Ecological, behavioral, and biochemical aspects of insect hydrocarbons

Effect of temperature on survival and cuticular composition of three different ant species

Climatic factors, such as temperature variation, interfere with the survival of insects. To respond to these variations, insects have some specific characteristics. These include water content of the body, thickness of the lipid layer, as well as the qualitative and quantitative characteristics of cuticular chemical components. This study hypothesizes that different ant species respond to temperature changes in different ways and that such differences may be associated with cuticle hydrocarbons (CHCs) and fatty acids. As model ant species, Atta sexdens, Odontomachus bauri and Ectatomma brunneum were used for experimental analyses. Ants were submitted to a water bath for 5 h at different temperatures, and their CHCs and fatty acids were identified and quantified, followed by correlating these chemical compounds with temperature variations and the survival. Temperatures below 30 °C did not affect the survival of the three species. E. brunneum had a higher percentage of survival at temperatures above 30 °C. O. bauri was the most sensitive species with 100% mortality at 40 °C. Survival was found to be unrelated to any of the identified fatty acids. However, CHCs underwent significant quantitative and qualitative variation, as shown by an increased percentage of CHCs with longer chain length of linear alkanes at temperatures above 30 °C. These increase enables these ants to maintain the integrity of their cuticle and survive at temperatures above 30 °C. It can be concluded that the forager ants studied respond differently to temperature variation and that changes in the conformation of CHCs are in line with the ecological characteristics of the different studied species because, they vary in terms of diurnal/nocturnal foraging and types of environments foraged. Among the three species, E. brunneum foragers were found to be more active under adverse conditions and more tolerant to temperature variation with the correspondingly appropriate changes in CHCs composition.

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.

Caught red-handed: behaviour of brood thieves in an Indian ant

Theft of resources is ubiquitous in the animal kingdom. An evolutionary arms race between thieves and their victims is expected. Although several studies have documented inter- and intraspecific theft of resources in different taxa, studies that delve into the behaviour of thieves and the factors that influence their behaviour have not been undertaken. In the current study on the primitively eusocial ant Diacamma indicum, we caught brood thieves red-handed: we observed them in the act of stealing brood and examined their behaviour. Thieves were persistent in their attempts despite facing aggression in the victim colony. Experiencing aggression or failure to steal in the previous attempt negatively impacted a thief's drive to reattempt. To avoid the risks associated with theft, successful thieves exited from victim nests about three times faster than others who were procuring brood from unguarded nests. In a series of experiments examining factors that caused thieves to increase their exit speed, we found that indirect cues of a foreign colony's presence, such as odour or the presence of foreign ants, did not induce these changes in thieves. Thus, we conclude that these ant thieves only respond to the direct threat posed by aggressive foreign ants. In this comprehensive study using behavioural experiments, we reveal the simple rules of engagement between victims and brood thieves.

RiboTag translatomic profiling of Drosophila oenocytes under aging and induced oxidative stress

BACKGROUND

Aging is accompanied with loss of tissue homeostasis and accumulation of cellular damages. As one of the important metabolic centers, liver shows age-related dysregulation of lipid metabolism, impaired detoxification pathway, increased inflammation and oxidative stress response. However, the mechanisms for these age-related changes still remain unclear. In the fruit fly, Drosophila melanogaster, liver-like functions are controlled by two distinct tissues, fat body and oenocytes. Compared to fat body, little is known about how oenocytes age and what are their roles in aging regulation. To characterize age- and stress-regulated gene expression in oenocytes, we performed cell-type-specific ribosome profiling (RiboTag) to examine the impacts of aging and oxidative stress on oenocyte translatome in Drosophila.

RESULTS

We show that aging and oxidant paraquat significantly increased the levels of reactive oxygen species (ROS) in adult oenocytes of Drosophila, and aged oenocytes exhibited reduced sensitivity to paraquat treatment. Through RiboTag sequencing, we identified 3324 and 949 differentially expressed genes in oenocytes under aging and paraquat treatment, respectively. Aging and paraquat exhibit both shared and distinct regulations on oenocyte translatome. Among all age-regulated genes, oxidative phosphorylation, ribosome, proteasome, fatty acid metabolism, and cytochrome P450 pathways were down-regulated, whereas DNA replication and immune response pathways were up-regulated. In addition, most of the peroxisomal genes were down-regulated in aged oenocytes, including genes involved in peroxisomal biogenesis factors and fatty acid beta-oxidation. Many age-related mRNA translational changes in oenocytes are similar to aged mammalian liver, such as up-regulation of innate immune response and Ras/MAPK signaling pathway and down-regulation of peroxisome and fatty acid metabolism. Furthermore, oenocytes highly expressed genes involving in liver-like processes (e.g., ketogenesis).

CONCLUSIONS

Our oenocyte-specific translatome analysis identified many genes and pathways that are shared between Drosophila oenocytes and mammalian liver, highlighting the molecular and functional similarities between the two tissues. Many of these genes were altered in both oenocytes and liver during aging. Thus, our translatome analysis provide important genomic resource for future dissection of oenocyte function and its role in lipid metabolism, stress response and aging regulation.

Phenotypic evolution shaped by current enzyme function in the bioluminescent courtship signals of sea fireflies

Mating behaviours are diverse and noteworthy, especially within species radiations where they may contribute to speciation. Studying how differences in mating behaviours arise between species can help us understand how diversity is generated at multiple biological levels. The bioluminescent courtship displays of cypridinid ostracods (or sea fireflies) are an excellent system for this because amazing variety evolves while using a conserved biochemical mechanism. We find that the evolution of one aspect in this behavioural phenotype-the duration of bioluminescent courtship pulses-is shaped by biochemical function. First, by measuring light production from induced bioluminescence in 38 species, we discovered differences between species in their biochemical reactions. Then, for 16 species for which biochemical, phylogenetic and behavioural data are all available, we used phylogenetic comparative models to show that differences in biochemical reaction are nonlinearly correlated with the duration of courtship pulses. This relationship indicates that changes to both enzyme (c-luciferase) function and usage have shaped the evolution of courtship displays, but that they differentially contribute to these phenotypic changes. This nonlinear dynamic may have consequences for the disparity of signalling phenotypes observed across species, and demonstrates how unappreciated diversity at the biochemical level can lead to inferences about behavioural evolution.

Chemistry of the Secondary Metabolites of Termites

Isolation, structure determination, synthesis, and biochemistry of the low-molecular-weight compounds of the secretion of exocrine glands of termites are described, with an emphasis on pheromones and defensive compounds.

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.

New method for estimating the post-mortem interval using the chemical composition of different generations of empty puparia: Indoor cases

Most flies of forensic importance are in two superfamilies, the Muscoidea and the Oestroidea, with similar life stages including the puparium. Upon completion of metamorphosis the adult fly emerges from the puparium, leaving behind an exuvia that is of potential significance in forensic investigation. The empty puparium is a durable piece of entomological evidence lasting several years. Through the study of chemical compounds, specifically the hydrocarbons of these puparia, it is possible to identify the species, in addition to how long they have been exposed to weathering and for this reason, these parameters can assist forensic entomologists in estimating long-term postmortem interval (minPMI). In corpses that take a relatively longer time to decompose, insects may use the same corpses for several oviposition cycles. Therefore, the aim of this study was to develop a new method to determine the PMI based on chemical compounds of the puparia from different oviposition cycles of the fly Chrysomya megacephala. The chemical composition of 50 puparia from different cycles of oviposition were evaluated by Gas Chromatography–Mass Spectrometry (GC-MS). In total, 60 compounds were identified ranging from C₁₈ to C₃₄, 38 of those were common to all generations. Our results demonstrate that chemical profiles can be used to differentiate puparia collected from successive cycles, and therefore valuable in the estimation of minPMI.

Variation in the cuticular hydrocarbons of the Mexican fruit fly Anastrepha ludens males between strains and age classes

In this study cuticular hydrocarbons (CHCs) were characterized from wings of individual unmated males of different Anastrepha ludens (Loew) mass-reared strains of different ages (3 and 19-day-old): (a) a standard mass-reared colony (control), (b) a genetic sexing strain, (c) a selected strain, (d) a hybrid strain, and (e) wild males. We found that the hydrocarbon profiles in all males included two n-alkanes, five monomethyl alkanes, and two alkenes. CHCs ranged from C₂₆ to C₃₁ . The most prominent peaks were 2-methyloctacosane (2-Me-C28), n-nonacosene (C29:1), 2-methyltriacontane (2-Me-C30), and n-hentriacontene (C31:1). Significant variations in the CHC amounts of the mass-reared strains were observed from Day 9 and thereafter. Comparison of CHCs using multivariate and canonical analyses across ages and among mass-reared strains and wild males revealed qualitative and quantitative differences. The relative amounts of C29:1 and 2-Me-C30 were significantly higher across age groups in the mass-reared strains than those in the wild males. In contrast, amounts of n-nonacosane (C29) significantly increased in wild males as they aged. Through statistical analyses, we inferred that CHC amounts vary with age. Wild males differed significantly from the mass-reared strains in the amount of C29, and the genetic sexing strain Tap-7 had significantly higher values for 2-methylhexacosane (2-Me-C26). In contrast the selected and control strain differed from the other strains in amounts of C29:1 and 2-Me-C30. We suggest that differential profiles in hydrocarbon composition among the strains may be mainly due to environmental pressures.

Effect of antibiotic treatment and gamma-irradiation on cuticular hydrocarbon profiles and mate choice in tsetse flies (Glossina m. morsitans)

BACKGROUND

Symbiotic microbes represent a driving force of evolutionary innovation by conferring novel ecological traits to their hosts. Many insects are associated with microbial symbionts that contribute to their host's nutrition, digestion, detoxification, reproduction, immune homeostasis, and defense. In addition, recent studies suggest a microbial involvement in chemical communication and mating behavior, which can ultimately impact reproductive isolation and, hence, speciation. Here we investigated whether a disruption of the microbiota through antibiotic treatment or irradiation affects cuticular hydrocarbon profiles, and possibly mate choice behavior in the tsetse fly, Glossina morsitans morsitans. Four independent experiments that differentially knock down the multiple bacterial symbionts of tsetse flies were conducted by subjecting tsetse flies to ampicillin, tetracycline, or gamma-irradiation and analyzing their cuticular hydrocarbon profiles in comparison to untreated controls by gas chromatography - mass spectrometry. In two of the antibiotic experiments, flies were mass-reared, while individual rearing was done for the third experiment to avoid possible chemical cross-contamination between individual flies.

RESULTS

All three antibiotic experiments yielded significant effects of antibiotic treatment (particularly tetracycline) on cuticular hydrocarbon profiles in both female and male G. m. morsitans, while irradiation itself had no effect on the CHC profiles. Importantly, tetracycline treatment reduced relative amounts of 15,19,23-trimethyl-heptatriacontane, a known compound of the female contact sex pheromone, in two of the three experiments, suggesting a possible implication of microbiota disturbance on mate choice decisions. Concordantly, both female and male flies preferred non-treated over tetracycline-treated flies in direct choice assays.

CONCLUSIONS

While we cannot exclude the possibility that antibiotic treatment had a directly detrimental effect on fly vigor as we are unable to recolonize antibiotic treated flies with individual symbiont taxa, our results are consistent with an effect of the microbiota, particularly the obligate nutritional endosymbiont Wigglesworthia, on CHC profiles and mate choice behavior. These findings highlight the importance of considering host-microbiota interactions when studying chemical communication and mate choice in insects.

Desiccation Resistance and Micro-Climate Adaptation: Cuticular Hydrocarbon Signatures of Different Argentine Ant Supercolonies Across California

Cuticular hydrocarbons (CHCs), the dominant fraction of the insects' epicuticle and the primary barrier to desiccation, form the basis for a wide range of chemical signaling systems. In eusocial insects, CHCs are key mediators of nestmate recognition, and colony identity appears to be maintained through a uniform CHC profile. In the unicolonial Argentine ant Linepithema humile, an unparalleled invasive expansion has led to vast supercolonies whose nestmates can still recognize each other across thousands of miles. CHC profiles are expected to display considerable variation as they adapt to fundamentally differing environmental conditions across the Argentine ant's expanded range, yet this variation would largely conflict with the vastly extended nestmate recognition based on CHC uniformity. To shed light on these seemingly contradictory selective pressures, we attempt to decipher which CHC classes enable adaptation to such a wide array of environmental conditions and contrast them with the overall CHC profile uniformity postulated to maintain nestmate recognition. n-Alkanes and n-alkenes showed the largest adaptability to environmental conditions most closely associated with desiccation, pointing at their function for water-proofing. Trimethyl alkanes, on the other hand, were reduced in environments associated with higher desiccation stress. However, CHC patterns correlated with environmental conditions were largely overriden when taking overall CHC variation across the expanded range of L. humile into account, resulting in conserved colony-specific CHC signatures. This delivers intriguing insights into the hierarchy of CHC functionality integrating both adaptation to a wide array of different climatic conditions and the maintenance of a universally accepted chemical profile.

Domestication modifies the volatile emissions produced by male Queensland fruit flies during sexual advertisement

Insects commonly undergo substantial changes during adaptation for laboratory or mass-rearing environments (‘domestication’) that may have significant implications for inferences from laboratory studies and utility for biological control. We assessed the effect of domestication on the amount and blend of volatiles released during sexual calling by laboratory-reared Bactrocera tryoni males using colonies from three regions of Australia: Brisbane, Cairns and Sydney. For each region, volatiles released by males from a young colony (five or fewer generations) and an old colony (20+ generations) during sexual calling was compared. Males from old colonies released more volatiles than males from young colonies. All components of the blend were more abundant in one or more of the older colonies, although differences varied by compound and by region. To assess changes over generations, the young and old colonies obtained from Brisbane were sampled at 5, 12 and 15 generations (young colony) and 25, 35 and 38 generations (old colony). While the old colony remained unchanged, flies from the young colony released more volatiles at each sequential sampling episode, and became increasingly similar to the old colony. Increased volatile production during domestication may be an adaptive response to crowded rearing conditions in which males need to overcome a chemically noisy environment to be sexually successful.

Exploring Interactions between the Gut Microbiota and Social Behavior through Nutrition

Microbes influence a wide range of host social behaviors and vice versa. So far, however, the mechanisms underpinning these complex interactions remain poorly understood. In social animals, where individuals share microbes and interact around foods, the gut microbiota may have considerable consequences on host social interactions by acting upon the nutritional behavior of individual animals. Here we illustrate how conceptual advances in nutritional ecology can help the study of these processes and allow the formulation of new empirically testable predictions. First, we review key evidence showing that gut microbes influence the nutrition of individual animals, through modifications of their nutritional state and feeding decisions. Next, we describe how these microbial influences and their social consequences can be studied by modelling populations of hosts and their gut microbiota into a single conceptual framework derived from nutritional geometry. Our approach raises new perspectives for the study of holobiont nutrition and will facilitate theoretical and experimental research on the role of the gut microbiota in the mechanisms and evolution of social behavior.

Observation of an anisotropic texture inside the wax layer of insect cuticle

The outermost part of insect cuticles is very often covered with wax, which prevents desiccation and serves for chemical communication in many species. Earlier studies on cuticular waxes have mainly focused on their chemical composition revealing complex mixtures of lipids. In the absence of information on their physical organization, cuticular waxes have been considered isotropic. Here we report the presence of parallel stripes in the wax layer of the carapace of the scarab beetle, Chrysina gloriosa, with a textural periodicity of ca. 28 nm, as revealed by electron microscopy of transverse sections. Observations at oblique incidence argue for a layered organization of the wax, which might be related to a layer-by-layer deposition of excreted wax. Our findings may lay the foundation for further studies on the internal structure of cuticular waxes for other insects.

Mountain pine beetle (Dendroctonus ponderosae) CYP4Gs convert long and short chain alcohols and aldehydes to hydrocarbons

Hydrocarbon biosynthesis in insects involves the elongation of fatty acyl-CoAs to very-long chain fatty acyl-CoAs that are then reduced and converted to hydrocarbon, with the last step involving the oxidative decarbonylation of an aldehyde to hydrocarbon and carbon dioxide. Cytochromes P450 in the 4G family decarbonylate aldehydes to hydrocarbon. All insect acyl-CoA reductases studied to date reduce fatty acyl-CoAs to alcohols. The results of the work reported herein demonstrate that CYP4G55 and CYP4G56 from the mountain pine beetle, Dendroctonus ponderosae, expressed as fusion proteins with house fly cytochrome P450 reductase (CPR), convert both long chain aldehydes and long chain alcohols to hydrocarbons. CYP4G55 and CYP4G56 appear to prefer primary alcohols to aldehydes as substrates. These data strongly suggest that hydrocarbon biosynthesis in insects occurs by the two-step reduction of very long chain fatty acyl-CoAs to alcohols, which are then oxidized to aldehydes and then oxidatively decarbonylated to hydrocarbon by CYP4G enzymes. In addition, both CYP4G55 and CYP4G56 fusion proteins convert C10 alcohols and aldehydes to hydrocarbons, including the conversion of (Z)-7-decenal, a putative intermediate in the exo-brevicomin pheromone biosynthetic pathway, to (Z)-3-nonene. These data demonstrate that the highly conserved CYP4G enzymes accept a broad range of carbon chain lengths, including C10 and C18, and have evolved to function in cuticular hydrocarbon biosynthesis and pheromone production.

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.

Social environment affects the transcriptomic response to bacteria in ant queens

Social insects have evolved enormous capacities to collectively build nests and defend their colonies against both predators and pathogens. The latter is achieved by a combination of individual immune responses and sophisticated collective behavioral and organizational disease defenses, that is, social immunity. We investigated how the presence or absence of these social defense lines affects individual-level immunity in ant queens after bacterial infection. To this end, we injected queens of the ant Linepithema humile with a mix of gram+ and gram- bacteria or a control solution, reared them either with workers or alone and analyzed their gene expression patterns at 2, 4, 8, and 12 hr post-injection, using RNA-seq. This allowed us to test for the effect of bacterial infection, social context, as well as the interaction between the two over the course of infection and raising of an immune response. We found that social isolation per se affected queen gene expression for metabolism genes, but not for immune genes. When infected, queens reared with and without workers up-regulated similar numbers of innate immune genes revealing activation of Toll and Imd signaling pathways and melanization. Interestingly, however, they mostly regulated different genes along the pathways and showed a different pattern of overall gene up-regulation or down-regulation. Hence, we can conclude that the absence of workers does not compromise the onset of an individual immune response by the queens, but that the social environment impacts the route of the individual innate immune responses.

From Acetoin to ( Z)-3-Hexen-1-ol: The Diversity of Volatile Organic Compounds that Induce Plant Responses

Evidence that plants can respond to volatile organic compounds (VOCs) was first presented 35 years ago. Since then, over 40 VOCs have been found to induce plant responses. These include VOCs that are produced not only by plants but also by microbes and insects. Here, we summarize what is known about how these VOCs are produced and how plants detect and respond to them. In doing so, we highlight notable observations we believe are worth greater consideration. For example, the VOCs that induce plant responses appear to have little in common. They are derived from many different biosynthetic pathways and have few distinguishing chemical or structural features. Likewise, plants appear to use several mechanisms to detect VOCs rather than a single dedicated "olfactory" system. Considering these observations, we advocate for more discovery-oriented experiments and propose that future research take a fresh look at the ways plants detect and respond to VOCs.

Both local presence and regional distribution of predator cues modulate prey colonisation in pond landscapes

Recent work on habitat selection has shown that the perceived quality of habitat patches may depend on the quality of adjacent patches. However, it is still unclear how local habitat selection cues can alter distribution patterns in metacommunities at a larger (regional) scale. We studied mosquito oviposition in pond landscapes that differed in the proportion of bad patches with fish predation risk. Our experiment provided conclusive evidence for two local and two regional types of habitat selection. Good patches near bad patches were avoided (local risk contagion) while more distant good patches experienced increased oviposition (regional compression). Oviposition in bad patches increased when located next to good patches (reward contagion) or when there were no good patches regionally present (regional compromise). This complex colonisation behaviour involving compromises at different spatial scales forces experimenters to reconsider the independence of spatial replicates and challenges available theories to predict species distribution patterns.

Interaction between two aggregation chemical signals in Triatoma infestans (Hemiptera: Reduviidae)

The nymphs and adults of Triatoma infestans spend much of their time aggregated among themselves within narrow and dark shelters. The search for a suitable shelter depends in part on the recognition of chemical signals coming from the feces and the cuticle of the other individuals who use the refuge. The aim of this study was determine the possible interaction between the chemical signals associated to the feces and to the cuticle of T. infestans. The results showed that the insects remained significantly more time on the feces that had contact with legs and the feces plus footprints than feces or footprints alone, demonstrating the interaction between evaluated signals. These results demonstrates also that feces extracted a chemical stimulus from the legs. Understanding the interaction feces-legs as an interaction feces-cuticle of legs, the results suggest that the feces could extract some cuticular compound with activity on the behavior of the insects. This is the first report of the interaction between the two aggregation signals recognized in T. infestans and of the increase in the behavioral response of insects exposed to feces that had contact with a cuticular structure.

Insect societies fight back: the evolution of defensive traits against social parasites

Insect societies face many social parasites that exploit their altruistic behaviours or their resources. Due to the fitness costs these social parasites incur, hosts have evolved various behavioural, chemical, architectural and morphological defence traits. Similar to bacteria infecting multicellular hosts, social parasites have to successfully go through several steps to exploit their hosts. Here, we review how social insects try to interrupt this sequence of events. They can avoid parasite contact by choosing to nest in parasite-free locales or evade attacks by adapting their colony structure. Once social parasites attack, hosts attempt to detect them, which can be facilitated by adjustments in colony odour. If social parasites enter the nest, hosts can either aggressively defend their colony or take their young and flee. Nest structures are often shaped to prevent social parasite invasion or to safeguard host resources. Finally, if social parasites successfully establish themselves in host nests, hosts can rebel by killing the parasite brood or by reproducing in the parasites' presence. Hosts of social parasites can therefore develop multiple traits, leading to the evolution of complex defence portfolios of co-dependent traits. Social parasites can respond to these multi-level defences with counter-adaptations, potentially leading to geographical mosaics of coevolution.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

Evolution of Caste-Specific Chemical Profiles in Halictid Bees

Chemical communication is crucial for the maintenance of colony organization in eusocial insects and chemical signals are known to mediate important aspects of their social life, including the regulation of reproduction. Sociality is therefore hypothesized to be accompanied by an increase in the complexity of chemical communication. However, little is known about the evolution of odor signals at the transition from solitary living to eusociality. Halictid bees are especially suitable models to study this question as they exhibit considerable variability in social behavior. Here we investigated whether the dissimilarities in cuticle chemical signals in females of different castes and life stages reflect the level of social complexity across halictid bee species. Our hypothesis was that species with a higher social behavior ergo obligate eusocial species possess a more distinct chemical profile between castes or female life stages. We analyzed cuticular chemical profiles of foundresses, breeding females and workers of ancestrally solitary species, facultative and obligate eusocial halictid species. We also tested whether social complexity was associated with a higher investment in chemical signals. Our results revealed higher chemical dissimilarity between castes in obligate than in facultative eusocial species, especially regarding macrocyclic lactones, which were the single common compound class overproduced in queens compared with workers. Chemical dissimilarities were independent of differences in ovarian status in obligate eusocial species but were dependent on ovarian status in facultative eusocial species, which we discuss in an evolutionary framework.

Lifetime inbreeding depression in a leaf beetle

Ongoing habitat loss and fragmentation result in rapid population size reductions, which can increase the levels of inbreeding. Consequently, many species are threatened by inbreeding depression, a loss of individual fitness following the mating of close relatives. Here, we investigated inbreeding effects on fitness-related traits throughout the lifetime of the mustard leaf beetle (Phaedon cochleariae) and mechanisms for the avoidance of inbreeding. Previously, we found that these beetles have family-specific cuticular hydrocarbon profiles, which are likely not used as recognition cue for precopulatory inbreeding avoidance. Thus, we examined whether adult beetles show postcopulatory inbreeding avoidance instead. For this purpose, we determined the larval hatching rate of eggs laid by females mated sequentially with two nonsiblings, two siblings, a nonsibling, and a sibling or vice versa. The beetles suffered from inbreeding depression throughout their entire ontogeny, as evinced by a prolonged larval development, a decreased larval and adult survival and a decreased reproductive output of inbred compared to outbred individuals. The highest larval hatching rates were detected when females were mated with two nonsiblings or first with a sibling and second with a nonsibling. Significantly lower hatching rates were measured in the treatments with a sibling as second male. Thus, the results do not support the existence of postcopulatory inbreeding avoidance in P. cochleariae, but revealed evidence for second male sperm precedence. Consequently, an alternative strategy to avoid inbreeding costs might exist in this beetle, such as a polyandrous mating system, potentially coupled with a specific dispersal behavior.

The genetic basis of female pheromone differences between Drosophila melanogaster and D. simulans

Chemical signals are one means by which many insect species communicate. Differences in the combination of surface chemicals called cuticular hydrocarbons (CHCs) can influence mating behavior and affect reproductive isolation between species. Genes influencing three CHC compounds have been identified in Drosophila melanogaster. However, the genetic basis of other CHC compounds, whether these genes affect species differences in CHCs, and the genes' resulting effect on interspecies mating, remains unknown. We used fine-scale deficiency mapping of the third chromosome to identify 43 genomic regions that influence production of CHCs in both D. melanogaster and Drosophila simulans females. We identified an additional 23 small genomic regions that affect interspecies divergence in CHCs between females of these two species, one of which spans two genes known to influence the production of multiple CHCs within D. melanogaster. By testing these genes individually, we determined that desat1 also affects interspecific divergence in one CHC compound, while desat2 has no effect on interspecific divergence. Thus, some but not all genes affecting intraspecific amounts of CHCs also affect interspecific divergence, but not all genes or all CHCs. Lastly, we find no evidence of a relationship between the CHC profile and female attractiveness or receptivity towards D. melanogaster males.

Volatile and Contact Chemical Cues Associated with Host and Mate Recognition Behavior of Sphenophorus venatus and Sphenophorus parvulus (Coleoptera: Dryophthoridae)

Beetles in the genus Sphenophorus Schönherr, or billbugs, potentially utilize both volatile and non-volatile behavior-modifying chemical signals. These insects are widely distributed across North America, often occurring in multi-species assemblages in grasses. However, details about their host- and mate-finding behavior are poorly understood. This study tested the hypothesis that volatile organic compounds from host-plants and conspecifics direct the dispersal behavior of hunting billbug S. venatus Say. Further, we characterized the cuticular hydrocarbon profiles of two widespread pest species, S. venatus and bluegrass billbug S. parvulus Gyllenhaal, to assess the potential role of contact pheromones in mate-recognition. In Y-tube olfactometer bioassays, S. venatus males were attracted to a combination of conspecifics and Cynodon dactylon host-plant material, as well as C. dactylon plant material alone. S. venatus females were attracted to a combination of male conspecifics and host-plants but were also attracted to male conspecifics alone. Field evaluation of a putative male-produced aggregation pheromone, 2-methyl-4-octanol, identified from two congeners, S. levis Vaurie and S. incurrens Gyllenhaal, did not support the hypothesis that S. venatus and S. parvulus were also attracted to this compound. Gas chromatography-mass spectrometry analysis of S. venatus and S. parvulus whole-body cuticular extracts indicated a series of hydrocarbons with qualitative and quantitative interspecific variation in addition to intraspecific quantitative variation between males and females. This study provides the first evidence that S. venatus orients toward host- and insect-derived volatile organic compounds and substantiates the presence of species-specific cuticular hydrocarbons that could serve as contact pheromones for sympatric Sphenophorus species.

Differences in the reliance on cuticular hydrocarbons as sexual signaling and species discrimination cues in parasitoid wasps

Background

Cuticular hydrocarbons (CHC) have been documented to play crucial roles as species- and sex-specific cues in the chemical communication systems of a wide variety of insects. However, whether they are sufficient by themselves as the sole cue triggering sexual behavior as well as preference of con- over heterospecific mating partners is rarely assessed. We conducted behavioral assays in three representative species of parasitoid wasps (Hymenoptera: Pteromalidae) to determine their reliance on CHC as species-specific sexual signaling cues.

Results

We found a surprising degree of either unspecific or insufficient sexual signaling when CHC are singled out as recognition cues. Most strikingly, the cosmopolitan species Nasonia vitripennis, expected to experience enhanced selection pressure to discriminate against other co-occurring parasitoids, did not discriminate against CHC of a partially sympatric species from another genus, Trichomalopsis sarcophagae. Focusing on the latter species, in turn, it became apparent that CHC are even insufficient as the sole cue triggering conspecific sexual behavior, hinting at the requirement of additional, synergistic sexual cues particularly important in this species. Finally, in the phylogenetically and chemically most divergent species Muscidifurax uniraptor, we intriguingly found both CHC-based sexual signaling as well as species discrimination behavior intact although this species is naturally parthenogenetic with sexual reproduction only occurring under laboratory conditions.

Conclusions

Our findings implicate a discrepancy in the reliance on and specificity of CHC as sexual cues in our tested parasitioid wasps. CHC profiles were not sufficient for unambiguous discrimination and preference behavior, as demonstrated by clear cross-attraction between some of our tested wasp genera. Moreover, we could show that only in T. sarcophagae, additional behavioral cues need to be present for triggering natural mating behavior, hinting at an interesting shift in signaling hierarchy in this particular species. This demonstrates the importance of integrating multiple, potentially complementary signaling modalities in future studies for a better understanding of their individual contributions to natural sexual communication behavior.

Electronic supplementary material

The online version of this article (10.1186/s12983-018-0263-z) contains supplementary material, which is available to authorized users.

Strategies for the construction of insect P450 fusion enzymes

Cytochrome P450 monooxygenases (P450s) are ubiquitous enzymes with a broad substrate spectrum. Insect P450s are known to catalyze reactions such as the detoxification of insecticides and the synthesis of hydrocarbons, which makes them useful for many industrial processes. Unfortunately, it is difficult to utilize P450s effectively because they must be paired with cytochrome P450 reductases (CPRs) to facilitate electron transfer from reduced nicotinamide adenine dinucleotide phosphate (NADPH). Furthermore, eukaryotic P450s and CPRs are membrane-anchored proteins, which means they are insoluble and therefore difficult to purify when expressed in their native state. Both challenges can be addressed by creating fusion proteins that combine the P450 and CPR functions while eliminating membrane anchors, allowing the production and purification of soluble multifunctional polypeptides suitable for industrial applications. Here we discuss several strategies for the construction of fusion enzymes combining insect P450 with CPRs.

Chemical Fertility Signaling in Termites: Idiosyncrasies and Commonalities in Comparison with Ants

Termites evolved eusociality independently from social Hymenoptera. As a common trait, reproductive monopoly is maintained through chemical communication. The queen (and in termites also a king) prevents workers from reproduction by conveying their reproductive status. In termites all soldiers are sterile, but workers' potential to reproduce differs between species. It ranges from totipotency in wood-dwelling lower termites where workers are a transient stage from which all other castes develop, to sterile workers in some higher termites. Intermediate are species in which workers can develop into replacement sexuals within the nest but not into winged sexuals. I summarize the patchy picture about fertility signaling that we currently have for termites, pointing also to potential conflicts over reproduction that differ from those in social Hymenoptera. Recent findings imply that, similar to many social Hymenoptera, wood-dwelling termites that live in confined nests use long-chain cuticular hydrocarbons (CHCs) as fertility signals. Yet other compounds are important as well, comprising proteinaceous secretions and especially volatiles. For a subterranean termite, two volatiles have been identified as primer pheromones that prevent reproductive differentiation of workers. It requires more data to test whether wood-dwelling termites use CHCs, while species with larger colonies and less confined nests use volatiles, or whether all species rely on multicomponent signals. Ultimately, we need more effort to model and test potential conflicts over reproduction between queens, kings and workers. Here results from social Hymenoptera cannot be transferred to termites as the latter are diploid and commonly inbred. This review illustrates promising future research avenues.

Review on nanoparticles and nanostructured materials: history, sources, toxicity and regulations

Nanomaterials (NMs) have gained prominence in technological advancements due to their tunable physical, chemical and biological properties with enhanced performance over their bulk counterparts. NMs are categorized depending on their size, composition, shape, and origin. The ability to predict the unique properties of NMs increases the value of each classification. Due to increased growth of production of NMs and their industrial applications, issues relating to toxicity are inevitable. The aim of this review is to compare synthetic (engineered) and naturally occurring nanoparticles (NPs) and nanostructured materials (NSMs) to identify their nanoscale properties and to define the specific knowledge gaps related to the risk assessment of NPs and NSMs in the environment. The review presents an overview of the history and classifications of NMs and gives an overview of the various sources of NPs and NSMs, from natural to synthetic, and their toxic effects towards mammalian cells and tissue. Additionally, the types of toxic reactions associated with NPs and NSMs and the regulations implemented by different countries to reduce the associated risks are also discussed.

Distinct physiological, biochemical and morphometric adjustments in the malaria vectors Anopheles gambiae and A. coluzzii as means to survive dry season conditions in Burkina Faso

Aestivation and dispersive migration are the two strategies evoked in the literature to explain the way in which malaria vectors Anopheles coluzzii and A. gambiae survive the harsh climatic conditions of the dry season in sub-Saharan Africa. However, the physiological mechanisms regulating these two strategies are unknown. In the present study, mosquito species were exposed to controlled environmental conditions mimicking the rainy and dry seasons of south western Burkina Faso. Survival strategies were studied through morphometric (wing length), ecophysiological (respiratory gas exchanges), biochemical (cuticular hydrocarbons composition) and molecular (AKH mRNA expression levels) parameters, variations of which are usually considered to be hallmarks of aestivation and dispersion mechanisms in various insects. Our results showed that ecophysiological and morphometric adjustments are made in both species to prevent water losses during the dry season. However, the usual metabolic rate modifications expected as signatures of aestivation and migration were not observed, highlighting specific and original physiological mechanisms sustaining survival in malaria mosquitoes during the dry season. Differences in epicuticular hydrocarbon composition and AKH levels of expression were found between the permanent and temporary A. coluzzii populations, illustrating the great phenotypic plasticity of this mosquito species. Altogether, our work underlines the diverse and complex pattern of changes occurring in the two mosquito species and at the population level to cope with the dry season and highlights potential targets of future control tools.

Identification of Floral Volatiles and Pollinator Responses in Kiwifruit Cultivars, Actinidia chinensis var. chinensis

Volatiles emitted from unpollinated in situ flowers were collected from two male cultivars, 'M33', 'M91', and one female cultivar 'Zesy002' (Gold3) of kiwifruit (Actinidia chinensis var. chinensis). The samples were found to contain 48 compounds across the three cultivars with terpenes and straight chain alkenes dominating the headspace. Electrophysiological responses of honey bees (Apis mellifera) and bumble bees (Bombus terrestris) to the headspace of the kiwifruit flowers were recorded. Honey bees consistently responded to 11 floral volatiles from Gold3 pistillate flowers while bumble bees consistently responded to only five compounds from the pistillate flowers. Nonanal, 2-phenylethanol, 4-oxoisophorone and (3E,6E)-α-farnesene from pistillate flowers elicited responses from both bee species. Overall, honey bees were more sensitive to the straight chain hydrocarbons of the kiwifruit flowers than the bumble bees, which represented one of the main differences between the responses of the two bee species. The floral volatiles from staminate flowers of the male cultivars 'M33' and 'M91' varied greatly from those of the pistillate flowers of the female cultivar Gold3, with most of the bee active compounds significantly different from those in the Gold3 flower headspace. The total floral emissions of 'M33' flowers were significantly less than those of the Gold3 flowers, while the total floral emissions of the 'M91' flowers were significantly greater than those of the Gold3 flowers.

Multivariate Analysis of Multiple Datasets: a Practical Guide for Chemical Ecology

Chemical ecology has strong links with metabolomics, the large-scale study of all metabolites detectable in a biological sample. Consequently, chemical ecologists are often challenged by the statistical analyses of such large datasets. This holds especially true when the purpose is to integrate multiple datasets to obtain a holistic view and a better understanding of a biological system under study. The present article provides a comprehensive resource to analyze such complex datasets using multivariate methods. It starts from the necessary pre-treatment of data including data transformations and distance calculations, to the application of both gold standard and novel multivariate methods for the integration of different omics data. We illustrate the process of analysis along with detailed results interpretations for six issues representative of the different types of biological questions encountered by chemical ecologists. We provide the necessary knowledge and tools with reproducible R codes and chemical-ecological datasets to practice and teach multivariate methods.

Phenotypic Plasticity of Cuticular Hydrocarbon Profiles in Insects

The insect integument is covered by cuticular hydrocarbons (CHCs) which provide protection against environmental stresses, but are also used for communication. Here we review current knowledge on environmental and insect-internal factors which shape phenotypic plasticity of solitary living insects, especially herbivorous ones. We address the dynamics of changes which may occur within minutes, but may also last weeks, depending on the species and conditions. Two different modes of changes are suggested, i.e. stepwise and gradual. A switch between two distinct environments (e.g. host plant switch by phytophagous insects) results in stepwise formation of two distinct adaptive phenotypes, while a gradual environmental change (e.g. temperature gradients) induces a gradual change of numerous adaptive CHC phenotypes. We further discuss the ecological and evolutionary consequences of phenotypic plasticity of insect CHC profiles by addressing the question at which conditions is CHC phenotypic plasticity beneficial. The high plasticity of CHC profiles might be a trade-off for insects using CHCs for communication. We discuss how insects cope with the challenge to produce and "understand" a highly plastic, environmentally dependent CHC pattern that conveys reliable and comprehensible information. Finally, we outline how phenotypic plasticity of CHC profiles may promote speciation in insects that rely on CHCs for mate recognition.

Cuticular hydrocarbons correlate with queen reproductive status in native and invasive Argentine ants (Linepithema humile, Mayr)

In insect societies, chemical communication plays an important role in colony reproduction and individual social status. Many studies have indicated that cuticular hydrocarbons (CHCs) are the main chemical compounds encoding reproductive status. However, these studies have largely focused on queenless or monogynous species whose workers are capable of egg laying and have mainly explored the mechanisms underlying queen-worker or worker-worker reproductive conflicts. Less is known about what occurs in highly polygynous ant species with permanently sterile workers. Here, we used the Argentine ant as a model to examine the role of CHCs in communicating reproductive information in such insect societies. The Argentine ant is unicolonial, highly polygynous, and polydomous. We identified several CHCs whose presence and levels were correlated with queen age, reproductive status, and fertility. Our results also provide new insights into queen executions in the Argentine ant, a distinctive feature displayed by this species in its introduced range. Each spring, just before new sexuals appear, workers eliminate up to 90% of the mated queens in their colonies. We discovered that queens that survived execution had different CHC profiles from queens present before and during execution. More specifically, levels of some CHCs were higher in the survivors, suggesting that workers could eliminate queens based on their chemical profiles. In addition, queen CHC profiles differed based on season and species range (native vs. introduced). Overall, the results of this study provide new evidence that CHCs serve as queen signals and do more than just regulate worker reproduction.

Desiccation resistance: effect of cuticular hydrocarbons and water content in Drosophila melanogaster adults

Background

The insect cuticle covers the whole body and all appendages and has bi-directionnal selective permeability: it protects against environmental stress and pathogen infection and also helps to reduce water loss. The adult cuticle is often associated with a superficial layer of fatty acid-derived molecules such as waxes and long chain hydrocarbons that prevent rapid dehydration. The waterproofing properties of cuticular hydrocarbons (CHs) depend on their chain length and desaturation number. Drosophila CH biosynthesis involves an enzymatic pathway including several elongase and desaturase enzymes.

Methods

The link between desiccation resistance and CH profile remains unclear, so we tested (1) experimentally selected desiccation-resistant lines, (2) transgenic flies with altered desaturase expression and (3) natural and laboratory-induced CH variants. We also explored the possible relationship between desiccation resistance, relative water content and fecundity in females.

Results

We found that increased desiccation resistance is linked with the increased proportion of desaturated CHs, but not with their total amount. Experimentally-induced desiccation resistance and CH variation both remained stable after many generations without selection. Conversely, flies with a higher water content and a lower proportion of desaturated CHs showed reduced desiccation resistance. This was also the case in flies with defective desaturase expression in the fat body.

Discussion

We conclude that rapidly acquired desiccation resistance, depending on both CH profile and water content, can remain stable without selection in a humid environment. These three phenotypes, which might be expected to show a simple relationship, turn out to have complex physiological and genetic links.

Fatty acid composition in native bees: Associations with thermal and feeding ecology

Fatty acid (FA) composition of lipids plays a crucial role in the functioning of lipid-containing structures in organisms and may be affected by the temperature an organism experiences, as well as its diet. We compared FA composition among four bee genera: Andrena, Bombus, Megachile, and Osmia which differ in their thermal ecology and diet. Fatty acid methyl esters (FAME) were prepared by direct transesterification with KOH and analyzed using gas-liquid chromatography with a flame ionization detector. Sixteen total FAs ranging in chain length from eight to 22 carbon atoms were identified. Linear discriminant analysis separated the bees based on their FA composition. Andrena was characterized by relatively high concentrations of polyunsaturated FAs, Bombus by high monounsaturated FAs and Megachilids (Megachile and Osmia) by relatively high amounts of saturated FAs. These differences in FA composition may in part be explained by variation in the diets of these bees. Because tongue (proboscis) length may be used as a proxy for the types of flowers bees may visit for nectar and pollen, we compared FA composition among Bombus that differed in proboscis length (but have similar thermal ecology). A clear separation in FA composition within Bombus with varying proboscis lengths was found using linear discriminant analysis. Further, comparing the relationship between each genus by cluster analysis revealed aggregations by genus that were not completely separated, suggesting potential overlap in dietary acquisition of FAs.

How do cuticular hydrocarbons evolve? Physiological constraints and climatic and biotic selection pressures act on a complex functional trait

Cuticular hydrocarbons (CHCs) cover the cuticles of virtually all insects, serving as a waterproofing agent and as a communication signal. The causes for the high CHC variation between species, and the factors influencing CHC profiles, are scarcely understood. Here, we compare CHC profiles of ant species from seven biogeographic regions, searching for physiological constraints and for climatic and biotic selection pressures. Molecule length constrained CHC composition: long-chain profiles contained fewer linear alkanes, but more hydrocarbons with disruptive features in the molecule. This is probably owing to selection on the physiology to build a semi-fluid cuticular layer, which is necessary for waterproofing and communication. CHC composition also depended on the precipitation in the ants' habitats. Species from wet climates had more alkenes and fewer dimethyl alkanes than those from drier habitats, which can be explained by different waterproofing capacities of these compounds. By contrast, temperature did not affect CHC composition. Mutualistically associated (parabiotic) species possessed profiles highly distinct from non-associated species. Our study is, to our knowledge, the first to show systematic impacts of physiological, climatic and biotic factors on quantitative CHC composition across a global, multi-species dataset. We demonstrate how they jointly shape CHC profiles, and advance our understanding of the evolution of this complex functional trait in insects.

Influence of microbial symbionts on insect pheromones

Symbiotic microorganisms can influence the fitness of their insect hosts by modulating pheromone production and perception.

Evidence of Adaptive Evolution and Relaxed Constraints in Sex-Biased Genes of South American and West Indies Fruit Flies (Diptera: Tephritidae)

Several studies have demonstrated that genes differentially expressed between sexes (sex-biased genes) tend to evolve faster than unbiased genes, particularly in males. The reason for this accelerated evolution is not clear, but several explanations have involved adaptive and nonadaptive mechanisms. Furthermore, the differences of sex-biased expression patterns of closely related species are also little explored out of Drosophila. To address the evolutionary processes involved with sex-biased expression in species with incipient differentiation, we analyzed male and female transcriptomes of Anastrepha fraterculus and Anastrepha obliqua, a pair of species that have diverged recently, likely in the presence of gene flow. Using these data, we inferred differentiation indexes and evolutionary rates and tested for signals of selection in thousands of genes expressed in head and reproductive transcriptomes from both species. Our results indicate that sex-biased and reproductive-biased genes evolve faster than unbiased genes in both species, which is due to both adaptive pressure and relaxed constraints. Furthermore, among male-biased genes evolving under positive selection, we identified some related to sexual functions such as courtship behavior and fertility. These findings suggest that sex-biased genes may have played important roles in the establishment of reproductive isolation between these species, due to a combination of selection and drift, and unveil a plethora of genetic markers useful for more studies in these species and their differentiation.