Cuticular Hydrocarbons Expression Research Articles

Geographic variation in cuticular hydrocarbon profiles in Pacific field crickets

1. Insect cuticular hydrocarbons (CHCs) serve a role in both desiccation resistance and sexual attractiveness. These two functions are thought to be maximised by different CHC compositions, which should lead to local adaptation between populations experiencing different natural and sexual selection pressures.2. The Pacific field cricket (Teleogryllus oceanicus) is distributed throughout Oceania, including the Hawaiian and Cook Islands. In Hawaii, these crickets face strong natural selection from a parasitoid fly that has promoted the evolution of a novel male reproductive morph, dramatically altering sexual selection in the Hawaiian populations. The parasitoid and male polymorphism do not occur in the Cook Islands.3. We captured male and female T. oceanicus on two Hawaiian Islands and three Cook Islands, and quantified their CHCs. We also measured temperature and RH at our collection sites to test for associations with cricket CHC profiles.4. We discovered significant sexual dimorphism in CHC profiles, as well as differences between both island chains and particular islands. The two male reproductive morphs in Hawaii showed distinct CHC expression. There were no associations between weather variables and CHC profiles.5. Observed differences in T. oceanicus CHC profiles appear to reflect geographic isolation rather than selection in response to local and environmental and social conditions.

Inheritance of pheromone profiles from aged D. melanogaster.

Through aging, D. melanogaster males and females change their social spacing. Flies are initially more social, but reduce sociability as they grow older. This preferred social space is inherited in their progeny. Here, we report that in females, the profiles of cuticular hydrocarbons (CHC), which are known to promote social interaction between individuals, similarly are affected by age. Importantly, for a subset of those CHC, the progeny’s CHC levels are comparable to those of their parents, suggesting that parental age influences offspring CHC expression. Those data establish a foundation to identify the relationship between CHC levels and social spacing, and to understand the mechanisms of the inheritance of complex traits.

Major Transitions in Cuticular Hydrocarbon Expression Coincide with Sexual Maturity in a Blowfly (Diptera: Calliphoridae).

In many animals, there is a prolonged pre-reproductive period prior to sexual maturity. To avoid premature mating attempts, it is common for phenotypic changes to occur during this period that signal the onset of reproductive viability. Among the insects, pre-reproductive phases can last for up to 50% of the adult lifespan, but little is known about the accompanying phenotypic changes that signal sexual maturity. Contact pheromones such as cuticular hydrocarbons (CHCs) may fulfil this role, as they are known to change rapidly with age in many insects. Despite this, few studies have investigated CHC development in the context of sexual maturity or considered differences in CHC development between sexes. The blowflies (Diptera: Calliphoridae) provide an ideal system for such studies because CHCs are known to change rapidly with age and likely play an important role in sexual behaviour. As such, using the small hairy maggot blowfly Chrysomya varipes, we investigate whether there are age- and sex-specific changes in CHCs over the course of adult blowfly maturation. We show that: (1) major qualitative transitions in CHC expression coincide with the onset of sexual maturity and (2) these changes occur more slowly in females - in line with their extended pre-reproductive phase. We suggest that CHCs may play an important role in signalling sexual maturity in the small hairy maggot blowfly and that this species will likely serve as a useful model for understanding the complex ontogeny of cuticular hydrocarbons in insects.

Mating has opposite effects on male and female sexually selected cuticular hydrocarbons

In Drosophila serrata flies, there is female choice for male cuticular hydrocarbon (CHC) profiles and male choice for female CHC profiles. Furthermore, both males and females can alter their CHCs: when there is more opportunity for mating, males express combinations of CHCs preferred by females; however, females appear to change CHC profiles to avoid male harassment. In this study, I investigate the effect of number of matings (0–4) on male and female sexually selected CHCs. Mating caused males to express CHCs associated with higher male mating success. Thus, successfully mating males are likely to have increased future mating success. Conversely, females that mated more times expressed CHC profiles that were associated with lower female mating success. Females maximized their offspring production by mating more than once, but additional matings did not provide additional benefits. Furthermore, number of matings did not affect female survival. In total, these results suggest that females alter CHC expression to discourage male courtship when additional matings are not beneficial. In conclusion, plasticity in male and female CHC expression can both increase variance in male mating success and decrease variance in female mating success, driving the evolution of sexually selected chemical signals.

Sex differences in nutrient intake can reduce the potential for sexual conflict over fitness maximization by female and male crickets.

As females and males have different roles in reproduction, they are expected to require different nutrients for the expression of reproductive traits. However, due to their shared genome, both sexes may be constrained in the regulation of nutrient intake that maximizes sex-specific fitness. Here, we used the Geometric Framework for nutrition to examine the effect of macronutrient and micronutrient intakes on lifespan, fecundity and cuticular hydrocarbons (CHCs) that signal mate quality to prospective mates in female field crickets, Teleogryllus oceanicus. In addition, we contrasted nutritional effects on life-history traits between males and females to determine how sex differences influence nutrient regulation. We found that carbohydrate intake maximized female lifespan and protein intake influenced CHC expression, while early life fecundity (cumulative fecundity at day 21) and lifetime fecundity were dependent on both macronutrient and micronutrient intakes. Fecundity required different nutrient blends to those required to optimize sperm viability in males, generating the potential for sexual conflict over macronutrient intake. The regulation of protein (P) and carbohydrate (C) intakes by virgin and mated females initially matched that of males, but females adjusted their intake to a higher P:C ratio, 1P:2C, that maximized fecundity as they aged. This suggests that a sex-specific, age-dependent change in intake target for sexually mature females, regardless of their mating status, adjusts protein consumption in preparation for oviposition. Sex differences in the regulation of nutrient intake to optimize critical reproductive traits in female and male T.oceanicus provide an example of how sexual conflict over nutrition can shape differences in foraging between the sexes.

A costly chemical trait: phenotypic condition dependence of cuticular hydrocarbons in a dung beetle.

Chemical traits are increasingly recognized as important cues used in mate choice. For example, the cuticular hydrocarbons (CHCs) of insects have been shown to influence mating success in a range of taxa. Less is known, however, about how CHCs are expressed in proportion to an individual's condition, and consequently whether CHCs can function as condition-dependent signals of quality. We investigated this question using the dung beetle, Onthophagus taurus. CHCs are subject to sexual selection in this species through mate choice. A dietary manipulation revealed condition dependence of CHC expression for both sexes: dietary restriction decreased overall CHC production and altered the composition of CHCs. Furthermore, CHC production was associated with a measure of condition in beetles fed a limited diet but not those fed adlibitum. These results implicate a resource cost to CHC production that is likely to result in trade-offs with other fitness components in this species, as these respond similarly to a dietary restriction. The CHC profiles showed sexual dimorphism: males produced more CHCs and the sexes differed in the blend of compounds they produced. There was evidence for a male dimorphism in the CHC profile, in line with the presence of the alternative reproductive tactics (minor sneaks and major fighters) in this species. However, rather than mimicking a female CHC profile, minor males differed more from females than major males. Our results suggest that CHCs are a costly trait in O.taurus that has the potential to act as a condition-dependent signal of quality.

The complex interplay between macronutrient intake, cuticular hydrocarbon expression and mating success in male decorated crickets.

The condition dependence of male sexual traits plays a central role in sexual selection theory. Relatively little, however, is known about the condition dependence of chemical signals used in mate choice and their subsequent effects on male mating success. Furthermore, few studies have isolated the specific nutrients responsible for condition-dependent variation in male sexual traits. Here, we used nutritional geometry to determine the effect of protein (P) and carbohydrate (C) intake on male cuticular hydrocarbon (CHC) expression and mating success in male decorated crickets (Gryllodes sigillatus). We show that both traits are maximized at a moderate-to-high intake of nutrients in a P:C ratio of 1:1.5. We also show that female precopulatory mate choice exerts a complex pattern of linear and quadratic sexual selection on this condition-dependent variation in male CHC expression. Structural equationmodelling revealed that although the effect of nutrient intake on mating success is mediated through condition-dependent CHC expression, it is not exclusively so, suggesting that other traits must also play an important role. Collectively, our results suggest that the complex interplay between nutrient intake, CHC expression and mating success plays an important role in the operation of sexual selection in G.sigillatus.

Crowd control: sex ratio affects sexually selected cuticular hydrocarbons in male Drosophila serrata.

Although it is advantageous for males to express costly sexually selected signals when females are present, they may also benefit from suppressing these signals to avoid costly interactions with rival males. Cuticular chemical profiles frequently function as insect sexual signals; however, few studies have asked whether males alter these signals in response to their social environment. In Drosophila serrata, an Australian fly, there is sexual selection for a multivariate combination of male cuticular hydrocarbons (CHCs). Here, we show that the ratio of females to males that an adult male experiences has a strong effect on his CHC expression, with female-biased adult sex ratios eliciting greater expression of CHC profiles associated with higher male mating success. Classical models predict that male reproductive investment should be highest when there is a small but nonzero number of rivals, but we found that males expressed the most attractive combination of CHCs when there were no rivals. We found that male CHCs were highly sensitive to adult sex ratio, with males expressing higher values of CHC profiles associated with greater mating success as the ratio of females to males increased. Moreover, sex ratio has a stronger effect on male CHC expression than adult density. Finally, we explore whether sex ratio affects the variance among a group of males in their CHC expression, as might be expected if individuals respond differently to a given social environment, but find little effect. Our results reveal that subtle differences in social environment can induce plasticity in male chemical signal expression.

Effects of temperature on transcriptome and cuticular hydrocarbon expression in ecologically differentiated populations of desert Drosophila.

We assessed the effects of temperature differences on gene expression using whole‐transcriptome microarrays and cuticular hydrocarbon variation in populations of cactophilic Drosophila mojavensis. Four populations from Baja California and mainland Mexico and Arizona were each reared on two different host cacti, reared to sexual maturity on laboratory media, and adults were exposed for 12 hr to 15, 25, or 35°C. Temperature differences influenced the expression of 3,294 genes, while population differences and host plants affected >2,400 each in adult flies. Enriched, functionally related groups of genes whose expression changed at high temperatures included heat response genes, as well as genes affecting chromatin structure. Gene expression differences between mainland and peninsular populations included genes involved in metabolism of secondary compounds, mitochondrial activity, and tRNA synthases. Flies reared on the ancestral host plant, pitaya agria cactus, showed upregulation of genes involved in metabolism, while flies reared on organ pipe cactus had higher expression of DNA repair and chromatin remodeling genes. Population × environment (G × E) interactions had widespread effects on the transcriptome where population × temperature interactions affected the expression of >5,000 orthologs, and there were >4,000 orthologs that showed temperature × host plant interactions. Adults exposed to 35°C had lower amounts of most cuticular hydrocarbons than those exposed to 15 or 25°C, including abundant unsaturated alkadienes. For insects adapted to different host plants and climatic regimes, our results suggest that temperature shifts associated with climate change have large and significant effects on transcriptomes of genetically differentiated natural populations.

Level up: the expression of male sexually selected cuticular hydrocarbons is mediated by sexual experience

The use of cuticular hydrocarbons (CHCs) in species recognition, sex identification and sexual selection is widespread in insects. However, few studies have studied plasticity in CHCs. Here we examine the effect of age and social environment on a suite of sexually selected CHCs in Drosophila serrata. We demonstrate that the combination of CHCs that is associated with increased male mating success (CHCβ) changes as males age, and this effect is mediated by social environment. When single males were housed with multiple females, their expression of CHCβ increased across the first few days of their adult life, after which expression declined with increasing age. In contrast, sexually selected CHCs of males housed with other males, males housed with other males and females, and males housed alone all decreased across days. To determine the long-term consequences of mating on CHC expression, we allowed males a single mating opportunity and subsequently found some indication of a brief spike in CHCβ. Finally, to determine whether visual and olfactory contact with females, copulation, or intromission causes males to express high values of CHCβ, we manipulated male access and physical contact with females. We found that although prolonged copulation causes a slight increase in male CHCβ, only a successful copulation with sperm transfer induced males to develop CHCs associated with high mating success. Taken as a whole, our results demonstrate that the expression of sexually selected CHCs in males varies with both age and social context, and suggest that the latter is mediated at least in part by successful matings with females. More generally, contextual plasticity in CHCs is likely to affect both the experimental design of CHC-based experiments and the evolution of CHC signals as naturally and sexually selected traits.

Sexual selection and population divergence I: The influence of socially flexible cuticular hydrocarbon expression in male field crickets (Teleogryllus oceanicus).

Debates about how coevolution of sexual traits and preferences might promote evolutionary diversification have permeated speciation research for over a century. Recent work demonstrates that the expression of such traits can be sensitive to variation in the social environment. Here, we examined social flexibility in a sexually selected male trait-cuticular hydrocarbon (CHC) profiles-in the field cricket Teleogryllus oceanicus and tested whether population genetic divergence predicts the extent or direction of social flexibility in allopatric populations. We manipulated male crickets' social environments during rearing and then characterized CHC profiles. CHC signatures varied considerably across populations and also in response to the social environment, but our prediction that increased social flexibility would be selected in more recently founded populations exposed to fluctuating demographic environments was unsupported. Furthermore, models examining the influence of drift and selection failed to support a role of sexual selection in driving population divergence in CHC profiles. Variation in social environments might alter the dynamics of sexual selection, but our results align with theoretical predictions that the role social flexibility plays in modulating evolutionary divergence depends critically on whether responses to variation in the social environment are homogeneous across populations, or whether gene by social environment interactions occur.

Female choice for male cuticular hydrocarbon profile in decorated crickets is not based on similarity to their own profile.

Indirect genetic benefits derived from female mate choice comprise additive (good genes) and nonadditive genetic benefits (genetic compatibility). Although good genes can be revealed by condition-dependent display traits, the mechanism by which compatibility alleles are detected is unclear because evaluation of the genetic similarity of a prospective mate requires the female to assess the genotype of the male and compare it to her own. Cuticular hydrocarbons (CHCs), lipids coating the exoskeleton of most insects, influence female mate choice in a number of species and offer a way for females to assess genetic similarity of prospective mates. Here, we determine whether female mate choice in decorated crickets is based on male CHCs and whether it is influenced by females' own CHC profiles. We used multivariate selection analysis to estimate the strength and form of selection acting on male CHCs through female mate choice, and employed different measures of multivariate dissimilarity to determine whether a female's preference for male CHCs is based on similarity to her own CHC profile. Female mating preferences were significantly influenced by CHC profiles of males. Male CHC attractiveness was not, however, contingent on the CHC profile of the choosing female, as certain male CHC phenotypes were equally attractive to most females, evidenced by significant linear and stabilizing selection gradients. These results suggest that additive genetic benefits, rather than nonadditive genetic benefits, accrue to female mate choice, in support of earlier work showing that CHC expression of males, but not females, is condition dependent.

Insect Cuticular Hydrocarbons as Dynamic Traits in Sexual Communication

Recent research has demonstrated extensive within-species variation in pheromone expression in insect species, contrary to the view that pheromones are largely invariant within species. In fact, many studies on insect cuticular hydrocarbons (CHCs) show that pheromones can be highly dynamic traits that can express significant short-term plasticity across both abiotic and social environments. It is likely that this variability in CHC expression contributes to their important role in sexual signaling and mate choice. In this review, I discuss CHC plasticity and how this might influence sexual communication. I also highlight two important avenues for future research: examining plasticity in how individuals respond to CHC signals, and testing how sexual communication varies across abiotic and social environments.

Chemical cues mediate species recognition in field crickets

Cuticular hydrocarbons (CHCs) are important in mate choice in many insects, and may be used for species recognition if CHC profiles differ between potentially hybridizing species. In the sibling field cricket species Gryllus campestris and G. bimaculatus, females of G. bimaculatus are tolerant towards G. campestris males and can mate with them. However, G. campestris females are highly aggressive towards heterospecific G. bimaculatus males, and matings between them never happen. We examined whether cricket females might use CHCs to determine the species identity of their potential mates. We firstly analyzed the cuticular chemical profile by gas chromatography and mass spectrometry to assess the potential of CHCs to be used for species recognition in these crickets. We then manipulated females’ ability to detect chemical cues by carrying out chemical ablation of the antennae, and measured changes in aggressive responses to heterospecific males. We show that there are significant interspecies differences in CHC expression for both sexes, and that females with chemically ablated antennae reduce aggressive behavior towards heterospecific males. Our findings support the prediction that cuticular semiochemicals can play a key role in reproductive isolation between closely related insect species.

Environmental heterogeneity, multivariate sexual selection and genetic constraints on cuticular hydrocarbons in Drosophila simulans

Sexual selection is responsible for the evolution of many elaborate traits, but sexual trait evolution could be influenced by opposing natural selection as well as genetic constraints. As such, the evolution of sexual traits could depend heavily on the environment if trait expression and attractiveness vary between environments. Here, male Drosophila simulans were reared across a range of diets and temperatures, and we examined differences between these environments in terms of (i) the expression of male cuticular hydrocarbons (CHCs) and (ii) which male CHC profiles were most attractive to females. Temperature had a strong effect on male CHC expression, whereas the effect of diet was weaker. Male CHCs were subject to complex patterns of directional, quadratic and correlational sexual selection, and we found differences between environments in the combination of male CHCs that were most attractive to females, with clearer differences between diets than between temperatures. We also show that genetic covariance between environments is likely to cause a constraint on independent CHC evolution between environments. Our results demonstrate that even across the narrow range of environmental variation studied here, predicting the outcome of sexual selection can be extremely complicated, suggesting that studies ignoring multiple traits or environments may provide an over-simplified view of the evolution of sexual traits.

Functional genomic and phenotypic responses to desiccation in natural populations of a desert drosophilid.

We used whole-transcriptome microarrays to assess changes in gene expression and monitored mortality rates and epicuticular hydrocarbons (CHCs) in response to desiccation stress in four natural populations of Drosophila mojavensis from Baja California and mainland Mexico. Desiccation had the greatest effect on gene expression, followed by biogeographical variation at regional and population levels. Genes involved in environmental sensing and cuticular structure were up-regulated in dry conditions, while genes involved in transcription itself were down-regulated. Flies from Baja California had higher expression of reproductive and mitochondrial genes, suggesting that these populations have greater fecundity and higher metabolic rates. Host plant differences had a surprisingly minor effect on the transcriptome. In most cases, desiccation-caused mortality was greater in flies reared on fermenting cactus tissues than that on laboratory media. Water content of adult females and males was significantly different and was lower in Baja California males. Different groups of CHCs simultaneously increased and decreased in amounts due to desiccation exposure of 9 and 18 h and were population-specific and dependent on larval rearing substrates. Overall, we observed that changes in gene expression involved a coordinated response of behavioural, cuticular and metabolic genes. Together with differential expression of cuticular hydrocarbons, this study revealed some of the mechanisms that have allowed D. mojavensis to exploit its harsh desert conditions. Certainly, for D. mojavensis that uses different host plants, population-level understanding of responses to stressors associated with future climate change in desert regions must be evaluated across geographical and local ecological scales.

Genotype‐by‐environment interactions for cuticular hydrocarbon expression in Drosophila simulans

Genotype-by-environment interactions (G × Es) describe genetic variation for phenotypic plasticity. Recent interest in the role of these interactions in sexual selection has identified G × Es across a diverse range of species and sexual traits. Additionally, theoretical work predicts that G × Es in sexual traits could help to maintain genetic variation, but could also disrupt the reliability of these traits as signals of mate quality. However, empirical tests of these theoretical predictions are scarce. We reared iso-female lines of Drosophila simulans across two axes of environmental variation (diet and temperature) in a fully factorial design and tested for G × Es in the expression of cuticular hydrocarbons (CHCs), a multivariate sexual trait in this species. We find sex-specific environmental, genetic and G × E effects on CHC expression, with G × Es for diet in both male and female CHC profile and a G × E for temperature in females. We also find some evidence for ecological crossover in these G × Es, and by quantifying variance components, genetic correlations and heritabilities, we show the potential for these G × Es to help maintain genetic variation and cause sexual signal unreliability in D. simulans CHC profiles.

Sex‐specific genotype‐by‐environment interactions for cuticular hydrocarbon expression in decorated crickets, Gryllodes sigillatus: implications for the evolution of signal reliability

Phenotypic traits that convey information about individual identity or quality are important in animal social interactions, and the degree to which such traits are influenced by environmental variation can have profound effects on the reliability of these cues. Using inbred genetic lines of the decorated cricket, Gryllodes sigillatus, we manipulated diet quality to test how the cuticular hydrocarbon (CHC) profiles of males and females respond across two different nutritional rearing environments. There were significant differences between lines in the CHC profiles of females, but the effect of diet was not quite statistically significant. There was no significant genotype-by-environment interaction (GEI), suggesting that environmental effects on phenotypic variation in female CHCs are independent of genotype. There was, however, a significant effect of GEI for males, with changes in both signal quantity and content, suggesting that environmental effects on phenotypic expression of male CHCs are dependent on genotype. The differential response of male and female CHC expression to variation in the nutritional environment suggests that these chemical cues may be under sex-specific selection for signal reliability. Female CHCs show the characteristics of reliable cues of identity: high genetic variability, low condition dependence and a high degree of genetic determination. This supports earlier work showing that female CHCs are used in self-recognition to identify previous mates and facilitate polyandry. In contrast, male CHCs show the characteristics of reliable cues of quality: condition dependence and a relatively higher degree of environmental determination. This suggests that male CHCs are likely to function as cues of underlying quality during mate choice and/or male dominance interactions.

Strong differences in chemical recognition cues between two closely related species of ants from the genus Lasius (Hymenoptera: Formicidae)

Cuticular hydrocarbons (CHCs) are increasingly recognized as important to insects and are used for constructing taxonomies. However, multiple parameters affect the expression of CHCs besides a genetic component. We propose that selection may act differently on the expression of CHCs, depending on the evolutionary context. To explore the influence of selection, the CHCs of two closely related ant species, Lasius niger and Lasius platythorax, were studied in a multidisciplinary approach. We characterized (i) CHCs and (ii) niches (through baiting, activity observations and foraging analysis). The species were distinct in both measures, although to a varying degree. Although they showed moderate niche partitioning along diet and environmental preferences, chemical differences were unexpectedly pronounced. This may be explained by divergent selection on mate recognition cues or by other influences on CHCs. Such striking chemical differences among closely related species may not be the rule and suggest that taxonomies based on CHCs should be interpreted cautiously; though, they remain useful tools for differentiating among cryptic species.

Male crickets alter the relative expression of cuticular hydrocarbons when exposed to different acoustic environments

Males often display signals from more than one sensory modality when producing courtship displays. In crickets, males produce a courtship song that conveys information about male quality. Although less well studied, male crickets also produce chemical cues that are used in mate choice. Plasticity in chemical cues has been recently reported, but empirical evidence for signal-mediated plasticity in these cues is primarily lacking. We tested whether exposure to acoustic sexual signals in the Australian field cricket, Teleogryllus oceanicus , affects the expression of another sexual signal, cuticular hydrocarbons. We found that the expression of male cuticular hydrocarbons was affected by acoustic experience; in the absence of singing males, males increased the relative concentration of a number of cuticular hydrocarbons that females have previously been shown to find attractive. Our results demonstrate that social cues relating to acoustic signals can stimulate a plastic change in cuticular hydrocarbon expression.