Laurent Dormont

Human skin volatiles : a review

Odors emitted by human skin are of great interest to biologists in many fields; applications range from forensic studies to diagnostic tools, the design of perfumes and deodorants, and the ecology of blood-sucking insect vectors of human disease. Numerous studies have investigated the chemical composition of skin odors, and various sampling methods have been used for this purpose. The literature shows that the chemical profile of skin volatiles varies greatly among studies, and the use of different sampling procedures is probably responsible for some of these variations. To our knowledge, this is the first review focused on human skin volatile compounds. We detail the different sampling techniques, each with its own set of advantages and disadvantages, which have been used for the collection of skin odors from different parts of the human body. We present the main skin volatile compounds found in these studies, with particular emphasis on the most frequently studied body regions, axillae, hands, and feet. We propose future directions for promising experimental studies on odors from human skin, particularly in relation to the chemical ecology of blood-sucking insects.

Influence of dung volatiles on the process of resource selection by coprophagous beetles

Summary.Most dung beetles colonize the faeces of several vertebrate species without much discrimination, and are thus often considered as polyphagous. Recent studies have provided evidence for clear feeding preferences in scarab beetles colonizing dung of herbivore species, but little is known about these insects’ abilities to discriminate among odours from faeces of various herbivores. In this study, trophic preferences were examined using blocks of pitfall traps baited with dung from four different herbivore species, i.e., sheep, cattle, horse, and red deer, in a mountainous area of south-central France. 4941 coprophagous scarabs, belonging to 27 species, were captured. Beetles were more attracted to dung of sheep (2257 individuals) than that of cattle (1294 individuals), followed by deer dung (768 individuals) and horse dung (622 individuals). Eleven of the 27 beetle species collected had significant feeding preferences for one of the four dung types. For each insect species, trophic habits did not vary between the two different sites of trapping, an open pasture and a wooded habitat. In laboratory olfactometer bioassays, scarab beetles orientated preferentially towards the dung volatiles from the dung type they preferred in the field. Trypocopris pyrenaeus, Anoplotrupes stercorosus, and Aphodius rufipes were more attracted to volatile compounds from sheep dung, Onthophagus fracticornis significantly preferred horse dung volatiles, and Aphodius haemorrhoidalis responded positively to deer dung odours. The role of dung olfactory cues in the process of resource selection by dung beetles is discussed.

Trophic Preferences Mediated by Olfactory Cues in Dung Beetles Colonizing Cattle and Horse Dung

Abstract Scarab beetles colonizing vertebrate feces are considered polyphagous insects, because they often use dung of several species. However, recent works have provided evidence for strong feeding preferences in some dung beetle species. The influence of dung odors in the process of resource selection by beetles has not received attention. The objective of this study was to investigate trophic preferences in Mediterranean dung beetles colonizing cattle and horse dung. The pattern of resource use by insects was studied using series of pitfall traps baited with cattle or horse dung in three different sites of south-central France. A total of 4,276 insects belonging to 39 species were captured. Cattle dung attracted more insects than horse feces (2,570 versus 1,706 individuals). Twenty-four of the 39 beetle species collected had clear feeding preferences for cattle dung (13 species) or horse dung (11 species). None of these species seemed linked exclusively to one kind of dung. The behavioral responses of seven scarab beetles to volatile compounds emitted by cattle and horse dung were compared in laboratory olfactometer bioassays. Three insect species (Aphodius erraticus, A. scrutator, Onthophagus vacca) were more attracted to volatile compounds from cattle dung. Two other species (Euonthophagus amyntas, Bubas bubalus) orientated preferentially toward horse dung volatiles. Except for two species, beetles were thus attracted to volatiles from the dung type they preferred in the field. The reasons why coprophagous beetles show local feeding preferences for particular dung types are discussed.

Innate olfactory preferences in dung beetles

SUMMARY The effects of insect larval diet on adult olfactory responses to host-plant or food volatiles are still debated. The induction of adult host preferences has been studied in insects with diverse ecologies, including parasitoids, flower-visitors and phytophagous species. We investigated this question for the first time in a coprophagous insect species. Larvae of the French scarab dung beetle Agrilinus constans were reared on four different artificial substrates containing dung from cattle, horse, sheep or wild boar, and responses of imagos to dung volatiles were then behaviourally tested in an olfactometer. We also reported the first analysis of the composition of different mammal dung volatiles. We showed that adult beetles were more attracted to cattle and sheep dung odours, and that larval feeding experience had no effect on the adult olfactory responses to dung volatiles. A second experiment showed that the presence of other insects inside the dung resource affects the process of dung selection by adults. We identified 64 chemical compounds from dung emissions, and showed that dung volatiles clearly differed among different mammal species, allowing olfactory discrimination by dung beetles. Our results suggest that resource selection in coprophagous insects may be based on innate olfactory preferences. Further experiments should examine whether Agrilinus adults can learn new dung odours, and whether larval diet may influence the behaviour of adults in other coprophagous species.

New methods for field collection of human skin volatiles and perspectives for their application in the chemical ecology of human–pathogen–vector interactions

SUMMARY Odours emitted by human skin are of great interest to biologists in many fields, with practical applications in forensics, health diagnostic tools and the ecology of blood-sucking insect vectors of human disease. Convenient methods are required for sampling human skin volatiles under field conditions. We experimentally compared four modern methods for sampling skin odours: solvent extraction, headspace solid-phase micro-extraction (SPME), and two new techniques not previously used for the study of mammal volatiles, contact SPME and dynamic headspace with a chromatoprobe design. These methods were tested and compared both on European subjects under laboratory conditions and on young African subjects under field conditions. All four methods permitted effective trapping of skin odours, including the major known human skin volatile compounds. In both laboratory and field experiments, contact SPME, in which the time of collection was restricted to 3 min, provided results very similar to those obtained with classical headspace SPME, a method that requires 45 min of collection. Chromatoprobe sampling also proved to be very sensitive, rapid and convenient for the collection of human-produced volatiles in natural settings. Both contact SPME and chromatoprobe design may considerably facilitate the study of human skin volatiles under field conditions, opening new possibilities for examining the olfactory cues mediating the host-seeking behaviour of mosquito vectors implicated in the transmission of major diseases.

Rare white‐flowered morphs increase the reproductive success of common purple morphs in a food‐deceptive orchid

How floral colour polymorphism can be maintained in evolutionary time is still debated. In rewardless orchids, it is unknown whether rare white-flowered morphs differ in scent chemistry from pigmented morphs, and whether such intraspecific variation in floral signals may have an impact on reproductive success. We compared the chemical composition of floral volatiles emitted by white- and purple-flowered morphs of Orchis mascula, and recorded the fruit set of both colour morphs. We also used white ping-pong balls to mimic white-flowered morphs in field bioassays. We found that colour polymorphism was not associated with floral odour polymorphism. Surprisingly, when populations of purple-flowered plants included a few white-flowered individuals, the fruit set of the purple morph increased significantly (from 6 to 27%), while that of the white morph remained low. We obtained the same fourfold increase in fruit set when using ping-pong balls as visual lures, demonstrating the association between colour variation and fruit set, and the key role of visual signals in pollinator attraction. Our results are incompatible with negative frequency-dependent selection, a hypothesis invoked to explain colour polymorphism in other rewardless orchids. We propose several hypotheses to explain the maintenance of white morphs in O. mascula.

Floral scent variation in two Antirrhinum majus subspecies influences the choice of naïve bumblebees

Two wild subspecies of snapdragon, Antirrhinum majus, subspecies pseudomajus and striatum, differ in floral color and can be visually discriminated by insect visitors. The extent to which olfactory cues derived from floral scents contribute to discrimination between snapdragon subspecies is however unknown. We tested whether these two subspecies differ in floral scent and whether these olfactory differences are used by bumblebees (Bombus terrestris) to discriminate between them. We grew individuals of both subspecies, collected from a total of seven wild populations, under controlled conditions. We quantified the volatile organic compounds (VOCs) emitted by the flowers using gas-chromatography/mass-spectrometry/flame-ionization-detection. We studied antennal detection of VOCs by bumblebees, by means of electroantennogram study (EAG). We also performed behavioral experiments in a Y-maze to determine the innate response of bumblebees to the main floral VOCs emitted by our snapdragon subspecies. The floral scent of Antirrhinum majus pseudomajus contained three volatile benzenoids absent in the floral scent of Antirrhinum majus striatum. One of them, acetophenone, contributed over 69% of the absolute emissions of A. majus pseudomajus. These benzenoids elicited a significantly higher EAG response compared with other VOCs. In the Y-maze, bumblebees were significantly less attracted by acetophenone, suggesting an aversive effect of this VOC. Our findings indicate that bumblebees are able to discriminate between the two Antirrhinum majus subspecies. Differences in flower scent between these subspecies and olfactory bumblebee preferences are discussed in the light of biochemical constraints on VOCs synthesis and of the role of flower scent in the evolutionary ecology of A. majus.

Colour-scent associations in a tropical orchid: Three colours but two odours

Colour and scent are the major pollinator attractants to flowers, and their production may be linked by shared biosynthetic pathways. Species with polymorphic floral traits are particularly relevant to study the joint evolution of floral traits. We used in this study the tropical orchid Calanthe sylvatica from Réunion Island. Three distinct colour varieties are observed, presenting lilac, white or purple flowers, and named respectively C. sylvaticavar.lilacina (hereafter referred as var. lilacina), C. sylvaticavar. alba (var. alba) and C. sylvatica var. purpurea (var. purpurea). We investigated the composition of the floral scent produced by these colour varieties using the non-invasive SPME technique in the wild. Scent emissions are dominated by aromatic compounds. Nevertheless, the presence of the terpenoid (E)-4,8-dimethylnona-1,3,7-triène (DMNT) is diagnostic of var. purpurea, with the volatile organic compounds (VOC) produced by some individuals containing up to 60% of DMNT. We evidence specific colour-scent associations in C. sylvatica, with two distinct scent profiles in the three colour varieties: the lilacina-like profile containing no or very little DMNT (<2%) and the purpurea-like profile containing DMNT (>2%). Calanthe sylvatica var. alba individuals group with one or the other scent profile independently of their population of origin. We suggest that white-flowered individuals have evolved at least twice, once from var. lilacina and at least once from var. purpurea after the colonisation of la Réunion. White-flowered individuals may have been favoured by the particular pollinator fauna characterising the island. These flowering varieties of C. sylvatica, which display three colours but two scents profiles prove that colour is not always a good indicator of odour and that colour-scent associations may be complex, depending on pollination ecology of the populations concerned.

Floral scent emitted by white and coloured morphs in orchids

Polymorphism of floral signals, such as colour and odour, is widespread in flowering plants and often considered to be adaptive, reflecting various pollinator preferences for particular floral traits. Several authors have recently hypothesized that particular associations exist between floral colour and scent, which would result from shared biochemistry between these two floral traits. In this study, we compared the chemical composition of floral volatiles emitted by white- and purple-flowered morphs of three different orchid species, including two food-deceptive species (Orchis mascula and Orchis simia) and a food-rewarding species (Anacamptis coriophora fragrans). We found clear interspecific differences in floral odours. As expected from their pollination strategy, the two deceptive orchids showed high inter-individual variation of floral volatiles, whereas the food-rewarding A. c. fragrans showed low variation of floral scent. Floral volatiles did not differ overall between white- and coloured-flowered morphs in O. mascula and A. c. fragrans, while O. simia exhibited different volatile profiles between the two colour morphs. However, a detailed analysis restricted to benzenoid compounds (which are associated with the production of floral anthocyanin pigments) showed that white inflorescences emitted more volatiles of the shikimic pathway than coloured ones, both for O. mascula and O. simia. These results are consistent with the current hypothesis that shared biochemistry creates pleiotropic links between floral colour and scent. Whether intraspecific variation of floral signals actually affects pollinator attraction and influences the reproductive success of these orchids remains to be determined.

Differentiation in a tropical deceptive orchid: colour polymorphism and beyond

Colour polymorphism has often been described among individuals of deceptive orchid species, and several studies have investigated reproductive success variations among colour morphs. However, whether colour morphs differed in other traits has received little attention in previous studies. Here, we report the case of a tropical deceptive orchid in Reunion Island that exhibits three different colour varieties. We investigated patterns of trait variation among colour varieties and found significant differences in floral and inflorescence morphology. Interestingly, we found that most populations included only individuals of a single variety, and that strong differences exist among varieties in spatial distribution and flowering phenology. This situation differs from previously reported cases of floral polymorphism in deceptive orchids where colour morphs co-occur and flower in the same populations. The spatio-temporal variation in flowering suggests that colour varieties have independent reproductive ecologies and are adapted to local conditions. We propose that potential variations in pollinator species abundance or diversity and the co-occurrence of nectar-producing species in the community may have driven the adaptation of each variety to its current pollination niche.