Chemical Communication and Reproduction Partitioning in Social Wasps

Paper wasps: A model clade for social cognition

Paper wasps are a highly intelligent group of socially flexible insects with complex lives and variation in social structures. They engage in sophisticated communication within their small societies using olfaction, vibration, and even visual signals of quality or individual identity in some species. Here we describe the social biology of paper wasps as well as the impressive visual and cognitive abilities seen in this group. We summarize the recent discoveries about where and how social information is processed in the wasp brain and highlight the potential of this clade to further our understanding of the neural underpinnings of complex social cognition, its development, and its evolution.

Assessment of the In Vivo and In Vitro Release of Chemical Compounds from Vespa velutina

Vespa velutina has been rapidly expanding throughout Galicia since 2012. It is causing human health risks and well-known losses in the beekeeping sector. Control methods are scarce, unspecific, and ineffective. Semiochemicals are insect-derived chemicals that play a role in communication and they could be used an integrated pest management tool alternative to conventional pesticides. A previous determination of the organic chemical profile should be the first step in the study of these semiochemicals. HS-SPME in living individuals and the sting apparatus extraction followed by GC-MS spectrometry were combined to extract a possible profile of these compounds in 43 hornets from Galicia. The identified compounds were hydrocarbons, ketones, terpenes, and fatty acid, and fatty acid esters. Nonanal aldehyde appeared in important concentrations in living individuals. While pentadecane, 8-hexyl- and ethyl oleate were mainly extracted from the venom apparatus. Ketones 2-nonanone, 2-undecanone and 7-nonen-2-one, 4,8-dimethyl- were identified by both procedures, as was 1,7-Nonadiene, 4,8-dimethyl-. Some compounds were detected for the first time in V. velutina such as naphthalene, 1,6-dimethyl-4-(1-methylethyl). The chemical profile by caste was also characterized.

Chemically Insignificant Social Parasites Exhibit More Anti-Dehydration Behaviors than Their Hosts

Simple Summary

Social parasites use a variety of deceptive mechanisms to avoid detection by their social-insect hosts and get tolerance in their colonies. One of these mechanisms is chemical insignificance, where social parasites have reduced amounts of recognition cues—hydrocarbons—on their cuticle, thus evading host chemical detection. This exposes social parasites to dehydration stress, as cuticular hydrocarbons also limit body water loss. By analyzing behavioral data from field observations, here we show that a Polistes wasp social parasite exhibits water-saving behaviors; parasites were less active than their cohabiting host foundresses, spent more time at the nest, and rested in the shadow, contradicting the rule that dominant individuals occupy prominent positions at the nest.

Abstract

Social parasites have evolved adaptations to overcome host resistance as they infiltrate host colonies and establish there. Among the chemical adaptations, a few species are chemically “insignificant”; they are poor in recognition cues (cuticular hydrocarbons) and evade host detection. As cuticular hydrocarbons also serve a waterproofing function, chemical insignificance is beneficial as it protects parasites from being detected but is potentially harmful because it exposes parasites to desiccation stress. Here I tested whether the social parasites Polistes atrimandibularis employ behavioral water-saving strategies when they live at Polistes biglumis colonies. Observations in the field showed that parasites were less active than their cohabiting host foundresses, spent more time at the nest, and rested in the shadowy, back face of the nest, rather than at the front face, which contradicted expectations for the use of space for dominant females—typically, dominants rest at the nest front-face. These data suggest that behavioral adaptations might promote resistance to desiccation stress in chemical insignificant social parasites.

Expansion and Accelerated Evolution of 9-Exon Odorant Receptors in Polistes Paper Wasps

Independent origins of sociality in bees and ants are associated with independent expansions of particular odorant receptor (OR) gene subfamilies. In ants, one clade within the OR gene family, the 9-exon subfamily, has dramatically expanded. These receptors detect cuticular hydrocarbons (CHCs), key social signaling molecules in insects. It is unclear to what extent 9-exon OR subfamily expansion is associated with the independent evolution of sociality across Hymenoptera, warranting studies of taxa with independently derived social behavior. Here, we describe OR gene family evolution in the northern paper wasp, Polistes fuscatus, and compare it to four additional paper wasp species spanning ∼40 million years of evolutionary divergence. We find 200 putatively functional OR genes in P. fuscatus, matching predictions from neuroanatomy, and more than half of these are in the 9-exon subfamily. Most OR gene expansions are tandemly arrayed at orthologous loci in Polistes genomes, and microsynteny analysis shows species-specific gain and loss of 9-exon ORs within tandem arrays. There is evidence of episodic positive diversifying selection shaping ORs in expanded subfamilies. Values of omega (dN/dS) are higher among 9-exon ORs compared to other OR subfamilies. Within the Polistes OR gene tree, branches in the 9-exon OR clade experience relaxed negative (relaxed purifying) selection relative to other branches in the tree. Patterns of OR evolution within Polistes are consistent with 9-exon OR function in CHC perception by combinatorial coding, with both natural selection and neutral drift contributing to interspecies differences in gene copy number and sequence.

Is It Possible to Obtain the Chemical Profile From Ethanol-Preserved Specimens? The Hydrocarbon and Fatty Acid Composition of the Social Wasp Polybia paulista (Hymenoptera: Vespidae: Epiponini)

Cuticular hydrocarbons perform multiple functions in insects such as protecting against desiccation and pathogenic infection, and signaling interactions. Evaluation of cuticular hydrocarbon (CHC) profiles of insects is commonly performed by extraction using a nonpolar solvent such as hexane. Specimens intended for CHC analysis are ideally handled by avoiding contact with solvents such as ethanol. However, insects are frequently stored in ethanol after collection, especially if intended for molecular analysis. To determine if it is possible that chemical compounds in the cuticles of specimens can withstand previous exposure to ethanol, we evaluated the efficiency of CHC extraction from specimens preserved in 95% ethanol. We extracted cuticular compounds from specimens of the social wasp Polybia paulista (Ihering) with no contact with ethanol solvents and compared them with those from specimens stored in 95% ethanol. We analyzed chemical composition from wasps and the 95% ethanol in which they had been stored by a gas chromatograph coupled to a mass spectrometer. In total, 56 compounds were detected: 50 that were classified as hydrocarbons which were mostly branched alkanes, followed by linear alkanes and alkenes. Three compounds were identified as fatty acids, and three compounds were unidentifiable. The ethanol-preserved specimens showed similar chemical profiles to those of specimens that had no contact with ethanol. Thus, we suggest that it is possible to study the chemical profiles of ethanol-preserved specimens.

Age and social experience induced plasticity across brain regions of the paper wasp Polistes fuscatus

Developmental studies of brain volumes can reveal which portions of neural circuits are sensitive to environmental inputs. In social insects, differences in relative investment across brain regions emerge as behavioural repertoires change during ontogeny or as a result of experience. Here, we test the effects of maturation and social experience on morphological brain development in Polistes fuscatus paper wasps, focusing on brain regions involved in visual and olfactory processing. We find that mature wasps regardless of social experience have relatively larger brains than newly emerged wasps and this difference is driven by changes to mushroom body calyx and visual regions but not olfactory processing neuropils. Notably, social wasps invest more in the anterior optic tubercle (AOT), a visual glomerulus involved in colour and object processing in other taxa, relative to other visual integration centres the mushroom body calyces compared with aged socially naive wasps. Differences in developmental plasticity between visual and olfactory neuropil volumes are discussed in light of behavioural maturation in paper wasps, especially as it relates to social recognition. Previous research has shown that P. fuscatus need social experience to develop specialized visual processing of faces, which is used to individually recognize conspecifics. The present study suggests that the AOT is a candidate brain region that could mediate facial processing in this species.

Reproductive inhibition among nestmate queens in the invasive Argentine ant

In social species, the presence of several reproductive individuals can generate conflict. In social insects, as queen number increases, individual oviposition rate may decrease because of direct and indirect behavioural and/or chemical interactions. Understanding the factors that mediate differences in queen fecundity should provide insight into the regulation and maintenance of highly polygynous insect societies, such as those of the invasive Argentine ant (Linepithema humile). In this study, we investigated (1) whether differences in the oviposition rates of Argentine ant queens exposed to polygynous conditions could result from interactions among them; (2) whether such differences in fecundity stemmed from differences in worker attention; and (3) whether polygynous conditions affected the cuticular hydrocarbon profiles of queens (CHCs). We found that differences in queen fecundity and CHC profiles observed under polygynous conditions disappeared when queens were exposed to monogynous conditions, suggesting some form of reproductive inhibition may exist when queens cohabit. These differences did not seem to arise from variation in worker attention because more fecund queens were not more attractive to workers. Levels of some CHCs were higher in more fecund queens. These CHCs are associated with greater queen productivity and survival. Our findings indicate that such compounds could be multifunctional queen pheromones.

Pheromonal Regulation of the Reproductive Division of Labor in Social Insects

The reproductive altruism in social insects is an evolutionary enigma that has been puzzling scientists starting from Darwin. Unraveling how reproductive skew emerges and maintains is crucial to understand the reproductive altruism involved in the consequent division of labor. The regulation of adult worker reproduction involves conspecific inhibitory signals, which are thought to be chemical signals by numerous studies. Despite the primary identification of few chemical ligands, the action modes of primer pheromones that regulate reproduction and their molecular causes and effects remain challenging. Here, these questions were elucidated by comprehensively reviewing recent advances. The coordination with other modalities of queen pheromones (QPs) and its context-dependent manner to suppress worker reproduction were discussed under the vast variation and plasticity of reproduction during colony development and across taxa. In addition to the effect of QPs, special attention was paid to recent studies revealing the regulatory effect of brood pheromones. Considering the correlation between pheromone and hormone, this study focused on the production and perception of pheromones under the endocrine control and highlighted the pivotal roles of nutrition-related pathways. The novel chemicals and gene pathways discovered by recent works provide new insights into the understanding of social regulation of reproductive division of labor in insects.

Context matters: plasticity in response to pheromones regulating reproduction and collective behavior in social Hymenoptera

Pheromones mediating social behavior are critical components in the cohesion and function of the colony and are instrumental in the evolution of eusocial insect species. However, different aspects of colony function, such as reproductive division of labor and colony maintenance (e.g. foraging, brood care, and defense), pose different challenges for the optimal function of pheromones. While reproductive communication is shaped by forces of conflict and competition, colony maintenance calls for enhanced cooperation and self-organization. Mechanisms that ensure efficacy, adaptivity and evolutionary stability of signals such as structure-to-function suitability, honesty and context are important to all chemical signals but vary to different degrees between pheromones regulating reproductive division of labor and colony maintenance. In this review, we will discuss these differences along with the mechanisms that have evolved to ensure pheromone adaptivity in reproductive and non-reproductive context.