Antifungal activity in thrips soldiers suggests a dual role for this caste

The social insect soldier is perhaps the most widely known caste, because it often exhibits spectacular weapons, such as highly enlarged jaws or reinforced appendages, which are used to defend the colony against enemies ranging in size from wasps to anteaters. We examined the function of the enlarged forelimbs of soldiers (both male and female) of the eusocial, gall-inhabiting insect Kladothrips intermedius, and discovered that they have little impact on their ability to repel the specialized invading thrips Koptothrips species. While the efficacy of the enlarged forelimb appears equivocal, we show that soldiers secrete strong antifungal compounds capable of controlling the specialized insect fungal pathogen, Cordyceps bassiana. Our data suggest that these thrips soldiers have evolved in response to selection by both macro- and micro-organisms. While it is unknown whether specialized fungal pathogens have been major selective agents in the evolution of the soldier caste in general, they were probably present when sociality first evolved and may have been the primordial enemies of social insects.     

Task-specific expression of the foraging gene in harvester ants

In social insects, groups of workers perform various tasks such as brood care and foraging. Transitions in workers from one task to another are important in the organization and ecological success of colonies. Regulation of genetic pathways can lead to plasticity in social insect task behaviour. The colony organization of advanced eusocial insects evolved independently in ants, bees, and wasps and it is not known whether the genetic mechanisms that influence behavioural plasticity are conserved across species. Here we show that a gene associated with foraging behaviour is conserved across social insect species, but the expression patterns of this gene are not. We cloned the red harvester ant (Pogonomyrmex barbatus) ortholog (Pbfor) to foraging, one of few genes implicated in social organization, and found that foraging behaviour in harvester ants is associated with the expression of this gene; young (callow) worker brains have significantly higher levels of Pbfor mRNA than foragers. Levels of Pbfor mRNA in other worker task groups vary among harvester ant colonies. However, foragers always have the lowest expression levels compared to other task groups. The association between foraging behaviour and the foraging gene is conserved across social insects but ants and bees have an inverse relationship between foraging expression and behaviour.     

Medium molecular weight polar substances of the cuticle as tools in the study of the taxonomy,systematics and chemical ecology of tropical hover wasps (Hymenoptera: Stenogastrinae)

The Stenogastrinae wasps have been proposed as a key group for an understanding of social evolution in insects, but the phylogeny of the group is still under discussion. The use of chemical characters, in particular cuticular hydrocarbons, for insect taxonomy is relatively recent and only a few studies have been conducted on the cuticular polar substances. In this work, we ascertain, by the matrix‐assisted laser desorption ionization‐time of flight mass spectrometry technique, that different species of primitively eusocial hover wasps have different compositions of the epicuticular polar compounds ranging from 900 to 3600 Da. General linear model analysis and discriminant analysis showed that the average spectral profiles of this fraction can be diagnostic for identification of the species. Moreover, for the first time we show population diversification in the medium MW polar cuticular mixtures in insects. In conclusion, the results demonstrate that the chemical characters are consistent with the physical characters and the study support the importance of medium MW polar substances as powerful tools for systematics (chemosystematics) and chemical ecology (fertility signal and population characterization) in a primitively social insect taxon.     

Ant sperm storage organs do not have phenoloxidase constitutive immune activity

The prophenoloxidase system (proPO-AS) is a primordial constituent of insect innate immunity. Its broad action spectrum, rapid response time, and cytotoxic by-products induced by phenoloxidase (PO) production contribute to the effective clearing of invading pathogens. However, such immune reactions may not be optimal for insect organs that evolved to have mutualistic interactions with non-self-cells. Ant queens are long-lived, but only mate early in adult life and store the sperm in a specialized organ, the spermatheca. They never re-mate so their life-time reproductive success is ultimately sperm-limited, which maintains strong selection for high sperm viability before and after storage. The proPO-AS may therefore be inappropriate for the selective clearing of sexually transmitted infections, as it might also target sperm cells that cannot be replaced.We measured PO enzymatic activity in the sperm storage organs of three ant species before and after mating. Our data show that no PO is produced in the sperm storage organs, relative to other somatic tissues as controls, and that these negative results are not due to non-detection in small volumes as non-immune-relevant catalase activity in single spermatheca fluid samples of both virgin and mated queens was significant. The lack of PO activity in sperm storage organs across three different ant species may represent an evolutionarily conserved adaptation to life-long sperm storage by ant queens. We expect that PO activity will be similarly suppressed in queen spermathecae of other eusocial Hymenoptera (bees and wasps) and, more generally, of insect females that store sperm for long periods.     

The influence of sociality on the conservation biology of social insects

Social insects (ants, bees, wasps and termites) as a group are species rich and ecologically dominant. Many are outstanding "ecological engineers", or providers of "ecosystem services", or potential bioindicator species. Few social insects are currently formally classified as Threatened, but this is almost certainly due to a lack of information on population sizes and trends in scarce species. The main influence that sociality has on threats faced by social insects is in reducing effective population sizes, increasing population genetic subdivision and possibly reducing levels of genetic variation relative to solitary species. The main influence that sociality has on threats from social insects is via its role in the ecological success of invasive species, which frequently pose a major hazard to native biotas. In some cases, social features underpinning ecological success in the original range almost certainly contribute to the success of invasive social insects. However, recent studies show or strongly suggest that, in some of the most notoriously invasive populations of ants, bees and wasps, novel social traits have arisen that greatly enhance the rate of spread and ecological competitiveness of these populations. Sociality can therefore represent either a liability or an asset in its contribution to the persistence of social insect populations.     

Parasitic wasp‐associated symbiont affects plant‐mediated species interactions between herbivores

Microbial mutualistic symbiosis is increasingly recognised as a hidden driving force in the ecology of plant–insect interactions. Although plant‐associated and herbivore‐associated symbionts clearly affect interactions between plants and herbivores, the effects of symbionts associated with higher trophic levels has been largely overlooked. At the third‐trophic level, parasitic wasps are a common group of insects that can inject symbiotic viruses (polydnaviruses) and venom into their herbivorous hosts to support parasitoid offspring development. Here, we show that such third‐trophic level symbionts act in combination with venom to affect plant‐mediated interactions by reducing colonisation of subsequent herbivore species. This ecological effect correlated with changes induced by polydnaviruses and venom in caterpillar salivary glands and in plant defence responses to herbivory. Because thousands of parasitoid species are associated with mutualistic symbiotic viruses in an intimate, specific relationship, our findings may represent a novel and widespread ecological phenomenon in plant–insect interactions.     

Sociality and the rate of molecular evolution

The molecular clock does not tick at a uniform rate in all taxa but may be influenced by species characteristics. Eusocial species (those with reproductive division of labor) have been predicted to have faster rates of molecular evolution than their nonsocial relatives because of greatly reduced effective population size; if most individuals in a population are nonreproductive and only one or few queens produce all the offspring, then eusocial animals could have much lower effective population sizes than their solitary relatives, which should increase the rate of substitution of "nearly neutral" mutations. An earlier study reported faster rates in eusocial honeybees and vespid wasps but failed to correct for phylogenetic nonindependence or to distinguish between potential causes of rate variation. Because sociality has evolved independently in many different lineages, it is possible to conduct a more wide-ranging study to test the generality of the relationship. We have conducted a comparative analysis of 25 phylogenetically independent pairs of social lineages and their nonsocial relatives, including bees, wasps, ants, termites, shrimps, and mole rats, using a range of available DNA sequences (mitochondrial and nuclear DNA coding for proteins and RNAs, and nontranslated sequences). By including a wide range of social taxa, we were able to test whether there is a general influence of sociality on rates of molecular evolution and to test specific predictions of the hypothesis: (1) that social species have faster rates because they have reduced effective population sizes; (2) that mitochondrial genes would show a greater effect of sociality than nuclear genes; and (3) that rates of molecular evolution should be correlated with the degree of sociality. We find no consistent pattern in rates of molecular evolution between social and nonsocial lineages and no evidence that mitochondrial genes show faster rates in social taxa. However, we show that the most highly eusocial Hymenoptera do have faster rates than their nonsocial relatives. We also find that social parasites (that utilize the workers from related species to produce their own offspring) have faster rates than their social relatives, which is consistent with an effect of lower effective population size on rate of molecular evolution. Our results illustrate the importance of allowing for phylogenetic nonindependence when conducting investigations of determinants of variation in rate of molecular evolution.     

The effect of juvenile hormone on temporal polyethism in the paper wasp Polistes dominulus

Juvenile hormone (JH) has an important role in the behavior of eusocial Hymenoptera. Previous work has shown that JH influences aggression and dominance behavior in primitive eusocial insects that lack discrete queen and worker castes (e.g. Bombus bees and Polistes wasps). In contrast, JH is one of the factors that mediates temporal polyethism among workers in advanced eusocial insects that have reproductive castes (e.g. Apis bees and Polybiawasps). Therefore, initial observations suggest that JH may have different roles in primitive and advanced eusocial taxa. Here, we use detailed behavioral observations of marked individuals to test whether JH influences temporal polyethism in the primitive eusocial wasp Polistes dominulus. First, we show that workers in P. dominulus have an age-related division of labor, as workers switch from nest work to foraging as they mature. Then, we show that application of JH accelerates the onset of foraging behavior.Workers treated with JH start foraging at a younger age than control workers. Therefore, JH mediates temporal polyethism in the primitively eusocial insect Polistes dominulus. Received 23 April 2008; revised 6 August 2008; accepted 11 August 2008     

Multiple paternity or multiple queens: two routes to greater intracolonial genetic diversity in the eusocial Hymenoptera

Understanding the evolution of multiple mating by females (polyandry) is an important question in behavioural ecology. Most leading explanations for polyandry by social insect queens are based upon a postulated fitness benefit from increased intracolonial genetic diversity, which also arises when colonies are headed by multiple queens (polygyny). An indirect test of the genetic diversity hypotheses is therefore provided by the relationship between polyandry and polygyny across species, which should be negative if the genetic diversity hypotheses are correct. Here, we conduct a powerful comparative investigation of the relationship between polyandry and polygyny for 241 species of eusocial Hymenoptera (ants, bees and wasps). We find a clear and significant negative relationship between polyandry and polygyny after controlling for phylogeny. These results strongly suggest that fitness benefits resulting from increased intracolonial genetic diversity have played an important role in the evolution of polyandry, and possibly polygyny, in social insects.     

Little effect of seasonal constraints on population genetic structure in eusocial paper wasps

Climate has long been suggested to affect population genetic structures of eusocial insect societies. For instance, Hamilton [Journal of Theoretical Biology 7 (1964) 17] discusses whether temperate and tropical eusocial insects may show differences in population‐level genetic structure and viscosity, and how this might relate to differences in the degree of synchrony in their life cycles or modes of nest founding. Despite the importance of Hamilton's 1964 papers, this specific idea has not been tested in actual populations of wasps, probably due to the paucity of studies on tropical species. Here, we compare colony and population genetic structures in two species of primitively eusocial paper wasps with contrasting ecologies: the tropical species Polistes canadensis and the temperate species P. dominulus. Our results provide important clarifications of Hamilton's discussion. Specifically, we show that the genetic structures of the temperate and tropical species were very similar, indicating that seasonality does not greatly affect population viscosity or inbreeding. For both species, the high genetic differentiation between nests suggests strong selection at the nest level to live with relatives, whereas low population viscosity and low genetic differentiation between nest aggregations might reflect balancing selection to disperse, avoiding competition with relatives. Overall, our study suggests no prevalence of seasonal constraints of the life cycle in affecting the population genetic structure of eusocial paper wasps. These conclusions are likely to apply also to other primitively eusocial insects, such as halictine bees. They also highlight how selection for a kin structure that promotes altruism can override potential effects of ecology in eusocial insects.     

Social complexity and nesting habits are factors in the evolution of antimicrobial defences in wasps

Microbial diseases are important selective agents in social insects and one major defense mechanism is the secretion of cuticular antimicrobial compounds. We hypothesized that given differences in group size, social complexity, and nest type the secretions of these antimicrobials will be under different selective pressures. To test this we extracted secretions from nine wasp species of varying social complexity and nesting habits and assayed their antimicrobial compounds against cultures of Staphylococcus aureus. These data were then combined with phylogenetic data to provide an evolutionary context. Social species showed significantly higher (18x) antimicrobial activity than solitary species and species with paper nests showed significantly higher (11x) antimicrobial activity than those which excavated burrows. Mud-nest species showed no antimicrobial activity. Solitary, burrow-provisioning wasps diverged at more basal nodes of the phylogenetic trees, while social wasps diverged from the most recent nodes. These data suggest that antimicrobial defences may have evolved in response to ground-dwelling pathogens but the most important variable leading to increased antimicrobial strength was increase in group size and social complexity.     

Systematic analysis of a wasp parasitism arsenal

Parasitoid wasps are among the most diverse insects on earth with many species causing major mortality in host populations. Parasitoids introduce a variety of factors into hosts to promote parasitism, including symbiotic viruses, venom, teratocytes and wasp larvae. Polydnavirus‐carrying wasps use viruses to globally suppress host immunity and prevent rejection of developing parasites. Although prior results provide detailed insights into the genes viruses deliver to hosts, little is known about other products. RNAseq and proteomics were used to characterize the proteins secreted by venom glands, teratocytes and larvae from Microplitis demolitor, which carries M. demolitor bracovirus (MdBV). These data revealed that venom glands and teratocytes secrete large amounts of a small number of products relative to ovaries and larvae. Venom and teratocyte products exhibited almost no overlap with one another or MdBV genes, which suggested that M. demolitor effector molecules are functionally partitioned according to their source. This finding was well illustrated in the case of MdBV and teratocytes. Many viral proteins have immunosuppressive functions that include disruption of antimicrobial peptide production, yet this study showed that teratocytes express high levels of the antimicrobial peptide hymenoptaecin, which likely compensates for MdBV‐mediated immunosuppression. A second key finding was the prevalence of duplications among genes encoding venom and teratocyte molecules. Several of these gene families share similarities with proteins from other species, while also showing specificity of expression in venom glands or teratocytes. Overall, these results provide the first comprehensive analysis of the proteins a polydnavirus‐carrying wasp introduces into its host.     

Hemolytic activities of stinging insect venoms

The direct hemolytic activities of the venoms from 21 species of stinging insects were determined. The activities spanned 3 1/2 orders of magnitude, ranging from a low of four to a high of 12,000 hemolytic units/mg dry venom, respectively, for the solitary wasp, Dasymutilla lepeletierii, and the social wasp, Polistes infuscatus. The latter activity is the highest reported for any insect venom and represents a level that is potentially harmful to humans stung by the wasp. The social wasps as a group generally possessed highly hemolytic venoms; the ants, poorly hemolytic venoms; and the solitary stinging species, venoms with extremely low activity. For the venoms, hemolytic activity correlated with neither lethal toxicity (LD₅₀) nor algogenicity. This finding suggests that hemolysins alone do not determine venom toxicity, and that the hemolysins of stinging insect venoms serve a variety of poorly understood roles. The range of activity of hemolysins from different venoms indicates they probably have different chemical structures and functions.     

Evolution of the neuronal substrate for kin recognition in social Hymenoptera

In evolutionary terms, life is about reproduction. Yet, in some species, individuals forgo their own reproduction to support the reproductive efforts of others. Social insect colonies for example, can contain up to a million workers that actively cooperate in tasks such as foraging, brood care and nest defence, but do not produce offspring. In such societies the division of labour is pronounced, and reproduction is restricted to just one or a few individuals, most notably the queen(s). This extreme eusocial organisation exists in only a few mammals, crustaceans and insects, but strikingly, it evolved independently up to nine times in the order Hymenoptera (including ants, bees and wasps). Transitions from a solitary lifestyle to an organised society can occur through natural selection when helpers obtain a fitness benefit from cooperating with kin, owing to the indirect transmission of genes through siblings. However, this process, called kin selection, is vulnerable to parasitism and opportunistic behaviours from unrelated individuals. An ability to distinguish kin from non-kin, and to respond accordingly, could therefore critically facilitate the evolution of eusociality and the maintenance of non-reproductive workers. The question of how the hymenopteran brain has adapted to support this function is therefore a fundamental issue in evolutionary neuroethology. Early neuroanatomical investigations proposed that social Hymenoptera have expanded integrative brain areas due to selection for increased cognitive capabilities in the context of processing social information. Later studies challenged this assumption and instead pointed to an intimate link between higher social organisation and the existence of developed sensory structures involved in recognition and communication. In particular, chemical signalling of social identity, known to be mediated through cuticular hydrocarbons (CHCs), may have evolved hand in hand with a specialised chemosensory system in Hymenoptera. Here, we compile the current knowledge on this recognition system, from emitted identity signals, to the molecular and neuronal basis of chemical detection, with particular emphasis on its evolutionary history. Finally, we ask whether the evolution of social behaviour in Hymenoptera could have driven the expansion of their complex olfactory system, or whether the early origin and conservation of an olfactory subsystem dedicated to social recognition could explain the abundance of eusocial species in this insect order. Answering this question will require further comparative studies to provide a comprehensive view on lineage-specific adaptations in the olfactory pathway of Hymenoptera.     

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.     

The genomic impact of 100 million years of social evolution in seven ant species

Ants (Hymenoptera, Formicidae) represent one of the most successful eusocial taxa in terms of both their geographic distribution and species number. The publication of seven ant genomes within the past year was a quantum leap for socio- and ant genomics. The diversity of social organization in ants makes them excellent model organisms to study the evolution of social systems. Comparing the ant genomes with those of the honeybee, a lineage that evolved eusociality independently from ants, and solitary insects suggests that there are significant differences in key aspects of genome organization between social and solitary insects, as well as among ant species. Altogether, these seven ant genomes open exciting new research avenues and opportunities for understanding the genetic basis and regulation of social species, and adaptive complex systems in general.     

The Problems of a Priori Categorisation of Agonism and Cooperation: Circle‐Tube Interactions in Two Allodapine Bees

Circle‐tube experiments have been widely used to both examine nestmate recognition in social and solitary insects, as well as to characterise interactions in terms of agonism, cooperation, and avoidance. Despite their use in studies of halictid bees, carpenter bees, adrenid bees, and wasps, they have never been used to explore social interactions in the bee tribe Allodapini, where sociality varies widely in terms of complexity. Here, we investigate behaviours displayed within the highly eusocial allodapine Exoneurella tridentata and the facultatively social Exoneura robusta, using a standardised circle‐tube apparatus. We present two broad and important results: (i) Discrimination between nestmates and non‐nestmates is much more strongly expressed in the facultatively social species and (ii) principal components analyses do not indicate suites of behaviours that permit clear interpretations as being agonistic, cooperative, or avoidance. Our first set of results suggests that nestmate recognition is not an essential ability for social species. Our second set of results raise important questions about a priori functional categorisations of behaviours and whether these can be used to reliably classify observed behaviours as either avoidance, cooperation, or aggression. Our analyses highlight the risks of interpreting social insect interactions and call for a more cautious approach.     

Juvenile hormone accelerates ovarian development and does not affect age polyethism in the primitively eusocial wasp,Ropalidia marginata

Juvenile hormone modulates post-imaginal reproductive division of labor in primitively eusocial species and promotes the production of queens (e.g., Polistes) while it modulates age polyethism and promotes the production of foragers in highly eusocial species (e.g., the honey bee). Ropalidia marginata is a primitively eusocial wasp that shows both post-imaginal regulation of reproductive division of labor as well as age polyethism. Hence, R.marginata is a particularly interesting model system to study the effect of juvenile hormone. We demonstrate here that a single, topical application of 100 micro g of juvenile hormone-III per female wasp accelerates ovarian development of wasps held in isolation. Similar application to wasps released back on to their natal nests has no effect on their rate of behavioral development as witnessed from the age of first performance of feed larva, build, bring pulp and bring food. We conclude therefore that in R.marginata, juvenile hormone has retained its function of modulating reproductive division of labor and has not acquired the function of modulating age polyethism.