The Chemical Basis of Host-Plant Recognition in a Specialized Bee Pollinator

Elevational shifts in reproductive ecology indicate the climate response of a model chasmophyte, Rainer's bellflower (Campanula raineri)

BACKGROUND AND AIMS

Elevation gradients provide 'natural experiments' for investigating plant climate change responses, advantageous for the study of protected species and life forms for which transplantation experiments are illegal or unfeasible, such as chasmophytes with perennial rhizomes pervading rock fissures. Elevational climatic differences impact mountain plant reproductive traits (pollen and seed quality, sexual vs. vegetative investment) and pollinator community composition; we investigated the reproductive ecology of a model chasmophyte, Campanula raineri Perp. (Campanulaceae), throughout its current elevational/climatic range to understand where sub-optimal conditions jeopardise survival. We hypothesised that: 1) reproductive fitness measures are positively correlated with elevation, indicative of the relationship between fitness and climate; 2) C. raineri, like other campanulas, is pollinated mainly by Hymenoptera; 3) potential pollinators shift with elevation.

METHODS

We measured pollen and seed quality, seed production, the relative investment in sexual vs. vegetative structures and vegetative (Grime's CSR) strategies at different elevations. Potential pollinators were assessed by combining molecular and morphological identification.

KEY RESULTS

Whereas CSR strategies were not linked to elevation, pollen and seed quality were positively correlated, as was seed production per fruit (Hypothesis 1 is supported). The main pollinators of C. raineri were Apidae, Andrenidae, Halictidae (Hymenoptera) and Syrphidae (Diptera), probably complemented by a range of occasional pollinators and visitors (Hypothesis 2 partially supported). Potential pollinator communities showed a taxonomic shift towards Diptera with elevation (particularly Anthomyiidae and Muscidae) and away from Hymenoptera (Hypothesis 3 was supported).

CONCLUSIONS

Pollinator availability is maintained at all elevations by taxon replacement. However, reduced pollen quality and seed production at lower elevations suggest an impact of climate change on reproduction (especially <1200 m a.s.l., where seed germination was limited). Aside from guiding targeted conservation actions for C. raineri, our results highlight problems that may be common to mountain chasmophytes worldwide.

Fields of flowers with few strikes: how oligolectic bees manage their foraging behavior on Calibrachoa elegans (Solanaceae)

In specialized plant-pollinator associations, partners may exhibit adaptive traits, which favor the maintenance of the interaction. The association between Calibrachoa elegans (Solanaceae) and its oligolectic bee pollinator, Hexantheda missionica (Colletidae), is mutualistic and forms a narrowly specialized pollination system. Flowers of C. elegans are pollinated exclusively by this bee species, and the bees restrict their pollen resources to this plant species. The pollen presentation schedules of C. elegans were evaluated at the population level to test the hypothesis that H. missionica females adjust their foraging behavior to the resource offering regime of C. elegans plants. For this, the number of new flowers and anthers opened per hour (as a proxy for pollen offering) was determined, and pollen advertisement was correlated with the frequency of flower visits during the day. Preferences of female bees for flowers of different stages were also investigated, and their efficiency as pollinators was evaluated. Pollen offering by C. elegans was found to be partitioned throughout the day through scattered flower openings. Females of H. missionica indeed adjusted their foraging activity to the most profitable periods of pollen availability. The females preferred new, pollen-rich flowers over old ones and gathered pollen and nectar selectively according to flower age. Such behaviors must optimize female bee foraging efficiency on flowers. Female bees set 93% of fruit after a single visit. These findings guarantee their importance as pollinators and the persistence of the specialized plant-pollinator association.

Semiochemical based integrated livestock pest control

The role of arthropods as livestock pests has been well established. Besides their biting habits causing nuisance in animals; they are important vectors for transmission of economically important livestock diseases worldwide. Various pests and vector control managemental programs that also make use of chemicals have variable success rates. Consequently, insecticide/acaricide resistance has been reported against most of the commonly used chemicals along with increased concern for environment and demand for clean and green, residue-free animal products. This calls for an urgent need to develop novel, alternate, effective strategies/technologies. This lays the foundation for the use of semiochemicals as alternatives along with other biological control agents. Current knowledge on semiochemical use in livestock is refined and limited; however, it has been widely exploited in the agricultural sector to control plant and food crop pests, surveillance, and monitoring. Semiochemicals have an added advantage of being natural and safe; however, knowledge of extraction and quantification by using assays needs to be explicit. Expertise is required in behavioral and electrophysiological studies of arthropods and their interactions with the host and environment targeting specific semiochemicals for promising results. A thorough prior understanding on aspects such as mechanism of action, the stimulus for the release, the effecter/target species, response produced, application methods, dose and concentration is required to develop any successful pest/vector control program. The current review provides essential and frontline information on semiochemicals and their potential applications in the livestock sector along with future challenges and interventions.

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.

Effect of ozone exposure on the foraging behaviour of Bombus terrestris

Tropospheric ozone (O₃) mixing ratios have increased substantially since preindustrial times and high O₃ peaks are increasingly common. Plant-pollinator interactions are central to natural ecosystem functioning and food production systems but could be negatively affected by unfavourable environmental conditions such as elevated O₃. Ecosystem functioning is threatened by O₃, which can degrade floral volatile organic compounds (VOCs) used by pollinators as olfactory cues during foraging. It can also exert oxidative stress on VOC-emitting plants and receiving organisms, potentially disturbing the sending and receiving of VOC signals. The aim of this study was to determine the effects of elevated ozone on the foraging behaviour of Bombus terrestris on three species of the Brassicaceae, with a particular focus on bumblebee choices and the mechanisms underpinning differences observed. Moreover, the study was designed to fill a gap between observations in small-scale laboratory experiments and large-scale modelling through empirical observations in polytunnels that represent a medium-large-scale artificial environment. Using 10 × 3 × 2 m polytunnels the effects of O₃ on pollinator foraging parameters on Sinapis alba, Sinapis arvensis and Raphanus raphanistrum were assessed. Significant effects of elevated O₃ (100 ± 10 ppb) on the time taken for the first bee to alight on a flower and the cumulative amount of time spent on flowers was observed. To further investigate the underlying mechanisms, a laboratory test was conducted to determine the effects of ozone on the VOC blend composition of S. alba flowers. Synthetic VOC blends representing O₃-altered and unaltered profiles were reconstituted and utilized in polytunnel and olfactometry experiments. The results indicated that a reduction of olfaction-mediated orientation, probably via VOC-degradation or direct effects of O₃ on bees, was responsible for the altered foraging parameters of B. terrestris, suggesting that the presence of elevated O₃ could have negative effects on the foraging efficiency of important pollinator species.

Synthetic fertilizers alter floral biophysical cues and bumblebee foraging behavior

The use of agrochemicals is increasingly recognized as interfering with pollination services due to its detrimental effects on pollinators. Compared to the relatively well-studied chemical toxicity of agrochemicals, little is known on how they influence various biophysical floral cues that are used by pollinating insects to identify floral rewards. Here, we show that widely used horticultural and agricultural synthetic fertilizers affect bumblebee foraging behavior by altering a complex set of interlinked biophysical properties of the flower. We provide empirical and model-based evidence that synthetic fertilizers recurrently alter the magnitude and dynamics of floral electrical cues, and that similar responses can be observed with the neonicotinoid pesticide imidacloprid. We show that biophysical responses interact in modifying floral electric fields and that such changes reduce bumblebee foraging, reflecting a perturbation in the sensory events experienced by bees during flower visitation. This unveils a previously unappreciated anthropogenic interference elicited by agrochemicals within the electric landscape that is likely relevant for a wide range of chemicals and organisms that rely on naturally occurring electric fields.

Floral Cues of Non-host Plants Attract Oligolectic Chelostoma rapunculi Bees

Oligolectic bees are highly dependent on the availability of the host plants to which they are specialized. Nevertheless, females of Chelostoma rapunculi have recently been monitored occasionally to visit Malva moschata and Geranium sanguineum flowers, in addition to their well-known Campanula spp. hosts. The questions therefore arise which floral cues promote visits to non-host plants. As host-specific floral cues are key attractants for oligolectic bees, we have studied the attractiveness of olfactory and visual cues of the established host Campanula trachelium in comparison to the non-host plants G. sanguineum and M. moschata in behavioral experiments. Chemical and electrophysiological analyses of the floral scent and spectral measurements of floral colors were used to compare and contrast host and non-host plants. The behavioral experiments showed that foraging-naïve bees, in particular, were attracted by olfactory cues of the non-host plants, and that they did not favor the Campanula host scent in choice experiments. Many electrophysiologically active floral volatiles were present in common in the studied plants, although each species produced an individual scent profile. Spiroacetals, the key components that enable C. rapunculi to recognize Campanula hosts, were detected in trace amounts in Geranium but could not be proved to occur in Malva. The visual floral cues of all species were particularly attractive for foraging-experienced bees. The high attractiveness of G. sanguineum and M. moschata flowers to C. rapunculi bees and the floral traits that are similar to the Campanula host plants can be a first step to the beginning of a host expansion or change which, however, rarely occurs in oligolectic bees.

Differential Evolutionary History in Visual and Olfactory Floral Cues of the Bee-Pollinated Genus Campanula (Campanulaceae)

Visual and olfactory floral signals play key roles in plant-pollinator interactions. In recent decades, studies investigating the evolution of either of these signals have increased considerably. However, there are large gaps in our understanding of whether or not these two cue modalities evolve in a concerted manner. Here, we characterized the visual (i.e., color) and olfactory (scent) floral cues in bee-pollinated Campanula species by spectrophotometric and chemical methods, respectively, with the aim of tracing their evolutionary paths. We found a species-specific pattern in color reflectance and scent chemistry. Multivariate phylogenetic statistics revealed no influence of phylogeny on floral color and scent bouquet. However, univariate phylogenetic statistics revealed a phylogenetic signal in some of the constituents of the scent bouquet. Our results suggest unequal evolutionary pathways of visual and olfactory floral cues in the genus Campanula. While the lack of phylogenetic signal on both color and scent bouquet points to external agents (e.g., pollinators, herbivores) as evolutionary drivers, the presence of phylogenetic signal in at least some floral scent constituents point to an influence of phylogeny on trait evolution. We discuss why external agents and phylogeny differently shape the evolutionary paths in floral color and scent of closely related angiosperms.

Chemical signal is in the blend: bases of plant-pollinator encounter in a highly specialized interaction

In several highly specialized plant-insect interactions, scent-mediated specificity of pollinator attraction is directed by the emission and detection of volatile organic compounds (VOCs). Although some plants engaged in such interactions emit singular compounds, others emit mixtures of VOCs commonly emitted by plants. We investigated the chemical ecological bases of host plant recognition in the nursery pollination mutualism between the dioecious Ficus carica and its specific pollinator Blastophaga psenes. Using Y-tube olfactometer tests, we show that B. psenes females are attracted by VOCs of receptive figs of both sexes and do not exhibit preference for VOCs of either male or female figs. Electrophysiological tests and chemical analysis revealed that of all the VOCs emitted by receptive figs, only five were found to be active on female antennae. Behavioural tests show that, in contrast to VOCs presented alone, only a blend with a particular proportion of four of these VOCs is as attractive as the odour of receptive figs, and that if there is a very small change in this blend proportion, the pollinator is no longer attracted. This study revealed that in highly specialized mutualistic interactions specificity could be mediated by a particular blend of common compounds emitted by plants.

Solitary bee pollinators adjust pollen foraging to the unpredictable flower opening of a species of Petunia (Solanaceae)

Details of the foraging patterns of solitary bees are much less well known than those of social species, and these patterns are often adjusted to exploit floral resources of one or only a few species. The specialized flower-visiting bees of Petunia are good models for investigating such foraging patterns. Here we analysed the floral biology and pollen presentation schedule of the endangered Petunia mantiqueirensis in mixed Araucaria forests of Serra da Mantiqueira, Brazil. Pollinators and their pollen foraging behaviour and food specialization were determined through analyses of scopa pollen loads. Flowers opened throughout the day and presented all their pollen resources within the first 30 min of anthesis, thus providing their pollen resources in an asynchronous fashion in one-flower packages throughout the day. Females of Pseudagapostemon fluminensis were the most frequent flower visitors, contacting stigmas in 96% of their visits, and were the unique effective pollinators of Petunia mantiqueirensis. These pollinators were responsible for the first three visits to 115 individually monitored flowers at any daylight hour, removing ~86% of a flower’s total pollen supply during the first visit. Although female bees harvest the majority of pollen resources of Petunia mantiqueirensis, analyses of scopa loads revealed that most of them also collect pollen from plants of other families while foraging for pollen in Petunia flowers.

Flowers of European pear release common and uncommon volatiles that can be detected by honey bee pollinators

Floral scents are important pollinator attractants, but there is limited knowledge about the importance of single components in plant–pollinator interactions. This especially is true in crop pollination systems. The aim of this study is to identify floral volatiles of several European pear cultivars (Pyrus communis L.), and to determine their potential in eliciting physiological responses in antennae of honey bees (Apis mellifera L.), the most important pollinators of pear. Volatiles were collected by dynamic headspace and analysed by (high resolution) gas chromatography coupled to mass spectrometry (GC/MS) and nuclear magnetic resonance spectroscopy. Antennal responses were investigated by GC coupled to electroantennographic detection (GC/EAD). We trapped in the mean 256 ng of scent per flower and hour (flower−1 h−1) from the different cultivars with either linalool + methyl benzoate or methyl 2-hydroxy-3-methylpentanoate as most abundant compounds. Of the 108 detected pear floral scent components, 17 were electrophysiologically active in honey bee antennae. Among these compounds were (E)-N-(2-methylbutyl)- and (E)-N-(3-methylbutyl)-1-(pyridin-3-yl)methanimine, which were not known from nature before to the best of our knowledge. Most other compounds identified as flower scent in pear are widespread compounds, known from flowers of various other species. Our results provide new insights in the floral volatile chemistry of an important insect-pollinated crop and show for the first time that honey bees have the olfactory ability to detect several pear floral volatiles. These data are an important basis for more detailed studies of the olfactory communication between honey bees and European pear flowers and might in the long term be used to manipulate the attractiveness of pear to obtain optimal fruit set.

Bumblebees distinguish floral scent patterns, and can transfer these to corresponding visual patterns

Flowers act as multisensory billboards to pollinators by using a range of sensory modalities such as visual patterns and scents. Different floral organs release differing compositions and quantities of the volatiles contributing to floral scent, suggesting that scent may be patterned within flowers. Early experiments suggested that pollinators can distinguish between the scents of differing floral regions, but little is known about how these potential scent patterns might influence pollinators. We show that bumblebees can learn different spatial patterns of the same scent, and that they are better at learning to distinguish between flowers when the scent pattern corresponds to a matching visual pattern. Surprisingly, once bees have learnt the spatial arrangement of a scent pattern, they subsequently prefer to visit novel unscented flowers that have an identical arrangement of visual marks, suggesting that multimodal floral signals may exploit the mechanisms by which learnt information is stored by the bee.

Total Bee Dependence on One Flower Species Despite Available Congeners of Similar Floral Shape

Extreme specialization is a common phenomenon in antagonistic biotic interactions but it is quite rare in mutualistic ones. Indeed, bee specialization on a single flower species (monolecty) is a questioned fact. Here, we provide multiple lines of evidence on true monolecty in a solitary bee (Flavipanurgus venustus, Andrenidae), which is consistent across space (18 sites in SW Iberian Peninsula) and time (three years) despite the presence of closely related congeneric plant species whose flowers are morphologically similar. The host flower (Cistus crispus, Cistaceae) is in turn a supergeneralist, visited by at least 85 insect species. We uncover ultraviolet light reflectance as a distinctive visual cue of the host flower, which can be a key mechanism because bee specialization has an innate basis to recognize specific signals. Moreover, we hypothesized that a total dependence on an ephemeral resource (i.e. one flower species) must lead to spatiotemporal matching with it. Accordingly, we prove that the bee’s flight phenology is synchronized with the blooming period of the host flower, and that the densities of bee populations mirror the local densities of the host flower. This case supports the ‘predictable plethora’ hypothesis, that is, that host-specialization in bees is fostered by plant species providing predictably abundant floral resources. Our findings, along with available phylogenetic information on the genus Cistus, suggest the importance of historical processes and cognitive constraints as drivers of specialization in bee-plant interactions.

Host choice in a bivoltine bee: how sensory constraints shape innate foraging behaviors

Background

Many insects have multiple generations per year and cohorts emerging in different seasons may evolve their own phenotypes if they are subjected to different selection regimes. The bivoltine bee Andrena bicolor is reported to be polylectic and oligolectic (on Campanula) in the spring and summer generations, respectively. Neurological constraints are assumed to govern pollen diet in bees. However, evidence comes predominantly from studies with oligolectic bees. We have investigated how sensory constraints influence the innate foraging behavior of A. bicolor and have tested whether bees of different generations evolved behavioral and sensory polyphenism to cope better with the host flowers available in nature when they are active.

Results

Behavioral and sensory polyphenisms were tested in choice assays and electroantennographic analyses, respectively. In the bioassays, we found that females of both generations (1) displayed a similar innate relative reliance on visual and olfactory floral cues irrespective of the host plants tested; (2) did not prefer floral cues of Campanula to those of Taraxacum (or vice versa) and (3) did not display an innate preference for yellow and lilac colors. In the electroantennographic analyses, we found that bees of both generations responded to the same set of compounds.

Conclusion

Overall, we did not detect seasonal polyphenism in any trait examined. The finding that bees of both generations are not sensory constrained to visit a specific host flower, which is in strict contrast to results from studies with oligolectic bees, suggest that also bees of the second generation have a flexibility in innate foraging behavior and that this is an adaptive trait in A. bicolor. We discuss the significance of our findings in context of the natural history of A. bicolor and in the broader context of host-range evolution in bees.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-016-0074-z) contains supplementary material, which is available to authorized users.

(E)-Caryophyllene and α-Humulene: Aedes aegypti Oviposition Deterrents Elucidated by Gas Chromatography-Electrophysiological Assay of Commiphora leptophloeos Leaf Oil

Aedes aegypti is responsible for the transmission of dengue, a disease that infects millions of people each year. Although essential oils are well recognized as sources of compounds with repellent and larvicidal activities against the dengue mosquito, much less is known about their oviposition deterrent effects. Commiphora leptophloeos, a tree native to South America, has important pharmacological properties, but the chemical profile and applicability of its essential oil in controlling the spread of the dengue mosquito have not been investigated. The aim of this study was to determine the composition of C. leptophloeos leaf oil and to evaluate its larvicidal and oviposition deterrent effects against A. aegypti. Fifty-five components of the essential oil were detected by gas chromatography (GC)-mass spectrometry, with α-phellandrene (26.3%), (E)-caryophyllene (18.0%) and β-phellandrene (12.9%) identified as the major constituents. Bioassays showed that the oil exhibited strong oviposition deterrent effects against A. aegypti at concentrations between 25 and 100 ppm, and possessed good larvicidal activity (LC50 = 99.4 ppm). Analysis of the oil by GC coupled with electroantennographic detection established that seven constituents could trigger antennal depolarization in A. aegypti gravid females. Two of these components, namely (E)-caryophyllene and α-humulene, were present in substantial proportions in the oil, and oviposition deterrence assays confirmed that both were significantly active at concentrations equivalent to those present in the oil. It is concluded that these sesquiterpenes are responsible, at least in part, for the deterrent effect of the oil. The oviposition deterrent activity of the leaf oil of C. leptophloeos is one of the most potent reported so far, suggesting that it could represent an interesting alternative to synthetic insecticides. The results of this study highlight the importance of integrating chemical and electrophysiological methods for screening natural compounds for their potential in combating vectors of insect-borne diseases.

Scent emission profiles from Darwin's orchid--Angraecum sesquipedale: Investigation of the aldoxime metabolism using clustering analysis

The display of scent is crucial for plants in attracting pollinating insects to flowers and ensuring successful pollination and reproduction. The large number of aldoxime volatile species present in the scent of the Madagascan orchid Angraecum sesquipedale has been suggested to play a primary role in attracting the sphingid moth Xanthopan morgani praedicta. By solid phase micro-extraction (SPME) coupled with gas chromatography-mass spectrometry (GC-MS), we monitored the scent release from different flowers of a single orchid, day and night throughout the entire flowering period. In separate experiments, the diurnal release was monitored in 3h intervals and the tissue specific release from the different floral parts was tracked. Numerous novel compounds related to the aldoxime metabolism not previously detected in A. sesquipedale were identified and positioned into a proposed pathway for aldoxime metabolism. From the results, we hypothesize that (E/Z)-phenylacetaldoxime and its derivatives could be important attractants for the pollinating moth X. morgani praedicta. By applying an untargeted Partitioning Around Medoids (PAM) cluster analysis to the metabolite profiles in the scent, the proposed pathways for the formation of aldoximes were substantiated. With this study, we demonstrate the powerful utility of a bioinformatics tool to aid in the elucidation of the routes of formation for volatiles and provide a benchmark and guidelines for future detailed observations of hawkmoth pollination of Angraecum species, and in particular A. sesquipedale, in the wild.

Improving our chemistry: challenges and opportunities in the interdisciplinary study of floral volatiles

The field of chemical ecology was established, in large part, through collaborative studies between biologists and chemists with common interests in the mechanisms that mediate chemical communication in ecological and evolutionary contexts. Pollination is one highly diverse and important category of such interactions, and there is growing evidence that floral volatiles play important roles in mediating pollinator behaviour and its consequences for plant reproductive ecology and evolution. Here we outline next-generation questions emerging in the study of plants and pollinators, and discuss the potential for strengthening collaboration between biologists and chemists in answering such questions.

Pollination biology in the dioecious orchid Catasetum uncatum: How does floral scent influence the behaviour of pollinators?

Catasetum is a neotropical orchid genus that comprises about 160 dioecious species with a remarkable sexual dimorphism in floral morphology. Flowers of Catasetum produce perfumes as rewards, which are collected only by male euglossine bees. Currently, floral scents are known to be involved in the selective attraction of specific euglossine species. However, sexual dimorphism in floral scent and its eventual role in the pollination of Catasetum species have never been investigated. Here, we have investigated the pollination of Catasetum uncatum and asked: (1) Is floral scent a sexual dimorphic trait? (2) Does pollinarium removal/deposition affect scent emission? (3) Does sexual dimorphism in floral scent and changed scent emission have implications with regard to the behaviour of the pollinators? The frequency and behaviour of floral visitors were observed in non-manipulated flowers (both flower sexes) and in manipulated flowers (pistillate only) in which pollinaria were deposited. Scents of staminate and pistillate flowers (both manipulated and non-manipulated) were collected by using dynamic headspace methods and analysed chemically. Electrophysiological analyses were performed to detect compounds triggering antennal depolarisation in the euglossine species. C. uncatum is pollinated mainly by males of Euglossa nanomelanotricha. Pollinators were more frequent in pistillate than in staminate inflorescences. Bees approaching staminate flowers frequently flew away without visiting them, a behavioural pattern not observed in pistillate flowers. In the chemical analyses, we recorded 99 compounds, 31 of which triggered antennal depolarisation in pollinators. Multivariate analyses with the electrophysiological-active compounds did not detect differences between the scent composition of staminate and pistillate flowers. Pollinarium removal or deposition resulted in diminished scent emission within 24h in staminate and pistillate flowers, respectively. Surprisingly, bees discriminated pollinated from non-pollinated pistillate flowers as early as 2h after pollination. The rapid loss in the attractiveness of flowers following pollinarium removal/deposition can be interpreted as a strategy to direct pollinators to non-pollinated flowers. We have found no evidence that euglossine males discriminate staminate from pistillate flowers by means of floral scent. Instead, we speculate that bees use visual cues, such as sex dimorphic traits, to discriminate flowers of different sexes. Together, our results provide interesting insights into the evolution of floral signals in gender-dimorphic species and into its significance in plant reproductive biology.

Visual and Olfactory Floral Cues of Campanula (Campanulaceae) and Their Significance for Host Recognition by an Oligolectic Bee Pollinator

Oligolectic bees collect pollen from a few plants within a genus or family to rear their offspring, and are known to rely on visual and olfactory floral cues to recognize host plants. However, studies investigating whether oligolectic bees recognize distinct host plants by using shared floral cues are scarce. In the present study, we investigated in a comparative approach the visual and olfactory floral cues of six Campanula species, of which only Campanula lactiflora has never been reported as a pollen source of the oligolectic bee Ch. rapunculi. We hypothesized that the flowers of Campanula species visited by Ch. rapunculi share visual (i.e. color) and/or olfactory cues (scents) that give them a host-specific signature. To test this hypothesis, floral color and scent were studied by spectrophotometric and chemical analyses, respectively. Additionally, we performed bioassays within a flight cage to test the innate color preference of Ch. rapunculi. Our results show that Campanula flowers reflect the light predominantly in the UV-blue/blue bee-color space and that Ch. rapunculi displays a strong innate preference for these two colors. Furthermore, we recorded spiroacetals in the floral scent of all Campanula species, but Ca. lactiflora. Spiroacetals, rarely found as floral scent constituents but quite common among Campanula species, were recently shown to play a key function for host-flower recognition by Ch. rapunculi. We conclude that Campanula species share some visual and olfactory floral cues, and that neurological adaptations (i.e. vision and olfaction) of Ch. rapunculi innately drive their foraging flights toward host flowers. The significance of our findings for the evolution of pollen diet breadth in bees is discussed.

Ecology and evolution of floral volatile-mediated information transfer in plants

Floral volatiles are complex, multi-functional signals that are often used by pollinators in combination with other signals, such as color. Floral visitors use floral scent to estimate the amount of reward present in flowers, to facilitate the identification of a specific host flower or as signals that chemically resemble those important for pollinator insects in other ecological contexts. There is good evidence that floral scent evolves under selection imposed by both mutualists and antagonists. Antagonists may often limit the amount of scent emitted by flowers, thus contributing to spatial population variation, and select for phenotypic plasticity after enemy attack. Floral scent is also an important component of pollinator-mediated reproductive isolation, as it often co-varies with color and morphology in sister species with different pollination systems.

Bees use honest floral signals as indicators of reward when visiting flowers

Pollinators visit flowers for rewards and should therefore have a preference for floral signals that indicate reward status, so called 'honest signals'. We investigated honest signalling in Brassica rapa L. and its relevance for the attraction of a generalised pollinator, the bumble bee Bombus terrestris (L.). We found a positive association between reward amount (nectar sugar and pollen) and the floral scent compound phenylacetaldehyde. Bumble bees developed a preference for phenylacetaldehyde over other scent compounds after foraging on B. rapa. When foraging on artificial flowers scented with synthetic volatiles, bumble bees developed a preference for those specific compounds that honestly indicated reward status. These results show that the honesty of floral signals can play a key role in their attractiveness to pollinators. In plants, a genetic constraint, resource limitation in reward and signal production, and sanctions against cheaters may contribute to the evolution and maintenance of honest signalling.

An aromatic volatile attracts oligolectic bee pollinators in an interdependent bee-plant relationship

Chemical signals emitted by the plant frequently mediate host-plant localization in specialized animal - plant associations. Studying the interdependent highly specialized association of the narrowly oligolectic bee pollinator Protodiscelis palpalis (Colletidae, Neopasiphaeinae) with Hydrocleys martii (Alismataceae) in ephemeral aquatic water bodies in semi-arid Caatinga of Brazil, we asked if specific volatile compounds produced by the flowers mediate pollinator attraction. The yellow Hydrocleys flowers are the sole pollen and nectar resources, and mating sites for the bees. We analyzed the floral scents of this species and of the closely related H. nymphoides, which is not visited by P. palpalis, and tested the main volatile compounds of both species under field conditions to evaluate their attractiveness to bees of P. palpalis. Methoxylated aromatics were the dominant floral scent components in both species, but each species exhibited a characteristic scent profile. Dual choice bioassays using artificial flowers made of yellow and blue adhesive paper clearly revealed that ρ-methylanisole alone, the dominant volatile of H. martii, attracted significantly more bees than unbaited flowers. This compound represents an olfactory communication channel used by the plant that lures its effective oligolectic pollinators to its flowers. Yellow artificial flowers baited significantly more bees than blue ones. Our study reinforces the recent findings that specific compounds in complex floral scent bouquets are crucial for host-plant location in oligolectic bees.

Discovery of pyrazines as pollinator sex pheromones and orchid semiochemicals: implications for the evolution of sexual deception

Sexually deceptive orchids employ floral volatiles to sexually lure their specific pollinators. How and why this pollination system has evolved independently on multiple continents remains unknown, although preadaptation is considered to have been important. Understanding the chemistry of sexual deception is a crucial first step towards solving this mystery. The combination of gas chromatography-electroantennographic detection (GC-EAD), GC-MS, synthesis and field bioassays allowed us to identify the volatiles involved in the interaction between the orchid Drakaea glyptodon and its sexually attracted male thynnine wasp pollinator, Zaspilothynnus trilobatus. Three alkylpyrazines and one novel hydroxymethyl pyrazine were identified as the sex pheromone of Z. trilobatus and are also used by D. glyptodon for pollinator attraction. Given that our findings revealed a new chemical system for plants, we surveyed widely across representative orchid taxa for the presence of these compounds. With one exception, our chemical survey failed to detect pyrazines in related genera. Collectively, no evidence for preadaptation was found. The chemistry of sexual deception is more diverse than previously known. Our results suggest that evolutionary novelty may have played a key role in the evolution of sexual deception and highlight the value of investigating unusual pollination systems for advancing our understanding of the role of chemistry in evolution.