Cuticular Hydrocarbons as Potential Close Range Recognition Cues in Orchid Bees

Evolution of Acquired Perfumes and Endogenous Lipid Secretions in Orchid Bees

Male orchid bees are unique in the animal kingdom for making perfumes that function as sex pheromone. Males collect volatile chemicals from the environment in the neotropical forests, including floral and non-floral sources, creating complex but species-specific blends. Male orchid bees exhibit several adaptations to facilitate perfume collection and storage. When collecting volatile compounds, males apply lipid substances that they secrete from cephalic labial glands onto the fragrant substrate. These lipids help dissolve and retain the volatiles, similar to the process of 'enfleurage' in the traditional perfume industry. We investigated how the chemical composition of acquired perfume and labial gland secretions varied across the phylogeny of orchid bees, including 65 species in five genera from Central and South America. Perfumes showed rapid evolution as revealed by low overall phylogenetic signal, in agreement with the idea that perfume compounds diverge rapidly and substantially among closely related species due to their role in species recognition. A possible exception were perfumes in the genus Eulaema, clustering closely in chemospace, partly mediated by high proportions of carvone and trans-carvone oxide. Labial gland secretions, in contrast, showed a strong phylogenetic signal at the genus level, with secretions of Eufriesea and Exaerete dominated by fatty acids and Eulaema dominated by saturated acetates of chain lengths 12 to 16 C-atoms. Secretions of the majority of Euglossa were heavily dominated by one unsaturated long chain diacetate, (9Z)-Eicosen-1,20-diyldiacetate. However, we also identified few highly divergent species of Euglossa in four subclades (11 species) that appear to have secondarily replaced the diacetate with other compounds. In comparison with environment-derived perfumes, the evolution of labial gland secretion is much slower, likely constrained by the underlying biochemical pathways, but perhaps influenced by perfume-solvent chemical interactions.

Age-dependent perfume development in male orchid bees, Euglossa imperialis

Male neotropical orchid bees concoct complex perfume blends by collecting exogenous volatiles from various sources, including orchids. These perfumes, stored in specialized hind-leg pouches and released during courtship, serve as inter-sexual signals. It has been hypothesized that male perfumes honestly indicate aspects of male fitness. If perfume traits such as quantity or complexity increase over individual lifetime, perfumes could reflect age (survival) and cumulative foraging success of males. We conducted a two-season mark-recapture study with Euglossa imperialis in Costa Rica, monitoring the balance of perfume uptake and expenditure over individual male lifetime. We sealed one hind-leg pouch upon initial capture, 'freezing' the perfume status on one side, and compared it with the other side at recapture to assess changes in perfume traits over time. Additionally, we used a novel method to estimate individual age by combining two parameters of wing degradation. Contrary to predictions, young to intermediate-aged bees had the highest quantities of perfume and the highest diversity of detected compounds. At the same time, the change in perfume between recaptures was positive (increase in amount and complexity) in young bees, whereas it was neutral to negative in older bees. Although these findings do not disprove an indicator function of male perfume, they shift the emphasis to non-cumulative fitness components such as sensory acuteness or cognitive capacity as likely targets of selection. Females preferring strong perfume signals in mates would maximize speed of foraging in offspring rather than their lifetime cumulative yield.

Function of environment-derived male perfumes in orchid bees

Perfume making in male orchid bees is a unique behavior that has given rise to an entire pollination syndrome in the neotropics.^1,2 Male orchid bees concoct and store species-specific perfume mixtures in specialized hind-leg pockets³ using volatiles acquired from multiple environmental sources, including orchid flowers.^4,5 However, the function and the ultimate causes of this behavior have remained elusive.^2,6 Although previous observations suggested that male perfumes serve as chemical signals, the attractiveness for females has not be shown.^7,8 Here, we demonstrate that the possession of perfume increases male mating success and paternity in Euglossa dilemma, a species of orchid bees recently naturalized in Florida. We supplemented males reared from trap-nests with perfume loads harvested from wild conspecifics. In dual-choice experiments, males supplemented with perfumes mated with more females, and sired more offspring, than untreated, equal-aged, control males. Although perfume supplementation had little effect on the intensity of male courtship display, it changed the dynamics of male-male interactions. Our results demonstrate that male-acquired perfumes are sexual signals that stimulate females for mating and suggest that sexual selection is key in shaping the evolution of perfume communication in orchid bees.

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.

Experimental disruption of social structure reveals totipotency in the orchid bee, Euglossa dilemma

Eusociality has evolved multiple times across the insect phylogeny. Social insects with greater levels of social complexity tend to exhibit specialized castes with low levels of individual phenotypic plasticity. In contrast, species with simple social groups may consist of totipotent individuals that transition among behavioral and reproductive states. However, recent work has shown that in simple social groups, there can still be constraint on individual plasticity, caused by differences in maternal nourishment or social interaction. It is not well understood how these constraints arise, ultimately leading to the evolution of nonreproductive workers. Some species of orchid bees form social groups of a dominant and-one to two subordinate helpers where all individuals are reproductive. Females can also disperse to start their own nest as a solitary foundress, which includes a nonreproductive phase characterized by ovary inactivation, not typically expressed by subordinates. Little is known about individual flexibility across these trajectories. Here, using the orchid bee Euglossa dilemma, we assess the plasticity of subordinate helpers, finding that they are capable of the same behavioral, physiological, transcriptomic, and chemical changes seen in foundresses. Our results suggest that the lack of nonreproductive workers in E. dilemma is not due to a lack of subordinate plasticity.

Social Behavior, Ovary Size, and Population of Origin Influence Cuticular Hydrocarbons in the Orchid Bee Euglossa dilemma

AbstractCuticular hydrocarbons (CHCs) are waxy compounds on the surface of insects that prevent desiccation and frequently serve as chemical signals mediating social and mating behaviors. Although their function in eusocial species has been heavily investigated, little is known about the evolution of CHC-based communication in species with simpler forms of social organization lacking specialized castes. Here we investigate factors shaping CHC variation in the orchid bee Euglossa dilemma, which forms casteless social groups of two to three individuals. We first assess geographic variation, examining CHC profiles of males and females from three populations. We also consider CHC variation in the sister species, Euglossa viridissima, which occurs sympatrically with one population of E. dilemma. Next, we consider variation associated with female behavioral phases, to test the hypothesis that CHCs reflect ovary size and social dominance. We uncover a striking CHC polymorphism in E. dilemma spanning populations. In addition, we identify a separate set of CHCs that correlate with ovary size, social dominance, and expression of genes associated with social behavior, suggesting that CHCs convey reproductive and social information in E. dilemma. Together, our results reveal complex patterns of variation in which a subset of CHCs reflect the social and reproductive status of nestmates.

Stored perfume dynamics and consequences for signal development in male orchid bees

Male orchid bees (Euglossini) collect volatiles from their environment to concoct species-specific "perfumes", which are later emitted at mating sites. Intensity, complexity or composition of perfumes may encode age (survival) of a male, but how the individual perfume develops over time needs to be clarified. We investigated chemical changes during storage in leg pockets. We injected a mixture of eight perfume compounds into pockets of Euglossa imperialis and only the two most volatile compounds decreased over 12 days. Using a different approach we found significant shifts in quantities of naturally occurring perfume compounds of Euglossa championi over 10 days, with the strongest decreases (up to 70% peak area) in highly volatile minor compounds, e.g. monoterpenes, and noteworthy increases (up to 40%) in some sesquiterpenoids. Corresponding shifts were observed in legs of dried bees, suggesting that no metabolic activity is required for the observed changes to occur. Our results confirm that male orchid bees are generally good at preserving collected perfumes. However, subtle shifts towards heavier compounds in blends may occur over the lifetime of individual bees, e.g. due to evaporation or in-pocket chemical reaction, with old males acquiring a more pronounced base note in their seasoned perfumes.

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

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