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

A volatile sex attractant of tsetse flies

Tsetse flies transmit trypanosomes-parasites that cause devastating diseases in humans and livestock-across much of sub-Saharan Africa. Chemical communication through volatile pheromones is common among insects; however, it remains unknown if and how such chemical communication occurs in tsetse flies. We identified methyl palmitoleate (MPO), methyl oleate, and methyl palmitate as compounds that are produced by the tsetse fly Glossina morsitans and elicit strong behavioral responses. MPO evoked a behavioral response in male-but not virgin female-G. morsitans. G. morsitans males mounted females of another species, Glossina fuscipes, when they were treated with MPO. We further identified a subpopulation of olfactory neurons in G. morsitans that increase their firing rate in response to MPO and showed that infecting flies with African trypanosomes alters the flies' chemical profile and mating behavior. The identification of volatile attractants in tsetse flies may be useful for reducing disease spread.

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.

Condition- and context-dependent alternative reproductive tactic in Drosophila prolongata

Species with intense male–male competition for access to females often show alternative reproductive tactics (ARTs) where males of lower competitive ability adopt a sneaky behavior to gain access to mates. These ARTs are usually associated with intrasexual dimorphisms, in that males with distinct morphologies show different tactics. In some cases, however, males adopt different tactics without being dimorphic. Male Drosophila prolongata exhibit continuous variation in body size and shape, with enlarged forelegs that they use in male-male contests and in courtship, including stimulation of the female’s abdomen. During this “leg vibration,” however, nearby males can intercept the courted female and mate without their own courtship. Here, we studied the causes and consequences of these different mating tactics in competitive mating trials between males varying in their size and shape. We found that male mating tactics were condition-dependent. Whereas large, high-condition males were more likely to show leg vibration in their courtship, smaller, lower-condition males were more likely to intercept. However, the number of offspring produced was independent of male condition and reproductive tactic. We discuss possible scenarios for the evolution and maintenance of the ARTs and some future directions for the study of ARTs in this species and in general.

Pure species discriminate against hybrids in the Drosophila melanogaster species subgroup

Introgression, the exchange of alleles between species, is a common event in nature. This transfer of alleles between species must happen through fertile hybrids. Characterizing the traits that cause defects in hybrids illuminates how and when gene flow is expected to occur. Inviability and sterility are extreme examples of fitness reductions but are not the only type of defects in hybrids. Some traits specific to hybrids are more subtle but are important to determine their fitness. In this report, we study whether F1 hybrids between two species pairs of Drosophila are as attractive as the parental species. We find that in both species pairs, the sexual attractiveness of the F1 hybrids is reduced and that pure species discriminate strongly against them. We also find that the cuticular hydrocarbon (CHC) profile of the female hybrids is intermediate between the parental species. Perfuming experiments show that modifying the CHC profile of the female hybrids to resemble pure species improves their chances of mating. Our results show that behavioral discrimination against hybrids might be an important component of the persistence of species that can hybridize.

Ant cuticular hydrocarbons are heritable and associated with variation in colony productivity

In social insects, cuticular hydrocarbons function in nest-mate recognition and also provide a waxy barrier against desiccation, but basic evolutionary features, including the heritability of hydrocarbon profiles and how they are shaped by natural selection are largely unknown. We used a new pharaoh ant (Monomorium pharaonis) laboratory mapping population to estimate the heritability of individual cuticular hydrocarbons, genetic correlations between hydrocarbons, and fitness consequences of phenotypic variation in the hydrocarbons. Individual hydrocarbons had low to moderate estimated heritability, indicating that some compounds provide more information about genetic relatedness and can also better respond to natural selection. Strong genetic correlations between compounds are likely to constrain independent evolutionary trajectories, which is expected, given that many hydrocarbons share biosynthetic pathways. Variation in cuticular hydrocarbons was associated with variation in colony productivity, with some hydrocarbons experiencing strong directional selection. Altogether, this study builds on our knowledge of the genetic architecture of the social insect hydrocarbon profile and indicates that hydrocarbon variation is shaped by natural selection.

Limits to environmental masking of genetic quality in sexual signals

There is considerable debate over the value of male sexual ornaments as signals of genetic quality. Studies alternately report that environmental variation enhances or diminishes the genetic signal, or leads to crossover where genotypes perform well in one environment but poorly in another. A unified understanding is lacking. We conduct a novel experimental test examining the dual effects of distinct categories of genetic (inbred vs. crossed parental lines) and environmental quality (low, through high to extreme larval food stress) on a condition-dependent male ornament. We find that differences in genetic quality signalled by the ornament (male eyespan in Diasemopsis meigenii stalk-eyed flies) become visible and are amplified under high stress but are overwhelmed in extreme-stress environments. Variance among independent genetic lines increases with environmental stress in both genetic quality classes, but at a slower rate in high quality outcrossed flies. Individual genetic lines generally maintain their ranks across environments, except among high quality lines under low environmental stress, where low genetic variance among lines precludes differentiation between ranks. Our results provide a conceptual advance, demonstrating a unified pattern for how genetic and environmental quality interact. They show when environmental conditions lead to the amplification of differences in signals of genetic quality and thereby enhance the potential indirect genetic benefits gained by female mate choice.

Sexual selection drives the evolution of male wing interference patterns

The seemingly transparent wings of many insects have recently been found to display unexpected structural coloration. These structural colours (wing interference patterns: WIPs) may be involved in species recognition and mate choice, yet little is known about the evolutionary processes that shape them. Furthermore, to date investigations of WIPs have not fully considered how they are actually perceived by the viewers' colour vision. Here, we use multispectral digital imaging and a model of Drosophila vision to compare WIPs of male and female Drosophila simulans from replicate populations forced to evolve with or without sexual selection for 68 generations. We show that WIPs modelled in Drosophila vision evolve in response to sexual selection and provide evidence that WIPs correlate with male sexual attractiveness. These findings add a new element to the otherwise well-described Drosophila courtship display and confirm that wing colours evolve through sexual selection.

Genetic and environmental variation in transcriptional expression of seminal fluid proteins

Seminal fluid proteins (SFPs) are crucial mediators of sexual selection and sexual conflict. Recent studies have chiefly focused on environmentally induced plasticity as one source of variation in SFP expression, particularly in response to differing sperm competition levels. However, understanding the evolution of a trait in heterogenous environments requires estimates of both environmental and genetic sources of variation, as well as their interaction. Therefore, we investigated how environment (specifically mating group size, a good predictor of sperm competition intensity), genotype and genotype-by-environment interactions affect seminal fluid expression. To do so, we reared 12 inbred lines of a simultaneously hermaphroditic flatworm Macrostomum lignano in groups of either two or eight worms and measured the expression levels of 58 putative SFP transcripts. We then examined the source of variation in the expression of each transcript individually and for multivariate axes extracted from a principal component analysis. We found that mating group size did not affect expression levels according to the single transcript analyses, nor did it affect the first principal component (presumably representing overall investment in seminal fluid production). However, mating group size did affect the relative expression of different transcripts captured by the second principal component (presumably reflecting variation in seminal fluid composition). Most transcripts were genetically variable in their expression level and several exhibited genotype-by-environment interactions; relative composition also showed high genetic variation. Collectively, our results reveal the tightly integrated nature of the seminal fluid transcriptome and provide new insights into the quantitative genetic basis of seminal fluid investment and composition.

The evolutionary potential of diet-dependent effects on lifespan and fecundity in a multi-parental population of Drosophila melanogaster

The nutritional conditions experienced by a population have a major role in shaping trait evolution in many taxa. Constraints exerted by nutrient limitation or nutrient imbalance can influence the maximal value that fitness components such as reproduction and lifespan attains, and organisms may shift how resources are allocated to different structures and functions in response to changes in nutrition. Whether the phenotypic changes associated with changes in nutrition represent an adaptive response is largely unknown. Further, it is unclear whether the response of fitness components to diet even has the potential to evolve in most systems. In this study, we use an admixed multi-parental population of Drosophila melanogaster reared in three different diet conditions to estimate quantitative genetic parameters for lifespan and fecundity. We find significant genetic variation for both traits in our population and show that lifespan has moderate to high heritabilities within diets. Genetic correlations for lifespan between diets were significantly less than one, demonstrating a strong genotype by diet interaction. These findings demonstrate substantial standing genetic variation in our population that is comparable to natural populations and highlights the potential for adaptation to changing nutritional environments.

Using Variation in Heritability Estimates as a Test of G × E in Behavioral Research: A Brief Research Note

Better characterization of the sources of phenotypic variation in human behavioural traits-stemming from genetic and environmental influences-will allow for more informed decisions about how to approach a range of challenges arising from variation, ranging from societal issues to the treatment of diseases. In particular, understanding how the environment moderates genetic influence on phenotypes (i.e., genotype-environment interactions, or G × E) is a central component of the behavioral sciences. Yet, understanding of this phenomenon is lagging somewhat, due in part to the difficulties of detecting G × E. We discuss the logic behind one of the primary ways to detect G × E: comparing heritability estimates across environments. Then, we highlight some pitfalls, with an emphasis on how very strong G × E can sometimes be undetectable using this method when high heritability is present in multiple environments. We conclude by forwarding some initial, yet tentative, suggestions for how best to address to the problem.

Phenotypic Plasticity of Cuticular Hydrocarbon Profiles in Insects

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

Factors Affecting Species Identifications of Blow Fly Pupae Based upon Chemical Profiles and Multivariate Statistics

Alternative methods for the identification of species of blow fly pupae have been developed over the years that consist of the analyses of chemical profiles. However, the effect of biotic and abiotic factors that could influence the predictive manner for the tests have not been evaluated. The lipids of blowfly pupae (Cochliomyia macellaria, Lucilia cuprina, Lucilia sericata, and Phormia regina) were extracted in pentane, derivatized, and analyzed by total-vaporization solid phase microextraction gas chromatography-mass spectrometry (TV-SPME GC-MS). Peak areas for 26 compounds were analyzed. Here we evaluated one biotic factor (colonization) on four species of blow flies to determine how well a model produced from lipid profiles of colonized flies predicted the species of flies of offspring of wild-caught flies and found very good species identification following 10 generations of inbreeding. When we evaluated four abiotic factors in our fly rearing protocols (temperature, humidity, pupation substrate, and diet), we found that the ability to assign the chemical profile to the correct species was greatly reduced.

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 equation modelling 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.

Signal Diversity, Sexual Selection, and Speciation

Communication is ubiquitous. Developing a framework for the diversity of signals has important consequences for understanding alternative models of sexual selection and the processes contributing to speciation. In this article we review how models of neutral evolution in the perceptual space of signal perceivers provide a first step toward constructing a framework for signal diversity. We discuss how the distinction between additive and multiplicative effects of multimodal signaling represents a second step. We then assess how signal efficiency, reliability, and the aesthetics of perceivers provide distinct mechanisms for signals to be effective, thereby partly explaining signal diversity. Understanding the relative contribution of each of these mechanisms to the effectiveness of mate choice signals unravels the relative importance of alternative models of sexual selection. It can also help to distinguish whether divergence of communication is a driver or a consequence of speciation. Throughout the review we emphasize the importance of verification and learning in repeated interactions for understanding variation in signals.

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.

Dissecting the Genetic Architecture of Behavior in Drosophila melanogaster

Variation in behaviors in natural populations arises from complex networks of multiple segregating polymorphic alleles whose expression can be modulated by the environment. Since behaviors reflect dynamic interactions between organisms and their environments, they are central targets for adaptive evolution. Drosophila melanogaster presents a powerful system for dissecting the genetic basis of behavioral phenotypes, since both the genetic background and environmental conditions can be controlled and behaviors accurately quantified. Single gene mutational analyses can identify the roles of individual genes within cellular pathways, whereas systems genetic approaches that exploit natural variation can construct genetic networks that underlie phenotypic variation. Combining these approaches with emerging technologies, such as genome editing, is likely to yield a comprehensive understanding of the neurogenetic underpinnings that orchestrate the manifestation of behaviors.

A population genomic scan in Chorthippus grasshoppers unveils previously unknown phenotypic divergence

Understanding the genetics of speciation and the processes that drive it is a central goal of evolutionary biology. Grasshoppers of the Chorthippus species group differ strongly in calling song (and corresponding female preferences) but are exceedingly similar in other characteristics such as morphology. Here, we performed a population genomic scan on three Chorthippus species (Chorthippus biguttulus, C. mollis and C. brunneus) to gain insight into the genes and processes involved in divergence and speciation in this group. Using an RNA-seq approach, we examined functional variation between the species by calling SNPs for each of the three species pairs and using FST -based approaches to identify outliers. We found approximately 1% of SNPs in each comparison to be outliers. Between 37% and 40% of these outliers were nonsynonymous SNPs (as opposed to a global level of 17%) indicating that we recovered loci under selection. Among the outliers were several genes that may be involved in song production and hearing as well as genes involved in other traits such as food preferences and metabolism. Differences in food preferences between species were confirmed with a behavioural experiment. This indicates that multiple phenotypic differences implicating multiple evolutionary processes (sexual selection and natural selection) are present between the species.

Are we underestimating the genetic variances of dimorphic traits?

Populations often contain discrete classes or morphs (e.g., sexual dimorphisms, wing dimorphisms, trophic dimorphisms) characterized by distinct patterns of trait expression. In quantitative genetic analyses, the different morphs can be considered as different environments within which traits are expressed. Genetic variances and covariances can then be estimated independently for each morph or in a combined analysis. In the latter case, morphs can be considered as separate environments in a bivariate analysis or entered as fixed effects in a univariate analysis. Although a common approach, we demonstrate that the latter produces downwardly biased estimates of additive genetic variance and heritability unless the quantitative genetic architecture of the traits concerned is perfectly correlated between the morphs. This result is derived for four widely used quantitative genetic variance partitioning methods. Given that theory predicts the evolution of genotype-by-environment (morph) interactions as a consequence of selection favoring different trait combinations in each morph, we argue that perfect correlations between the genetic architectures of the different morphs are unlikely. A sampling of the recent literature indicates that the majority of researchers studying traits expressed in different morphs recognize this and do estimate morph-specific quantitative genetic architecture. However, ca. 16% of the studies in our sample utilized only univariate, fixed-effects models. We caution against this approach and recommend that it be used only if supported by evidence that the genetic architectures of the different morphs do not differ.

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.

Between-sex genetic covariance constrains the evolution of sexual dimorphism in Drosophila melanogaster

Males and females share much of their genome, and as a result, intralocus sexual conflict is generated when selection on a shared trait differs between the sexes. This conflict can be partially or entirely resolved via the evolution of sex-specific genetic variation that allows each sex to approach, or possibly achieve, its optimum phenotype, thereby generating sexual dimorphism. However, shared genetic variation between the sexes can impose constraints on the independent expression of a shared trait in males and females, hindering the evolution of sexual dimorphism. Here, we examine genetic constraints on the evolution of sexual dimorphism in Drosophila melanogaster cuticular hydrocarbon (CHC) expression. We use the extended G matrix, which includes the between-sex genetic covariances that constitute the B matrix, to compare genetic constraints on two sets of CHC traits that differ in the extent of their sexual dimorphism. We find significant genetic constraints on the evolution of further dimorphism in the least dimorphic traits, but no such constraints for the most dimorphic traits. We also show that the genetic constraints on the least dimorphic CHCs are asymmetrical between the sexes. Our results suggest that there is evidence both for resolved and ongoing sexual conflict in D. melanogaster CHC profiles.

Transient and permanent experience with fatty acids changes Drosophila melanogaster preference and fitness

Food and host-preference relies on genetic adaptation and sensory experience. In vertebrates, experience with food-related cues during early development can change adult preference. This is also true in holometabolous insects, which undergo a drastic nervous system remodelling during their complete metamorphosis, but remains uncertain in Drosophila melanogaster. We have conditioned D. melanogaster with oleic (C18:1) and stearic (C18:0) acids, two common dietary fatty acids, respectively preferred by larvae and adult. Wild-type individuals exposed either during a transient period of development-from embryo to adult-or more permanently-during one to ten generation cycles-were affected by such conditioning. In particular, the oviposition preference of females exposed to each fatty acid during larval development was affected without cross-effect indicating the specificity of each substance. Permanent exposure to each fatty acid also drastically changed oviposition preference as well as major fitness traits (development duration, sex-ratio, fecundity, adult lethality). This suggests that D. melanogaster ability to adapt to new food sources is determined by its genetic and sensory plasticity both of which may explain the success of this generalist-diet species.

The heritability of mating behaviour in a fly and its plasticity in response to the threat of sperm competition

Phenotypic plasticity is a key mechanism by which animals can cope with rapidly changeable environments, but the evolutionary lability of such plasticity remains unclear. The socio-sexual environment can fluctuate very rapidly, affecting both the frequency of mating opportunities and the level of competition males may face. Males of many species show plastic behavioural responses to changes in social environment, in particular the presence of rival males. For example, Drosophila pseudoobscura males respond to rivals by extending mating duration and increasing ejaculate size. Whilst such responses are predicted to be adaptive, the extent to which the magnitude of response is heritable, and hence selectable, is unknown. We investigated this using isofemale lines of the fruit fly D. pseudoobscura, estimating heritability of mating duration in males exposed or not to a rival, and any genetic basis to the change in this trait between these environments (i.e. degree of plasticity). The two populations differed in population sex ratio, and the presence of a sex ratio distorting selfish chromosome. We find that mating duration is heritable, but no evidence of population differences. We find no significant heritability of plasticity in mating duration in one population, but borderline significant heritability of plasticity in the second. This difference between populations might be related to the presence of the sex ratio distorting selfish gene in the latter population, but this will require investigation in additional populations to draw any conclusions. We suggest that there is scope for selection to produce an evolutionary response in the plasticity of mating duration in response to rivals in D. pseudoobscura, at least in some populations.

Sexual selection on cuticular hydrocarbons of male sagebrush crickets in the wild

Cuticular hydrocarbons (CHCs) play an essential role in mate recognition in insects but the form and intensity of sexual selection on CHCs has only been evaluated in a handful of studies, and never in a natural population. We quantified sexual selection operating on CHCs in a wild population of sagebrush crickets, a species in which nuptial feeding by females imposes an unambiguous phenotypic marker on males. Multivariate selection analysis revealed a saddle-shaped fitness surface, suggesting a complex interplay between the total abundance of CHCs and specific CHC combinations in their influence on female choice. The fitness surface resulting from two axes of disruptive selection reflected a trade-off between short- and long-chained CHCs, suggesting that males may be sacrificing some level of desiccation resistance in favour of increased attractiveness. There was a significant correlation between male body size and total CHC abundance, suggesting that male CHCs provide females with a reliable cue for maximizing benefits obtained from males. Notwithstanding the conspicuousness of males' acoustic signals, our results suggest that selection imposed on males via female mating preferences may be far more complex than previously appreciated and operating in multiple sensory modalities.

Sexual selection in complex environments

Sexual selection has resulted in some of the most captivating features of insects, including flashy colors, bizarre structures, and complex pheromones. These features evolve in dynamic environments, where conditions can change rapidly over space and time. However, only recently has ecological complexity been embraced by theory and practice in sexual selection. We review replicated selection studies as well as studies on variation in the agents of selection to delineate gaps in current knowledge and clarify exciting new directions for research. Existing work suggests that fluctuations in sexual selection may be extremely common, though work on the ecological factors influencing these fluctuations is scarce. We suggest that deeper ecological perspectives on sexual selection may alter some of the fundamental assumptions of sexual selection theory and rapidly lead to new discoveries.

Sex-specific responses to sexual familiarity, and the role of olfaction in Drosophila

Studies of mating preferences have largely neglected the potential effects of individuals encountering their previous mates (‘directly sexually familiar’), or new mates that share similarities to previous mates, e.g. from the same family and/or environment (‘phenotypically sexually familiar’). Here, we show that male and female Drosophila melanogaster respond to the direct and phenotypic sexual familiarity of potential mates in fundamentally different ways. We exposed a single focal male or female to two potential partners. In the first experiment, one potential partner was novel (not previously encountered) and one was directly familiar (their previous mate); in the second experiment, one potential partner was novel (unrelated, and from a different environment from the previous mate) and one was phenotypically familiar (from the same family and rearing environment as the previous mate). We found that males preferentially courted novel females over directly or phenotypically familiar females. By contrast, females displayed a weak preference for directly and phenotypically familiar males over novel males. Sex-specific responses to the familiarity of potential mates were significantly weaker or absent in Orco¹ mutants, which lack a co-receptor essential for olfaction, indicating a role for olfactory cues in mate choice over novelty. Collectively, our results show that direct and phenotypic sexual familiarity is detected through olfactory cues and play an important role in sex-specific sexual behaviour.

Genotype-by-environment interactions for female mate choice of male cuticular hydrocarbons in Drosophila simulans

Recent research has highlighted the potential importance of environmental and genotype-by-environment (G×E) variation in sexual selection, but most studies have focussed on the expression of male sexual traits. Consequently, our understanding of genetic variation for plasticity in female mate choice is extremely poor. In this study we examine the genetics of female mate choice in Drosophila simulans using isolines reared across two post-eclosion temperatures. There was evidence for G×Es in female choosiness and preference, which suggests that the evolution of female mate choice behaviour could differ across environments. However, the ranked order of preferred males was consistent across females and environments, so the same males are favoured by mate choice in spite of G×Es. Our study highlights the importance of taking cross-environment perspectives in order to gain a more comprehensive understanding of the operation of sexual selection.

Heritability of male attractiveness persists despite evidence for unreliable sexual signals in Drosophila simulans

Sexual signals can be used to attract mates, but to be honest indicators of signaller quality they need to convey information reliably. However, environmental variation and genotype-by-environment (G × E) interactions have the potential to compromise the reliability of sexual signals. Here, we test the reliability of cuticular hydrocarbons (CHCs) as signals of heritable aspects of male attractiveness in Drosophila simulans. We examined the heritability of male attractiveness and a measure of the difference between fathers' and sons' CHC profiles across dietary and temperature environments. Our results show that environmental heterogeneity disrupts the similarity of some components of father and son CHC profile. However, overall male attractiveness is heritable within and across environments, so that sire attractiveness is a good predictor of son attractiveness even with environmental heterogeneity. This suggests that although some male CHC signals are unreliable, attractive genotypes retain their attractiveness across environments on average.