Evolution of Cuticular Hydrocarbons in the Hymenoptera: a Meta-Analysis

Cuticular Hydrocarbons as Putative Sex Pheromones and Mating Peculiarity of Cuckoo Wasps Chrysis Angustula, Chrysis fulgida and Chrysis Iris (Hymenoptera: Chrysididae)

Cuckoo wasps, also known as jewel or gold wasps (Hymenoptera: Chrysididae), are kleptoparasites and parasitoids that impose their offspring on the breeding efforts of other wasp species. Chrysidids oviposit in the nests of predatory wasps, and the hatched larva kills the host's larva and consumes the resources collected by the host. When a cuckoo wasp is detected by the host wasp, the host may abandon the nest or take other measures to prevent the development of the kleptoparasite. Chemical camouflage, particularly involving cuticlar hydrocarbons (CHCs), is crucial for cuckoo wasp females. There is a conflict because females need chemical signals that are easily detectable by males but not by the host wasps. It was demonstrated that virgin females of Chrysis angustula, Chrysis fulgida, and Chrysis iris contain CHCs that stimulate behavioral reactions in conspecific males. Coupled gas chromatography-electroantennogram detection analysis revealed 16 hydrocarbons with carbon chain lengths of C23-27 in virgin females: 6 in C. angustula, 8 in C. fulgida, and 13 in C. iris. A specific ratio of CHC compounds, including putative sex pheromones, was established for these three chrysidid species. Observations of re-mating refusals led to the hypothesis that virgin females of cuckoo wasps cease production of some unsaturated CHCs, functioning as sex pheromones, following mating. This change could reduce the risk of detection of CHCs traces left while entering the host's nest.

Variation in Ants' Chemical Recognition Signals across Vineyard Agroecosystems

Ant evolutionary success depends mainly on the coordination of colony members, who recognize nestmates based on the cuticular hydrocarbon (CHC) profile of their epicuticle. While several studies have examined variations in this crucial factor for colony identity, few have investigated the anthropic impact on CHC profiles, and none have focused on Lasius paralienus. Here, we surveyed the changes in L. paralienus CHC assemblages across agroecosystems and assessed whether different vineyard management influences these profiles. Soil sampling within ant nests and in close surroundings was performed to measure microhabitat variations. Our results show that the cuticular chemical composition of Lasius paralienus is mainly affected by the differences between areas, with an existing but unclear anthropic influence on them. Normalized soil respiration partially explains these interarea variations. Irrespective of the conventional or organic management, human activities in agroecosystems mostly impacted L. paralienus linear alkanes, a specific class of CHCs known to play a major role against dehydration, but also affected the abundance of compounds that can be pivotal for maintaining the colony identity. Our findings suggest that vineyard practices primarily affect features of the ant cuticle, potentially enhancing microclimate adaptations. Still, the potential effects as disruptive factors need further investigation through the implementation of behavioral bioassays.

Evolution of odorant receptor repertoires across Hymenoptera is not linked to the evolution of eusociality

Communication is essential for social organisms. In eusocial insects, olfaction facilitates communication and recognition between nestmates. The study of certain model organisms has led to the hypothesis that odorant receptors are expanded in eusocial Hymenoptera. This has become a widely mentioned idea in the literature, albeit with conflicting reports, and has not been tested with a broad comparative analysis. Here we combined existing genomic and new neuroanatomical data, including from an approximately 100 Myr old fossil ant, across a phylogenetically broad sample of hymenopteran lineages. We find no evidence that variation in the size and evolutionary tempo of odorant receptor repertoires is related to eusociality. Post hoc exploration of our data hinted at loss of flight as a possible factor shaping some of the variation in OR repertoires in Hymenoptera. Nevertheless, our analyses revealed a complex pattern of evolutionary variation, and raise new questions about the ecological, behavioural and social factors that shape olfactory abilities.

Chemical and transcriptomic diversity do not correlate with ascending levels of social complexity in the insect order Blattodea

Eusocial insects, such as ants and termites, are characterized by high levels of coordinated social organization. This is contrasted by solitary insects that display more limited forms of collective behavior. It has been hypothesized that this gradient in sociobehavioral sophistication is positively correlated with chemical profile complexity, due to a potentially increased demand for diversity in chemical communication mechanisms in insects with higher levels of social complexity. However, this claim has rarely been assessed empirically. Here, we compare different levels of chemical and transcriptomic complexity in selected species of the order Blattodea that represent different levels of social organization, from solitary to eusocial. We primarily focus on cuticular hydrocarbon (CHC) complexity, since it has repeatedly been demonstrated that CHCs are key signaling molecules conveying a wide variety of chemical information in solitary as well as eusocial insects. We assessed CHC complexity and divergence between our studied taxa of different social complexity levels as well as the differentiation of their respective repertoires of CHC biosynthesis gene transcripts. Surprisingly, we did not find any consistent pattern of chemical complexity correlating with social complexity, nor did the overall chemical divergence or transcriptomic repertoire of CHC biosynthesis genes reflect on the levels of social organization. Our results challenge the assumption that increasing social complexity is generally reflected in more complex chemical profiles and point toward the need for a more cautious and differentiated view on correlating complexity on a chemical, genetic, and social level.

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.

Pathogen-Mediated Alterations of Insect Chemical Communication: From Pheromones to Behavior

Pathogens can influence the physiology and behavior of both animal and plant hosts in a manner that promotes their own transmission and dispersal. Recent research focusing on insects has revealed that these manipulations can extend to the production of pheromones, which are pivotal in chemical communication. This review provides an overview of the current state of research and available data concerning the impacts of bacterial, viral, fungal, and eukaryotic pathogens on chemical communication across different insect orders. While our understanding of the influence of pathogenic bacteria on host chemical profiles is still limited, viral infections have been shown to induce behavioral changes in the host, such as altered pheromone production, olfaction, and locomotion. Entomopathogenic fungi affect host chemical communication by manipulating cuticular hydrocarbons and pheromone production, while various eukaryotic parasites have been observed to influence insect behavior by affecting the production of pheromones and other chemical cues. The effects induced by these infections are explored in the context of the evolutionary advantages they confer to the pathogen. The molecular mechanisms governing the observed pathogen-mediated behavioral changes, as well as the dynamic and mutually influential relationships between the pathogen and its host, are still poorly understood. A deeper comprehension of these mechanisms will prove invaluable in identifying novel targets in the perspective of practical applications aimed at controlling detrimental insect species.

Cuticular hydrocarbons as weathering biomarkers of empty puparia of the forensically important blowfly Calliphora vicina Robineau-Desvoidy, 1830 (Diptera: Calliphoridae) in soil v/s under room conditions

Forensic entomology uses the age of insects, such as blow flies, to determine a minimum post-mortem interval (PMImin). Recent research has focused on using the analysis of specific cuticular hydrocarbons (CHCs) in adult insects and their empty puparia to estimate their age, as it has been shown that their profile changes are consistent with age. The current work is based on the weathering of five CHCs from empty puparia of Calliphora vicina that were stored in soil (field/outdoor) and non-soil (room/indoor conditions) based pupariation media for a total of six months. The experiment was conducted in a controlled environment chamber at a constant temperature of 25 ± 2 °C under constant darkness. Gas chromatography-mass spectrometry (GC-MS) was used to analyze the cuticular hydrocarbons after they were extracted in n-Hexane. n-Pentacosane, n-Hexacosane, n-Heptacosane, n-Octacosane, and n-Nonacosane were the five CHCs investigated. Results showed that CHCs weathered more quickly in the soil than in the non-soil environment. It was also found that the abundance of Heptacosane increased in the samples during the fifth month when stored in a non-soil medium, while the abundances of all five CHCs were not detected after eight weeks onwards in soil pupation medium.

Decoding the genetic and chemical basis of sexual attractiveness in parasitic wasps

Attracting and securing potential mating partners is of fundamental importance for reproduction. Therefore, signaling sexual attractiveness is expected to be tightly coordinated in communication systems synchronizing senders and receivers. Chemical signaling has permeated through all taxa of life as the earliest and most widespread form of communication and is particularly prevalent in insects. However, it has been notoriously difficult to decipher how exactly information related to sexual signaling is encoded in complex chemical profiles. Similarly, our knowledge of the genetic basis of sexual signaling is very limited and usually restricted to a few case studies with comparably simple pheromonal communication mechanisms. The present study jointly addresses these two knowledge gaps by characterizing two fatty acid synthase genes that most likely evolved by tandem gene duplication and that simultaneously impact sexual attractiveness and complex chemical surface profiles in parasitic wasps. Gene knockdown in female wasps dramatically reduces their sexual attractiveness coinciding with a drastic decrease in male courtship and copulation behavior. Concordantly, we found a striking shift of methyl-branching patterns in the female surface pheromonal compounds, which we subsequently demonstrate to be the main cause for the greatly reduced male mating response. Intriguingly, this suggests a potential coding mechanism for sexual attractiveness mediated by specific methyl-branching patterns in complex cuticular hydrocarbon (CHC) profiles. So far, the genetic underpinnings of methyl-branched CHCs are not well understood despite their high potential for encoding information. Our study sheds light on how biologically relevant information can be encoded in complex chemical profiles and on the genetic basis of sexual attractiveness.

Neuroendocrine mechanisms contributing to the coevolution of sociality and communication

Communication is inherently social, so signaling systems should evolve with social systems. The 'social complexity hypothesis' posits that social complexity necessitates communicative complexity and is generally supported in vocalizing mammals. This hypothesis, however, has seldom been tested outside the acoustic modality, and comparisons across studies are confounded by varying definitions of complexity. Moreover, proximate mechanisms underlying coevolution of sociality and communication remain largely unexamined. In this review, we argue that to uncover how sociality and communication coevolve, we need to examine variation in the neuroendocrine mechanisms that coregulate social behavior and signal production and perception. Specifically, we focus on steroid hormones, monoamines, and nonapeptides, which modulate both social behavior and sensorimotor circuits and are likely targets of selection during social evolution. Lastly, we highlight weakly electric fishes as an ideal system in which to comparatively address the proximate mechanisms underlying relationships between social and signal diversity in a novel modality.

Cuticular hydrocarbons of alpine bumble bees (Hymenoptera: Bombus) are species-specific, but show little evidence of elevation-related climate adaptation

Alpine bumble bees are the most important pollinators in temperate mountain ecosystems. Although they are used to encounter small-scale successions of very different climates in the mountains, many species respond sensitively to climatic changes, reflected in spatial range shifts and declining populations worldwide. Cuticular hydrocarbons (CHCs) mediate climate adaptation in some insects. However, whether they predict the elevational niche of bumble bees or their responses to climatic changes remains poorly understood. Here, we used three different approaches to study the role of bumble bees’ CHCs in the context of climate adaptation: using a 1,300 m elevational gradient, we first investigated whether the overall composition of CHCs, and two potentially climate-associated chemical traits (proportion of saturated components, mean chain length) on the cuticle of six bumble bee species were linked to the species’ elevational niches. We then analyzed intraspecific variation in CHCs of Bombus pascuorum along the elevational gradient and tested whether these traits respond to temperature. Finally, we used a field translocation experiment to test whether CHCs of Bombus lucorum workers change, when translocated from the foothill of a cool and wet mountain region to (a) higher elevations, and (b) a warm and dry region. Overall, the six species showed distinctive, species-specific CHC profiles. We found inter- and intraspecific variation in the composition of CHCs and in chemical traits along the elevational gradient, but no link to the elevational distribution of species and individuals. According to our expectations, bumble bees translocated to a warm and dry region tended to express longer CHC chains than bumble bees translocated to cool and wet foothills, which could reflect an acclimatization to regional climate. However, chain lengths did not further decrease systematically along the elevational gradient, suggesting that other factors than temperature also shape chain lengths in CHC profiles. We conclude that in alpine bumble bees, CHC profiles and traits respond at best secondarily to the climate conditions tested in this study. While the functional role of species-specific CHC profiles in bumble bees remains elusive, limited plasticity in this trait could restrict species’ ability to adapt to climatic changes.

Intrasexual cuticular hydrocarbon dimorphism in a wasp sheds light on hydrocarbon biosynthesis genes in Hymenoptera

Cuticular hydrocarbons (CHCs) cover the cuticle of insects and serve as desiccation barrier and as semiochemicals. While the main enzymatic steps of CHC biosynthesis are well understood, few of the underlying genes have been identified. Here we show how exploitation of intrasexual CHC dimorphism in a mason wasp, Odynerus spinipes, in combination with whole-genome sequencing and comparative transcriptomics facilitated identification of such genes. RNAi-mediated knockdown of twelve candidate gene orthologs in the honey bee, Apis mellifera, confirmed nine genes impacting CHC profile composition. Most of them have predicted functions consistent with current knowledge of CHC metabolism. However, we found first-time evidence for a fatty acid amide hydrolase also influencing CHC profile composition. In situ hybridization experiments furthermore suggest trophocytes participating in CHC biosynthesis. Our results set the base for experimental CHC profile manipulation in Hymenoptera and imply that the evolutionary origin of CHC biosynthesis predates the arthropods' colonization of land.

Sensory plasticity in a socially plastic bee

The social Hymenoptera have contributed much to our understanding of the evolution of sensory systems. Attention has focussed chiefly on how sociality and sensory systems have evolved together. In the Hymenoptera, the antennal sensilla are important for optimizing the perception of olfactory social information. Social species have denser antennal sensilla than solitary species, which is thought to enhance social cohesion through nestmate recognition. In the current study, we test whether sensilla numbers vary between populations of the socially plastic sweat bee Halictus rubicundus from regions that vary in climate and the degree to which sociality is expressed. We found population differences in both olfactory and hygro/thermoreceptive sensilla numbers. We also found evidence that olfactory sensilla density is developmentally plastic: when we transplanted bees from Scotland to the south-east of England, their offspring (which developed in the south) had more olfactory hairs than the transplanted individuals themselves (which developed in Scotland). The transplanted bees displayed a mix of social (a queen plus workers) and solitary nesting, but neither individual nor nest phenotype was related to sensilla density. We suggest that this general, rather than caste-specific sensory plasticity provides a flexible means to optimize sensory perception according to the most pressing demands of the environment. Sensory plasticity may support social plasticity in H. rubicundus but does not appear to be causally related to it.

Why do ants differ in acclimatory ability? Biophysical mechanisms behind cuticular hydrocarbon acclimation across species

Maintaining water balance is vital for terrestrial organisms. Insects protect themselves against desiccation via cuticular hydrocarbons (CHCs). CHC layers are complex mixtures of solid and liquid hydrocarbons, with a surprisingly diverse composition across species. This variation may translate to differential phase behaviour, and hence varying waterproofing capacity. This is especially relevant when temperatures change, which requires acclimatory CHC changes to maintain waterproofing. Nevertheless, the physical consequences of CHC variation are still little understood. We studied acclimatory responses and their consequences for CHC composition, phase behaviour, and drought survival in three congeneric ant species. Colony fragments were kept under cool, warm, and fluctuating temperature regimes. Lasius niger and platythorax, both of which are rich in methyl-branched alkanes, showed largely predictable acclimatory changes of the CHC profile. In both species, warm acclimation increased drought resistance. Warm acclimation increased the proportion of solid compounds in L. niger but not in L. platythorax. In both species, the CHC layer formed a liquid matrix of constantly low viscosity, which contained highly viscous and solid parts. This phase heterogeneity may be adaptive, increasing robustness to temperature fluctuations. In L. brunneus, which is rich in unsaturated hydrocarbons, acclimatory CHC changes were less predictable, and warm acclimation did not enhance drought survival. The CHC layer was more homogenous, but matrix viscosity changed with acclimation. We showed that ant species use different physical mechanisms to enhance waterproofing during acclimation. Hence, the ability to acclimate, and thus climatic niche breadth, may strongly depend on species-specific CHC profile.

Genetic and genomic architecture of species-specific cuticular hydrocarbon variation in parasitoid wasps

Cuticular hydrocarbons (CHCs) serve two fundamental functions in insects: protection against desiccation and chemical signalling. How the interaction of genes shapes CHC profiles, which are essential for insect survival, adaptation and reproductive success, is still poorly understood. Here we investigate the genetic and genomic basis of CHC biosynthesis and variation in parasitoid wasps of the genus Nasonia. We mapped 91 quantitative trait loci (QTL) explaining the variation of a total of 43 CHCs in F₂ hybrid males from interspecific crosses between three Nasonia species. To identify candidate genes, we localized orthologues of CHC biosynthesis-related genes in the Nasonia genomes. We discovered multiple genomic regions where the location of QTL coincides with the location of CHC biosynthesis-related candidate genes. Most conspicuously, on a region close to the centromere of chromosome 1, multiple CHC biosynthesis-related candidate genes co-localize with several QTL explaining variation in methyl-branched alkanes. The genetic underpinnings behind this compound class are not well understood so far, despite their high potential for encoding chemical information as well as their prevalence in hymenopteran CHC profiles. Our study considerably extends our knowledge on the genetic architecture governing this important compound class, establishing a model for methyl-branched alkane genetics in the Hymenoptera in general.

Cuticular hydrocarbons as caste-linked cues in Neotropical swarm-founding wasps

Wasps (Vespidae) are important organisms to understand the evolution of social behaviour. Wasps show different levels of sociality, which includes solitary to highly eusocial organisms. In social insect species, queens and workers differ in physiology and morphology. The Neotropical swarm-founding wasps (Epiponini) show a variety of caste syndromes. In this clade, the caste-flexibility is a unique characteristic, in which workers can become queens and swarm to start a new nest. The investigation of the caste system comparing several Epiponini species show a clear-cut morphological distinction between queens and workers, with a morphological continuum between queens and workers. However, whether cuticular hydrocarbons (CHCs) are used as cues for caste recognition in swarm-founding wasps is still unknown. We studied whether CHCs may display caste-linked differences in eleven species of Epiponini wasps and if CHCs differences would follow morphological patterns. Our results suggest that queens and workers of Epiponini wasps are chemically different from each other at two levels, qualitatively and quantitatively, or merely quantitatively. This variation seems to exist regardless of their morphological traits and may be useful to help us understanding how chemical communication evolved differently in these species.

Inter- and Intrasexual Variation in Cuticular Hydrocarbons in Trichrysis cyanea (Linnaeus, 1758) (Hymenoptera: Chrysididae)

Simple Summary

The biology of many species of cuckoo wasps (Chrysididae) is largely unknown, except for, if at all, the host species of these brood parasites. Cuticular hydrocarbons (CHCs), which form a waxy layer on the body surface of insects, have been shown to be mimicked by host-specific cuckoo wasp species. We studied the CHC profiles of a rather generalist brood parasitic cuckoo wasp species, Trichrysis cyanea, in detail. We found sex-related differences in CHCs and three different chemotypes among females. Genetic data reject the possibility that these different chemotypes represent different (cryptic) species. The CHC polymorphism could be an adaptation for females to sneak into nesting holes of hosts with the different female chemotypes representing adaptations to a broader host range. However, since information on the CHC profiles of the hosts is missing, it remains unclear whether these different CHC profiles are used for chemical mimicry or are simply age-related, reflecting the females’ reproductive state.

Abstract

Cuckoo wasps (Chrysididae, Hymenoptera) are known for their parasitoid or cleptoparasitic life histories. Indeed, the biology of only a few species has been studied in detail and often only little more is known than the host species. By mimicking their hosts’ cuticular hydrocarbon (CHC) profiles, species that parasitize single (or a few closely related) host species manage to deceive their hosts. However, the variability of the CHC profile in generalist cuckoo-wasp species is still unknown. Here, we used gas chromatography—mass spectrometry (GC-MS) and DNA barcoding to study intraspecific variation in cuticular hydrocarbons of one less host-specific species of cuckoo wasps, Trichrysis cyanea. Cuticular hydrocarbon (CHC) patterns were found to differ between males and females. Additionally, we found chemical polymorphism among females, which formed three distinct chemical subgroups characterized by different alkene patterns. A lack of divergence in the DNA barcoding region suggests that these different chemotypes do not represent cryptic species. Whether this intrasexual CHC-profile variation is an adaptation (mimicry) to different host species, or simply signaling the reproductive status, remains unclear.

Neglected Very Long-Chain Hydrocarbons and the Incorporation of Body Surface Area Metrics Reveal Novel Perspectives for Cuticular Profile Analysis in Insects

Simple Summary

The waxy layer covering the surface of most terrestrial insects is mainly composed of non-polar lipids termed cuticular hydrocarbons (CHCs). These have a long research history as important dual traits for both desiccation prevention and chemical communication. We analyzed CHC profiles of seven species of the insect order Blattodea (termites and cockroaches) with the most commonly applied chromatographic method, gas-chromatography coupled with mass spectrometry (GC-MS), and the more novel approach of silver-assisted laser desorption/ionization mass spectrometry (Ag-LDI-MS). Comparing these two analytical methods, we demonstrated that the conventional GC-MS approach does not provide enough information on the entire CHC profile range in the tested species. Ag-LDI-MS was able to detect very long-chain CHCs ranging up to C58, which remained undetected when solely relying on standard GC-MS analysis. Additionally, we measured the body surface areas of each tested species applying 3D scanning technology to assess their respective CHC amounts per mm². When adjusting for body surface areas, proportional CHC quantity distributions shifted considerably between our studied species, suggesting the importance of including this factor when conducting quantitative CHC comparisons, particularly in insects that vary substantially in body size.

Abstract

Most of our knowledge on insect cuticular hydrocarbons (CHCs) stems from analytical techniques based on gas-chromatography coupled with mass spectrometry (GC-MS). However, this method has its limits under standard conditions, particularly in detecting compounds beyond a chain length of around C40. Here, we compare the CHC chain length range detectable by GC-MS with the range assessed by silver-assisted laser desorption/ionization mass spectrometry (Ag-LDI-MS), a novel and rarely applied technique on insect CHCs, in seven species of the order Blattodea. For all tested species, we unveiled a considerable range of very long-chain CHCs up to C58, which are not detectable by standard GC-MS technology. This indicates that general studies on insect CHCs may frequently miss compounds in this range, and we encourage future studies to implement analytical techniques extending the conventionally accessed chain length range. Furthermore, we incorporate 3D scanned insect body surface areas as an additional factor for the comparative quantification of extracted CHC amounts between our study species. CHC quantity distributions differed considerably when adjusted for body surface areas as opposed to directly assessing extracted CHC amounts, suggesting that a more accurate evaluation of relative CHC quantities can be achieved by taking body surface areas into account.

Courtship behavior and identification of a sex pheromone in Ibalia leucospoides (Hymenoptera: Ibaliidae), a larval parasitoid of Sirex noctilio (Hymenoptera: Siricidae)

BACKGROUND

Ibalia leucospoides (Hymenoptera: Ibaliidae) is a larval parasitoid that has been widely introduced as a biological control agent for the invasive woodwasp,Sirex noctilio (Hymenoptera: Siricidae) in the Southern Hemisphere. In this study, the courtship behavior and identificaion of sex pheromones are described for I. leucospoides under laboratory conditions.

METHODS

For courtship behavior, both sexes were observed in a wire mesh observation cylinder (75 cm length ×10 cm diameter) for 15 minutes. The female body washes were analyzed using Gas Chromatography- Electroantennographic Detection (GC-EAD). Then the EAD-active compounds were tentatively identified using GC-Mass Spectrometry (GC-MS) and examined in olfactometer assays.

RESULTS

The courtship behavior included rhythmic lateral movements, mounting, head-nodding cycles in males, and wing-fanning in females. GC-EAD analysis of female body washes with male antennae revealed seven compounds which elicited antennal responses, four of which are straight-chain alkanes (C23, C25, C26, and C27). The identities of these alkanes were confirmed by matching the retention times, mass spectra, and male antennal activity to those of commercially obtained chemicals. In olfactometer assays, a blend of the four straight-chain alkanes was attractive to I. leucospoides males, and there was no response to blends that lacked any of these four compounds. Female body wash was no more attractive than the four-component blend. The ratios of EAD-active components differ between hydrocarbon profiles from males and females.

CONCLUSION

This study is the first investigation of cuticular hydrocarbons in the family Ibaliidae. It provides evidence that the ubiquitous alkanes (C23, C25, C26, and C27) in sex-specific ratios attract I. leucospoides males.

Olfactory System Morphology Suggests Colony Size Drives Trait Evolution in Odorous Ants (Formicidae: Dolichoderinae)

In social insects colony fitness is determined in part by individual worker phenotypes. Across ant species, colony size varies greatly and is thought to affect worker trait variation in both proximate and ultimate ways. Little is known about the relationship between colony size and worker trait evolution, but hypotheses addressing the role of social structure in brain evolution suggest workers of small-colony species may have larger brains or larger brain regions necessary for complex behaviors. In previous work on odorous ants (Formicidae: Dolichoderinae) we found no correlation between colony size and these brain properties, but found that relative antennal lobe size scaled negatively with colony size. Therefore, we now test whether sensory systems scale with colony size, with particular attention to olfactory components thought to be involved in nestmate recognition. Across three species of odorous ants, Forelius mccooki, Dorymyrmex insanus, and D. bicolor, which overlap in habitat and foraging ecology but vary in colony size, we compare olfactory sensory structures, comparing those thought to be involved in nestmate recognition. We use the visual system, a sensory modality not as important in social communication in ants, as a control comparison. We find that body size scaling largely explains differences in eye size, antennal length, antennal sensilla density, and total number of olfactory glomeruli across these species. However, sensilla basiconica and olfactory glomeruli in the T6 cluster of the antennal lobe, structures known to be involved in nestmate recognition, do not follow body size scaling observed for other structures. Instead, we find evidence from the closely related Dorymyrmex species that the larger colony species, D. bicolor, invests more in structures implicated in nestmate recognition. To test for functional consequences, we compare nestmate and non-nestmate interactions between these two species and find D. bicolor pairs of either type engage in more interactions than D. insaus pairs. Thus, we do not find evidence supporting a universal pattern of sensory system scaling associated with changes in colony size, but hypothesize that observed differences in the olfactory components in two closely related Dorymyrmex species are evidence of a link between colony size and sensory trait evolution.

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.

Cuticular Hydrocarbon Profile of Parasitic Beetles, Aethina tumida (Coleoptera: Nitidulidae)

Simple Summary

Social insects use cuticular hydrocarbons for chemical recognition and communication. Cuticular hydrocarbons can also be exploited by parasites to their advantage for undermining host recognition systems. The small hive beetle (SHB) is a parasite of honey bee colonies but can also infest nests of other bee species. However, its chemical profile is still not known. For the first time, the present study investigated the SHB chemical profile and compared it with that of its honey bee host. The results show that the SHB has a low chemical profile that is similar to its honey bee host’s. However, while honey bees had a clear colony-specific chemical profile, SHBs did not. The generic chemical profile of the SHB is most likely linked to its free-flying behaviour in the field as these parasites are known to switch between host colonies, possibly limiting the acquisition of a colony specific chemical profile. Our findings also suggest that SHBs do not exploit any finely tuned chemical strategy to conceal their presence inside host colonies and probably rely on behavioural adaptations.

Abstract

Cuticular hydrocarbons (CHCs) cover insects’ bodies and play important roles in chemical communication, including nestmate recognition, for social insects. To enter colonies of a social host species, parasites may acquire host-specific CHCs or covertly maintain their own CHC profile by lowering its quantity. However, the chemical profile of small hive beetles (SHBs), Aethina tumida, which are parasites of honey bee, Apis mellifera, colonies, and other bee nests, is currently unknown. Here, adults of SHB and honey bee host workers were collected from the same field colonies and their CHC profiles were analysed using GC-MS. The chemical profiles of field-sampled SHBs were also compared with those of host-naive beetles reared in the laboratory. Laboratory-reared SHBs differed in their CHC profiles from field-sampled ones, which showed a more similar, but ten-fold lower, generic host CHC profile compared to host workers. While the data confirm colony-specific CHCs of honey bee workers, the profile of field-collected SHBs was not colony-specific. Adult SHBs often commute between different host colonies, thereby possibly preventing the acquisition of a colony-specific CHC profiles. An ester was exclusive to both groups of SHBs and might constitute an intraspecific recognition cue. Our data suggest that SHBs do not use any finely tuned chemical strategy to conceal their presence inside host colonies and instead probably rely on their hard exoskeleton and defence behaviours.

Cuticular Hydrocarbon Plasticity in Three Rice Planthopper Species

Insect cuticular hydrocarbons (CHCs) are organic compounds of the surface lipid layer, which function as a barrier against water loss and xenobiotic penetration, while also serving as chemical signals. Plasticity of CHC profiles can vary depending upon numerous biological and environmental factors. Here, we investigated potential sources of variation in CHC profiles of Nilaparvata lugens, Laodelphax striatellus and Sogatella furcifera, which are considered to be the most important rice pests in Asia. CHC profiles were quantified by GC/MS, and factors associated with variations were explored by conducting principal component analysis (PCA). Transcriptomes were further compared under different environmental conditions. The results demonstrated that CHC profiles differ among three species and change with different developmental stages, sexes, temperature, humidity and host plants. Genes involved in cuticular lipid biosynthesis pathways are modulated, which might explain why CHC profiles vary among species under different environments. Our study illustrates some biological and ecological variations in modifying CHC profiles, and the underlying molecular regulation mechanisms of the planthoppers in coping with changes of environmental conditions, which is of great importance for identifying potential vulnerabilities relating to pest ecology and developing novel pest management strategies.

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.

Interactions of ants with native and invasive lady beetles and the role of chemical cues in intraguild interference

The predator-predator naïveté hypothesis suggests that non-native predators benefit from being unknown to native predators, resulting in reduced intraguild interference with native predators. This novelty advantage should depend on the ability of native predators to recognize cues of non-native predators. Here, we compared ant aggression and lady beetle reaction in four native and the invasive lady beetle species Harmonia axyridis. In addition, we tested whether lady beetle cuticular hydrocarbons (CHCs) are involved in species recognition, which might explain naïveté if the invasive species has a specific CHC profile. To this end, we conducted behavioral assays confronting two native ant species with both living lady beetles and lady beetle elytra bearing or lacking CHCs of different lady beetle species. Finally, we characterized CHC profiles of the lady beetles using GC–MS. In general, the aggression of Lasius niger was more frequent than that of Myrmica rubra and L. niger aggression was more frequent towards most native lady beetle species compared to H. axyridis. The removal of CHCs from lady beetle elytra reduced aggression of both ant species. If CHCs of respective lady beetle species were added on cue-free elytra, natural strength of L. niger aggression could be restored. CHC analyses revealed a distinct cue composition for each lady beetle species. Our experiments demonstrate that the presence of chemical cues on the surface of lady beetles contribute to the strength of ant aggression against lady beetles. Reduced aggression of L. niger towards H. axyridis and reduced avoidance behavior in H. axyridis compared to the equally voracious C. septempunctata might improve the invasive lady beetle’s access to ant-tended aphids.

Cuticular and Dufour's Gland Chemistry Reflect Reproductive and Social State in the Facultatively Eusocial Sweat Bee Megalopta genalis (Hymenoptera: Halictidae)

Queen pheromones evolved independently in multiple eusocial insect lineages, in which they mediate reproductive conflict by inhibiting worker ovarian development. Although fundamentally important for reproductive division of labor - the hallmark of eusociality - their evolutionary origins are enigmatic. Here, we analyze cuticular and Dufour's gland chemistries across alternative social and reproductive phenotypes in Megalopta genalis bees (tribe Augochlorini, family Halictidae) that facultatively express simple eusociality. Reproductive bees have distinct overall glandular and cuticular chemical phenotypes compared with non-reproductive workers. On the cuticle, a likely site of signal transmission, reproductives are enriched for certain alkenes, most linear alkanes, and are heavily enriched for all methyl-branched alkanes. Chemicals belonging to these compound classes are known to function as fertility signals in other eusocial insect taxa. Some macrocyclic lactones, compounds that serve as queen pheromones in the other eusocial halictid tribe (Halictini), are also enriched among reproductives relative to workers. The intra-population facultative eusociality of M. genalis permits direct comparisons between individuals expressing alternative reproductive phenotypes - females that reproduce alone (solitary reproductives) and social queens - to highlight traits in the latter that may be important mediators of eusociality. Compared with solitary reproductives, the cuticular chemistries of queens are more strongly differentiated from those of workers, and furthermore are especially enriched for methyl-branched alkanes. Determining the pheromonal function(s) and information content of the candidate signaling compounds we identify will help illuminate the early evolutionary history of queen pheromones, chemical signals central to the organization of insect eusocial behavior.

Distinct chemical blends produced by different reproductive castes in the subterranean termite Reticulitermes flavipes

The production of royal pheromones by reproductives (queens and kings) enables social insect colonies to allocate individuals into reproductive and non-reproductive roles. In many termite species, nestmates can develop into neotenics when the primary king or queen dies, which then inhibit the production of additional reproductives. This suggests that primary reproductives and neotenics produce royal pheromones. The cuticular hydrocarbon heneicosane was identified as a royal pheromone in Reticulitermes flavipes neotenics. Here, we investigated the presence of this and other cuticular hydrocarbons in primary reproductives and neotenics of this species, and the ontogeny of their production in primary reproductives. Our results revealed that heneicosane was produced by most neotenics, raising the question of whether reproductive status may trigger its production. Neotenics produced six additional cuticular hydrocarbons absent from workers and nymphs. Remarkably, heneicosane and four of these compounds were absent in primary reproductives, and the other two compounds were present in lower quantities. Neotenics therefore have a distinct 'royal' blend from primary reproductives, and potentially over-signal their reproductive status. Our results suggest that primary reproductives and neotenics may face different social pressures. Future studies of these pressures should provide a more complete understanding of the mechanisms underlying social regulation in termites.

Advances in deciphering the genetic basis of insect cuticular hydrocarbon biosynthesis and variation

Cuticular hydrocarbons (CHCs) have two fundamental functions in insects. They protect terrestrial insects against desiccation and serve as signaling molecules in a wide variety of chemical communication systems. It has been hypothesized that these pivotal dual traits for adaptation to both desiccation and signaling have contributed to the considerable evolutionary success of insects. CHCs have been extensively studied concerning their variation, behavioral impact, physiological properties, and chemical compositions. However, our understanding of the genetic underpinnings of CHC biosynthesis has remained limited and mostly biased towards one particular model organism (Drosophila). This rather narrow focus has hampered the establishment of a comprehensive view of CHC genetics across wider phylogenetic boundaries. This review attempts to integrate new insights and recent knowledge gained in the genetics of CHC biosynthesis, which is just beginning to incorporate work on more insect taxa beyond Drosophila. It is intended to provide a stepping stone towards a wider and more general understanding of the genetic mechanisms that gave rise to the astonishing diversity of CHC compounds across different insect taxa. Further research in this field is encouraged to aim at better discriminating conserved versus taxon-specific genetic elements underlying CHC variation. This will be instrumental in greatly expanding our knowledge of the origins and variation of genes governing the biosynthesis of these crucial phenotypic traits that have greatly impacted insect behavior, physiology, and evolution.

Chemoreceptor Diversity in Apoid Wasps and Its Reduction during the Evolution of the Pollen-Collecting Lifestyle of Bees (Hymenoptera: Apoidea)

Chemoreceptors help insects to interact with their environment, to detect and assess food sources and oviposition sites, and to aid in intra- and interspecific communication. In Hymenoptera, species of eusocial lineages possess large chemoreceptor gene repertoires compared with solitary species, possibly because of their additional need to recognize nest-mates and caste. However, a critical piece of information missing so far has been the size of chemoreceptor gene repertoires of solitary apoid wasps. Apoid wasps are a paraphyletic group of almost exclusively solitary Hymenoptera phylogenetically positioned between ant and bee, both of which include eusocial species. We report the chemosensory-related gene repertoire sizes of three apoid wasps: Ampulex compressa, Cerceris arenaria, and Psenulus fuscipennis. We annotated genes encoding odorant (ORs), gustatory, and ionotropic receptors and chemosensory soluble proteins and odorant-binding proteins in transcriptomes of chemosensory tissues of the above three species and in early draft genomes of two species, A. compressa and C. arenaria. Our analyses revealed that apoid wasps possess larger OR repertoires than any bee lineage, that the last common ancestor of Apoidea possessed a considerably larger OR repertoire (∼160) than previously estimated (73), and that the expansion of OR genes in eusocial bees was less extensive than previously assumed. Intriguingly, the evolution of pollen-collecting behavior in the stem lineage of bees was associated with a notable loss of OR gene diversity. Thus, our results support the view that herbivorous Hymenoptera tend to possess smaller OR repertoires than carnivorous, parasitoid, or kleptoparasitic species.

Red Imported Fire Ant (Solenopsis invicta) Chemosensory Proteins Are Expressed in Tissue, Developmental, and Caste-Specific Patterns

The red imported fire ant, Solenopsis invicta, is a eusocial invasive insect that has spread worldwide. Chemosensory proteins (CSPs) are ligand-binding proteins that participate in a diverse range of physiological processes that include olfaction and chemical transport. Here, we performed a systematic survey of the expression of the 21 gene S. invicta CSP family that includes at least two groups of apparent S. invicta-specific gene expansions. These data revealed caste, tissue, and developmental stage-specific differential expression of the SiCSPs. In general, moderate to high SiCSP expression was seen in worker antennae and abdomen tissues with lower expression in head/thorax regions. Male and female alates showed high antennal expression of fewer SiCSPs, with the female alate thorax showing comparatively high SiCSP expression. SiCSP expression was lower in male alates tissues compared to workers and female alates, albeit with some highly expressed SiCSPs. SiCSP expression was low during development including in eggs, larvae (early and late instars), and pupae. Global analyses revealed examples of conserved, divergent, and convergent SiCSP expression patterns linked to phylogenetic relationships. The developmental and caste-specific variation seen in SiCSP expression patterns suggests specific functional diversification of CSPs that may translate into differential chemical recognition and communication among individuals and/or reflect other cellular roles of CSPs. Our results support a model for CSPs acting as general ligand carriers involved in a wide range of physiological processes beyond olfaction. As compared to the expression patterns of the S. invicta odorant binding proteins (OBPs), an inverse correlation between SiOBP and SiCSP expression was seen, suggesting potential complementary and/or compensatory functions between these two classes of ligand carriers.

The evolution of sexually dimorphic cuticular hydrocarbons in blowflies (Diptera: Calliphoridae)

Cuticular hydrocarbons (CHCs) are organic compounds found on the cuticles of all insects which can act as close-contact pheromones, while also providing a hydrophobic barrier to water loss. Given their widespread importance in sexual behaviour and survival, CHCs have likely contributed heavily to the adaptation and speciation of insects. Despite this, the patterns and mechanisms of their diversification have been studied in very few taxa. Here, we perform the first study of CHC diversification in blowflies, focussing on wild populations of the ecologically diverse genus Chrysomya. We convert CHC profiles into qualitative and quantitative traits and assess their inter- and intra-specific variation across 10 species. We also construct a global phylogeny of Chrysomya, onto which CHCs were mapped to explore the patterns of their diversification. For the first time, we demonstrate that blowflies express an exceptional diversity of CHCs, which have diversified in a nonphylogenetic and punctuated manner, are species-specific and sexually dimorphic. It is likely that both ecological and sexual selection have shaped these patterns of CHC diversification, and our study now provides a comprehensive framework for testing such hypotheses.

Different lethal treatments induce changes in piperidine (1,1'-(1,2-ethanediyl)bis-) in the epidermal compounds of red imported fire ants and affect corpse-removal behavior

Corpse-removal behavior of the red imported fire ant (RIFA) and the effects of lethal substances on RIFA signal communication were investigated in this study. The RIFA corpses, obtained through freezing, ether, 0.25 mg/L thiamethoxam, and starvation to death treatments, and naturally dead red fire ants were subjected to gas chromatography-mass spectrometry to identify the cuticular hydrocarbon profiles that had an effect on the corpse-removal behavior. The results showed that lethal toxic substances altered the epidermal compounds of RIFA and affected their corpse-removal behavior. Lethal toxic substances increased the number of worker touches with corpses and identification time of corpses. In addition, the content of piperidine (1,1'-(1,2-ethanediyl)bis-) on the surface of the corpse was different following the various treatments. Contamination with toxic substances resulted in the increased secretion of piperidine and led to increased identification time of corpses, number of touch with corpses, and total time for removal of corpses. Piperidine content was higher under conditions of natural death (4.67 ± 0.55%) and with thiamethoxam (10.43 ± 0.78%), freezing (0.83 ± 0.25%), and ether treatment (12.50 ± 0.70%) than under starvation treatment (0). The higher content of piperidine led to a longer number of touches with corpses and identification time. Piperidine compounds may be an element in warning information, which could affect the occurrence of different corpse-removal behaviors.

Convergent evolution of small molecule pheromones in Pristionchus nematodes

The small molecules that mediate chemical communication between nematodes-so-called 'nematode-derived-modular-metabolites' (NDMMs)-are of major interest because of their ability to regulate development, behavior, and life-history. Pristionchus pacificus nematodes produce an impressive diversity of structurally complex NDMMs, some of which act as primer pheromones that are capable of triggering irreversible developmental switches. Many of these NDMMs have only ever been found in P. pacificus but no attempts have been made to study their evolution by profiling closely related species. This study brings a comparative perspective to the biochemical study of NDMMs through the systematic MS/MS- and NMR-based analysis of exo-metabolomes from over 30 Pristionchus species. We identified 36 novel compounds and found evidence for the convergent evolution of complex NDMMs in separate branches of the Pristionchus phylogeny. Our results demonstrate that biochemical innovation is a recurrent process in Pristionchus nematodes, a pattern that is probably typical across the animal kingdom.

Distinct Roles of Cuticular Aldehydes as Pheromonal Cues in Two Cotesia Parasitoids

Cuticular compounds (CCs) that cover the surface of insects primarily serve as protection against entomopathogens, harmful substances, and desiccation. However, CCs may also have secondary signaling functions. By studying the role of CCs in intraspecific interactions, we may advance our understanding of the evolution of pheromonal communication in insects. We previously found that the gregarious parasitoid, Cotesia glomerata (L.), uses heptanal as a repellent pheromone to help avoid mate competition among sibling males, whereas another cuticular aldehyde, nonanal, is part of the female-produced attractive sex pheromone. Here, we show that the same aldehydes have different pheromonal functions in a related solitary parasitoid, Cotesia marginiventris (Cresson). Heptanal enhances the attractiveness of the female's sex pheromone, whereas nonanal does not affect a female's attractiveness. Hence, these common aldehydes are differentially used by the two Cotesia species to mediate, synergistically, the attractiveness of the main constituents of their respective sex pheromones. The specificity of the complete sex pheromone blend is apparently regulated by two specific, less volatile compounds, which evoke strong electroantennographic (EAG) responses. This is the first demonstration that volatile CCs have evolved distinct pheromonal functions to aid divergent mating strategies in closely related species. We discuss the possibility that additional compounds are involved in attraction and that, like the aldehydes, they are likely oxidative products of unsaturated cuticular hydrocarbons.

Bacterial Semiochemicals and Transkingdom Interactions with Insects and Plants

A peculiar feature of all living beings is their capability to communicate. With the discovery of the quorum sensing phenomenon in bioluminescent bacteria in the late 1960s, it became clear that intraspecies and interspecies communications and social behaviors also occur in simple microorganisms such as bacteria. However, at that time, it was difficult to imagine how such small organisms-invisible to the naked eye-could influence the behavior and wellbeing of the larger, more complex and visible organisms they colonize. Now that we know this information, the challenge is to identify the myriad of bacterial chemical signals and communication networks that regulate the life of what can be defined, in a whole, as a meta-organism. In this review, we described the transkingdom crosstalk between bacteria, insects, and plants from an ecological perspective, providing some paradigmatic examples. Second, we reviewed what is known about the genetic and biochemical bases of the bacterial chemical communication with other organisms and how explore the semiochemical potential of a bacterium can be explored. Finally, we illustrated how bacterial semiochemicals managing the transkingdom communication may be exploited from a biotechnological point of view.

Partner choice correlates with fine scale kin structuring in the paper wasp Polistes dominula

Cooperation among kin is common in animal societies. Kin groups may form by individuals directly discriminating relatives based on kin recognition cues, or form passively through natal philopatry and limited dispersal. We describe the genetic landscape for a primitively eusocial wasp, Polistes dominula, and ask whether individuals choose cooperative partners that are nearby and/or that are genetic relatives. Firstly, we genotyped an entire sub-population of 1361 wasps and found genetic structuring on an extremely fine scale: the probability of finding genetic relatives decreases exponentially within just a few meters of an individual's nest. At the same time, however, we found a lack of genetic structuring between natural nest aggregations within the population. Secondly, in a separate dataset where ~2000 wasps were genotyped, we show that wasps forced experimentally to make a new nest choice tended to choose new nests near to their original nests, and that these nests tended to contain some full sisters. However, a significant fraction of wasps chose nests that did not contain sisters, despite sisters being present in nearby nests. Although we cannot rule out a role for direct kin recognition or natal nest-mate recognition, our data suggest that kin groups may form via a philopatric rule-of-thumb, whereby wasps simply select groups and nesting sites that are nearby. The result is that most subordinate helpers obtain indirect fitness benefits by breeding cooperatively.

Advertisement of unreceptivity - Perfume modifications of mason bee females (Osmia bicornis and O. cornuta) and a non-existing antiaphrodisiac

Females of many monandrous insect species announce their receptivity either by specialised sex-pheromones or by a signature mixture of cuticular hydrocarbons (CHCs). The trigger that shuts down the sex-pheromone release or initialises a change in CHC bouquet is thought to be either the mating per se or male pheromones transferred during copulation. Besides a conversion of female volatiles, the application of antiaphrodisiacs, male derived pheromones that render mated females unattractive to competitors, is another strategy to protect females from further sexual chasings. This simple pattern becomes more complicated in the monandrous mason bees Osmia bicornis (syn: O. rufa) and O. cornuta due to a post-copulation phase in their mating sequence. Males display a stereotypic behaviour right after the intromission that induces females’ unreceptivity. This post-copulatory display is predestined both to trigger a transition of the CHC profile and for the application of an antiaphrodisiac. However, the postulated antiaphrodisiac was not detectable even on freshly mated females. Moreover, the male’s post-copulatory display did not trigger a change in the CHC bouquet and neither did the insemination. Instead the CHC profile of freshly emerged females changes into the bouquet of nesting females simply by age as an ontogenetic process in both Osmia species. This autonomous change in the CHC profile coincides with an age-specific decrease of young female’s willingness to mate. How the resulting short period of female receptivity without back coupling by storage of sperm and the lack of an antiaphrodisiac fit into the behavioural ecology of the studied mason bee species is discussed.

Natural biocide disrupts nestmate recognition in honeybees

Honeybee colonies are under the threat of many stressors, biotic and abiotic factors that strongly affect their survival. Recently, great attention has been directed at chemical pesticides, including their effects at sub-lethal doses on bee behaviour and colony success; whereas the potential side effects of natural biocides largely used in agriculture, such as entomopathogenic fungi, have received only marginal attention. Here, we report the impact of the fungus Beauveria bassiana on honeybee nestmate recognition ability, a crucial feature at the basis of colony integrity. We performed both behavioural assays by recording bee guards’ response towards foragers (nestmate or non-nestmate) either exposed to B. bassiana or unexposed presented at the hive entrance, and GC-MS analyses of the cuticular hydrocarbons (CHCs) of fungus-exposed versus unexposed bees. Our results demonstrated that exposed bees have altered cuticular hydrocarbons and are more easily accepted into foreign colonies than controls. Since CHCs are the main recognition cues in social insects, changes in their composition appear to affect nestmate recognition ability at the colony level. The acceptance of chemically unrecognizable fungus-exposed foragers could therefore favour forager drift and disease spread across colonies.

When a male perceives a female: the effect of waxy components on the body surface on decision-making in the invasive pest weevil

Insects use various semiochemicals for sexual communication and mate recognition; these can therefore be used to govern the behaviours of harmful pest species, and several candidate chemicals have been explored for this purpose. For the West Indian sweet potato weevil, Euscepes postfasciatus, which is one of the most serious pests of sweet potato, no effective capture techniques, such as sex pheromone lures, exist. Toward exploring promising procedures for monitoring these weevils, we assessed the effect of secretions on the body surface on the recognition of congeners and on courtship behaviour in the weevils. Our study clearly demonstrated that weevils responded to extracts from the body surface, and the behaviour adopted by the weevils varied significantly depending on the condition of the extracts. Furthermore, we found a significantly prolonged retention time for males on glass beads covered with extracts of females based on survival analysis. These findings are, as far as we are aware, the first to show the effect of lipid components of the body surface on decision-making in these economically important pest weevils.

Communication vs. waterproofing: the physics of insect cuticular hydrocarbons

Understanding the evolution of complex traits is among the major challenges in biology. One such trait is the cuticular hydrocarbon (CHC) layer in insects. It protects against desiccation and provides communication signals, especially in social insects. CHC composition is highly diverse within and across species. To understand the adaptive value of this chemical diversity, we must understand how it affects biological functionality. So far, CHCs received ample research attention, but their physical properties were little studied. We argue that these properties determine their biological functionality, and are vital to understand how CHC composition affects their adaptive value. We investigated melting behaviour and viscosity of CHCs from eleven ant species using differential scanning calorimetry and a novel microrheological technique.

Cuticular hydrocarbons began melting below -45°C, and often were entirely liquid only above 30°C. Thus, they formed a solid-liquid mixture at ambient conditions, which contrasts to previous assumptions of entirely solid layers in many species. This may be adaptive since only biphasic CHC layers ensure uniform coating of the insect body, which is necessary for waterproofing. CHC viscosity was mostly between 0.1 and 0.2 Pa.s, thus similar to motor oils. Surprisingly, chemically different CHC profiles had similar viscosities, suggesting that a certain viscosity level is adaptive and ensures that communication signals can be perceived.

With this study, we draw attention to the importance of studying the physics of CHC layers. Only by understanding how chemical and physical mechanisms enable CHC functionality can we understand the causes and consequences of CHC diversification.

Hydrocarbons mediate seed dispersal: a new mechanism of vespicochory

Vespicochory, seed dispersal by hornets, is an uncommon seed dispersal pattern in angiosperms. To date, this phenomenon has been recorded in only four families. Because of its rarity, the causes and consequences of vespicochory remain unclear. Hence, this seed dispersal syndrome is often regarded as anecdotal. Through field investigations, chemical analyses, electrophysiological tests, identification of chemosensory proteins from the antennae of hornets, and behavioral assays, we investigated whether olfactory and/or visual cues of the diaspores of Stemona tuberosa mediate the behavior of the social hornets and maintain their mutualism. This study demonstrated that the elaiosome of S. tuberosa emits hydrocarbons, which are attractive to hornets. However, these compounds, which induce responses in the antennae of naive hornets, are ubiquitous substances on insect cuticle surfaces. Innate preference and experienced foraging behavior of hornets can increase their seed dispersal efficiency. This is the first example in which hydrocarbons have been identified as a diaspore odour involved in the attraction of hornets. Given that the ubiquity of hornets, and the communication function of hydrocarbons in insects, we predict that this rare seed dispersal mechanism may be an overlooked mechanism of insect-plant mutualism.

Crozier's Effect and the Acceptance of Intraspecific Brood Parasites

Organisms can often benefit by distinguishing between different classes of individuals. An example is kin recognition, whereby individuals preferentially associate with or aid genetic relatives that bear matching recognition cues but reject others. Despite its potential benefits, however, kin recognition using genetically based cues is often weak or absent [1-4]. A general explanation, termed "Crozier's effect," is that when individuals interact randomly, rarer cue alleles less often match cues of other individuals, and so are involved predominantly in "reject"-type interactions. If such interactions are more costly, positive frequency-dependent selection will erode the cue diversity upon which discrimination depends [4, 5]. Although widely cited [1, 2, 4, 6-9], this idea lacks rigorous testing in the field. Here, we show how Crozier's effect applies to interactions between hosts and conspecific parasites, and measure it using field data. In the wasp we studied, conspecific parasitism fits a key assumption of Crozier's model: the same females act as both hosts and parasites. By exchanging offspring between nests experimentally, we find no evidence that females respond to genetically based cues associated with foreign offspring. Through measuring costs and benefits, however, we demonstrate a strong Crozier effect: because more parental investment is wasted when foreign offspring are rejected, interactions involving rejection have substantially lower payoffs than interactions involving acceptance. Costly rejection can thus eliminate cue diversity by causing selection against rare cue alleles, consistent with the absence of genetically based recognition that we observe. Females instead appear to rely on non-genetic cues that enable them to detect less than half of parasitic offspring.

Deceptive signals and behaviors of a cleptoparasitic beetle show local adaptation to different host bee species

Chemosensory signals play a key role in species recognition and mate location in both invertebrate and vertebrate species. Closely related species often produce similar but distinct signals by varying the ratios or components in pheromone blends to avoid interference in their communication channels and minimize cross-attraction among congeners. However, exploitation of reproductive signals by predators and parasites also may provide strong selective pressure on signal phenotypes. For example, bolas spiders mimic the pheromones of several moth species to attract their prey, and parasitic blister beetle larvae, known as triungulins, cooperatively produce an olfactory signal that mimics the sex pheromone of their female host bees to attract male bees, as the first step in being transported by their hosts to their nests. In both cases, there is strong selection pressure on the host to discriminate real mates from aggressive mimics and, conversely, on the predator, parasite, or parasitoid to track and locally adapt to the evolving signals of its hosts. Here we show local adaptation of a beetle, Meloe franciscanus (Coleoptera: Meloidae), to the pheromone chemistry and mate location behavior of its hosts, two species of solitary bees in the genus Habropoda We report that Mfranciscanus' deceptive signal is locally host-adapted in its chemical composition and ratio of components, with host bees from each allopatric population preferring the deceptive signals of their sympatric parasite population. Furthermore, in different locales, the triungulin aggregations have adapted their perching height to the height at which local male bees typically patrol for females.