Floral biology and reproductive isolation by floral scent in three sympatric aroid species in French Guiana

We studied the reproductive biology of three sympatric Araceae species, Anthurium sagittatum, A. thrinax and Spathiphyllum humboldtii in French Guiana. The plants flowered simultaneously and were visited by scent‐collecting male euglossine bees, which were apparently their major pollinators. In total, each species was visited by 3–7 euglossine species, and 2–3 euglossine species accounted for at least 80% of all flower visits, with visits being plant species‐specific. Floral scent consisted of 6–10 main compounds, which made up 76–94% of the total amount of volatiles and were specific in these high amounts to each plant species. We suggest that the different floral scents lead to clear separation of the main pollinating euglossine species, providing a directed and efficient intraspecific pollen flow that results in high reproductive success. Since the simple floral (inflorescence) morphology of the studied plants does not support any morphological mechanisms to exclude visitors, as for example in euglossine‐pollinated perfume orchids, floral scent might be of major importance for the reproductive isolation and sympatric occurrence of these plants.     

The role of nectar production, flower pigments and odour in the pollination of four species of Passiflora (Passifloraceae) in south-eastern Brazil

The pollination biology of four species of passionflower was studied in south-eastern Brazil, specifically the importance of chemical features of floral nectar, pigments and odours. All species required pollinators to produce fruits: P. alata was pollinated by bees, P. speciosa by hummingbirds, and P. galbana and P. mucronata by bats. Pollinators consumed nectar as a food source. The activity of vertebrate pollinators reflected resource availability: they foraged when large amounts of nectar were available and when quantitative resource predictability was greater. The nectar of the vertebrate-pollinated species was richer in cholesterol and phospholipids, and had a potassium-sodium ratio higher than 1.0. For all species, the light absorption of flowers was paralleled by the pollinators' visual spectral sensitivity. This first report on Passiflora floral volatile compounds showed that there was a greater chemical class diversity among the species pollinated by animals with an acute olfactory sense, such as bees and bats. Benzenoid alcohols were the most represented compounds. The fragrances contained compounds that occur in other plant species and in the exocrine secretions of bees. This study shows a strong association between pollinators and the attracting and rewarding features of flowers.     

Specificity and mutual dependency of the orchid-euglossine bee interaction

Seasonal and geographic relationships, and host pollinator specificities are examined for indications of interdependency in the orchid-euglossine bee interaction. The orchids are dependent on the bees for pollination, and their flowering seasonality corresponds well with peak activity of their pollinators. However, there is little evidence that the bees are dependent on these fragrance hosts. The orchids tap the majority of euglossine species and individuals for pollinator services during any given season, but most of those bee species that temporarily lack orchid fragrance hosts persist in the area, continually emerge from nests, and seek floral fragrance compounds. Pollinator specificity occurs in less than half of the orchids, and host specificity is rare. Geographic distributions of nearly all orchid-pollinator pairs are not mutually inclusive. Moreover, nearly a third of the local male euglossine bee species censused are not pollinators of any fragrance orchids in the area. Local alternative fragrance sources occur. The orchid-male euglossine bee interaction does not appear to represent a mutually obligatory relationship. The orchids may have exploited a preexisting behavioural phenomenon of the bees, and reciprocal evolutionary responses may not have occurred.     

Are deception-pollinated species more variable than those offering a reward?

In most pollination systems, animals transfer pollen among plants of a given species. Pollinator visitations do not come without cost, so plants usually offer a reward. However, the flowers of some plant species, mostly orchids, lack rewards and deceive animals into visiting their flowers. Deceptive species are thought to have high levels of variation in traits associated with advertisement and pollinator attraction, which have been attributed to genetic drift, or disruptive selection due to pollinator behavior. Rewarding species are assumed to have less variation due to stabilizing selection. We compared variability in floral morphology and fragrance composition between deceptive and rewarding species. Because both suites of traits are often linked with floral advertisement and pollinator attraction, we expected variation to be greater in species with deceptive pollination systems than in those offering rewards. We obtained floral morphology metrics for 20 deceptive species and 41 rewarding species native or naturalized in Puerto Rico, Venezuela, and Ecuador. Floral fragrances were sampled from eight deceptive species and four rewarding species. We found that the amplitude of variation in floral morphology and fragrance composition covaries significantly. Comparison of coefficients of variation for morphology indicated that, overall, deceptive species show significantly higher variation than rewarding species, and this pattern was also found among just orchids or just nonorchids. There were no statistical differences in morphological variation between orchids and nonorchids within a functional pollination group. Fragrance variation, measured by Jaccard distance, tended to be greater for deceptive species than for rewarding species. Although overlap in measures of variation occurs between the two groups, the data support the hypothesis that populations of deception-pollinated species are more variable than rewarding species in traits associated with pollinator attraction.     

Floral scent compounds of Amazonian Annonaceae species pollinated by small beetles and thrips

Chemical analysis (GC-MS) yielded a total of 58 volatile compounds in the floral scents of six species of Annonaceae distributed in four genera (Xylopia, Anaxagorea, Duguetia, and Rollinia), Xylopia aromatica is pollinated principally by Thysanoptera and secondarily by small beetles (Nitidulidae and Staphylinidae), whereas the five other species were pollinated by Nitidulidae and Staphylinidae only. Although the six Annonaceae species attract a similar array of pollinator groups, the major constituents of their floral scents are of different biochemical origin. The fragrances of flowers of Anaxagorea brevipes and Anaxagorea dolichocarpa were dominated by esters of aliphatic acids (ethyl 2-methylbutanoate, ethyl 3-methylbutanoate), which were not detected in the other species. Monoterpenes (limonene, p-cymene, alpha-pinene) were the main scent compounds of Duguetia asterotricha, and naphthalene prevailed in the scent of Rollinia insignis flowers. The odors of X. aromatica and Xylopia benthamii flowers were dominated by high amounts of benzenoids (methylbenzoate, 2-phenylethyl alcohol).     

Pollinator attraction in a sexually deceptive orchid by means of unconventional chemicals

Ophrys flowers mimic virgin females of their pollinators, and thereby attract males for pollination. Stimulated by scent, the males attempt to copulate with flower labella and thereby ensure pollination. Here, we show for the first time, to our knowledge, that pollinator attraction in sexually deceptive orchids may be based on a few specific chemical compounds. Ophrys speculum flowers produce many volatiles, including trace amounts of (omega-1)-hydroxy and (omega-1)-oxo acids, especially 9-hydroxydecanoic acid. These compounds, which are novel in plants, prove to be the major components of the female sex pheromone in the scoliid wasp Campsoscolia ciliata, and stimulate male copulatory behaviour in this pollinator species. The specificity of the signal depends primarily on the structure and enantiomeric composition of the oxygenated acids, which is the same in wasps and in the orchids. The overall composition of the blend differs significantly between the orchid and its pollinator and is of secondary importance. 9-Hydroxydecanoic acid is a rarely occurring compound that until now has been identified only in honeybees. Contrary to the standard hypothesis that Ophrys flowers produce only 'second-class attractivity compounds' and are neglected once the pollinator females are present, we show that flowers are more attractive to the males than are their own females.     

NEGATIVE FREQUENCY-DEPENDENT SELECTION BY POLLINATORS ON ARTIFICIAL FLOWERS WITHOUT REWARDS

Many species of nonmodel deceptively pollinated orchids are polymorphic for corolla color. These species are pollinated by naive insects searching for nectar, and are not mimics. It has been suggested that the foraging behavior of insect pollinators during the avoidance learning process results in these stable corolla color polymorphisms; for this to occur pollinators must induce negative frequency-dependent selection on corolla color. Therefore the hypothesis that pollinator behavior results in a preference for rare color morphs of deceptive species was tested experimentally. Bumblebees (Bombus terrestris) foraged in the laboratory on arrays of artificial flowers with different corolla color morphs. Morphs were varied in frequency, and bumblebee preferences were recorded on arrays where morphs did and did not contain sucrose solution rewards. Bumblebees preferred the most common color morph when flowers contained sucrose solution rewards, but overvisited rare morphs when sampling flowers that contained no rewards. Bumblebees also tended to move between unlike color morphs when these were unrewarding, suggesting that a probabilistic sampling strategy was adopted. Thus experiments demonstrated that pollinator behavior could result in a selective advantage for rare color morphs of plant species that are pollinated by deception without mimicry, which would induce negative frequency-dependent selection on corolla color. The observed pollinator behavior could allow stable corolla color polymorphisms to be maintained by selection in nonmodel deceptively pollinated species.     

Smells like aphids: orchid flowers mimic aphid alarm pheromones to attract hoverflies for pollination

Most insects are dependent on chemical communication for activities such as mate finding or host location. Several plants, and especially orchids, mimic insect semiochemicals to attract insects for unrewarded pollination. Here, we present a new case of pheromone mimicry found in the terrestrial orchid Epipactis veratrifolia. Flowers are visited and pollinated by several species of aphidophagous hoverflies, the females of which also often lay eggs in the flowers. The oviposition behaviour of these hoverflies is mainly guided by aphid-derived kairomones. We show that the flowers produce α- and β-pinene, β-myrcene and β-phellandrene, and that these compounds attract and induce oviposition behaviour in female hoverflies. This floral odour profile is remarkably similar to the alarm pheromone released by several aphid species, such as Megoura viciae. We therefore suggest that E. veratrifolia mimics aphid alarm pheromones to attract hoverflies for pollination; this is the first time, to our knowledge, that such a case of mimicry has been demonstrated.     

Long-term ecology of euglossine orchid-bees (Apidae: Euglossini) in Panama

Summary Abundance patterns during 6–7 years and orchid visitation were determined for 51 species of the 57 local euglossine bees. Male bees were counted at 3 chemical attractants presented in the same manner each month. Sites were separated by 75 km but included wet Atlantic forest at 500 m elevation, moist forest at 180 m near Barro Colorado Island, and cloud forest at 900 m near the Pacific ocean. 1. From 15 to 30 euglossine species of 4 genera were active in each month and site; monthly species number and general bee abundance were positively correlated. Many species had 3 annual abundance peaks (range 1–4) and were active throughout the year, but peak annual abundances rarely occurred during late wet or early dry seasons. In contrast, Eufriesea generally were present as adults only 1–2 months in a year. 2. Euglossine populations were exceptionally stable. Species at each site were more stable than any known insect population, and stability and abundance were positively associated. However, year-to-year population stability and the degree of seasonality were not correlated. Among the three sites, the more diverse (species rich) bee assemblages displayed lower stability; these were the wetter and upland sites. 3. The most abundant bees visited more orchid species. Eg. and El. each visited and average of 4 orchid species (range 0–13); Ex. and Ef. visited 0–3. Stable populations did not visit more or fewer orchid species than did unstable populations. 4. Less than 68% of species at each site visited orchid flowers; less than a few dozen of the 100–800 bees counted in a day carried orchid pollinaria. Over 20% of the euglossine species never were seen with pollinaria at any site and probably seldom visit orchids in central Panama. 5. Most bee species visited 1 or no fragrance orchids in a given habitat. Orchids tended to utilize common pollinators that seldom included more than 1 species, and they utilized stable or unstable, seasonal or aseasonal bees. However, the most stable and abundant bee, Eg. imperialis, rarely pollinated orchids; fewer than 10 of ca. 20000 bees carried pollinaria. 6. Orchids may interact primarily with discrete seasonal bee population peaks-probably the emerging adults. Although specialized orchid preferences are implicated for species that visit few or no local orchids but pollinate other species and carry pollinaria in other areas, euglossine bees do not need orchids to survive or reproduce.     

Effects of floral display and plant abundance on fruit production of Ryncholaelia glauca (Orchidaceae)

Flowering plant density can increase number of visits and fruit set in multi-flowering plants, however this aspect has not been studied on few flower species. We studied the effects of individual floral display and plant density on the fruit production of the epiphytic, moth-pollinated orchid, Ryncholaelia glauca, in an oak forest of Chavarrillo, Veracruz, Mexico. Species is non-autogamous, and produced one flower per flowering shoot each flowering season. We hypothesized that orchids with more flowering shoots and those on trees with clumps of conspecific should develop more fruits than isolated ones. R. glauca population flowers synchronouly, and individual flowers last up to 18 days, with flowers closing rapidly after pollination. Individuals produced few flowers per year, although some plants developed flowers in both seasons and fewer of them developed fruits both years. There was no relationship between flower number per orchid, or per host tree, with the number of fruits developed per plant. Host trees with flowering and fruiting orchids were randomly dispersed and the pattern of distribution of flowering and fruiting plants was not related. Apparently, pollinators visit the flowers randomly, with no evidence of density dependence. The fruit set of R. glauca was as low as fruit set of multi-flowered orchids moth pollinated, suggesting that fruit set on moth pollinated orchids could be independent of the number of flowers displayed.     

Evolution and diversity of floral scent chemistry in the euglossine bee-pollinated orchid genus Gongora

•Background and Aims Animal-pollinated angiosperms have evolved a variety of signalling mechanisms to attract pollinators. Floral scent is a key component of pollinator attraction, and its chemistry modulates both pollinator behaviour and the formation of plant–pollinator networks. The neotropical orchid genus Gongora exhibits specialized pollinator associations with male orchid bees (Euglossini). Male bees visit orchid flowers to collect volatile chemical compounds that they store in hind-leg pouches to use subsequently during courtship display. Hence, Gongora floral scent compounds simultaneously serve as signalling molecules and pollinator rewards. Furthermore, because floral scent acts as the predominant reproductive isolating barrier among lineages, it has been hypothesized that chemical traits are highly species specific. A comparative analysis of intra- and inter-specific variation of floral scent chemistry was conducted to investigate the evolutionary patterns across the genus.•Methods Gas chromatography–mass spectrometry (GC-MS) was used to analyse the floral scent of 78 individuals belonging to 28 different species of Gongora from two of the three major lineages sampled across the neotropical region. Multidimensional scaling and indicator value analyses were implemented to investigate the patterns of chemical diversity within and among taxonomic groups at various geographic scales. Additionally, pollinator observations were conducted on a sympatric community of Gongora orchids exhibiting distinct floral scent phenotypes.•Key Results A total of 83 floral volatiles, mainly terpenes and aromatic compounds, were detected. Many of the identified compounds are common across diverse angiosperm families (e.g. cineole, eugenol, β-ocimene, β-pinene and terpinen-4-ol), while others are relatively rare outside euglossine bee-pollinated orchid lineages. Additionally, 29 volatiles were identified that are known to attract and elicit collection behaviour in male bees. Floral scent traits were less variable within species than between species, and the analysis revealed exceptional levels of cryptic diversity. Gongora species were divided into 15 fragrance groups based on shared compounds. Fragrance groups indicate that floral scent variation is not predicted by taxonomic rank or biogeographic region.•Conclusions Gongora orchids emit a diverse array of scent molecules that are largely species specific, and closely related taxa exhibit qualitatively and quantitatively divergent chemical profiles. It is shown that within a community, Gongora scent chemotypes are correlated with near non-overlapping bee pollinator assemblies. The results lend support to the hypothesis that floral scent traits regulate the architecture of bee pollinator associations. Thus, Gongora provides unique opportunities to examine the interplay between floral traits and pollinator specialization in plant–pollinator mutualisms.     

POLLINATION OF TWO SYMPATRIC SPECIES OF DALECHAMPIA (EUPHORBIACEAE) IN SURINAME BY MALE EUGLOSSINE BEES

Dalechampia brownsbergensis and D. fragrans co-occur in Suriname, and both are pollinated by fragrance-collecting male euglossine bees. Dalechampia brownsbergensis appears to bloom year-round and is pollinated by relatively few species of bees, including Euglossa tridentata and E. gaianii. In contrast, D. fragrans appears to bloom from late October through early December and is visited and pollinated by at least 13 species of euglossines. Field observations of pollination indicated that the two species did not share pollinators. However, when the flowers of D. fragrans were “transplanted” into a population of D. brownsbergensis, the main pollinator of D. brownsbergensis also visited the flowers of D. fragrans. The pollinators of D. fragrans, however, did not visit the flowers of D. brownsbergensis. Partial sharing of pollinators may have only a small negative impact on the two sympatric plant species at this site because they flower simultaneously only part of the year, and they are often spatially separated from one another.     

Variation in scent emission among floral parts and inflorescence developmental stages in beetle-pollinated Protea species (Proteaceae)

Floral fragrances are an important component for pollinator attraction in beetle-pollinated flowers. Several genera in the Proteaceae contain beetle-pollinated species. However, there is no information on the floral scent chemistry of beetle-pollinated members of the family. In this paper we report on the spatial variation and differences between developmental stages in emission of inflorescence (flowerhead) volatiles of four South African Protea species (P. caffra, P. dracomontana, P. simplex, and P. welwitschii) that are pollinated by cetoniine beetles. The scents from different inflorescence parts (bracts, perianth, styles, and nectar) and from successive anthesis stages of whole inflorescences were sampled using dynamic headspace collection and identified using GC–MS. Although the four species shared many scent compounds, possibly reflecting their close phylogenetic relationships and common pollinators, they showed significant differences in overall scent composition due to various species-specific compounds, such as the unique tiglate esters found in the scent of P. welwitschii. The strongest emissions and largest number of volatiles, especially monoterpenes, were from inflorescences at full pollen dehiscence. Senescing inflorescences of two species and nectars of all species emitted proportionally high amounts of acetoin (3-hydroxy-2-butanone) and aromatic alcohols, typical fermentation products. As a consequence, the scent composition of nectar was much more similar among species than was the scent composition of other parts of the inflorescence. These results illustrate how the blends of compounds that make up the overall floral scent are a dynamic consequence of emissions from various plant parts.     

Phylogeny of the orchid bees (Hymenoptera: Apinae: Euglossini): DNA and morphology yield equivalent patterns

Orchid bees (Euglossini) are spectacular long-tongued Neotropical bees important in the pollination of Neotropical long-corolla flowers, particularly some orchids. Besides remarkably long tongues, males in particular exhibit other flower-related adaptations, including setal brushes on the foretarsi used for rasping the petals of orchids while collecting aromatic compounds. These compounds are stored in large swollen tibiae and are thought to play an important role in courtship behavior. Euglossini are also unusual in lacking sociality; they are the only tribe among the corbiculate bees that are not eusocial, and two of the genera are cleptoparasitic. Each genus exhibits distinct behavioral traits including nest architecture and host-parasite interactions, yet their evolution is unknown. Despite previous phylogenetic studies of on morphological characters, the relationships among the five euglossine genera remain under debate. We investigate euglossine generic relationships using DNA sequence data from four genes and new morphological characters. The morphological and molecular data yield congruent evolutionary patterns, and combining the data gives a fully resolved and well supported phylogeny of Euglossini.     

Fragrances, male display and mating behaviour of Euglossa hemichlora: a flight cage experiment

Abstract. Male euglossine bees (Apidae: Euglossini) collect volatile substances (fragrances) from floral and nonfloral sources and store them in hair‐filled cavities in their hind tibiae. Over time, males accumulate large quantities of complex and species‐specific blends of fragrances. Various hypotheses have been put forward to explain this behaviour, including the idea that fragrance stores reflect the genetic quality of individual males and have evolved through sexual selection and female choice. Clear support of this hypothesis is lacking, largely because male–female interactions are both rare and difficult to observe in nature. Here, we report a flight cage experiment performed in Panama that permitted mating between virgin females (raised from brood cells) and males captured in the forest at fragrance baits. In the cage, eight individually marked males defended small territories around vertical perch sites and showed a characteristic display, which included a previously unreported ‘leg‐crossing’ movement, possibly related to fragrance release. A total of six copulations and three copulatory attempts by Euglossa hemichlora were observed and partly recorded on video. The copulations, all of which were initiated by the female landing on a male perch, were short (4–10 s) and showed no signs of the transfer of chemical substances from male to female. In some cases, the male hovered directly over the female before descending to mount her, possibly facilitating fragrance evaluation by the female. After the experiment, the contents of the males' hind legs were analysed by gas chromatography‐mass spectroscopy, which detected complex mixtures of terpenoids and aromatics (totalling 70 different compounds) dominated by hexahydrofarnesyl acetone, farnesene epoxide, ocimene and p‐dimethoxy benzene. Individual total amounts of fragrances were neither related to display activity or perch occupancy by given males, nor to the frequency of matings achieved. Display activity was the only positive correlate of mating frequency. Generally, individuals had uniformly large amounts of stored fragrances in comparison to a previous study of three other species of Panamanian Euglossa.     

Notes on pollination ecology and floral scent chemistry of the rare neotropical orchid Catasetum galeritum Rchb.f.

The neotropical orchid genus Catasetum embraces about 180 species that produce perfume as reward for pollinators (i.e. male euglossine bees). Among the ca. 1000 perfume‐rewarding plants, Catasetum species are the best studied with respect to their natural history. Nevertheless, the pollination ecology of most species (> 80%) remains unknown. Here, we investigated the pollination ecology and floral scent chemistry of C. galeritum, a rare species endemic to the poorly investigated Brazilian Amazon. Flowers of C. galeritum were visited only by male bees of Eufriesea superba. Its perfume bouquet was composed of six volatiles, with 1,4‐dimethoxybenzene accounting for about 85% of the total scent discharge. Curiously, this compound is a potent attractant of more than 40 euglossine species. The absence of euglossine species other than Ef. superba on flowers of C. galeritum might be, therefore, be mediated by a modifier effect of another compound(s) in its floral scent bouquet.     

Mechanisms that limit pollinator range in Ericaceae

Studies were conducted in 2001-2003 at Valdai National Park (Novgorod region) and at the Zvenigorod biological station of Moscow State University. The morphology of flowers, flowering dynamics and composition of insect visiting flowers of Ericaceae species: Andromeda polifolia, Chamaedaphne calyculata, Ledum palustre, Oxycoccus palustris, Vaccinium myrtillus, V. uliginosum, and V. vitis-idaea L. were studied. Some species of insects visiting flowers were excluded from the list of pollinators on the basis of observation on their behavior. L. palustre was visited mainly by flies where as other investigated species were visited mainly by bumblebees. In some cases bumblebees were the only visitors of the investigated plants. Mechanisms that protect flowers from flies and short-tongued solitary bees visits and ensure a best pollination by bumblebees are various among different species of Ericaceae. Efficiency of nectary protection also differs among different plant species and is defined by particularities of their habitats and flowering phenology. As far as all species of this family during the flowering are dominants in typical habitats, a competition for the pollination with species of other families in most cases is megligible. Flowering periods of V. vitis-idaea and V. myrtillus in forest ecosystems overlapped weakly. Moreover, V. myrtillus is pollinated mainly by bumblebee queens where as pollinators of V. vitis-idaea are bumblebee workers, solitary bees and horse flies. The other investigated plant species inhabit only oligotrophic peat bogs. Thery are pollinated by bumblebees but periods of flowering are not overlapped and consequently follow one after another. L. palustre and V. uliginosum flower simultaneosly but they are pollinated by different pollinators.     

At daybreak--reproductive biology and isolating mechanisms of Cirrhaea dependens (Orchidaceae)

Floral biology, reproduction, pollinator specificity, and fruit set of Cirrhaea dependens were recorded in forest areas of Southeastern Brazil. Cirrhaea dependens is a lithophytic or epiphytic herb occurring very sparsely below dense canopies. Nearly all the flowers of a single plant open simultaneously before dawn. They are short-lasting and offer floral fragrances as rewards, which are collected by male euglossine bees. Observations carried out in mesophytic forests at Serra do Japi revealed that Euglossa VIRIDIS is their principal pollinator, with Eufriesea violacea a sporadic co-pollinator. Visitation started soon after flower opening, and attractiveness remained high for about 2 h, decreasing abruptly at sunrise. Flower anthesis with subsequent fragrance release seems to be correlated with bee attraction. Observations using chemical baits were carried out at Serra do Japi, and in the Atlantic Rain Forest of Picinguaba. Three euglossine species were captured with pollinaria of C. dependens in Picinguaba, whereas only E. violacea was attracted in Serra do Japi. In Picinguaba, C. dependens occurs sympatrically with C. saccata and C. loddigesii. Each species attracted different pollinators. The specificity and resulting reproductive isolation are due to the production of different odours by each orchid species. Cirrhaea dependens is self-compatible but pollinator-dependent. The reproductive success was low and appears to result from a combination of factors discussed here, such as the production of short-lived flowers, presence of floral mechanisms avoiding self-pollination, non-synchronization of flower phases among plants, and populations with few and sparsely distributed individuals.     

Fragrance analyses,an aid to taxonomic relationships of the genusCoryanthes (Orchidaceae)

All members of the investigated genusCoryanthes (subtribeStanhopeinae) are pollinated by male euglossine bees. The different fragrance profiles are the primary reproductive isolating mechanisms, because the flowers are interfertile. The fragrance patterns of 17 species ofCoryanthes were analyzed by gas chromatography as a means to improve the classification of this genus. A first amine (2-N-methylaminobenzaldehyde) was found to be the main fragrance compound of an as yet unclassifiedCoryanthes species.     

A pollinators' eye view of a shelter mimicry system

Background and Aims

‘Human-red’ flowers are traditionally considered to be rather unpopular with bees, yet some allogamous species in the section Oncocyclus (genus Iris, Iridaceae) have evolved specialized interactions with their pollinators, a narrow taxonomic range of male solitary bees. The dark-red, tubular flowers of these irises are nectarless but provide protective shelters (i.e. a non-nutritive form of reward) primarily to male solitary bees (Apidae, Eucerini) that pollinate the flowers while looking for a shelter. An earlier study on orchids suggested that species pollinated predominantly by male solitary bees produce significantly larger amounts and larger numbers of different n-alkenes (unsaturated cuticular hydrocarbons). Whether or not this also applies to the Oncocyclus irises and whether pollinators are attracted by specific colours or scents of these flowers is unknown.

Methods

Using Iris atropurpurea, recording of pollinator preferences for shelters with different spatial parameters was combined with analyses of floral colours (by spectrophotometry) and scents (by gas chromatography–mass spectrometry) to test the hypotheses that (a) pollinators significantly prefer floral tunnels facing the rising sun (floral heat-reward hypothesis), and that (b) flowers pollinated predominantly by male solitary bees produce significantly larger amounts and larger numbers of unsaturated cuticular hydrocarbons (n-alkenes) in their floral scent (preadaptation to sexual-deception hypothesis).

Key Results

Male bees do not significantly prefer shelters facing the rising sun or with the presence of high absolute/relative amounts and numbers of n-alkenes in the floral scent.

Conclusions

The results suggest that the flowers of I. atropurpurea probably evolved by pollinator-mediated selection acting primarily on floral colours to mimic large achromatic (‘bee-black’) protective shelters used preferentially by male solitary bees, and that pollinator visits are presumably not the result of an odour-based sexual stimulation or motivated by an increased morning floral heat reward in tunnels facing the rising sun.