Trophallaxis and prophylaxis: social immunity in the carpenter ant Camponotus pennsylvanicus

In social insects, group behaviour can increase disease resistance among nest-mates and generate social prophylaxis. Stomodeal trophallaxis, or mutual feeding through regurgitation, may boost colony-level immunocompetence. We provide evidence for increased trophallactic behaviour among immunized workers of the carpenter ant Camponotus pennsylvanicus, which, together with increased antimicrobial activity of the regurgitate droplet, help explain the improved survival of droplet recipient ants relative to controls following an immune challenge. We have identified a protein related to cathepsin D, a lysosomal protease, as a potential contributor to the antimicrobial activity. The combined behavioural and immunological responses to infection in these ants probably represent an effective mechanism underlying the social facilitation of disease resistance, which could potentially produce socially mediated colony-wide prophylaxis. The externalization and sharing of an individual''s immune responses via trophallaxis could be an important component of social immunity, allowing insect colonies to thrive under high pathogenic pressures.     

Mimetic host shifts in an endangered social parasite of ants

An emerging problem in conservation is whether listed morpho-species with broad distributions, yet specialized lifestyles, consist of more than one cryptic species or functionally distinct forms that have different ecological requirements. We describe extreme regional divergence within an iconic endangered butterfly, whose socially parasitic young stages use non-visual, non-tactile cues to infiltrate and supplant the brood in ant societies. Although indistinguishable morphologically or when using current mitochondrial and nuclear sequence-, or microsatellite data, Maculinea rebeli from Spain and southeast Poland exploit different Myrmica ant species and experience 100 per cent mortality with each other''s hosts. This reflects major differences in the hydrocarbons synthesized from each region by the larvae, which so closely mimic the recognition profiles of their respective hosts that nurse ants afford each parasite a social status above that of their own kin larvae. The two host ants occupy separate niches within grassland; thus, conservation management must differ in each region. Similar cryptic differentiation may be common, yet equally hard to detect, among the approximately 10 000 unstudied morpho-species of social parasite that are estimated to exist, many of which are Red Data Book listed.     

Rapid anti-pathogen response in ant societies relies on high genetic diversity

Social organisms are constantly exposed to infectious agents via physical contact with conspecifics. While previous work has shown that disease susceptibility at the individual and group level is influenced by genetic diversity within and between group members, it remains poorly understood how group-level resistance to pathogens relates directly to individual physiology, defence behaviour and social interactions. We investigated the effects of high versus low genetic diversity on both the individual and collective disease defences in the ant Cardiocondyla obscurior. We compared the antiseptic behaviours (grooming and hygienic behaviour) of workers from genetically homogeneous and diverse colonies after exposure of their brood to the entomopathogenic fungus Metarhizium anisopliae. While workers from diverse colonies performed intensive allogrooming and quickly removed larvae covered with live fungal spores from the nest, workers from homogeneous colonies only removed sick larvae late after infection. This difference was not caused by a reduced repertoire of antiseptic behaviours or a generally decreased brood care activity in ants from homogeneous colonies. Our data instead suggest that reduced genetic diversity compromises the ability of Cardiocondyla colonies to quickly detect or react to the presence of pathogenic fungal spores before an infection is established, thereby affecting the dynamics of social immunity in the colony.     

Strongyloides ratti: Dissociation of the rat's protective immunity into systemic and intestinal components

Analysis of the early stages of a challenge infection with Strongyloides ratti has shown that protection is expressed against the developing third-stage larval worms (L₃) and prevents the maturation to adulthood of most larvae. Challenge after an immunizing infection that was restricted to the parenteral L₃ migratory phase showed that some 10–40% of overall protection could be ascribed to systemic antilarval immunity. Some larvae were trapped in the skin at the site of injection whereas others failed to migrate to the head and lung of immune rats. Larvae arriving in the intestine at Days 3, 4, and 5 did not persist beyond Day 7 and 8. Studies using [⁷⁵Se]methionine-labeled L₃ showed a significant increase in fecal label in rats immunized by a complete infection. This loss did not occur to the same extent in rats immunized only with parenteral larvae. Significant rejection of worms transplanted to the intestine also indicated intestinal protection. The possible existence of large numbers of worms in a state of “arrested development” was excluded by their failure to appear after cortisone treatment and the absence of worm accumulation in radiolabeling studies. It is concluded that at least two responses operate against larval S. ratti, one is systemic and the other operates in the intestine against larvae in a manner that resembles the “rapid expulsion” rejection of Trichinella spiralis in immune rats.     

Adaptive social immunity in leaf-cutting ants

Social insects have evolved a suite of sophisticated defences against parasites. In addition to the individual physiological immune response, social insects also express ‘social immunity’ consisting of group-level defences and behaviours that include allogrooming. Here we investigate whether the social immune response of the leaf-cutting ant Acromyrmex echinatior reacts adaptively to the virulent fungal parasite, Metarhizium anisopliae. We ‘immunized’ mini-nests of the ants by exposing them twice to the parasite and then compared their social immune response with that of naive mini-nests that had not been experimentally exposed to the parasite. Ants allogroomed individuals exposed to the parasite, doing this both for those freshly treated with the parasite, which were infectious but not yet infected, and for those treated 2 days previously, which were already infected but no longer infectious. We found that ants exposed to the parasite received more allogrooming in immunized mini-nests than in naive mini-nests. This increased the survival of the freshly treated ants, but not those that were already infected. The results thus indicate that the social immune response of this leaf-cutting ant is adaptive, with the group exhibiting a greater and more effective response to a parasite that it has previously been exposed to.     

Translocation of bacteria from the gut to the eggs triggers maternal transgenerational immune priming in Tribolium castaneum

Invertebrates can be primed to enhance their protection against pathogens they have encountered before. This enhanced immunity can be passed maternally or paternally to the offspring and is known as transgenerational immune priming. We challenged larvae of the red flour beetle Tribolium castaneum by feeding them on diets supplemented with Escherichia coli, Micrococcus luteus or Pseudomonas entomophila, thus mimicking natural exposure to pathogens. The oral uptake of bacteria induced immunity-related genes in the offspring, but did not affect the methylation status of the egg DNA. However, we observed the translocation of bacteria or bacterial fragments from the gut to the developing eggs via the female reproductive system. Such translocating microbial elicitors are postulated to trigger bacterial strain-specific immune responses in the offspring and provide an alternative mechanistic explanation for maternal transgenerational immune priming in coleopteran insects.     

The developmental high wire: Balancing resource investment in immunity and reproduction

The strategic allocation of resources into immunity poses a unique challenge for individuals, where infection at different stages of development may result in unique trade‐offs with concurrent physiological processes or future fitness‐enhancing traits. Here, we experimentally induced an immune challenge in female Gryllus firmus crickets to test whether illness at discrete life stages differentially impacts fitness. We injected heat‐killed Serratia marcescens bacteria into antepenultimate juveniles, penultimate juveniles, sexually immature adults, and sexually mature adults, and then measured body growth, instar duration, mating rate, viability of stored sperm, egg production, oviposition rate, and egg viability. Immune activation significantly impacted reproductive traits, where females that were immune challenged as adults had decreased mating success and decreased egg viability compared to healthy individuals or females that were immune challenged as juveniles. Although there was no effect of an immune challenge on the other traits measured, the stress of handling resulted in reduced mass gain and smaller adult body size in females from the juvenile treatments, and females in the adult treatments suffered from reduced viability of sperm stored within their spermatheca. In summary, we found that an immune challenge does have negative impacts on reproduction, but also that even minor acute stressors can have significant impacts on fitness‐enhancing traits. These findings highlight that the factors affecting fitness can be complex and at times unpredictable, and that the consequences of illness are specific to when during an individual''s life an immune challenge is induced.     

Trichomes as dangerous lollipops: Do lizards also use caterpillar body and frass odor to optimize their foraging?

When attacked by herbivores, plants produce toxic secondary metabolites that function as direct defenses, as well as indirect defenses that attract and reward predators of the offending herbivores. These indirect defenses include both nutritive rewards such as extra floral nectar, as well as informational rewards, such as the production and release of volatile compounds that betray the location of feeding herbivores to predators. Herbivory of Nicotiana attenuata by the tobacco hornworm (Manduca larvae) alters the volatile profiles of both the plant and larval headspace. Herbivory-elicited specific changes in the volatile profiles are detected by arthropod predators of Manduca larvae. The known predators that perceive volatile cues induced by Manduca herbivory of N. attenuata are insects that target Manduca at early developmental stages, when the larvae are still small; large, late-instar larvae may have outgrown these predation risks. However, here we offer evidence that branched chain aliphatic acids derived from the digestion of plant O-acyl sugars from trichomes may betray Manduca larvae to lizard predators during late developmental stages as well.     

Experimental increase in fecundity causes upregulation of fecundity and body maintenance genes in the fat body of ant queens

In most organisms, fecundity and longevity are negatively associated and the molecular regulation of these two life-history traits is highly interconnected. In addition, nutrient intake often has opposing effects on lifespan and reproduction. In contrast to solitary insects, the main reproductive individual of social hymenopterans, the queen, is also the most long-lived. During development, queen larvae are well-nourished, but we are only beginning to understand the impact of nutrition on the queens'' adult life and the molecular regulation and connectivity of fecundity and longevity. Here, we used two experimental manipulations to alter queen fecundity in the ant Temnothorax rugatulus and investigated associated changes in fat body gene expression. Egg removal triggered a fecundity increase, leading to expression changes in genes with functions in fecundity such as oogenesis and body maintenance. Dietary restriction lowered the egg production of queens and altered the expression of genes linked to autophagy, Toll signalling, cellular homeostasis and immunity. Our study reveals that an experimental increase in fecundity causes the co-activation of reproduction and body maintenance mechanisms, shedding light on the molecular regulation of the link between longevity and fecundity in social insects.     

Perception of infection: disease-related social cues influence immunity in songbirds

While avoidance of sick conspecifics is common among animals, little is known about how detecting diseased conspecifics influences an organism''s physiological state, despite its implications for disease transmission dynamics. The avian pathogen Mycoplasma gallisepticum (MG) causes obvious visual signs of infection in domestic canaries (Serinus canaria domestica), including lethargy and conjunctivitis, making this system a useful tool for investigating how the perception of cues from sick individuals shapes immunity in healthy individuals. We tested whether disease-related social information can stimulate immune responses in canaries housed in visual contact with either healthy or MG-infected conspecifics. We found higher complement activity and higher heterophil counts in healthy birds viewing MG-infected individuals around 6–12 days post-inoculation, which corresponded with the greatest degree of disease pathology in infected stimulus birds. However, we did not detect the effects of disease-related social cues on the expression of two proinflammatory cytokines in the blood. These data indicate that social cues of infection can alter immune responses in healthy individuals and suggest that public information about the disease can shape how individuals respond to infection.     

Social learning about egg-laying substrates in fruitflies

Social learning, defined as learning from other individuals, has had dramatic effects on some species, including humans, in whom it has generated a rich culture. As a first step in examining the evolution of and mechanisms underlying social learning in insects, we tested for social learning in fruitflies (Drosophila melanogaster). Focal females (observers) that experienced novel food together with mated females (models), who had laid eggs on that food, subsequently exhibited a stronger preference for laying eggs on that food over another novel food compared with focal females that experienced the food alone. We observed no social learning, however, when observers experienced food with potentially more ambiguous social information provided by the presence of either virgin models or aggregation pheromone. This first documentation of social learning about egg-laying substrates in fruitflies builds on recent data indicating intricate use of social information by fruitflies and opens up exciting avenues for research on the evolution and neurogenetics of social learning using biology''s major model system.     

Anti-pathogen protection versus survival costs mediated by an ectosymbiont in an ant host

The fitness effects of symbionts on their hosts can be context-dependent, with usually benign symbionts causing detrimental effects when their hosts are stressed, or typically parasitic symbionts providing protection towards their hosts (e.g. against pathogen infection). Here, we studied the novel association between the invasive garden ant Lasius neglectus and its fungal ectosymbiont Laboulbenia formicarum for potential costs and benefits. We tested ants with different Laboulbenia levels for their survival and immunity under resource limitation and exposure to the obligate killing entomopathogen Metarhizium brunneum. While survival of L. neglectus workers under starvation was significantly decreased with increasing Laboulbenia levels, host survival under Metarhizium exposure increased with higher levels of the ectosymbiont, suggesting a symbiont-mediated anti-pathogen protection, which seems to be driven mechanistically by both improved sanitary behaviours and an upregulated immune system. Ants with high Laboulbenia levels showed significantly longer self-grooming and elevated expression of immune genes relevant for wound repair and antifungal responses (β-1,3-glucan binding protein, Prophenoloxidase), compared with ants carrying low Laboulbenia levels. This suggests that the ectosymbiont Laboulbenia formicarum weakens its ant host by either direct resource exploitation or the costs of an upregulated behavioural and immunological response, which, however, provides a prophylactic protection upon later exposure to pathogens.     

Rapid Decision-Making with Side-Specific Perceptual Discrimination in Ants

Background

Timely decision making is crucial for survival and reproduction. Organisms often face a speed-accuracy trade-off, as fully informed, accurate decisions require time-consuming gathering and treatment of information. Optimal strategies for decision-making should therefore vary depending on the context. In mammals, there is mounting evidence that multiple systems of perceptual discrimination based on different neural circuits emphasize either fast responses or accurate treatment of stimuli depending on the context.

Methodology/Principal Findings

We used the ant Camponotus aethiops to test the prediction that fast information processing achieved through direct neural pathways should be favored in situations where quick reactions are adaptive. Social insects discriminate readily between harmless group-members and dangerous strangers using easily accessible cuticular hydrocarbons as nestmate recognition cues. We show that i) tethered ants display rapid aggressive reactions upon presentation of non-nestmate odor (120 to 160 ms); ii) ants'' aggressiveness towards non-nestmates can be specifically reduced by exposure to non-nestmate odor only, showing that social interactions are not required to alter responses towards non-nestmates; iii) decision-making by ants does not require information transfer between brain hemispheres, but relies on side-specific decision rules.

Conclusions/Significance

Our results strongly suggest that first-order olfactory processing centers (up to the antennal lobes) are likely to play a key role in ant nestmate recognition. We hypothesize that the coarse level of discrimination achieved in the antennal lobes early in odor processing provides enough information to determine appropriate behavioral responses towards non-nestmates. This asks for a reappraisal of the mechanisms underlying social recognition in insects.     

Variation in Butterfly Larval Acoustics as a Strategy to Infiltrate and Exploit Host Ant Colony Resources

About 10,000 arthropods live as ants'' social parasites and have evolved a number of mechanisms allowing them to penetrate and survive inside the ant nests. Many of them can intercept and manipulate their host communication systems. This is particularly important for butterflies of the genus Maculinea, which spend the majority of their lifecycle inside Myrmica ant nests. Once in the colony, caterpillars of Maculinea “predatory species” directly feed on the ant larvae, while those of “cuckoo species” are fed primarily by attendance workers, by trophallaxis. It has been shown that Maculinea cuckoo larvae are able to reach a higher social status within the colony''s hierarchy by mimicking the acoustic signals of their host queen ants. In this research we tested if, when and how myrmecophilous butterflies may change sound emissions depending on their integration level and on stages of their life cycle. We studied how a Maculinea predatory species (M. teleius) can acoustically interact with their host ants and highlighted differences with respect to a cuckoo species (M. alcon). We recorded sounds emitted by Maculinea larvae as well as by their Myrmica hosts, and performed playback experiments to assess the parasites'' capacity to interfere with the host acoustic communication system. We found that, although varying between and within butterfly species, the larval acoustic emissions are more similar to queens'' than to workers'' stridulations. Nevertheless playback experiments showed that ant workers responded most strongly to the sounds emitted by the integrated (i.e. post-adoption) larvae of the cuckoo species, as well as by those of predatory species recorded before any contact with the host ants (i.e. in pre-adoption), thereby revealing the role of acoustic signals both in parasite integration and in adoption rituals. We discuss our findings in the broader context of parasite adaptations, comparing effects of acoustical and chemical mimicry.     

Trichinella spiralis: mediation of the intestinal component of protective immunity in the rat by multiple, phase-specific, antiparasitic responses.

Rats infected orally with Trichinella spiralis developed an immunity that was induced by and expressed against separate phases of the parasite's enteral life cycle. Infectious muscle larvae generated an immune response (rapid expulsion) that was directed against the very early intestinal infection and resulted in the expulsion of worms within 24 hr. This response eliminated more than 95% of worms in an oral challenge inoculum. Developing larvae (preadults) also induced an immune response that was expressed against adult worms. The effect on adults was dependent upon continuous exposure of worms to the immune environment throughout their enteral larval development. Immunity induced by preadult T. spiralis was not expressed against adult worms transferred from nonimmune rats. While adult worms were resistant to the immunity engendered by preadults they induced an efficient immunity that was autospecific. Both “preadult” and “adult” immunities were expressed in depression of worm fecundity as well as in the expulsion of adults from the gut. However, the two reactions differed in respect to their kinetics and their efficiency against various worm burdens. Preadult immunity was directed mainly against fecundity whereas adult immunity favored worm expulsion. All responses (rapid expulsion, preadult and adult immunity, and antifecundity) acted synergistically to produce sterile immunity against challenge infections of up to 5000 muscle larvae. These findings indicate that the host protective response to T. spiralis is a complex, multifactorial process that operates sequentially and synergistically to protect the host against reinfection.     

Elastase B of Pseudomonas aeruginosa stimulates the humoral immune response in the greater wax moth, Galleria mellonella

The role of Pseudomonas aeruginosa elastase B in activation of the humoral immune response in Galleria mellonella larvae was investigated. The results of our study showed that elastase B injected at a sublethal concentration was responsible for eliciting the humoral immune response in G. mellonella larvae. The insects exhibited increased antibacterial activity, namely, we observed appearance of antimicrobial peptides and a higher level of lysozyme in cell-free hemolymph. Elastase B seems to be a more potent elicitor than thermolysin because similar maximal antibacterial activity levels were observed at a 5-fold lower concentration. We also demonstrated that there were differences in the kinetics of induction of antimicrobial activity between thermolysin and elastase B. The maximum level was observed 18 h post challenge of thermolysin and 38 h after injection of elastase B. It was also shown that, 24 h after elastase injection, the relative levels of apoLp-III in the hemolymph significantly increased in comparison with control G. mellonella larvae. The activation of immune responses in metalloproteinase-challenged larvae involved synthesis of metalloproteinase inhibitors which increased the survival rates of insects both against the lethal dose of thermolysin as well as against viable pathogenic bacterial cells of P. aeruginosa.     

Acquisition of epibiotic bacteria along the life cycle of the hydrothermal shrimp Rimicaris exoculata

The caridean shrimp Rimicaris exoculata dominates the fauna at several Mid-Atlantic Ridge hydrothermal vent sites. This shrimp has an enlarged gill chamber, harboring a dense ectosymbiotic community of chemoautotrophic bacteria associated with mineral oxide deposits. Until now, their acquisition is not fully understood. At three hydrothermal vent sites, we analyzed the epibionts diversity at different moult stages and also in the first stages of the shrimp life (eggs, hatched eggs (with larvae) and juveniles). Hatched eggs associated with young larvae were collected for the first time directly from gravid females at the Logachev vent site during the Serpentine cruise. An approach using 16S rRNA clone libraries, scanning and transmission electron microscopy, and fluorescent in situ hybridization was used. Molecular results and microscope observations indicated a switch in the composition of the bacterial community between early R. exoculata life cycle stage (egg libraries dominated by the Gammaproteobacteria) and later stages (juvenile/adult libraries dominated by the Epsilonproteobacteria). We hypothesized that the epibiotic phylotype composition could vary according to the life stage of the shrimp. Our results confirmed the occurrence of a symbiosis with Gammaproteobacteria and Epsilonproteobacteria, but more complex than previously assumed. We revealed the presence of active type-I methanotrophic bacteria colonizing the cephalothorax of shrimps from the Rainbow site. They were also present on the eggs from the Logachev site. This could be the first ‘epibiotic'' association between methanotrophic bacteria and hydrothermal vent crustacean. We discuss possible transmission pathways for epibionts linked to the shrimp life cycle.     

The roles of IL-12 and IL-23 in CD8 T cell-mediated immunity against Listeria monocytogenes: Insights from a DC vaccination model

Listeria monocytogenes infection induces a strong inflammatory response characterized by the production of IL-12 and IFN-γ and protective immunity against this pathogen is dependent on CD8⁺ T cells (CTL). Recent studies have suggested that these inflammatory cytokines affect the rate of memory CD8⁺ T cell generation as well as the number of short-lived effector cells generated. The role of the closely related cytokine, IL-23, in this response has not been examined. We hypothesized that IL-12 and IL-23 produced by dendritic cells collectively enhance the generation and function of memory cells. To test this hypothesis, we employed a DC vaccination approach. Mice lacking IL-12 and IL-23 were vaccinated with wild-type (WT), IL-12^−/−, or IL-12/23^−/− DC and protection to Lm was monitored. Mice vaccinated with WT and IL-12^−/− DC were resistant to lethal challenge with Lm. Surprisingly, mice vaccinated with IL-12/23^−/− DC exhibited significantly reduced protection when challenged. Protection correlated with the relative size of the memory pools generated. In summary, these data indicate that IL-23 can partially compensate for the lack of IL-12 in the generation protective immunity against Lm.