Ecological, behavioral, and biochemical aspects of insect hydrocarbons

Integrating lipid metabolism, pheromone production and perception by Fruitless and Hepatocyte nuclear factor 4

Sexual attraction and perception, governed by separate genetic circuits in different organs, are crucial for mating and reproductive success, yet the mechanisms of how these two aspects are integrated remain unclear. In Drosophila , the male-specific isoform of Fruitless (Fru), Fru M , is known as a master neuro-regulator of innate courtship behavior to control perception of sex pheromones in sensory neurons. Here we show that the non-sex specific Fru isoform (Fru COM ) is necessary for pheromone biosynthesis in hepatocyte-like oenocytes for sexual attraction. Loss of Fru COM in oenocytes resulted in adults with reduced levels of the cuticular hydrocarbons (CHCs), including sex pheromones, and show altered sexual attraction and reduced cuticular hydrophobicity. We further identify Hepatocyte nuclear factor 4 ( Hnf4 ) as a key target of Fru COM in directing fatty acid conversion to hydrocarbons in adult oenocytes. fru - and Hnf4 -depletion disrupts lipid homeostasis, resulting in a novel sex-dimorphic CHC profile, which differs from doublesex - and transformer -dependent sexual dimorphism of the CHC profile. Thus, Fru couples pheromone perception and production in separate organs for precise coordination of chemosensory communication that ensures efficient mating behavior.

Teaser

Fruitless and lipid metabolism regulator HNF4 integrate pheromone biosynthesis and perception to ensure robust courtship behavior.

Consistency of Cuticular Hydrocarbon Mixtures of Five Reticulitermes (Blattodea: Rhinotermitidae) Taxa From Northern California: Similarity Among Colonies and Seasonal Variation

Cuticular hydrocarbon (CHC) mixtures from workers of five distinct CHC phenotypes of Reticulitermes Holmgren 1913 from two locations in northern California were examined from monthly collections taken over a 3-yr period. The objectives of this study were (1) to identify and quantify variations of the CHCs of multiple colonies of each of these phenotypes (= species or subspecies) to demonstrate consistency, (2) to assess the potential of CHC mixtures to separate or identify colonies within each phenotype, and (3) to detect any temporal changes in each of the hydrocarbons in the CHC mixtures. Nonmetric multidimensional scaling of all CHC mixtures of all samples collected at both locations separated the samples into five clearly visible, different groups of CHC phenotypes (taxa or species) of Reticulitermes. The degree of variability of the CHC mixtures among colonies of each phenotype was such that nonmetric multidimensional scaling did not separate or identify colonies. Strong seasonal fluctuations were evident in some of the CHCs of all five phenotypes and were significantly consistent with a sine curve. Maximum proportions of seasonal CHCs within a phenotype occurred in all seasons of the year but occurred mostly in the winter and summer. In general, the CHCs displaying maximum values in the winter were short-chained (C23-C27) methyl-branched alkanes, whereas the CHCs displaying maximum values in the summer were long-chained (C35-C43) methyl-branched alkanes, which likely influences water retention. These consistent chemical fingerprints are probably responsible for inter-phenotype recognition patterns and are thus useful for chemical taxonomy.

Geographical Variation of Cuticular Hydrocarbon Profiles of Adult Flies and Empty Puparia Amongst Three Populations of Calliphora vicina (Diptera: Calliphoridae)

Blowflies (Diptera: Calliphoridae) are of great importance in forensic entomology and in determining the minimum post-mortem interval, as they may be the first group of insects to colonize decomposing remains. Reliable species identification is an essential prerequisite. Classically, morphological characters or DNA sequences are used for this purpose. However, depending on the species and the condition of the specimen, this can be difficult, e.g., in the case of empty fly puparia. Recent studies have shown that cuticular hydrocarbon (CHC) profiles are species-specific in necrophagous taxa and represent another promising tool for identification. However, the population-specific variability of these substances as a function of e.g., local climatic parameters has not yet been sufficiently investigated. The aim of this study was to determine the geographical variation of CHC profiles of the blowfly Calliphora vicina (Robineau-Desvoidy, 1830) depending on different countries of origin. Flies were reared in the United Kingdom, Germany, and Turkey in common garden experiments under ambient conditions. CHC profiles of the resulting adult flies and their empty puparia were analyzed using gas chromatography-mass spectrometry. Data were visualized by principal component analysis and clustered by population. The populations of the United Kingdom and Germany, both having similar climates and being geographically close to each other, showed greater similarities in CHC profiles. However, the CHC profile of the Turkish population, whose climate is significantly different from the other two populations, was very different. Our study confirms the high potential of CHC analysis in forensic entomology but highlights the need to investigate geographical variability in chemical profiles.

Many parasitoids lack adult fat accumulation, despite fatty acid synthesis: A discussion of concepts and considerations for future research

Fat reserves, specifically the accumulation of triacylglycerols, are a major energy source and play a key role for life histories. Fat accumulation is a conserved metabolic pattern across most insects, yet in most parasitoid species adults do not gain fat mass, even when nutrients are readily available and provided ad libitum. This extraordinary physiological phenotype has evolved repeatedly in phylogenetically dispersed parasitoid species. This poses a conundrum because it could lead to significant constraints on energy allocation toward key adult functions such as survival and reproduction. Recent work on the underlying genetic and biochemical mechanisms has spurred a debate on fat accumulation versus fat production, because of incongruent interpretation of results obtained using different methodologies. This debate is in part due to semantics, highlighting the need for a synthetic perspective on fat accumulation that reconciles previous debates and provides new insights and terminology. In this paper, we propose updated, unambiguous terminology for future research in the field, including "fatty acid synthesis" and "lack of adult fat accumulation", and describe the distinct metabolic pathways involved in the complex process of lipogenesis. We then discuss the benefits and drawbacks of the main methods available to measure fatty acid synthesis and adult fat accumulation. Most importantly, gravimetric/colorimetric and isotope tracking methods give complementary information, provided that they are applied with appropriate controls and interpreted correctly. We also compiled a comprehensive list of fat accumulation studies performed during the last 25 years. We present avenues for future research that combine chemistry, ecology, and evolution into an integrative approach, which we think is needed to understand the dynamics of fat accumulation in parasitoids.

Cuticular Hydrocarbon Polymorphism in a Parasitoid Wasp

Cuticular hydrocarbons (CHCs) are major constituents of the cuticular lipid layer of insects. They serve not only as a barrier to desiccation, but often additionally mediate communication at close range. The compositions of the CHC profiles, i.e., the specific compounds and their relative amounts, usually differ between species. Additional intraspecific variability can be found between different populations, between colonies and castes of social insects, and between the sexes. Thus, such groups can often be distinguished based on distinctive compounds and/or specific compound ratios. The CHC profile may further be influenced by biotic and abiotic factors, which therefore can impact, e.g., nestmate recognition or mate choice. However, consistent intrasexual variation seems to be rare. Here, we investigated a case of intrasexual CHC variability within a single population of a parasitoid wasp. While wasps of both sexes produced the same set of compounds, the relative amounts of specific compound classes revealed the presence of intrasexual chemical phenotypes. This is, to our knowledge, the first report of three distinct female CHC profile patterns within a population of a solitary insect that uses CHCs for mate recognition. Additionally, male CHC profiles, while overall very similar, could be separated into two chemotypes by multivariate analysis. The study of species exhibiting such intraspecific and intrasexual CHC variation will advance our understanding of the effects of CHC variability on both, desiccation resistance and intraspecific communication.

A pleiotropic chemoreceptor facilitates the production and perception of mating pheromones

Optimal mating decisions depend on the robust coupling of signal production and perception because independent changes in either could carry a fitness cost. However, since the perception and production of mating signals are often mediated by different tissues and cell types, the mechanisms that drive and maintain their coupling remain unknown for most animal species. Here, we show that in Drosophila, behavioral responses to, and the production of, a putative inhibitory mating pheromone are co-regulated by Gr8a, a member of the Gustatory receptor gene family. Specifically, through behavioral and pheromonal data, we found that Gr8a independently regulates the behavioral responses of males and females to a putative inhibitory pheromone, as well as its production in the fat body and oenocytes of males. Overall, these findings provide a relatively simple molecular explanation for how pleiotropic receptors maintain robust mating signaling systems at the population and species levels.

Field Confirmation of (Z)-9-Heptacosene and (3Z,6Z,9Z)-Tricosatriene as Key Sex Pheromone Components of Korean Conogethes punctiferalis Guenée (Lepidoptera: Crambidae)

Recently, insufficient attractiveness of Conogethes punctiferalis Guenée adult males to sex pheromone, -(E)-10-hexadecenal and (Z)-10-hexadecenal, has been reported. To identify the other essential components of sex pheromone, male and female body extracts were analyzed. Two hydrocarbon components, (Z)-9-heptacosene (Z9-27:HC) and (3Z,6Z,9Z)-tricosatriene (Z3,Z6,Z9-23:HC), were identified from only female body extract. There was a significant difference in the electroantennogram (EAG) response of male antennae to Z3,Z6,Z9-23:HC and Z9-27:HC at all test concentrations compared to the response to the hexane control. In field attraction testing, the addition of Z9-27:HC and Z3,Z6,Z9-23:HC to binary aldehyde pheromones significantly increased trap catches of C. punctiferalis male adults. Based on the female and male body extract analysis and field attraction test, Z9-27:HC and Z3,Z6,Z9-23:HC were determined to be other essential sex pheromone components of the Korean C. punctiferalis population. No significant difference was observed in the number of male captures between the bucket trap and delta trap. Pheromone traps with a color close to yellow shade attracted more male adults than traps with a color close to blue shade.

Characterization of male-biased fatty acid desaturase in the jewel wasp Nasonia vitripennis and its role in pheromone production

Linoleic acid is the material for biosynthesis of sex attracting and blocking (postmating) pheromones in Nasonia vitripennis, it is synthesized from oleic acid by a male-biased fatty acid desaturase (SCD5a). In this study, we developed a specific antibody and further characterized the expression patterns of SCD5a in males at different mating stages by western blot. SCD5a was mainly expressed in male heads rather than in abdomens. Along with the aging process (from Day 1 to Day 3), SCD5a increased significantly. Compared with virgin males, mated males showed higher levels of SCD5a. Likewise, abdomen dipping frequency, during which males release attracting pheromone, increased with age and mating. Moreover, real-time quantitative PCR revealed that genes responsible for the first three steps of attracting pheromone biosynthesis were more highly expressed in head than in abdomen, but the final gene for transformation of attracting pheromone was more highly expressed in abdomen than in head. These results suggest that linoleic acid for biosynthesis of attracting pheromones may also originate from the head rather than only synthesized at the rectal vesicles.

Fatty acyl-CoA reductase influences wax biosynthesis in the cotton mealybug, Phenacoccus solenopsis Tinsley

Mealybugs are highly aggressive to a diversity of plants. The waxy layer covering the outermost part of the integument is an important protective defense of these pests. However, the molecular mechanisms underlying wax biosynthesis in mealybugs remain largely unknown. Here, we analyzed multi-omics data on wax biosynthesis by the cotton mealybug, Phenacoccus solenopsis Tinsley, and found that a fatty acyl-CoA reductase (PsFAR) gene, which was highly expressed in the fat bodies of female mealybugs, contributed to wax biosynthesis by regulating the production of the dominant chemical components of wax, cuticular hydrocarbons (CHCs). RNA interference (RNAi) against PsFAR by dsRNA microinjection and allowing mealybugs to feed on transgenic tobacco expressing target dsRNA resulted in a reduction of CHC contents in the waxy layer, and an increase in mealybug mortality under desiccation and deltamethrin treatments. In conclusion, PsFAR plays crucial roles in the wax biosynthesis of mealybugs, thereby contributing to their adaptation to water loss and insecticide stress.

The role of a fatty acyl-CoA reductase (PsFAR) in wax biosynthesis of cotton mealybug is investigated, RNAi against PsFAR resulted in insects with lower generation of waxy filaments and higher mortality under desiccation and deltamethrin treatments.

Chasing Flies: The Use of Wingbeat Frequency as a Communication Cue in Calyptrate Flies (Diptera: Calyptratae)

The incidental sound produced by the oscillation of insect wings during flight provides an opportunity for species identification. Calyptrate flies include some of the fastest and most agile flying insects, capable of rapid changes in direction and the fast pursuit of conspecifics. This flight pattern makes the continuous and close recording of their wingbeat frequency difficult and limited to confined specimens. Advances in sound editor and analysis software, however, have made it possible to isolate low amplitude sounds using noise reduction and pitch detection algorithms. To explore differences in wingbeat frequency between genera and sex, 40 specimens of three-day old Sarcophaga crassipalpis, Lucilia sericata, Calliphora dubia, and Musca vetustissima were individually recorded in free flight in a temperature-controlled room. Results showed significant differences in wingbeat frequency between the four species and intersexual differences for each species. Discriminant analysis classifying the three carrion flies resulted in 77.5% classified correctly overall, with the correct classification of 82.5% of S. crassipalpis, 60% of C. dubia, and 90% of L. sericata, when both mean wingbeat frequency and sex were included. Intersexual differences were further demonstrated by male flies showing significantly higher variability than females in three of the species. These observed intergeneric and intersexual differences in wingbeat frequency start the discussion on the use of the metric as a communication signal by this taxon. The success of the methodology demonstrated differences at the genus level and encourages the recording of additional species and the use of wingbeat frequency as an identification tool for these flies.

Species recognition limits mating between hybridizing ant species

Identifying mechanisms limiting hybridization is a central goal of speciation research. Here, we studied premating and postmating barriers to hybridization between two ant species, Formica selysi and Formica cinerea. These species hybridize in the Rhône valley in Switzerland, where they form a mosaic hybrid zone, with limited introgression from F. selysi into F. cinerea. There was no sign of temporal isolation between the two species in the production of queens and males. With choice experiments, we showed that queens and males strongly prefer to mate with conspecifics. Yet, we did not detect postmating barriers caused by genetic incompatibilities. Specifically, hybrids of all sexes and castes were found in the field and F1 hybrid workers did not show reduced viability compared to nonhybrid workers. To gain insights into the cues involved in species recognition, we analyzed the cuticular hydrocarbons (CHCs) of queens, males, and workers and staged dyadic encounters between workers. CHC profiles differed markedly between species, but were similar in F. cinerea and hybrids. Accordingly, workers also discriminated species, but they did not discriminate F. cinerea and hybrids. We discuss how the CHC-based recognition system of ants may facilitate the establishment of premating barriers to hybridization, independent of hybridization costs.

Near infrared spectroscopy (NIRS) coupled with chemometric methods to identify and estimate taxonomic relationships of flies with forensic potential (Diptera: Calliphoridae and Sarcophagidae)

Infrared spectroscopy has been gaining prominence in entomology, such as for solving taxonomic problems, sexing adult specimens, determining the age of immature specimens, detecting drugs of abuse in fly larvae, and can be an important technique in Forensic Entomology. In order to help identify the species of Calliphoridae and Sarcophagidae families, the present study aimed to evaluate the use of near infrared spectroscopy (NIRS) coupled with chemometric methods for separating fly specimens into taxonomic categories and understanding the taxonomic relationship between them. Spectra collected from nine species of flies were subjected to unsupervised principal component analysis (PCA) and hierarchical cluster analysis (HCA), in which we sought to visualize the relationship between the samples (segregation of genera and families) with subsequent identification. In PCA, the best model was achieved using five principal components (PCs), which explained 99.16% of total variance of the original data set. The first principal component (PC1) and the fourth principal component (PC4) provided the best segregation, the latter being more important in the segregation of the species Chrysomya albiceps, Lucilia eximia, and Ravinia belforti from the others. In the HCA dendrogram, there was a clear separation between the specimens by family (Calliphoridae and Sarcophagidae) and genera (Chrysomya, Lucilia, Oxysarcodexia, Peckia and Ravinia). This study shows that NIRS is efficient to identify flies' taxonomic properties, such as family and genera, providing quick evidence for the tested species identity.

Comparative Analysis of Epicuticular Lipids in Locusta migratoria and Calliptamus italicus: A Possible Role in Susceptibility to Entomopathogenic Fungi

Cuticular lipids protect insects from desiccation and may determine resistance to fungal pathogens. Nonetheless, the trade-off between these lipid functions is still poorly understood. The migratory locust Locusta migratoria and the Italian locust Calliptamus italicus have dissimilar hygrothermal preferences: L. migratoria inhabits areas near water bodies with a reed bed, and C. italicus exploits a wide range of habitats and prefers steppes and semideserts with the predominance of sagebrushes. This paper presents significant differences between these species' nymphs in epicuticular lipid composition (according to gas chromatography with mass spectrometry) and in susceptibility to Metarhizium robertsii and Beauveria bassiana. The main differences in lipid composition are shifts to longer chain and branched hydrocarbons (di- and trimethylalkanes) in C. italicus compared to L. migratoria. C. italicus also has a slightly higher n-alkane content. Fatty acids showed low concentrations in the extracts, and L. migratoria has a wider range of fatty acids than C. italicus does. Susceptibility to M. robertsii and the number of conidia adhering to the cuticle proved to be significantly higher in C. italicus, although conidia germination percentages on epicuticular extracts did not differ between the species. We propose that the hydrocarbon composition of C. italicus may be an adaptation to a wide range of habitats including arid ones but may make the C. italicus cuticle more hospitable for fungi.

Live Drosophila Melanogaster Larvae Deter Oviposition by Drosophila suzukii

Simple Summary

The invasive insect pest, Drosophila suzukii Matsumura or spotted-wing drosophila (SWD) lays its eggs in soft and stone fruit. Eggs hatch into larvae, which feed on fruit, causing fruit collapse and significant economic losses worldwide. Current control methods rely primarily on foliar insecticide applications, which are not sustainable long-term solutions. In nature, D. suzukii interacts with and encounters other Drosophila species, especially towards the end of the growing season when ripening fruits are scarce. We showed previously that D. suzukii were deterred from laying eggs on artificial media exposed to egg laying Drosophila melanogaster, its sister species. It was hypothesized that a signal was left by D. melanogaster which deterred D. suzukii from laying eggs. This study aimed to identify from which D. melanogaster life stage the egg laying deterrent signal originated and we showed that the presence of live D. melanogaster larvae on the egg laying media deter D. suzukii from laying eggs. Drosophila melanogaster cuticular hydrocarbons were examined as the signal source, but no evidence was found for their involvement. These results have improved our understanding of the interspecific interactions between D. suzukii and other Drosophila species and could provide new innovative approaches to D. suzukii management strategies.

Abstract

The worldwide invasive insect pest, Drosophila suzukii Matsumura (spotted-wing Drosophila), lays eggs in soft and stone fruit before harvest. Hatched larvae cause fruit collapse and significant economic losses. Current control methods rely primarily on foliar insecticide applications, which are not sustainable long-term solutions due to regulatory restrictions and the risk of insecticide resistance developing. We showed before that D. suzukii were deterred from laying eggs on artificial media previously visited by its sister species—Drosophila melanogaster. In the current study, laboratory choice test experiments were conducted to identify which D. melanogaster life stage (eggs, larvae, or adult) deterred D. suzukii oviposition. We demonstrated that the presence of live D. melanogaster larvae on the egg-laying media consistently deterred D. suzukii oviposition. Drosophila melanogaster cuticular hydrocarbons (CHCs) were examined as candidate for the oviposition deterrent. CHCs of larval and adult D. melanogaster and D. suzukii were analyzed. In both species, the composition of the CHCs of larvae was similar to that of adults, although quantities present were much lower. Furthermore, the CHC profiles of the two species were markedly different. However, when assayed as deterrents in the laboratory choice test experiment, CHC extracts from D. melanogaster did not deter oviposition by D. suzukii.

Molecular Evidence of Chemical Disguise by the Socially Parasitic Spiny Ant Polyrhachis lamellidens (Hymenoptera: Formicidae) When Invading a Host Colony

While most ant species establish a colony independently, some socially parasitic ants build the foundation of their colony by invading other ant (host) colonies and utilizing their labor forces. Many socially parasitic ants disguise their cuticular hydrocarbon (CHC) profile, which is also known as signature mixture for nestmate discrimination, when invading the host colony. Since the strategy of chemical disguise is widespread in socially parasitic ants, elucidating the mechanism of chemical disguise will promote knowledge on the evolutionary history of social parasitism. However, detailed knowledge is still lacking, as the relevant information has only originated from circumstantial evidence, which was obtained from ecological observations. In this study, we investigated the mechanism of chemical disguise in a new queen of a temporary socially parasitic spiny ant (Polyrhachis lamellidens) by measuring its CHC profile, performing a tracing assay with labeled substances, and analyzing gene expression levels. First, after rubbing behavior was observed against the host workers, the CHC profile in P. lamellidens shifted to pronounced peaks that closely resembling that of the host workers. We also observed a reduction in aggressive behaviors by the host ant against P. lamellidens after rubbing behavior was performed. In addition, P. lamellidens acquired artificially-applied labeling substances from host workers through their rubbing behaviors, while gene expression profiling showed the expression of CHC synthesis-related genes did not change during this behavior. These results suggest that P. lamellidens directly obtains host CHCs through rubbing behavior, and these host CHCs enables P. lamellidens to remain disguised during colony invasion.

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.

Olfactory Strategies in the Defensive Behaviour of Insects

Most animals must defend themselves in order to survive. Defensive behaviour includes detecting predators or intruders, avoiding them by staying low-key or escaping or deterring them away by means of aggressive behaviour, i.e., attacking them. Responses vary across insect species, ranging from individual responses to coordinated group attacks in group-living species. Among different modalities of sensory perception, insects predominantly use the sense of smell to detect predators, intruders, and other threats. Furthermore, social insects, such as honeybees and ants, communicate about danger by means of alarm pheromones. In this review, we focus on how olfaction is put to use by insects in defensive behaviour. We review the knowledge of how chemical signals such as the alarm pheromone are processed in the insect brain. We further discuss future studies for understanding defensive behaviour and the role of olfaction.

Land-use stress alters cuticular chemical surface profile and morphology in the bumble bee Bombus lapidarius

Pollinators and other insects are currently undergoing a massive decline. Several stressors are thought to be of importance in this decline, with those having close relationships to agricultural management and practice seemingly playing key roles. In the present study, we sampled Bombus lapidarius L. workers in grasslands differing in their management intensity and management regime across three different regions along a north-south gradient in Germany. We analyzed the bees with regard to (1) their cuticular hydrocarbon profile (because of its important role in communication in social insects) and amount of scent by using gas chromatography and (2) the size of each individual by using wing distances as a proxy for body size. Our analysis revealed changes related to land-use intensity and temperature in the cuticular scent profile of bumble bees. Decreasing body size and increasing total scent amount were explained by an interaction of land-use intensity and study region, but not by land-use intensity alone. Thus, land-use intensity and temperature influence intracolonial communication and size, both of which can have strong effects on foraging. Land management and climate are therefore probably detrimental for colony maintenance and the reproductive success of bumble bees.

Chemical and behavioural strategies along the spectrum of host specificity in ant-associated silverfish

Background

Host range is a fundamental trait to understand the ecological and evolutionary dynamics of symbionts. Increasing host specificity is expected to be accompanied with specialization in different symbiont traits. We tested this specificity-specialization association in a large group of 16 ant-associated silverfish species by linking their level of host specificity to their degree of behavioural integration into the colony and to their accuracy of chemically imitating the host’s recognition system, i.e. the cuticular hydrocarbon (CHC) profile.

Results

As expected, facultative associates and host generalists (targeting multiple unrelated ants) tend to avoid the host, whereas host-specialists (typically restricted to Messor ants) were bolder, approached the host and allowed inspection. Generalists and host specialists regularly followed a host worker, unlike the other silverfish. Host aggression was extremely high toward non-ant-associated silverfish and modest to low in ant-associated groups. Surprisingly, the degree of chemical deception was not linked to host specificity as most silverfish, including facultative ant associates, imitated the host’s CHC profile. Messor specialists retained the same CHC profile as the host after moulting, in contrast to a host generalist, suggesting an active production of the cues (chemical mimicry). Host generalist and facultative associates flexibly copied the highly different CHC profiles of alternative host species, pointing at passive acquisition (chemical camouflage) of the host’s odour.

Conclusions

Overall, we found that behaviour that seems to facilitate the integration in the host colony was more pronounced in host specialist silverfish. Chemical deception, however, was employed by all ant-associated species, irrespective of their degree of host specificity.

Rapid Changes in Composition and Contents of Cuticular Hydrocarbons in Sitobion avenae (Hemiptera: Aphididae) Clones Adapting to Desiccation Stress

Cuticular hydrocarbons (CHCs) are diverse in insects, and include variable classes of cuticular lipids, contributing to waterproofing for insects under desiccation environments. However, this waterproofing function of CHCs is still not well characterized in aphids. In this study, we compared CHC profiles for desiccation-resistant and nonresistant genotypes of the grain aphid, Sitobion avenae (Fabricius), in responses to desiccation. Our result showed that a total of 27 CHCs were detected in S. avenae, and linear alkanes (e.g., n-C29) were found to be the predominant components. Long-chain monomethyl alkanes were found to associate closely with water loss rates in S. avenae in most cases. Resistant genotypes of both wing morphs had higher contents of short-chain n-alkanes under control than nonresistant genotypes, showing the importance of short-chain n-alkanes in constitutive desiccation resistance. Among these, n-C25 might provide a CHC signature to distinguish between desiccation-resistant and nonresistant individuals. Compared with linear alkanes, methyl-branched CHCs appeared to display higher plasticity in rapid responses to desiccation, especially for 2-MeC26, implying that methyl-branched CHCs could be more sensitive to desiccation, and play more important roles in induced desiccation-resistance. Thus, both constitutive and induced CHCs (linear or methyl-branched) can contribute to adaptive responses of S. avenae populations under desiccation environments. Our results provide substantial evidence for adaptive changes of desiccation resistance and associated CHCs in S. avenae, and have significant implications for aphid evolution and management in the context of global climate change.

Enantiomeric Discrimination in Insects: The Role of OBPs and ORs

Olfaction is a complex recognition process that is critical for chemical communication in insects. Though some insect species are capable of discrimination between compounds that are structurally similar, little is understood about how this high level of discrimination arises. Some insects rely on discriminating between enantiomers of a compound, demonstrating an ability for highly selective recognition. The role of two major peripheral olfactory proteins in insect olfaction, i.e., odorant-binding proteins (OBPs) and odorant receptors (ORs) has been extensively studied. OBPs and ORs have variable discrimination capabilities, with some found to display highly specialized binding capability, whilst others exhibit promiscuous binding activity. A deeper understanding of how odorant-protein interactions induce a response in an insect relies on further analysis such as structural studies. In this review, we explore the potential role of OBPs and ORs in highly specific recognition, specifically enantiomeric discrimination. We summarize the state of research into OBP and OR function and focus on reported examples in the literature of clear enantiomeric discrimination by these proteins.

The Effect of Diet on Colony Recognition and Cuticular Hydrocarbon Profiles of the Invasive Argentine Ant, Linepithema humile

Simple Summary

The membership of social insect colonies is defined by chemical pheromones on the bodies of colony members. In nearly all ant species that have been studied, these pheromones have been shown to be genetically based. In some cases, however, environmentally derived odors have been implicated as colony recognition cues. The widely introduced and invasive Argentine ant is well known for forming massive “supercolonies” in its introduced ranges. Previous studies have implicated both genetic and diet-derived chemicals in the colony recognition systems of introduced populations. Here, we perform feeding experiments, in both realistic field settings and the lab, and show that dietary changes do not cause behavioral changes in the field, as well as under most laboratory conditions. However, one exception was found, in which reduced aggression was recorded in one of the laboratory feeding treatments (with crickets as the dietary item), but, nevertheless, not of sufficient magnitude to explain the unusual colony structure of introduced Argentine ants. We conclude that dietary shifts during introduction to new ranges do not account for the origin of Argentine ant supercolonies.

Abstract

Ants are some of the most abundant and ecologically successful terrestrial organisms, and invasive ants rank among the most damaging invasive species. The Argentine ant is a particularly well-studied invader, in part, because of the extreme social structure, known as unicoloniality, that occurs in introduced populations. Unicoloniality is characterized by the formation of geographically vast supercolonies, within which territorial behavior and intraspecific aggression are absent. Although there is considerable evidence supporting a genetic basis for the odor cues involved in colony recognition, some studies have suggested that diet may also influence colony recognition cues and, thus, colony structure. Here, we test the role for insect-derived recognition cues by performing a diet supplementation experiment in a natural field setting, and a more extreme dietary manipulation experiment in the lab. After one month, in both the field and the lab, we found that aggressive supercolonies remained aggressive toward each other and non-aggressive nests (from the same supercolony) remained non-aggressive, regardless of dietary treatment. In one lab treatment, we did observe a significant decrease in the level of aggression between different supercolonies that were fed the same diet, but aggression was still frequent. We did not see any evidence for cuticular hydrocarbon odor cues being transferred from prey to ants in any of the field treatments. In the more extreme lab treatment, however, several cuticular hydrocarbons were acquired from both roach and cricket insect prey (but not Drosophila). Based on these data, we conclude that dietary changes are unlikely to underlie changes in behavior or colony structure in Argentine ants in real-world settings. However, these results indicate that caution is warranted when interpreting the behaviors of animals that have been reared on diets that are substantially different from natural populations.

Metalloenzymes for Fatty Acid-Derived Hydrocarbon Biosynthesis: Nature's Cryptic Catalysts

Waning resources, massive energy consumption, everdeepening global warming crisis, and climate change have raised grave concerns regarding continued dependence on fossil fuels as the predominant source of energy and generated tremendous interest for developing biofuels, which are renewable. Hydrocarbon-based 'drop-in' biofuels can be a proper substitute for fossil fuels such as gasoline or jet fuel. In Nature, hydrocarbons are produced by diverse organisms such as insects, plants, bacteria, and cyanobacteria. Metalloenzymes play a crucial role in hydrocarbons biosynthesis, and the past decade has witnessed discoveries of a number of metalloenzymes catalyzing hydrocarbon biosynthesis from fatty acids and their derivatives employing unprecedented mechanisms. These discoveries elucidated the enigma related to the divergent chemistries involved in the catalytic mechanisms of these metalloenzymes. There is substantial diversity in the structure, mode of action, cofactor requirement, and substrate scope among these metalloenzymes. Detailed structural analysis along with mutational studies of some of these enzymes have contributed significantly to identifying the key amino acid residues that dictate substrate specificity and catalytic intricacy. In this Review, we discuss the metalloenzymes that catalyze fatty acid-derived hydrocarbon biosynthesis in various organisms, emphasizing the active site architecture, catalytic mechanism, cofactor requirements, and substrate specificity of these enzymes. Understanding such details is essential for successfully implementing these enzymes in emergent biofuel research through protein engineering and synthetic biology approaches.

Phenotypic disruption of cuticular hydrocarbon production in hybrids between sympatric species of Hawaiian picture-wing Drosophila

Interspecies hybrids can express phenotypic traits far outside the range of parental species. The atypical traits of hybrids provide insight into differences in the factors that regulate the expression of these traits in the parental species. In some cases, the unusual phenotypic traits of hybrids can lead to phenotypic dysfunction with hybrids experiencing reduced survival or reproduction. Cuticular hydrocarbons (CHCs) in insects are important phenotypic traits that serve several functions, including desiccation resistance and pheromones for mating. We used gas chromatography mass spectrometry to investigate the differences in CHC production between two closely related sympatric Hawaiian picture-wing Drosophila species, Drosophila heteroneura and D. silvestris, and their F1 and backcross hybrid offspring. CHC profiles differed between males of the two species, with substantial sexual dimorphism in D. silvestris but limited sexual dimorphism in D. heteroneura. Surprisingly, F1 hybrids did not produce three CHCs, and the abundances of several other CHCs occurred outside the ranges present in the two parental species. Backcross hybrids produced all CHCs with greater variation than observed in F1 or parental species. Overall, these results suggest that the production of CHCs was disrupted in F1 and backcross hybrids, which may have important consequences for their survival or reproduction.

Cellulosic hydrocarbons production by engineering dual synthesis pathways in Corynebacterium glutamicum

Background

Lignocellulose provides the only practical carbohydrates feedstock for sustainable bioproduction of hydrocarbons as future alternative of fossil fuels. Production of hydrocarbons from lignocellulose is achieved by a biorefinery process chain including pretreatment to breakdown the crystalline structure for cellulase-catalyzed hydrolysis, detoxification of inhibitory compounds generated during pretreatment, enzymatic hydrolysis to fermentable monosaccharide sugars, and fermentation to hydrocarbon products. The major barriers on fermentative production of hydrocarbons from lignocellulose include two aspects: one is the inherent stress of pretreatment-derived inhibitors on microbial cells, the other is the toxicity of hydrocarbons to cell membranes. The microbial cell factory should be tolerant to both inhibitor stress and hydrocarbons toxicity.

Results

Corynebacterium glutamicum was selected as the starting strain of hydrocarbons synthesis since it is well adapted to lignocellulose hydrolysate environment. The dual hydrocarbon synthesis pathways were constructed in an industrial C. glutamicum S9114 strain. The first pathway was the regular one in microalgae composed of fatty acyl-acyl carrier protein (fatty acyl-ACP) reductase (AAR) and aldehyde deformylating oxygenase (ADO) with fatty acyl-ACP as precursor. The second pathway was the direct decarboxylation of free fatty acid by fatty acid decarboxylase (OleT) using the rich fatty acids from the disruption of the transcriptional regulator fasR gene. The transmembrane transportation of hydrocarbon products was avoided by secretively expressing the fatty acid decarboxylase (OleT) to the extracellular space. The hydrocarbons generation from glucose reached 29.2 mg/L, in which the direct decarboxylation pathway contributed more than 70% of the total hydrocarbons generation, and the AAR–ADO pathway contributed the rest 30%.

Conclusion

The dual hydrocarbon synthesis pathways (OleT and AAR–ADO pathways) were constructed in the inhibitors tolerant C. glutamicum S9114 strain for hydrocarbon production using lignocellulose feedstock as the starting feedstock. When corn stover was used for hydrocarbons production after dry acid pretreatment and biodetoxification, the hydrocarbons generation reached 16.0 mg/L. This study provided a new strategy for hydrocarbons synthesis using microbial cell factory suitable for lignocellulose feedstock.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02129-7.

Reproductive Isolation in the Cryptic Species Complex of a Key Pest: Analysis of Mating and Rejection Behaviour of Onion Thrips (Thrips tabaci Lindeman)

Simple Summary

The onion thrips (Thrips tabaci), which is a key pest worldwide, includes three characteristic, distinct groups (i.e., lineages) under the same species name. In the current report, we addressed the question of whether individuals from these lineages recognize and assess each other as potential mating partners. We demonstrated that at least two of these lineages do not belong to the same species, since under our laboratory conditions no mating occurred between them. Moreover, specimens from these cross pairings often exhibited an escape response upon contact with the other thrips, while most of the pairs from the same lineages readily mated at their first interaction. The behaviour of males clearly indicated that they can assess the mating status of a female and usually only try to copulate with virgins. Our findings are important not only because in agriculture it is vital to know whether morphologically similar entities belong to the same species, but also because knowledge about the communication of insects and the possible role of the cues involved can help the development of new plant protection techniques.

Abstract

Thrips tabaci Lindeman is a serious pest of various cultivated plants, with three, distinct lineages within a cryptic species complex. Despite the well-known significance of this pest, many attributes of these lineages are not yet fully understood, including their reproductive behaviour. We performed no-choice-design cross-mating experiments under a controlled laboratory environment with virgin adult individuals from all three lineages. The behaviour of thrips was recorded with a camera mounted on a stereomicroscope, and the recordings were analysed in detail. We found that the so-called leek-associated lineages of this cryptic species complex are reproductively isolated from the tobacco-associated lineage; therefore, they represent different species. Divergence in the behaviour of conspecific and heterospecific pairs became evident only after contact. There were no marked differences between the lineages in their precopulatory and copulatory behaviour, except in the duration of the latter. We confirmed mating between thelytokous females and arrhenotokous males; however, we assume some form of loss of function in the sexual traits of asexual females. The post-mating behaviour of males indicated the presence and role of an anti-aphrodisiac pheromone. We also demonstrated differences between lineages regarding their activity and their propensity for exhibiting an escape response upon interaction with heterospecific thrips.

Convergent adaptation of ootheca formation as a reproductive strategy in Polyneoptera

Insects have evolved numerous adaptations and colonized diverse terrestrial environments. Several polyneopterans, including dictyopterans (cockroaches and mantids) and locusts, have developed oothecae, but little is known about the molecular mechanism, physiological function, and evolutionary significance of ootheca formation. Here, we demonstrate that the cockroach asymmetric colleterial glands produce vitellogenins, proline-rich protein, and glycine-rich protein as major ootheca structural proteins (OSPs) that undergo sclerotization and melanization for ootheca formation through the cooperative protocatechuic acid pathway and dopachrome and dopaminechrome subpathway. Functionally, OSP sclerotization and melanization prevent eggs from losing water at warm and dry conditions, and thus effectively maintain embryo viability. Dictyopterans and locusts convergently evolved vitellogenins, apolipoprotein D, and laminins as OSPs, whereas within Dictyoptera, cockroaches and mantids independently developed glycine-rich protein and fibroins as OSPs. Highlighting the ecological-evolutionary importance, convergent ootheca formation represents a successful reproductive strategy in Polyneoptera that promoted the radiation and establishment of cockroaches, mantids, and locusts.

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.

Aging-Related Variation of Cuticular Hydrocarbons in Wild Type and Variant Drosophila melanogaster

The cuticle of all insects is covered with hydrocarbons which have multiple functions. Cuticular hydrocarbons (CHCs) basically serve to protect insects against environmental harm and reduce dehydration. In many species, some CHCs also act as pheromones. CHCs have been intensively studied in Drosophila species and more especially in D. melanogaster. In this species, flies produce about 40 CHCs forming a complex sex- and species-specific bouquet. The quantitative and qualitative pattern of the CHC bouquet was characterized during the first days of adult life but remains unexplored in aging flies. Here, we characterized CHCs during the whole-or a large period of-adult life in males and females of several wild type and transgenic lines. Both types of lines included standard and variant CHC profiles. Some of the genotypes tested here showed very dramatic and unexpected aging-related variation based on their early days' profile. This study provides a concrete dataset to better understand the mechanisms underlying the establishment and maintenance of CHCs on the fly cuticle. It could be useful to determine physiological parameters, including age and response to climate variation, in insects collected in the wild.

Identification of Cuticular and Web Lipids of the Spider Argiope bruennichi

Emerging evidence shows that the cuticular and silk lipids of spiders are structurally more diverse than those of insects, although only a relatively low number of species have been investigated so far. As in insects, such lipids might play a role as signals in various contexts. The wasp spider Argiope bruennichi has probably the best investigated chemical communication system within spiders, including the known structure of the female sex pheromone. Recently we showed that kin-recognition in A. bruennichi could be mediated through the cuticular compounds consisting of hydrocarbons and, to a much larger proportion, of wax esters. By use of mass spectrometry and various derivatization methods, these were identified as esters of 2,4-dimethylalkanoic acids and 1-alkanols of varying chain lengths, such as tetradecyl 2,4-dimethylheptadecanoate. A representative enantioselective synthesis of this compound was performed which proved the identifications and allowed us to postulate that the natural enantiomer likely has the (2R,4R)-configuration. Chemical profiles of the silk and cuticular lipids of females were similar, while male cuticular profiles differed from those of females. Major components of the male cuticular lipids were tridecyl 2,4-dimethyl-C_17-19 alkanoates, whereas those of females were slightly longer, comprising tridecyl 2,4-dimethyl-C_19-21 alkanoates. In addition, minor female-specific 4-methylalkyl esters were detected.

Sublethal pesticide exposure induces larval removal behavior in honeybees through chemical cues

We were intrigued by reported observations of reduced brood production and a high number of empty brood cells in bee colonies exposed to sublethal pesticide doses, which could suggest an active removal of larvae. Higher numbers of oenocytes, insect cells responsible for lipid processing and detoxification, were also found in pesticide-exposed larvae. Oenocytes are involved in hydrocarbon metabolism and chemical communication, and we hypothesized that these larvae could display altered cuticular hydrocarbon (CHC) profiles when exposed to pesticides as compared to control larvae. In addition, we proposed that these chemical cues could trigger specific behavioral responses in colony nurses. To test these hypotheses, we analyzed the CHC profiles of artificially reared larvae that had been fed sublethal doses of either dimethoate or clothianidin or fed on lipopolysaccharide (LPS) using gas chromatography-mass spectrometry. We found significant differences in the CHC profiles of these differently treated larvae. In a subsequent behavioral experiment, we transferred clothianidin-treated or LPS-treated larvae into the brood combs of surrogate colonies. Larvae that had been fed either the pesticide or LPS were removed at a significantly higher rate than control larvae. Our results demonstrate that larvae exposed to clothianidin possess altered CHC profiles, are detected in the colony by nurse bees via chemical cues and are actively removed.

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

Simple Summary

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

Abstract

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

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.

Epicuticular hydrocarbons of the redbanded stink bug Piezodorus guildinii (Heteroptera: Pentatomidae): sexual dimorphism and alterations in insects collected in insecticide-treated soybean crops

BACKGROUND

The redbanded stink bug Piezodorus guildinii (Heteroptera: Pentatomidae) is one of the most important species affecting soybean crops in southern South America. Capillary gas chromatography coupled to mass spectrometry was used to characterize the epicuticular hydrocarbon profiles of field-collected insects, and to identify differences in their composition between fifth-instar nymphs and adults, males and females, and between bugs collected in insecticide-treated and insecticide-free soybean crops.

RESULTS

Straight chain saturated n-C27 and n-C29, and monomethyl and dimethyl chains of C31 and C33 were the most abundant compounds. A group of volatile hydrocarbons with n-C13 and n-C15 as the predominant compounds were also detected. The hydrocarbon pattern was different between nymphs and adults, either males or females. Heneicosene was almost exclusively detected in adult males and was the most important component to differentiate between both sexes, followed by tricosadiene. The total hydrocarbon amount was significantly higher in nymphs, males and females collected in insecticide-treated fields compared with insects obtained from untreated fields.

CONCLUSION

Differences were found in the epicuticular hydrocarbon pattern among nymphs and adults, as well as sexual dimorphism in adult stink bugs. Interestingly, an alteration was also found in the hydrocarbon profile of insects collected in insecticide-treated soybean crops and its relevance is discussed within a pest management context.

Hygiene-Eliciting Brood Semiochemicals as a Tool for Assaying Honey Bee (Hymenoptera: Apidae) Colony Resistance to Varroa (Mesostigmata: Varroidae)

Despite numerous interventions, the ectoparasitic mite Varroa (Varroa destructor Anderson and Trueman [Mesostigmata: Varroidae]) and the pathogens it vectors remain a primary threat to honey bee (Apis mellifera Linnaeus [Hymenoptera: Apidae]) health. Hygienic behavior, the ability to detect, uncap, and remove unhealthy brood from the colony, has been bred for selectively for over two decades and continues to be a promising avenue for improved Varroa management. Although hygienic behavior is expressed more in Varroa-resistant colonies, hygiene does not always confer resistance to Varroa. Additionally, existing Varroa resistance selection methods trade efficacy for efficiency, because those achieving the highest levels of Varroa resistance can be time-consuming, and thus expensive and impractical for apicultural use. Here, we tested the hypothesis that hygienic response to a mixture of semiochemicals associated with Varroa-infested honey bee brood can serve as an improved tool for predicting colony-level Varroa resistance. In support of our hypothesis, we demonstrated that a mixture of the compounds (Z)-10-tritriacontene, (Z)-8-hentriacontene, (Z)-8-heptadecene, and (Z)-6-pentadecene triggers hygienic behavior in a two-hour assay, and that high-performing colonies (hygienic response to ≥60% of treated cells) have significantly lower Varroa infestations, remove significantly more introduced Varroa, and are significantly more likely to survive the winter compared to low-performing colonies (hygienic response to <60% of treated cells). We discuss the relative efficacy and efficiency of this assay for facilitating apiary management decisions and selection of Varroa-resistant honey bees, as well as the relevance of these findings to honey bee health, pollination services, and social insect communication.

Integrating genomics and multivariate evolutionary quantitative genetics: a case study of constraints on sexual selection in Drosophila serrata

In evolutionary quantitative genetics, the genetic variance-covariance matrix, G, and the vector of directional selection gradients, β, are key parameters for predicting multivariate selection responses and genetic constraints. Historically, investigations of G and β have not overlapped with those dissecting the genetic basis of quantitative traits. Thus, it remains unknown whether these parameters reflect pleiotropic effects at individual loci. Here, we integrate multivariate genome-wide association study (GWAS) with G and β estimation in a well-studied system of multivariate constraint: sexual selection on male cuticular hydrocarbons (CHCs) in Drosophila serrata. In a panel of wild-derived re-sequenced lines, we augment genome-based restricted maximum likelihood to estimate G alongside multivariate single nucleotide polymorphism (SNP) effects, detecting 532 significant associations from 1 652 276 SNPs. Constraint was evident, with β lying in a direction of G with low evolvability. Interestingly, minor frequency alleles typically increased male CHC-attractiveness suggesting opposing natural selection on β. SNP effects were significantly misaligned with the major eigenvector of G, gmax, but well aligned to the second and third eigenvectors g2 and g3. We discuss potential factors leading to these varied results including multivariate stabilizing selection and mutational bias. Our framework may be useful as researchers increasingly access genomic methods to study multivariate selection responses in wild populations.

Aphids harbouring different endosymbionts exhibit differences in cuticular hydrocarbon profiles that can be recognized by ant mutualists

Cuticular hydrocarbons (CHCs) have important communicative functions for ants, which use CHC profiles to recognize mutualistic aphid partners. Aphid endosymbionts can influence the quality of their hosts as ant mutualists, via effects on honeydew composition, and might also affect CHC profiles, suggesting that ants could potentially use CHC cues to discriminate among aphid lines harbouring different endosymbionts. We explored how several strains of Hamiltonella defensa and Regiella insecticola influence the CHC profiles of host aphids (Aphis fabae) and the ability of aphid-tending ants (Lasius niger) to distinguish the profiles of aphids hosting different endosymbionts. We found significant compositional differences between the CHCs of aphids with different infections. Some endosymbionts changed the proportions of odd-chain linear alkanes, while others changed primarily methyl-branched compounds, which may be particularly important for communication. Behavioural assays, in which we trained ants to associate CHC profiles of endosymbiont infected or uninfected aphids with food rewards, revealed that ants readily learned to distinguish differences in aphid CHC profiles associated with variation in endosymbiont strains. While previous work has documented endosymbiont effects on aphid interactions with antagonists, the current findings support the hypothesis that endosymbionts also alter traits that influence communicative interactions with ant mutualists.

The cell invasion preference of Varroa destructor between the original and new honey bee hosts

Honey bees (Apis) are important pollinators for food crops and wild plants, but are facing great threats from pathogens and parasites, especially an obligate ectoparasitic mite, Varroa destructor. Cell invasion is a key step for V. destructor to reproduce, and the parasite displays remarkable host preference in this process. Varroa destructor made its host-shift from its original host, the Asian honey bee Apis cerana, to the new host, the European honey bee Apis mellifera several decades ago. However, it remains largely unstudied whether V. destructor shows a cell invasion preference between the two host species. Using cell invasion bioassays on a modified four-well arena, we showed that V. destructor significantly preferred to invade the worker and drone larvae of A. mellifera rather than A. cerana, suggesting that the new host is much more attractive to the parasite than the original one. Using gas chromatography-mass spectrometry (GC-MS), we revealed significant differences between the cuticular hydrocarbon (CHC) profiles of worker and drone larvae of the two bee hosts. The amounts of methyl-branched alkanes and alkenes (unsaturated CHCs), but not n-alkanes, were significantly different, and A. mellifera worker and drone larvae were found to express significantly higher amounts of methyl-alkanes, while A. cerana larvae produced higher amounts of alkenes. Cell invasion bioassays with glass dummies showed that the mites preferred the glass dummies coated with the CHCs of A. mellifera worker or drone larvae, which indicates a role of larval CHCs in mediating the preferential cell invasion of Varroa. The findings from this study extend our understanding of the host preference of V. destructor, and can potentially contribute to the development of effective strategies for mite control.

Cuticular Hydrocarbon Trails Released by Host Larvae Lose their Kairomonal Activity for Parasitoids by Solidification

Successful host search by parasitic wasps is often mediated by host-associated chemical cues. The ectoparasitoid Holepyris sylvanidis is known to follow chemical trails released by host larvae of the confused flour beetle, Tribolium confusum, for short-range host location. Although the hexane-extractable trails consist of stable, long-chain cuticular hydrocarbons (CHCs) with low volatility, the kairomonal activity of a trail is lost two days after release. Here, we studied whether this loss of kairomonal activity is due to changes in the chemical trail composition induced by microbial activity. We chemically analyzed trails consisting of hexane extracts of T. confusum larvae after different time intervals past deposition under sterile and non-sterile conditions. GC-MS analyses revealed that the qualitative and quantitative pattern of the long-chain CHCs of larval trails did not significantly change over time, neither under non-sterile nor sterile conditions. Hence, our results show that the loss of kairomonal activity of host trails is not due to microbially induced changes of the CHC pattern of a trail. Interestingly, the kairomonal activity of trails consisting of host larval CHC extracts was recoverable after two days by applying hexane to them. After hexane evaporation, the parasitoids followed the reactivated host trails as they followed freshly laid ones. Cryo-scanning electron microscopy showed that the trails gradually formed filament-shaped microstructures within two days. This self-assemblage of CHCs was reversible by hexane application. Our study suggests that the long-chain CHCs of a host trail slowly undergo solidification by a self-assembling process, which reduces the accessibility of CHCs to the parasitoid’s receptors as such that the trail is no longer eliciting trail-following behavior.

Trait Plasticity among Invasive Populations of the Ant Technomyrmex brunneus in Japan

Simple Summary

Invasive ants are a global economic and ecological problem. Understanding what makes them tick is thus an active area of scientific research. Many invasive ant species form large networks of inter-connected colonies (‘supercolonies’) which can span many hundreds of kilometres. Supercolonies are typically a characteristic of invasive populations, and studies have also identified shifts in other traits including diet in invasive populations compared to populations in the native range. This suggests that the ability to plastically change behaviour in this way may facilitate invasiveness. In this study, we assess trait plasticity in the invasive range of the ant Technomyrmex brunneus. This species is native to southern Asia and has invaded many islands of the southern Japanese archipelago, allowing us to compare traits among different invasive populations. We find a supercolony in one of the three invasive populations studied, though interestingly, this population did not show the expected pattern of lower genetic diversity. The supercolony population did differ in diet compared to other populations. While it is unclear if variation in these traits is linked, our findings add weight to arguments that plasticity is important in facilitating invasiveness.

Abstract

Characters in invasive populations often differ from those in the native range, and the ability to express different characters may enhance invasive potential. Ants are among the most pervasive and damaging invasive species, by virtue of their transportability and broad-ranging ecological interactions. Their success is often attributed to the ability to exhibit different characteristics in invasive populations, including the formation of large, unicolonial associations (‘supercolonies’). It remains unclear, however, if such characteristics are a product or cause of the ecological dominance of invasive ants, and the advancement of our understanding has likely been restrained by the fact that studies to date have focused on a few globally important species with well-established invasions. In this study, we take advantage of an ongoing invasion of the tramp ant Technomyrmex brunneus in Japan to assess trait plasticity in the invasive range of this species. We find evidence for plasticity in social structure among island populations, with a supercolony evident on one of the three islands studied. Interestingly, we found no evidence of lower genetic diversity in this population, though natural isotope data indicate it was operating at a lower trophic level than other populations. These findings add weight to arguments that invasive species may benefit from the capacity to adaptively mould themselves to new ecological contexts.

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.

Lipophorin receptor regulates the cuticular hydrocarbon accumulation and adult fecundity of the pea aphid Acyrthosiphon pisum

Cuticular hydrocarbons form a barrier that protects terrestrial insects from water loss via the epicuticle. Lipophorin loads and transports lipids, including hydrocarbons, from one tissue to another. In some insects, the lipophorin receptor (LpR), which binds to lipophorin and accepts its lipid cargo, is essential for female fecundity because it mediates the incorporation of lipophorin by developing oocytes. However, it is unclear whether LpR is involved in the accumulation of cuticular hydrocarbons and its precise role in aphid reproduction remains unknown. We herein present the results of our molecular characterization, phylogenetic analysis, and functional annotation of the pea aphid (Acyrthosiphon pisum) LpR gene (ApLpR). This gene was transcribed throughout the A. pisum life cycle, but especially during the embryonic stage and in the abdominal cuticle. Furthermore, we optimized the RHA interference (RNAi) parameters by determining the ideal dose and duration for gene silencing in the pea aphid. We observed that the RNAi-based ApLpR suppression significantly decreased the internal and cuticular hydrocarbon contents as well as adult fecundity. Additionally, a deficiency in cuticular hydrocarbons increased the susceptibility of aphids to desiccation stress, with decreased survival rates under simulated drought conditions. Moreover, ApLpR expression levels significantly increased in response to the desiccation treatment. These results confirm that ApLpR is involved in transporting hydrocarbons and protecting aphids from desiccation stress. Furthermore, this gene is vital for aphid reproduction. Therefore, the ApLpR gene of A. pisum may be a novel RNAi target relevant for insect pest management.

Modulation of fatty acid elongation in cockroaches sustains sexually dimorphic hydrocarbons and female attractiveness

Insect cuticular hydrocarbons (CHCs) serve as important intersexual signaling chemicals and generally show variation between the sexes, but little is known about the generation of sexually dimorphic hydrocarbons (SDHCs) in insects. In this study, we report the molecular mechanism and biological significance that underlie the generation of SDHC in the German cockroach Blattella germanica. Sexually mature females possess more C29 CHCs, especially the contact sex pheromone precursor 3,11-DimeC29. RNA interference (RNAi) screen against the fatty acid elongase family members combined with heterologous expression of the genes in yeast revealed that both BgElo12 and BgElo24 were involved in hydrocarbon (HC) production, but BgElo24 is of wide catalytic activities and is able to provide substrates for BgElo12, and only the female-enriched BgElo12 is responsible for sustaining female-specific HC profile. Repressing BgElo12 masculinized the female CHC profile, decreased contact sex pheromone level, and consequently reduced the sexual attractiveness of female cockroaches. Moreover, the asymmetric expression of BgElo12 between the sexes is modulated by sex differentiation cascade. Specifically, male-specific BgDsx represses the transcription of BgElo12 in males, while BgTra is able to remove this effect in females. Our study reveals a novel molecular mechanism responsible for the formation of SDHCs and also provide evidences on shaping of the SDHCs by sexual selection, as females use them to generate high levels of contact sex pheromone.

Sexual dimorphism of body waxes is prevalent in insects; this study reveals that the sex-differentiation pathway regulates fatty acid elongation, ensuring production of the sexually dimorphic cuticular hydrocarbons needed for high levels of sex pheromone and sexual attractiveness in female cockroaches.

Cuticular hydrocarbons are associated with mating success and insecticide resistance in malaria vectors

Anopheles coluzzii females, important malaria vectors in Africa, mate only once in their lifetime. Mating occurs in aerial swarms with a high male-to-female ratio, where traits underlying male mating success are largely unknown. Here, we investigated whether cuticular hydrocarbons (CHCs) influence mating success in natural mating swarms in Burkina Faso. As insecticides are widely used in this area for malaria control, we also determined whether CHCs affect insecticide resistance levels. We find that mated males have higher CHC abundance than unmated controls, suggesting CHCs could be determinants of mating success. Additionally, mated males have higher insecticide resistance under pyrethroid challenge, and we show a link between resistance intensity and CHC abundance. Taken together, our results suggest that CHC abundance may be subject to sexual selection in addition to selection by insecticide pressure. This has implications for insecticide resistance management, as these traits may be sustained in the population due to their benefits in mating even in the absence of insecticides.

In this study, Adams et al. investigate the effect of cuticular hydrocarbons on mating success in natural mosquito mating swarms. These hydrocarbons confer both higher mating success and increased resistance to pyrethroid, suggesting sexual selection for insecticide resistance in this population secondary to mating success.

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.

Antiparasitic Effects of Potentially Toxic Beetles (Tenebrionidae and Meloidae) from Steppe Zones

Arthropods and specifically beetles can synthesize and/or sequester metabolites from dietary sources. In beetle families such as Tenebrionidae and Meloidae, a few studies have reported species with toxic defensive substances and antiparasitic properties that are consumed by birds. Here we have studied the antiparasitic activity of extracts from beetle species present in the habitat of the Great Bustard (Otis tarda) against four pathogen models (Aspergillus niger, Meloidogyne javanica, Hyalomma lusitanicum, and Trichomonas gallinae). The insect species extracted were Tentyria peiroleri, Scaurus uncinus, Blaps lethifera (Tenebrionidae), and Mylabris quadripunctata (Meloidae). M. quadripunctata exhibited potent activity against M. javanica and T. gallinae, while T. peiroleri exhibited moderate antiprotozoal activity. The chemical composition of the insect extracts was studied by gas chromatography coupled with mass spectrometry (GC-MS) analysis. The most abundant compounds in the four beetle extracts were hydrocarbons and fatty acids such as palmitic acid, myristic acid and methyl linoleate, which are characteristic of insect cuticles. The presence of cantharidin (CTD) in the M. quadripunctata meloid and ethyl oleate (EO) in T. peiroleri accounted for the bioactivity of their extracts.

The Inquiline Ant Myrmica karavajevi Uses Both Chemical and Vibroacoustic Deception Mechanisms to Integrate into Its Host Colonies

Social parasitism represents a particular type of agonistic interaction in which a parasite exploits an entire society instead of a single organism. One fascinating form of social parasitism in ants is the "inquilinism", in which a typically worker-less parasitic queen coexists with the resident queen in the host colony and produces sexual offspring. To bypass the recognition system of host colonies, inquilines have evolved a repertoire of deceiving strategies. We tested the level of integration of the inquiline Myrmica karavajevi within the host colonies of M. scabrinodis and we investigated the mechanisms of chemical and vibroacoustic deception used by the parasite. M. karavajevi is integrated into the ant colony to such an extent that, in rescue experiments, the parasite pupae were saved prior to the host's brood. M. karavajevi gynes perfectly imitated the cuticular hydrocarbon profiles of M. scabrinodis queens and the parasite vibroacoustic signals resembled those emitted by the host queens eliciting the same levels of attention in the host workers during playback experiments. Our results suggest that M. karavajevi has evolved ultimate deception strategies to reach the highest social status in the colony hierarchy, encouraging the use of a combined molecular and behavioural approach when studying host-parasite interactions.

Recent advances in chemical ecology: complex interactions mediated by molecules

Chemical ecology is the highly interdisciplinary study of biochemicals that mediate the behavior of organisms and the regulation of physiological changes that alter intraspecific and/or interspecific interactions. Significant advances are often achieved through the collaboration of chemists and biologists working to understand organismal survival strategies with an eye on the development of targeted technologies for controlling agricultural, forestry, medical, and veterinary pests in a sustainable world. We highlight recent advances in chemical ecology from multiple viewpoints and discuss future prospects for applications.