibalism as well as signs of severe moribundity. Our study provides new mechanistic insight into the emergent collective behaviors involved in the disease defense of a termite society.

Control of pest ants by pathogenic fungi: state of the art

Pest ants are known for their damage to biodiversity, harm to agriculture, and negative impact on human welfare. Ants thrive when environmental opportunities arise, becoming pests and/or invading non-native areas. As social insects, they are extremely difficult to control using sustainable methods like biological control. The latter, although safer to the environment, acts slowly allowing the ants to use their individual and social defenses. Among biocontrol agents, fungal pathogens were proposed as promising, however, it is difficult to ascertain their success when the bibliography has not been reviewed and condensed. Therefore, this paper is the first in performing such task by analyzing publications mainly from 2000 to 2022 about the control of pest ants by fungi. From 85 publications selected, 77% corresponded to laboratory studies. Beauveria and Metarhizium were the genera most used in laboratory and field studies. Most of them included Acromyrmex and Atta leaf-cutter ants (LCA), and Solenopsis fire ants. From laboratory experiments, we evaluated how ant net mortality was affected by ant and fungal species, and also by origin, concentration, and inoculation technique of the fungal strains tested. Beauveria bassiana and Metarhizium anisopliae produced the greatest mortality, along with the inoculation spray technique and fungal strains collected from ants. There was a positive relationship between ant mortality and fungal concentration only for those studies which evaluated more than one concentration. Twenty field experimental studies were found, covering 13 pest species, mainly LCA and Solenopsis invicta. Only B. bassiana was tested on Solenopsis, M. anisopliae was mostly used for Acromyrmex, and M. anisopliae or Trichoderma were mainly used with Atta species. The median control field efficiency varied from 20% to 85% for different fungi and ant genera. When grouping all fungal species together, the median control efficiency seemed to be better for Acromyrmex (67%) than for Atta and Solenopsis (both 43%). Our review shows that, at this stage of knowledge, it is very difficult to extrapolate any result. We offer suggestions to improve and standardize laboratory and field experimental studies in order to advance more efficiently in the fungal control of pest ants.

Group size rather than social status influences personal immune efficacy in a socially polymorphic bee

The evolution of group living is associated with increased pressure from parasites and pathogens. This can be offset by greater investment in personal immune defences and/or the development of cooperative immune defences (social immunity). An enduring question in evolutionary biology is whether social-immune benefits arose in response to an increased need in more complex societies, or arose early in group living and helped facilitate the evolution of more complex societies. In this study, we shed light on this question through investigating how immunity varies intraspecifically in a socially polymorphic bee. Using a novel immune assay, we show that personal antibacterial efficacy in individuals from social nests is higher than that of solitary individuals, but that this can be explained by higher densities in social nests. We conclude that personal immune effects are likely to play a role in the social/solitary transition in this species. These patterns are consistent with the idea that social immunity evolved secondarily, following the evolution of group living. The flexibility of the individual immune system may have favoured a reliance on its use during the facultative phase early in social evolution.

Pollen as Bee Medicine: Is Prevention Better than Cure?

To face environmental stressors such as infection, animals may display behavioural plasticity to improve their physiological status through ingestion of specific food. In bees, the significance of medicating pollen may be limited by their ability to exploit it. Until now, studies have focused on the medicinal effects of pollen and nectar after forced-feeding experiments, overlooking spontaneous intake. Here, we explored the medicinal effects of different pollen on Bombus terrestris workers infected by the gut parasite Crithidia bombi. First, we used a forced-feeding experimental design allowing for the distinction between prophylactic and therapeutic effects of pollen, considering host tolerance and resistance. Then, we assessed whether bumble bees favoured medicating resources when infected to demonstrate potential self-medicative behaviour. We found that infected bumble bees had a lower fitness but higher resistance when forced to consume sunflower or heather pollen, and that infection dynamics was more gradual in therapeutic treatments. When given the choice between resources, infected workers did not target medicating pollen, nor did they consume more medicating pollen than uninfected ones. These results emphasize that the access to medicating resources could impede parasite dynamics, but that the cost-benefit trade-off could be detrimental when fitness is highly reduced.

Breeding honey bees (Apis mellifera L.) for low and high Varroa destructor population growth: Gene expression of bees performing grooming behavior

Introduction

Social organisms, including honey bees (Apis mellifera L.), have defense mechanisms to control the multiplication and transmission of parasites and pathogens within their colonies. Self-grooming, a mechanism of behavioral immunity, seems to contribute to restrain the population growth of the ectoparasitic mite Varroa destructor in honey bee colonies. Because V. destructor is the most damaging parasite of honey bees, breeding them for resistance against the mite is a high priority of the beekeeping industry.

Methods

A bidirectional breeding program to select honey bee colonies with low and high V. destructor population growth (LVG and HVG, respectively) was conducted. Having high and low lines of bees allowed the study of genetic mechanisms underlying self-grooming behavior between the extreme genotypes. Worker bees were classified into two categories: 'light groomers' and 'intense groomers'. The brains of bees from the different categories (LVG-intense, LVG-light, HVG-intense, and HVG-light) were used for gene expression and viral quantification analyses. Differentially expressed genes (DEGs) associated with the LVG and HVG lines were identified.

Results

Four odorant-binding proteins and a gustatory receptor were identified as differentially expressed genes. A functional enrichment analysis showed 19 enriched pathways from a list of 219 down-regulated DEGs in HVG bees, including the Kyoto Encyclopedia of Genes and Genomes (KEGG) term of oxidative phosphorylation. Additionally, bees from the LVG line showed lower levels of Apis rhabdovirus 1 and 2, Varroa destructor virus -1 (VDV-1/DWV-B), and Deformed wing virus-A (DWV-A) compared to bees of the HVG line. The difference in expression of odorant-binding protein genes and a gustatory receptor between bee lines suggests a possible link between them and the perception of irritants to trigger rapid self-grooming instances that require the activation of energy metabolic pathways.

Discussion

These results provide new insights on the molecular mechanisms involved in honey bee grooming behavior. Differences in viral levels in the brains of LVG and HVG bees showed the importance of investigating the pathogenicity and potential impacts of neurotropic viruses on behavioral immunity. The results of this study advance the understanding of a trait used for selective breeding, self-grooming, and the potential of using genomic assisted selection to improve breeding programs.

Advances in the Immune Regulatory Role of Non-Coding RNAs (miRNAs and lncRNAs) in Insect-Pathogen Interactions

Insects are by far the most abundant and diverse living organisms on earth and are frequently prone to microbial attacks. In other to counteract and overcome microbial invasions, insects have in an evolutionary way conserved and developed immune defense mechanisms such as Toll, immune deficiency (Imd), and JAK/STAT signaling pathways leading to the expression of antimicrobial peptides. These pathways have accessory immune effector mechanisms, such as phagocytosis, encapsulation, melanization, nodulation, RNA interference (RNAi), lysis, autophagy, and apoptosis. However, pathogens evolved strategies that circumvent host immune response following infections, which may have helped insects further sophisticate their immune response mechanisms. The involvement of ncRNAs in insect immunity is undeniable, and several excellent studies or reviews have investigated and described their roles in various insects. However, the functional analyses of ncRNAs in insects upon pathogen attacks are not exhaustive as novel ncRNAs are being increasingly discovered in those organisms. This article gives an overview of the main insect signaling pathways and effector mechanisms activated by pathogen invaders and summarizes the latest findings of the immune modulation role of both insect- and pathogen-encoded ncRNAs, especially miRNAs and lncRNAs during insect–pathogen crosstalk.

Sociality and parasite transmission

Parasites and their social hosts form many different relationships. But what kind of selection regimes are important? A look at the parameters that determine fitness of the two parties suggests that social hosts differ from solitary ones primarily in the structure of transmission pathways. Because transmission is, both, the physical encounter of a new host and infecting it, several different elements determine parasite transmission success. These include spatial distance, genetic distance, or the temporal and ecological niche overlaps. Combing these elements into a ‘generalized transmission distance’ that determines parasite fitness aids in the identification of the critical steps. For example, short-distance transmission to genetically similar hosts within the social group is the most frequent process under sociality. Therefore, spatio-genetical distances are the main driver of parasite fitness. Vice versa, the generalized distance identifies the critical host defences. In this case, host defences should be primarily selected to defend against the within-group spread of an infection, especially among closely related group members.

Early queen infection shapes developmental dynamics and induces long-term disease protection in incipient ant colonies

Infections early in life can have enduring effects on an organism's development and immunity. In this study, we show that this equally applies to developing 'superorganisms'--incipient social insect colonies. When we exposed newly mated Lasius niger ant queens to a low pathogen dose, their colonies grew more slowly than controls before winter, but reached similar sizes afterwards. Independent of exposure, queen hibernation survival improved when the ratio of pupae to workers was small. Queens that reared fewer pupae before worker emergence exhibited lower pathogen levels, indicating that high brood rearing efforts interfere with the ability of the queen's immune system to suppress pathogen proliferation. Early-life queen pathogen exposure also improved the immunocompetence of her worker offspring, as demonstrated by challenging the workers to the same pathogen a year later. Transgenerational transfer of the queen's pathogen experience to her workforce can hence durably reduce the disease susceptibility of the whole superorganism.

Parasite defense mechanisms in bees: behavior, immunity, antimicrobials, and symbionts

Parasites are linked to the decline of some bee populations; thus, understanding defense mechanisms has important implications for bee health. Recent advances have improved our understanding of factors mediating bee health ranging from molecular to landscape scales, but often as disparate literatures. Here, we bring together these fields and summarize our current understanding of bee defense mechanisms including immunity, immunization, and transgenerational immune priming in social and solitary species. Additionally, the characterization of microbial diversity and function in some bee taxa has shed light on the importance of microbes for bee health, but we lack information that links microbial communities to parasite infection in most bee species. Studies are beginning to identify how bee defense mechanisms are affected by stressors such as poor-quality diets and pesticides, but further research on this topic is needed. We discuss how integrating research on host traits, microbial partners, and nutrition, as well as improving our knowledge base on wild and semi-social bees, will help inform future research, conservation efforts, and management.

Adult Mosquitoes Infected with Bacteria Early in Life Have Stronger Antimicrobial Responses and More Hemocytes after Reinfection Later in Life

The immunological strategies employed by insects to overcome infection vary with the type of infection and may change with experience. We investigated how a bacterial infection in the hemocoel of the African malaria mosquito, Anopheles gambiae, prepares the immune system to face a subsequent bacterial infection. For this, adult female mosquitoes were separated into three groups—unmanipulated, injured, or infected with Escherichia coli—and five days later all the mosquitoes were infected with a different strain of E. coli. We found that an injury or a bacterial infection early in life enhances the ability of mosquitoes to kill bacteria later in life. This protection results in higher mosquito survival and is associated with an increased hemocyte density, altered phagocytic activity by individual hemocytes, and the increased expression of nitric oxide synthase and perhaps prophenoloxidase 6. Protection from a second infection likely occurs because of heightened immune awareness due to an already existing infection instead of memory arising from an earlier, cured infection. This study highlights the dynamic nature of the mosquito immune response and how one infection prepares mosquitoes to survive a subsequent infection.

Pathogen avoidance and prey discrimination in ants

Insect societies have developed sanitary strategies, one of which is the avoidance of infectious food resources as a primary line of defence. Using binary choices, we investigated whether Myrmica rubra ants can identify prey that has been artificially infected with the entomopathogenic fungus, Metarhizium brunneum. We compared the ants' foraging behaviour towards infected prey at three different stages of fungus development: (i) prey covered with fungal conidia, (ii) prey freshly killed by the fungus and (iii) sporulating prey. Most foragers retrieved a corpse covered with a high number of spores but they consistently avoided a sporulating prey and collected less prey that had recently died from fungal infection. Furthermore, ant responses were highly variable, with some individuals retrieving the first prey they encountered while others inspected both available prey before making a decision. Workers were not repelled by the simple presence of fungal conidia but nevertheless, they avoided retrieving cadavers at later stages of fungal infection. We discuss how these different avoidance responses could be related to: differences in the ants' perceptive abilities; physico-chemical cues characterizing fungus-infected prey or in the existence of physiological or behavioural defences that limit sanitary risks associated with potentially contaminated resources.

No evidence of queen immunisation despite transgenerational immunisation in Crematogaster scutellaris ants

Like vertebrates, invertebrates evolved acquired immunity based on memory-like mechanisms, known as immunisation. Immunisation and its transmission among individuals are phylogenetically ancestral and conserved characters that have been reported in different insect orders. Physiological mechanisms are still largely unknown, and the high variability in responses in different host-parasite systems led to different conclusions. In social insect species, the complex organisation of colonies further complicates the interpretation of the immune responses. In ants, it has been shown that the expression of immunisation depends on species, caste and physiological status of individuals. In this study, we investigate the occurrence of immunisation in queens of Crematogaster scutellaris ants using the fungus Metarhizium anisopliae as elicitor. Foundation in C. scutellaris is claustral and monogynic, allowing us to test the existence of the phenomenon in two distinct physiological conditions, corresponding to the claustral and colonial phases of queens. Queens and foundresses challenged with heavy doses of the pathogen showed higher mortality if previously exposed to light doses, indicating the absence of immunisation in our experimental settings. On the other hand, evidence of the trans-generational immunisation in the same host-parasite system has been recently found, where workers produced by queens exposed to light doses of M. anisopliae survived longer than those belonging to the control group. These results indicate that foundresses exposed to M. anisopliae can elicit an increased resistance in the offspring without providing themselves with a similar increased immune response and that immunisation and trans-generational immunisation are uncoupled phenomena in this host-parasite system.

Trans-generational Immune Priming in Invertebrates: Current Knowledge and Future Prospects

Trans-generational immune priming (TGIP) refers to the transfer of the parental immunological experience to its progeny. This may result in offspring protection from repeated encounters with pathogens that persist across generations. Although extensively studied in vertebrates for over a century, this phenomenon has only been identified 20 years ago in invertebrates. Since then, invertebrate TGIP has been the focus of an increasing interest, with half of studies published during the last few years. TGIP has now been tested in several invertebrate systems using various experimental approaches and measures to study it at both functional and evolutionary levels. However, drawing an overall picture of TGIP from available studies still appears to be a difficult task. Here, we provide a comprehensive review of TGIP in invertebrates with the objective of confronting all the data generated to date to highlight the main features and mechanisms identified in the context of its ecology and evolution. To this purpose, we describe all the articles reporting experimental investigation of TGIP in invertebrates and propose a critical analysis of the experimental procedures performed to study this phenomenon. We then investigate the outcome of TGIP in the offspring and its ecological and evolutionary relevance before reviewing the potential molecular mechanisms identified to date. In the light of this review, we build hypothetical scenarios of the mechanisms through which TGIP might be achieved and propose guidelines for future investigations.

Staphylococcus aureus-induced sepsis in the lobster cockroach Nauphoeta cinerea

Invertebrates have been instrumental in understanding the mechanisms involved in infectious diseases, considering the idea to replace, reduce and refine the use of mammals as well as to understand the basic principles of immune response in insect. We evaluated the consequences of Staphylococcus aureus-induced sepsis in the last instar nymphs of Nauphoeta cinerea injected with different concentrations of bacteria preserved in two culture media. Infected groups had a decrease in hemolymph metabolites (glucose, amino acids, total proteins, and cholesterol), in contrast to the proteins in the fat body. Higher concentrations of S. aureus caused permanent morphological alterations in adults, decrease in food consumption, increase in isolation, and increase in CFU until death of the cockroaches. Survival and protection of nymphs against a repeated and stronger challenge with the same bacteria varied according to the medium they were conserved. N. cinerea proves to be a suitable and promising model for studies related to bacterial infections.

Transstadial immune activation in a mosquito: Adults that emerge from infected larvae have stronger antibacterial activity in their hemocoel yet increased susceptibility to malaria infection

Larval and adult mosquitoes mount immune responses against pathogens that invade their hemocoel. Although it has been suggested that a correlation exists between immune processes across insect life stages, the influence that an infection in the hemocoel of a larva has on the immune system of the eclosed adult remains unknown. Here, we used Anopheles gambiae to test whether a larval infection influences the adult response to a subsequent bacterial or malaria parasite infection. We found that for both female and male mosquitoes, a larval infection enhances the efficiency of bacterial clearance following a secondary infection in the hemocoel of adults. The adults that emerge from infected larvae have more hemocytes than adults that emerge from naive or injured larvae, and individual hemocytes have greater phagocytic activity. Furthermore, mRNA abundance of immune genes-such as cecropin A, Lysozyme C1, Stat-A, and Tep1-is higher in adults that emerge from infected larvae. A larval infection, however, does not have a meaningful effect on the probability that female adults will survive a systemic bacterial infection, and increases the susceptibility of females to Plasmodium yoelii, as measured by oocyst prevalence and intensity in the midgut. Finally, immune proficiency varies by sex; females exhibit increased bacterial killing, have twice as many hemocytes, and more highly express immune genes. Together, these results show that a larval hemocoelic infection induces transstadial immune activation-possibly via transstadial immune priming-but that it confers both costs and benefits to the emerged adults.

The Mechanisms of Social Immunity Against Fungal Infections in Eusocial Insects

Entomopathogenic fungus as well as their toxins is a natural threat surrounding social insect colonies. To defend against them, social insects have evolved a series of unique disease defenses at the colony level, which consists of behavioral and physiological adaptations. These colony-level defenses can reduce the infection and poisoning risk and improve the survival of societal members, and is known as social immunity. In this review, we discuss how social immunity enables the insect colony to avoid, resist and tolerate fungal pathogens. To understand the molecular basis of social immunity, we highlight several genetic elements and biochemical factors that drive the colony-level defense, which needs further verification. We discuss the chemosensory genes in regulating social behaviors, the antifungal secretions such as some insect venoms in external defense and the immune priming in internal defense. To conclude, we show the possible driving force of the fungal toxins for the evolution of social immunity. Throughout the review, we propose several questions involved in social immunity extended from some phenomena that have been reported. We hope our review about social 'host-fungal pathogen' interactions will help us further understand the mechanism of social immunity in eusocial insects.

Trans-generational immunization in the acrobat ant Crematogaster scutellaris

Trans-generational immunization is defined as the transmission of an enhanced resistance to a pathogen from parents to offspring. By using the host-parasite system of the ant Crematogaster scutellaris and the entomopathogenic fungus Metarhizium anisopliae, we describe this phenomenon for the first time in ants. We exposed four groups of hibernating queens to different treatments (i) a non-lethal dose of live conidiospores in Triton, (ii) a dose of heat-killed conidiospores in Triton, (iii) a control Triton solution, and (iv) a naive control. We exposed their first workers to a high dose of conidiospores and measured mortality rates. Workers produced by queens exposed to live conidiospores survived longer than those belonging to the other groups, while exposure to Triton and dead spores had no effect. Starved workers showed a significantly higher mortality. The treatments did not influence queen mortality, nor the number of offspring they produced at the emergence of the first worker, showing no evidence of immunization costs-at least for these parameters in the first year of colony development. We propose that trans-generational immunization represents an important component of social immunity that could affect colony success, particularly during the critical phase of claustral foundation.

Social cues trigger differential immune investment strategies in a non-social insect, Tenebrio molitor

Social immunization (SI) is a horizontal transfer of immunity that protects naive hosts against infection following exposure to infected nestmates. While mainly documented in eusocial insects, non-social species also share similar ecological features which favour the development of group-level immunity. Here, we investigate SI in Tenebrio molitor by pairing naive females with a pathogen-challenged conspecific for 72 h before measuring a series of immune and fitness traits. We found no evidence for SI, as beetles who cohabited with a live pathogen-challenged conspecific were not better protected against bacterial challenge. However, exposure to a heat-killed-bacteria-challenged conspecific appeared to increase pathogen tolerance, which manifested in differential fitness investment. Our results together suggest that T. molitor do respond to immune-related cues in the social environment, despite not showing a classic immunization response as predicted.

Pre-exposure to non-pathogenic bacteria does not protect Drosophila against the entomopathogenic bacterium Photorhabdus

Immune priming in insects involves an initial challenge with a non-pathogenic microbe or exposure to a low dose of pathogenic microorganisms, which provides a certain degree of protection against a subsequent pathogenic infection. The protective effect of insect immune priming has been linked to the activation of humoral or cellular features of the innate immune response during the preliminary challenge, and these effects might last long enough to promote the survival of the infected animal. The fruit fly Drosophila melanogaster is a superb model to dissect immune priming processes in insects due to the availability of molecular and genetic tools, and the comprehensive understanding of the innate immune response in this organism. Previous investigations have indicated that the D. melanogaster immune system can be primed efficiently. Here we have extended these studies by examining the result of immune priming against two potent entomopathogenic bacteria, Photorhabdus luminescens and P. asymbiotica. We have found that rearing D. melanogaster on diet containing a non-pathogenic strain of Escherichia coli alone or in combination with Micrococcus luteus upregulates the antibacterial peptide immune response in young adult flies, but it does not prolong fly life span. Also, subsequent intrathoracic injection with P. luminescens or P. asymbiotica triggers the Immune deficiency and Toll signaling pathways in flies previously exposed to a live or heat-killed mix of the non-pathogenic bacteria, but the immune activation fails to promote fly survival against the pathogens. These findings suggest that immune priming in D. melanogaster, and probably in other insects, is determined by the type of microbes involved as well as the mode of microbial exposure, and possibly requires a comprehensive and precise alteration of immune signaling and function to provide efficient protection against pathogenic infection.

A Visual Guide for Studying Behavioral Defenses to Pathogen Attacks in Leaf-Cutting Ants

The complex lifestyle, evolutionary history of advanced cooperation, and disease defenses of leaf-cutting ants are well studied. Although numerous studies have described the behaviors connected with disease defense, and the associated use of chemicals and antimicrobials, no common visual reference has been made. The main aim of this study was to record short clips of behaviors involved in disease defense, both prophylactically and directly targeted towards an antagonist of the colony following infection. To do so we used an infection experiment, with sub-colonies of the leaf-cutting ant species Acromyrmex echinatior, and the most significant known pathogenic threat to the ants' fungal crop (Leucoagaricus gongylophorus), a specialized pathogenic fungus in the genus Escovopsis. We filmed and compared infected and uninfected colonies, at both early and more advanced stages of infection. We quantified key defensive behaviors across treatments and show that the behavioral response to pathogen attack likely varies between different castes of worker ants, and between early and late detection of a threat. Based on these recordings we have made a library of behavioral clips, accompanied by definitions of the main individual defensive behaviors. We anticipate that such a guide can provide a common frame of reference for other researchers working in this field, to recognize and study these behaviors, and also provide greater scope for comparing different studies to ultimately help better understand the role these behaviors play in disease defense.

Social immunity: why we should study its nature, evolution and functions across all social systems

Mounting defences against pathogens is a necessity for all animals. Although these defences have long been known to rely on individual processes such as the immune system, recent studies have emphasized the importance of social defences for group-living hosts. These defences, called social immunity, have been mostly studied in eusocial insects such as bees, termites and ants, and include, for instance, mutual cleaning and waste management. Over the last few years, however, a growing number of works called for a broader exploration of social immunity in non-eusocial species. In this review, we summarize the rationales of this call and examine why it may provide major insights into our current understanding of the role of pathogens in social evolution. We start by presenting the original conceptual framework of social immunity developed in eusocial insects and shed light on its importance in highly derived social systems. We then clarify three major misconceptions possibly fostered by this original framework and demonstrate why they made necessary the shift towards a broader definition of social immunity. Because a broader definition still needs boundaries, we finally present three criteria to discriminate what is a form of social immunity, from what is not. Overall, we argue that studying social immunity across social systems does not only provide novel insights into how pathogens affect the evolution of eusociality, but also of the emergence and maintenance of social life from a solitary state. Moreover, this broader approach offers new scopes to disentangle the common and specific anti-pathogen defences developed by eusocial and non-eusocial hosts, and to better understand the dependent and independent evolutionary drivers of social and individual immunity.

The Wisdom of Honeybee Defenses Against Environmental Stresses

As one of the predominant pollinator, honeybees provide important ecosystem service to crops and wild plants, and generate great economic benefit for humans. Unfortunately, there is clear evidence of recent catastrophic honeybee colony failure in some areas, resulting in markedly negative environmental and economic effects. It has been demonstrated that various environmental stresses, including both abiotic and biotic stresses, functioning singly or synergistically, are the potential drivers of colony collapse. Honeybees can use many defense mechanisms to decrease the damage from environmental stress to some extent. Here, we synthesize and summarize recent advances regarding the effects of environmental stress on honeybees and the wisdom of honeybees to respond to external environmental stress. Furthermore, we provide possible future research directions about the response of honeybees to various form of stressors.

Ants avoid superinfections by performing risk-adjusted sanitary care

Being cared for when sick is a benefit of sociality that can reduce disease and improve survival of group members. However, individuals providing care risk contracting infectious diseases themselves. If they contract a low pathogen dose, they may develop low-level infections that do not cause disease but still affect host immunity by either decreasing or increasing the host's vulnerability to subsequent infections. Caring for contagious individuals can thus significantly alter the future disease susceptibility of caregivers. Using ants and their fungal pathogens as a model system, we tested if the altered disease susceptibility of experienced caregivers, in turn, affects their expression of sanitary care behavior. We found that low-level infections contracted during sanitary care had protective or neutral effects on secondary exposure to the same (homologous) pathogen but consistently caused high mortality on superinfection with a different (heterologous) pathogen. In response to this risk, the ants selectively adjusted the expression of their sanitary care. Specifically, the ants performed less grooming and more antimicrobial disinfection when caring for nestmates contaminated with heterologous pathogens compared with homologous ones. By modulating the components of sanitary care in this way the ants acquired less infectious particles of the heterologous pathogens, resulting in reduced superinfection. The performance of risk-adjusted sanitary care reveals the remarkable capacity of ants to react to changes in their disease susceptibility, according to their own infection history and to flexibly adjust collective care to individual risk.

Social Immunity: Emergence and Evolution of Colony-Level Disease Protection

Social insect colonies have evolved many collectively performed adaptations that reduce the impact of infectious disease and that are expected to maximize their fitness. This colony-level protection is termed social immunity, and it enhances the health and survival of the colony. In this review, we address how social immunity emerges from its mechanistic components to produce colony-level disease avoidance, resistance, and tolerance. To understand the evolutionary causes and consequences of social immunity, we highlight the need for studies that evaluate the effects of social immunity on colony fitness. We discuss the roles that host life history and ecology have on predicted eco-evolutionary dynamics, which differ among the social insect lineages. Throughout the review, we highlight current gaps in our knowledge and promising avenues for future research, which we hope will bring us closer to an integrated understanding of socio-eco-evo-immunology.

Social dilemma in the external immune system of the red flour beetle? It is a matter of time

Sociobiology has revolutionized our understanding of interactions between organisms. Interactions may present a social dilemma where the interests of individual actors do not align with those of the group as a whole. Viewed through a sociobiological lens, nearly all interactions can be described regarding their costs and benefits, and a number of them then resemble a social dilemma. Numerous experimental systems, from bacteria to mammals, have been proposed as models for studying such dilemmas. Here, we make use of the external immune system of the red flour beetle, Tribolium castaneum, to investigate how the experimental duration can affect whether the external secretion comprises a social dilemma or not. Some beetles (secretors) produce a costly quinone-rich external secretion that inhibits microbial growth in the surrounding environment, providing the secretors with direct personal benefits. However, as the antimicrobial secretion acts in the environment of the beetle, it is potentially also advantageous to other beetles (nonsecretors), who avoid the cost of producing the secretion. We test experimentally if the secretion qualifies as a public good. We find that in the short term, costly quinone secretion can be interpreted as a public good presenting a social dilemma where the presence of secretors increases the fitness of the group. In the long run, the benefit to the group of having more secretors vanishes and becomes detrimental to the group. Therefore, in such seminatural environmental conditions, it turns out that qualifying a trait as social can be a matter of timing.

Making New Connections: Insights from Primate-Parasite Networks

Social interactions are important in everyday life for primates and many other group-living animals; however, these essential exchanges also provide opportunities for parasites to spread through social groups. Network analysis is a unique toolkit for studying pathogen transmission in a social context, and recent primate-parasite network studies shed light on linkages between behavior and infectious disease dynamics, providing insights for conservation and public health. We review existing literature on primate-parasite networks, examining determinants of infection risk, issues of network scale and temporal dynamics, and applications for disease control. We also discuss analytical and conceptual gaps that should be addressed to improve our understanding of how individual and group-level factors affect infection risk, while highlighting interesting areas for future research.

Inside Honeybee Hives: Impact of Natural Propolis on the Ectoparasitic Mite Varroa destructor and Viruses

Social immunity is a key factor for honeybee health, including behavioral defense strategies such as the collective use of antimicrobial plant resins (propolis). While laboratory data repeatedly show significant propolis effects, field data are scarce, especially at the colony level. Here, we investigated whether propolis, as naturally deposited in the nests, can protect honeybees against ectoparasitic mites Varroa destructor and associated viruses, which are currently considered the most serious biological threat to European honeybee subspecies, Apis mellifera, globally. Propolis intake of 10 field colonies was manipulated by either reducing or adding freshly collected propolis. Mite infestations, titers of deformed wing virus (DWV) and sacbrood virus (SBV), resin intake, as well as colony strength were recorded monthly from July to September 2013. We additionally examined the effect of raw propolis volatiles on mite survival in laboratory assays. Our results showed no significant effects of adding or removing propolis on mite survival and infestation levels. However, in relation to V. destructor, DWV titers increased significantly less in colonies with added propolis than in propolis-removed colonies, whereas SBV titers were similar. Colonies with added propolis were also significantly stronger than propolis-removed colonies. These findings indicate that propolis may interfere with the dynamics of V. destructor-transmitted viruses, thereby further emphasizing the importance of propolis for honeybee health.

Immune memory in invertebrates

Evidence for innate immune memory (or 'priming') in invertebrates has been accumulating over the last years. We here provide an in-depth review of the current state of evidence for immune memory in invertebrates, and in particular take a phylogenetic viewpoint. Invertebrates are a very heterogeneous group of animals and accordingly, evidence for the phenomenon of immune memory as well as the hypothesized molecular underpinnings differ largely for the diverse invertebrate taxa. The majority of research currently focuses on Arthropods, while evidence from many other groups of invertebrates is fragmentary or even lacking. We here concentrate on immune memory that is induced by pathogenic challenges, but also extent our view to a non-pathogenic context, i.e. allograft rejection, which can also show forms of memory and can inform us about general principles of specific self-nonself recognition. We discuss definitions of immune memory and a number of relevant aspects such as the type of antigens used, the route of exposure, and the kinetics of reactions following priming.

Insect immunology and hematopoiesis

Insects combat infection by mounting powerful immune responses that are mediated by hemocytes, the fat body, the midgut, the salivary glands and other tissues. Foreign organisms that have entered the body of an insect are recognized by the immune system when pathogen-associated molecular patterns bind host-derived pattern recognition receptors. This, in turn, activates immune signaling pathways that amplify the immune response, induce the production of factors with antimicrobial activity, and activate effector pathways. Among the immune signaling pathways are the Toll, Imd, Jak/Stat, JNK, and insulin pathways. Activation of these and other pathways leads to pathogen killing via phagocytosis, melanization, cellular encapsulation, nodulation, lysis, RNAi-mediated virus destruction, autophagy and apoptosis. This review details these and other aspects of immunity in insects, and discusses how the immune and circulatory systems have co-adapted to combat infection, how hemocyte replication and differentiation takes place (hematopoiesis), how an infection prepares an insect for a subsequent infection (immune priming), how environmental factors such as temperature and the age of the insect impact the immune response, and how social immunity protects entire groups. Finally, this review highlights some underexplored areas in the field of insect immunobiology.

Infections by Pasteuria do not protect its natural host Daphnia magna from subsequent infections

The existence of immunological memory in invertebrates remains a contentious topic. Exposure of Daphnia magna crustaceans to a noninfectious dose of the bacterium Pasteuria ramosa has been reported to reduce the chance of future infection upon exposure to higher doses. Using clonal hosts and parasites, we tested whether initial exposure of the host to the parasite (priming), followed by clearing of the parasite with antibiotic, protects the host from a second exposure (challenge). Our experiments included three treatments: priming and challenge with the same or with a different parasite clone, or no priming. Two independent experiments showed that both the likelihood of infection and the degree of parasite proliferation did not differ between treatments, supporting the conclusion that there is no immunological memory in this system. We discuss the possibility that previous discordant reports could result from immune or stress responses that did not fade following initial priming.

Cloning and purification of the first termicin-like peptide from the cockroach Eupolyphaga sinensis

Background

Termicin is an antimicrobial peptide with six cysteines forming three disulfide bridges that was firstly isolated from the salivary glands and hemocytes of the termite Pseudacanthotermes spiniger. In contrast to many broad-spectrum antimicrobial peptides, termicin is most active against filamentous fungi. Although more than one hundred complementary DNAs (cDNAs) encoding termicin-like peptides have been reported to date, all these termicin-like peptides were obtained from Isoptera insects.

Methods

The cDNA was cloned by combination of cDNA library construction kit and DNA sequencing. The polypeptide was purified by gel filtration and reversed-phase high performance liquid chromatography (RP-HPLC). Its amino acid sequence was determined by Edman degradation and mass spectrometry. Antimicrobial activity was tested against several bacterial and fungal strains. The minimum inhibitory concentration (MIC) was determined by microdilution tests.

Results

A novel termicin-like peptide with primary structure ACDFQQCWVTCQRQYSINFISARCNGDSCVCTFRT was purified from extracts of the cockroach Eupolyphaga sinensis (Insecta: Blattodea). The cDNA encoding Es-termicin was cloned by cDNA library screening. This cDNA encoded a 60 amino acid precursor which included a 25 amino acid signal peptide. Amino acid sequence deduced from the cDNA matched well with the result of protein Edman degradation. Susceptibility test indicated that Es-termicin showed strong ability to kill fungi with a MIC of 25 μg/mL against Candida albicans ATCC 90028. It only showed limited potency to affect the growth of Gram-positive bacteria with a MIC of 200 μg/mL against Enterococcus faecalis ATCC 29212. It was inactive against gram-negative bacteria at the highest concentration tested (400 μg/mL). Es-termicin showed high sequence similarity with termicins from many species of termites (Insecta: Isoptera).

Conclusions

This is the first report of a termicin-like peptide isolated from E. sinensis that belongs to the insect order Blattodea. Our results demonstrate the diversity of termicin-like peptides, as well as antimicrobial peptides in insects.

Downregulation of the evolutionary capacitor Hsp90 is mediated by social cues

The relationship between robustness and evolvability is a long-standing question in evolution. Heat shock protein 90 (HSP90), a molecular chaperone, has been identified as a potential capacitor for evolution, since it allows for the accumulation and release of cryptic genetic variation, and also for the regulation of novel genetic variation through transposon activity. However, to date, it is unknown whether Hsp90 expression is regulated upon demand (i.e. when the release of cryptic genetic variation is most needed). Here, we show that Hsp90 has reduced transcription under conditions where the mobilization of genetic variation could be advantageous. We designed a situation that indicates a stressful environment but avoids the direct effects of stress, by placing untreated (focal) red flour beetles, Tribolium castaneum, into groups together with wounded conspecifics, and found a consistent reduction in expression of two Hsp90 genes (Hsp83 and Hsp90) in focal beetles. We moreover observed a social transfer of immunity in this non-eusocial insect: there was increased activity of the phenoloxidase enzyme and downregulation of the immune regulator, imd. Our study poses the exciting question of whether evolvability might be regulated through the use of information derived from the social environment.

Wolbachia and the insect immune system: what reactive oxygen species can tell us about the mechanisms of Wolbachia-host interactions

Wolbachia are intracellular bacteria that infect a vast range of arthropod species, making them one of the most prevalent endosymbionts in the world. Wolbachia's stunning evolutionary success is mostly due to their reproductive parasitism but also to mutualistic effects such as increased host fecundity or protection against pathogens. However, the mechanisms underlying Wolbachia phenotypes, both parasitic and mutualistic, are only poorly understood. Moreover, it is unclear how the insect immune system is involved in these phenotypes and why it is not more successful in eliminating the bacteria. Here we argue that reactive oxygen species (ROS) are likely to be key in elucidating these issues. ROS are essential players in the insect immune system, and Wolbachia infection can affect ROS levels in the host. Based on recent findings, we elaborate a hypothesis that considers the different effects of Wolbachia on the oxidative environment in novel vs. native hosts. We propose that newly introduced Wolbachia trigger an immune response and cause oxidative stress, whereas in coevolved symbioses, infection is not associated with oxidative stress, but rather with restored redox homeostasis. Redox homeostasis can be restored in different ways, depending on whether Wolbachia or the host is in charge. This hypothesis offers a mechanistic explanation for several of the observed Wolbachia phenotypes.

Experimental verification and molecular basis of active immunization against fungal pathogens in termites

Termites are constantly exposed to many pathogens when they nest and forage in the field, so they employ various immune strategies to defend against pathogenic infections. Here, we demonstrate that the subterranean termite Reticulitermes chinensis employs active immunization to defend against the entomopathogen Metarhizium anisopliae. Our results showed that allogrooming frequency increased significantly between fungus-treated termites and their nestmates. Through active social contact, previously healthy nestmates only received small numbers of conidia from fungus-treated individuals. These nestmates experienced low-level fungal infections, resulting in low mortality and apparently improved antifungal defences. Moreover, infected nestmates promoted the activity of two antioxidant enzymes (SOD and CAT) and upregulated the expression of three immune genes (phenoloxidase, transferrin, and termicin). We found 20 differentially expressed proteins associated with active immunization in R. chinensis through iTRAQ proteomics, including 12 stress response proteins, six immune signalling proteins, and two immune effector molecules. Subsequently, two significantly upregulated (60S ribosomal protein L23 and isocitrate dehydrogenase) and three significantly downregulated (glutathione S-transferase D1, cuticle protein 19, and ubiquitin conjugating enzyme) candidate immune proteins were validated by MRM assays. These findings suggest that active immunization in termites may be regulated by different immune proteins.

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.

Sociality and health: impacts of sociality on disease susceptibility and transmission in animal and human societies

This paper introduces a theme issue presenting the latest developments in research on the impacts of sociality on health and fitness. The articles that follow cover research on societies ranging from insects to humans. Variation in measures of fitness (i.e. survival and reproduction) has been linked to various aspects of sociality in humans and animals alike, and variability in individual health and condition has been recognized as a key mediator of these relationships. Viewed from a broad evolutionary perspective, the evolutionary transitions from a solitary lifestyle to group living have resulted in several new health-related costs and benefits of sociality. Social transmission of parasites within groups represents a major cost of group living, but some behavioural mechanisms, such as grooming, have evolved repeatedly to reduce this cost. Group living also has created novel costs in terms of altered susceptibility to infectious and non-infectious disease as a result of the unavoidable physiological consequences of social competition and integration, which are partly alleviated by social buffering in some vertebrates. Here, we define the relevant aspects of sociality, summarize their health-related costs and benefits, and discuss possible fitness measures in different study systems. Given the pervasive effects of social factors on health and fitness, we propose a synthesis of existing conceptual approaches in disease ecology, ecological immunology and behavioural neurosciences by adding sociality as a key factor, with the goal to generate a broader framework for organismal integration of health-related research.

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

Background

Social 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.

Results

Using 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.

Conclusion

These 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.

Eaten alive: cannibalism is enhanced by parasites

Cannibalism is ubiquitous in nature and especially pervasive in consumers with stage-specific resource utilization in resource-limited environments. Cannibalism is thus influential in the structure and functioning of biological communities. Parasites are also pervasive in nature and, we hypothesize, might affect cannibalism since infection can alter host foraging behaviour. We investigated the effects of a common parasite, the microsporidian Pleistophora mulleri, on the cannibalism rate of its host, the freshwater amphipod Gammarus duebeni celticus. Parasitic infection increased the rate of cannibalism by adults towards uninfected juvenile conspecifics, as measured by adult functional responses, that is, the rate of resource uptake as a function of resource density. This may reflect the increased metabolic requirements of the host as driven by the parasite. Furthermore, when presented with a choice, uninfected adults preferred to cannibalize uninfected rather than infected juvenile conspecifics, probably reflecting selection pressure to avoid the risk of parasite acquisition. By contrast, infected adults were indiscriminate with respect to infection status of their victims, probably owing to metabolic costs of infection and the lack of risk as the cannibals were already infected. Thus parasitism, by enhancing cannibalism rates, may have previously unrecognized effects on stage structure and population dynamics for cannibalistic species and may also act as a selective pressure leading to changes in resource use.

Infection-avoidance behaviour in humans and other animals

Compared with living free, the parasitic way of life has many attractions. Parasites create problems for all animals. Potential hosts can respond by learning to live with parasites (tolerance), actively fighting them (resistance), or they can avoid becoming infected in the first place (avoidance). I propose here a new classification of avoidance behaviour according to the epidemiology of infection risk, where animals must avoid (i) conspecifics, (ii) parasites and their vectors, (iii) parasite-rich environments, and (iv) niche infestation. I further explore how the disgust adaptive system, which coordinates avoidance behaviour, may form a continuum with the immune system through the sharing of signalling pathways, sites of action, and evolutionary history.