Subtle Chemical Variations with Strong Ecological Significance: Stereoselective Responses of Male Orchid Bees to Stereoisomers of Carvone Epoxide

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.

Complexity of the Male Perfume of Eulaema nigrita from Forest and Woody Physiognomies of the Brazilian Savanna: Is There a Relationship with Body Size and Wing Wear?

Male orchid bees collect volatile and semi-volatile compounds from the environment for storage and accumulation in specialized hind legs. Later, these compounds form a perfume blend used during courtship to lure conspecific females for mating. Male orchid bees perfume has been suggested to play an important role as a sexual signaling trait involved in pre-mating isolation of species, functioning as an indicator of male genetic quality. Eulaema nigrita Lepeletier (Apidae: Euglossini) is a common species in both forested and woody savanna (Cerrado stricto sensu) physiognomies of the Brazilian savanna biome. By identifying the chemical composition of male E. nigrita perfume, we tested for differences in the bouquet chemical profile in populations from remnants of seasonal semideciduous forest and woody savanna. In addition, we assessed the relation between perfume complexity and morphological traits associated with size and age of males. Our analysis showed a low effect of physiognomies on differences in the perfume chemical profile of sampled males. Nevertheless, we observed significant differences in the chemical profile of individuals from two seasonal semideciduous forest remnants, which suggests an environmental effect in individual bouquet. Wing wear measurements were positively related to perfume complexity, consistent with the premise that perfumes from older individuals are indicators of survival capacity in male orchid bees.

Chemistry, biosynthesis and biology of floral volatiles: roles in pollination and other functions

Covering: 2010 to 2023Floral volatiles are a chemically diverse group of plant metabolites that serve multiple functions. Their composition is shaped by environmental, ecological and evolutionary factors. This review will summarize recent advances in floral scent research from chemical, molecular and ecological perspectives. It will focus on the major chemical classes of floral volatiles, on notable new structures, and on recent discoveries regarding the biosynthesis and the regulation of volatile emission. Special attention will be devoted to the various functions of floral volatiles, not only as attractants for different types of pollinators, but also as defenses of flowers against enemies. We will also summarize recent findings on how floral volatiles are affected by abiotic stressors, such as increased temperatures and drought, and by other organisms, such as herbivores and flower-dwelling microbes. Finally, this review will indicate current research gaps, such as the very limited knowledge of the isomeric pattern of chiral compounds and its importance in interspecific interactions.

The volatile chemistry of orchid pollination

Covering: up to September 2022Orchids are renowned not only for their diversity of floral forms, but also for their many and often highly specialised pollination strategies. Volatile semiochemicals play a crucial role in the attraction of a wide variety of insect pollinators of orchids. The compounds produced by orchid flowers are as diverse as the pollinators they attract, and here we summarise some of the chemical diversity found across orchid taxa and pollination strategies. We focus on compounds that have been experimentally demonstrated to underpin pollinator attraction. We also highlight the structural elucidation and synthesis of a select subset of important orchid pollinator attractants, and discuss the ecological significance of the discoveries, the gaps in our current knowledge of orchid pollination chemistry, and some opportunities for future research in this field.

Antennae of psychodid and sphaerocerid flies respond to a high variety of floral scent compounds of deceptive Arum maculatum L

Insect-pollinated plants often release complex mixtures of floral scents to attract their pollinators. Yet scent compounds eliciting physiological or behavioural responses in pollinators have only been identified in few plant species. The sapromyiophilous aroid Arum maculatum releases a highly diverse dung-like scent with overall more than 300 different compounds recorded so far to attract its psychodid and other fly pollinators. The volatiles’ role in pollinator attraction is mostly unknown. To identify potential behaviourally active compounds, we recorded electroantennographic responses of four Psychodidae and one Sphaeroceridae species to (1) inflorescence scents of A. maculatum and (2) the scents released by cow dung, likely imitated by the plant species. Here we show that these flies are sensitive to 78 floral volatiles of various chemical classes, 18 of which were also found in cow dung. Our study, which for the first time determined physiologically active compounds in the antennae of Psychoda spp. and Sphaeroceridae, identified various volatiles not known to be biologically active in any floral visitors so far. The obtained results help deciphering the chemical basis that enables A. maculatum and other plants, pollinated by psychodids and sphaerocerids, to attract and deceive their pollinators.

Chemical Attraction of Gall Midge Pollinators (Cecidomyiidae: Cecidomyiinae) to Anthurium acutangulum (Araceae)

Flowering plants often use chemical signals to attract their pollinators, and compounds that elicit attraction are known for several groups of pollinators. For other pollinators such as gall midges, however, compounds responsible for their attraction to flowers are largely unknown. Here, we describe the pollination biology of Anthurium acutangulum, a Neotropical aroid species found to be attractive to gall midges. We collected and analyzed its floral scent by dynamic headspace collections and gas chromatography coupled to mass spectrometry, and identified compounds responsible for pollinator attraction. The inflorescences were almost exclusively visited by gall midges (females; Cecidomyiidae: Cecidomyiinae) and released a strong scent reminiscent of freshly cut cucumber, mainly (5S,7S)-trans-conophthorin, (E2,Z6)-2,6-nonadienal, and cis-conophthorin. Behavioral assays with the two most abundant compounds identified (E2,Z6)-2,6-nonadienal as being highly attractive to the female gall midge pollinators, whereas (5S,7S)-trans-conophthorin was not attractive. Overall, we introduce a new specialized gall midge pollination system and identify the chemical mediating communication between the pollinators and their host plants.

Unraveling the Olfactory Biases of Male Euglossine Bees: Species-Specific Antennal Responses and Their Evolutionary Significance for Perfume Flowers

Male euglossine bees exhibit unique adaptations for the acquisition and accumulation of chemical compounds from “perfume flowers” and other sources. During courtship display, male bees expose perfume mixtures, presumably to convey species-specific recognition and/or mate choice signals to females. Because olfaction regulates both signal production (in males) and signal detection (in females) in this communication system, strong selective pressures are expected to act on the olfactory system, which could lead to sensory specialization in favor of an increased sensitivity to specific chemical compounds. The floral scents of euglossine-pollinated plants are hypothesized to have evolved in response to the preexisting sensory biases of their male euglossine bee pollinators. However, this has never been investigated at the peripheral olfactory circuitry of distinct pollinating genera. Here, we present a comparative analysis using electroantennography (EAG) of males across the phylogeny of 29 euglossine bee species, among them Euglossa and Eulaema species. First, we tested whether antennal responses differ among different euglossine genera, subgenera and species. Secondly, we conducted a comparative phylogenetic analysis to investigate the macroevolutionary patterns of antennal responses across the euglossine bee phylogeny. We found that antennal response profiles are very unique on the species level and differ on the subgenus and the genus level. The differences can be explained by chemical compounds typically found in the floral scent bouquets of perfume flowers and specific compounds of species either pollinated by Euglossa (e.g., ipsdienol) or Eulaema bees (e.g., (−)-(E)-carvone epoxide). Also, we detected a phylogenetic signal in mean antennal responses and found that especially at the species level of our simulation the overall antennal responses exhibit greater disparity relative to a null model of pure Brownian-motion across the phylogeny. Altogether, our results suggest that (1) euglossine bee species exhibit species-specific antennal responses that differ among euglossine genera and subgenera, (2) antennal responses diverge early after speciation events, and (3) scent composition of perfume flowers evolved in response to pollinator-mediated selection imposed by preexisting sensory biases in euglossine bees.

Pollination of the strongly scented Sarcoglottis acaulis (Orchidaceae) by male orchid bees: nectar as resource instead of perfume

The Neotropical orchid genus Sarcoglottis comprises ~40 species, which emit strong floral scents, presumably involved in pollinator attraction. Information on basic aspects of its natural history is scant, with the few studies available so far pointing to nectar-seeking orchid bees as pollinators. Here, we investigated the reproductive biology of Sarcoglottis acaulis, addressing the ecological meaning of its floral scent. In Atlantic Forest fragments of NE Brazil, we described the floral biology, determined the breeding system and recorded the pollinators of S. acaulis. Additionally, we chemically characterized its floral scent and assessed its role on pollinator attraction. Although self-compatible, S. acaulis depends on pollinators to boost fruit set. Male orchid bees of Eulaema atleticana and E. niveofasciata were the only recorded pollinators. They foraged for nectar only, in spite of the strong scent emitted by S. acaulis flowers. The floral scent is composed of six compounds, of which geraniol and nerol elicited electroantennographic responses in Eulaema bees. A synthetic mixture of these compounds attracted Eulaema bees in field assays but did not trigger the stereotyped scent-gathering behaviour. The floral scent of S. acaulis acts in signalling and nectar is the sole reward for pollinators. Despite the low pollinator frequency, S. acaulis shows a high fruit set (77%), particularly when compared to other orchids. Attributes such as pollinia that release small pollen loads, allowing a single pollinia-carrying bee to pollinate several flowers, low amount of nectar, steady-state flowering and traplining behaviour of pollinators, might act together to assure this outstanding fruit set.

Natural and Artificial Chiral-Based Systems for Separation Applications

Chiral separation has attracted much attention for basic research and industrial applications in analytical chemistry. Generally, chiral separations use natural or artificial chiral-based materials as adsorbents. To improve the precision and efficiency of chiral separation, focus has shifted from natural and synthetic adsorbents to binary combinations of materials. This review specifically summarizes the significant advancements made in natural and artificial chiral adsorbents as promising candidates for diverse drug and biomolecule separation applications as well as the remaining drawbacks and challenges for research on chiral separations. The mechanisms of chiral-based recognition and separation and history and development of natural and artificial chiral-based systems are the focus of this review. Future directions in natural and artificial chiral-based systems for practical separations and other applications are also presented.

Methyl acetate, a highly volatile floral semiochemical mediating specialized plant-beetle interactions

Olfactory signaling is key to the reproductive biology of entomophilous palms. Both pollinating and specialized herbivorous insects are attracted to fragrance-emitting palm inflorescences that function as reliable food sources, as well as mating and oviposition sites. In the present study, we characterized the floral scent chemistry of the acuri palm (Attalea phalerata), assessing its role in the attraction of flower-visiting insects associated with this species over its natural distribution range. We sampled insects from staminate inflorescences of A. phalerata (n = 6) at four different sites in the Brazilian Atlantic Forest and Cerrado, and Colombian Amazon basin. Dynamic headspace scent samples of both pistillate and staminate inflorescences of A. phalerata (n = 3♀, 3♂) were collected and analyzed by gas chromatography-mass spectrometry. Methyl acetate, a rare floral scent compound, was identified as the almost exclusive constituent (> 99.8% relative percentage) in all the samples. Flight-interception traps baited with methyl acetate, installed in one of the sites in the Brazilian Cerrado, were attractive to beetles associated with inflorescences of A. phalerata across all four sampling sites (9 spp. in total), including the putative main pollinators (Mystrops spp., Nitidulidae; Andranthobius spp., Curculionidae) and various palm borers (Paratenthras martinsi, Cerambycidae; Parisoschoenus sp.1 and Belopoeus sp.1; Curculionidae). Methyl acetate is highly volatile and we hypothesize its efficacy relies on profuse emission by the inflorescences of A. phalerata, as specialized pollinating insects respond to high concentrations of the attractant, perhaps before odor plumes rapidly disperse. Such a strategy could prove particularly effective in dense populations of A. phalerata.