Colour evolution within orchids depends on whether the pollinator is a bee or a fly

• Orchids are a classic angiosperm model for understanding biotic pollination. We studied orchid species within two species‐rich herbaceous communities that are known to have either hymenopteran or dipteran insects as the dominant pollinators, in order to understand how flower colour relates to pollinator visual systems.
• We analysed features of the floral reflectance spectra that are significant to pollinator visual systems and used models of dipteran and hymenopteran colour vision to characterise the chromatic signals used by fly‐pollinated and bee‐pollinated orchid species.
• In contrast to bee‐pollinated flowers, fly‐pollinated flowers had distinctive points of rapid reflectance change at long wavelengths and a complete absence of such spectral features at short wavelengths. Fly‐pollinated flowers also had significantly more restricted loci than bee‐pollinated flowers in colour space models of fly and bee vision alike.
• Globally, bee‐pollinated flowers are known to have distinctive, consistent colour signals. Our findings of different signals for fly pollination is consistent with pollinator‐mediated selection on orchid species that results from the distinctive features of fly visual systems.

     

Specialized pollination by carpenter bees in Calanthe striata (Orchidaceae), with a review of carpenter bee pollination in orchids

Calanthe striata has nectarless flowers that are self‐compatible, but pollinator dependent. Field observations showed that the flowers were pollinated exclusively by the carpenter bee Xylocopa appendiculata circumvolans, although the bees occasionally wasted pollen by delivering to the stigmatic surface pollinaria that retained their anther caps. Fruit set ratios at the population level varied spatiotemporally, but were generally low (8.3–17.3%). Calanthe striata blooms in spring when post‐overwintering carpenter bees have not yet started foraging for brood production. It can therefore exploit an abundance of opportunistic/naïve foragers. This timing may also increase the possibility of pollinator visits, because no rewarding co‐flowering plants are available in the orchid habitats. A literature review of Orchidaceae pollinated by carpenter bees revealed that at least 14 species of Orchidoideae and Epidendroideae have evolved flowers specialized for carpenter bee pollination. They typically have shallow pink/magenta flowers with a foothold for pollinators; pollinaria are attached to the head, ventral thorax or base of the middle legs of carpenter bees when they insert their heads and/or proboscises into flowers; pollination success is generally low, a probable consequence of the deceptive pollination systems. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013 , 171 , 730–743.     

Pollination biology of the endangered orchid Cypripedium japonicum in a fragmented forest of Japan

Pollination biology studies of the endangered orchid Cypripedium japonicum were conducted in its natural habitat using pollinator observation and hand‐pollination experiments. The observed fruit set was as follows: artificial outcross‐pollinated, 100%; artificial self‐pollinated, 100%; pollinator‐excluded, 0%; and emasculated flowers, 0%. These results show that this species, although self‐compatible, is neither autogamous nor agamospermous. The fruit set for open‐pollinated flowers was 14.9%, which suggests that the study population was subject to pollinator limitation. The nectarless flowers of C. japonicum were exclusively visited and pollinated by the queens of two bumblebee species (Bombus ardens and B. diversus diversus). It is probable that the nectarless flowers of C. japonicum attract pollinators through a generalized food deceptive system.     

Functional Significance of Labellum Pattern Variation in a Sexually Deceptive Orchid (Ophrys heldreichii): Evidence of Individual Signature Learning Effects

Mimicking female insects to attract male pollinators is an important strategy in sexually deceptive orchids of the genus Ophrys, and some species possess flowers with conspicuous labellum patterns. The function of the variation of the patterns remains unresolved, with suggestions that these enhance pollinator communication. We investigated the possible function of the labellum pattern in Ophrys heldreichii, an orchid species in which the conspicuous and complex labellum pattern contrasts with a dark background. The orchid is pollinated exclusively by males of the solitary bee, Eucera berlandi. Comparisons of labellum patterns revealed that patterns within inflorescences are more similar than those of other conspecific plants. Field observations showed that the males approach at a great speed and directly land on flowers, but after an unsuccessful copulation attempt, bees hover close and visually scan the labellum pattern for up to a minute. Learning experiments conducted with honeybees as an accessible model of bee vision demonstrated that labellum patterns of different plants can be reliably learnt; in contrast, patterns of flowers from the same inflorescence could not be discriminated. These results support the hypothesis that variable labellum patterns in O. heldreichii are involved in flower-pollinator communication which would likely help these plants to avoid geitonogamy.     

What We Think We Know vs. What We Need to Know About Orchid Pollination and Conservation: <Emphasis Type="Italic">Cypripedium</Emphasis> L. as a Model Lineage

While Darwin (1862, 1877) showed that reproductive success in orchid populations depended on adaptive floral morphology coupled with pollinator visitation a more recent review of the literature (Tremblay et al., 2005) confirmed that many out-breeding species are pollinator-limited because most orchid species showing low fecundity also lack rewards. The absence of rewards depresses both pollinator fidelity and the frequency of pollinator visits to an orchid population even though orchid flowers that lack rewards retain the same interlocking floral structures for precise pollinia removal and deposition found in related species that offer rewards. Using the genus, Cypripedium, as a model lineage of non-rewarding flowers this study also shows that the correlation between low fruit set in a Cypripedium sp. and its specific pollinator(s) is insufficient to predict specific frequencies of low fecundity. Annual rates of fruit set often vary broadly between populations of the same species and within the same population over several seasons. We speculate that fruit-set rates also decline when orchid demography and additional biotic and abiotic factors interrupt rates of pollinator activity (pre-zygotic) and fertilization/fruit maturation (post-zygotic). We suggest that that traditional field studies on pollination ecology and breeding systems be combined with data sets recording genetic variation and orchid flower demography in relation to seasonal variation in climate. We also propose that the same information be collected in regard to genetic variation, demography and phenology of populations of known orchid pollinators and co-blooming angiosperm species native to orchid habitats.     

Reproductive biology of Cyrtopodium punctatum in situ: implications for conservation of an endangered Florida orchid

Cyrtopodium punctatum (Linnaeus) Lindley is an endangered epiphytic orchid restricted in the USA to southern Florida. This species has been extensively collected from the wild since the early 1900s, and today only a few plants remain in protected areas. As part of a conservation plan, a reproductive biology study was conducted to better understand the ecology of this species in Florida. Cyrtopodium punctatum relies on a deceit pollination system using aromatic compounds to attract pollinators. Nine aromatic compounds were identified as components of the fragrance of C. punctatum inflorescences, including two compounds that are known to be Euglossine bee attractants. However, this group of bees is not native to Florida. Of the four bee species observed to visit C. punctatum flowers in the present study, carpenter bees ( Xylocopa spp.) are likely to be the main pollinators. Pollination experiments demonstrated that C. puntatum is self-compatible, but requires a pollinator and thus does not exhibit spontaneous autogamy. In addition, the rates of fruit set were significantly higher for flowers that were outcrossed (xenogamy) than for those that were self-crossed. Thus, the species has evolved a degree of incompatibility. Examples of natural pollination and fruit set were observed during the present study (2007–2008), but the rates of reproduction were modest as a consequence of the low plant numbers and possible changes in insect densities as a result of anthropogenic influences.     

Floral and pollination biology of the critically endangered insular orchid Calanthe amamiana: Implications for in situ conservation

Calanthe amamiana is a terrestrial orchid that thrives on the shady forest floors of broad-leaved forests on Amami Island, central Ryukyus, Japan. Here, I report on the floral and pollination biology of this critically endangered orchid based on a 7-year study conducted in the species' natural habitats. As the nectarless flowers are probably self-compatible but are unable to autonomously self-pollinate, they need to attract pollen vectors by deceit for pollination. To advertise themselves, the flowering inflorescences, which comprise a few to dozens of flowers, use whitish floral colors that are comparatively conspicuous against the shaded background. The exclusive pollinator was the long-horned bee Eucera okinawae, which nests in soils of the forest floor, although bee abundance varied considerably among study years. Natural fruit-set ratios were generally low but showed interannual variation. The annual fruit-set ratios were positively correlated with annual bee abundance, thereby suggesting that under natural conditions, the orchid is pollinator-limited. Given that C. amamiana is dependent exclusively on E. okinawae for pollination and is also probably pollinator-limited, conservation plans for this orchid should take into consideration the welfare of long-horned bees. In particular, as nectariferous flowers were unavailable within broad-leaved forests, to satisfy the foraging requirements of long-horned bees, conservation managers should be cognizant of conservation values of flowering plants distributed in sun-lit sites near orchid habitats, and manage those in an appropriate manner.     

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.     

Solitary invasive orchid bee outperforms co-occurring native bees to promote fruit set of an invasive Solanum

Our understanding of the effects of introduced invasive pollinators on plants has been exclusively drawn from studies on introduced social bees. One might expect, however, that the impacts of introduced solitary bees, with much lower population densities and fewer foragers, would be small. Yet little is known about the potential effects of naturalized solitary bees on the environment. We took advantage of the recent naturalization of an orchid bee, Euglossa viridissima, in southern Florida to study the effects of this solitary bee on reproduction of Solanum torvum, an invasive shrub. Flowers of S. torvum require specialized buzz pollination. Through timed floral visitor watches and two pollination treatments (control and pollen supplementation) at three forest edge and three open area sites, we found that the fruit set of S. torvum was pollen limited at the open sites where the native bees dominate, but was not pollen limited at the forest sites where the invasive orchid bees dominate. The orchid bee’s pollination efficiency was nearly double that of the native halictid bees, and was also slightly higher than that of the native carpenter bee. Experiments using small and large mesh cages (to deny or allow E. viridissima access, respectively) at one forest site indicated that when the orchid bee was excluded, the flowers set one-quarter as many fruit as when the bee was allowed access. The orchid bee was the most important pollinator of the weed at the forest sites, which could pose additional challenges to the management of this weed in the fragmented, endangered tropical hardwood forests in the region. This specialized invasive mutualism may promote populations of both the orchid bee and this noxious weed. Invasive solitary bees, particularly species that are specialized pollinators, appear to have more importance than has previously been recognized. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

STUDIES ON THE POLLINATION ECOLOGY OF TIPULARIA DISCOLOR (ORCHIDACEAE)

Tipularia discolor, a woodland orchid, flowers in mid-summer when reproductive activity is minimal within the herb synusia. Tipularia is insect-pollinated, and artificial crosses showed that seeds are produced after self-pollination, intra-inflorescence pollination, and outcrossing. The single nocturnal pollinator, Pseudaletia unipuncta (Noctuidae), located Tipularia populations within a day or two of anthesis. Pollinators were shown to be capable of utilizing portions of the inflorescences that contained the most nectar. After total nectar resources declined, pollinators were no longer active on the inflorescences, even though flowers and nectar were still available. The mode of pollinator activity seems to be closely related to floral morphology, although the moths are able, early in the flowering phenophase, to successfully obtain nectar without effecting any change in the reproductive status of flowers.     

Decadal trends in the pollinator assemblage of Eucryphia cordifolia in Chilean rainforests

Long-term studies of plant–pollinator interactions are almost nonexistent in the scientific literature. The objective of the present study was to determine changes and trends in the pollinator assemblage of ulmo (Eucryphia cordifolia; Cunoniaceae), a canopy-emergent tree found in Chilean temperate rainforests. We assessed the temporal variability of the pollinator assemblage and identified possible modulators of the observed temporal shifts. We sampled insect visitors to the flowers of 16 individual trees of E. cordifolia during 10 consecutive flowering seasons (2000–2009), recording a total of 137 pollinator species with a mean number of species per year of 44. Only three pollinator species (2.2 %) were recorded every year. Two bee species accounted for 50 % of all insect visits to flowers. One bee species, Bombus dahlbomii (native), was dominant in one season, whereas Apis mellifera (exotic) dominated during the next season. These interannual shifts in population abundances presented first-order dynamics that were characterized by oscillations with a period of 2 years. Changes in the abundances of the dominant pollinators, as well as differences in temperature and precipitation during insect emergence and flowering, led to a nested temporal structure of pollinator composition. Furthermore, the abundances of less common pollinators were sensitive to the abundance of the dominant bee species and to monthly maximum temperatures and the average precipitation during spring and summer. Based on our results and those from other studies, we predict a decline in the numbers of Bombus dahlbomii and nondominant native pollinators in response to new exotic arrivals.     

Pollination and floral ecology of Arundina graminifolia (Orchidaceae) at the northern border of the species’ natural distribution

Arundina graminifolia is an early successional plant on Iriomote Island, the Ryukyus, Japan, where it is endangered. Populations flower for more than half a year, and many inflorescences bloom for one to several months. The nectarless gullet flowers, which open for up to six days, are self-compatible but cannot self-pollinate spontaneously; thus they rely on pollinating agents for capsule production. Field observations at two habitats identified at least six species of bees and wasps, primarily mate-seeking males of Megachile yaeyamaensis and Thyreus takaonis, as legitimate pollinators. Thus, this orchid is a pollinator generalist, probably owing to its long blooming period and simple flower morphology. Carpenter bees, which were previously reported to pollinate this orchid, frequently visited flowers but were too large to crawl into the labellum chamber and never pollinated the flowers. Extrafloral nectaries on inflorescences attracted approximately 40 insect taxa but were not involved with pollination. Fruit-set ratios at the population level varied spatiotemporally but were generally low (5.2–12.4 %), presumably owing to infrequent flower visits by mate-seeking pollinators and the lack of food rewards to pollinators.     

Environmental determinants of genetic diversity in Caragana microphylla (Fabaceae) in northern China

Non‐rewarding orchids rely on various ruses to attract their pollinators. One of the most common is for them to resemble flowers sought by insects as food sources. This can range from generalized food deception to the mimicry of specific sympatric food plants. We investigated the basis of pollinator deception in the European food‐deceptive orchid Traunsteinera globosa, which has unusually compact flowerheads resembling those of sympatric rewarding species of Knautia and Scabiosa (Dipsacaceae), and Valeriana (Caprifoliaceae). Visual signals of T. globosa are similar in both fly and bee vision models to those of the sympatric food plants used in the choice experiments, but scent signals are divergent. Field experiments conducted in Austria and the Czech Republic showed that both naive and experienced (with respect to visitation of T. globosa) insect species approached the orchids at the same rate as food plants, but direct contact with orchid flowers was taxon specific. Flies were most easily duped into probing the orchid, and, in doing so, frequently received and deposited pollinaria, whereas most bees and butterflies avoided landing on orchid flowers. We conclude that T. globosa is a mimic of a guild of fly‐pollinated plants, but the ecological dependence of the orchid on its models remains to be fully tested. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2016, 180 , 269–294.     

Pollination biology of Eulophia alta (Orchidaceae) in Amazonia: effects of pollinator composition on reproductive success in different populations

Background and Aims

Spatial variation in pollinator composition and abundance is a well-recognized phenomenon. However, a weakness of many studies claiming specificity of plant–pollinator interactions is that they are often restricted to a single locality. The aim of the present study was to investigate pollinator effectiveness of the different flower visitors to the terrestrial orchid Eulophia alta at three different localities and to analyse whether differences in pollinator abundance and composition effect this plant''s reproductive success.

Methods

Natural pollination was observed in vivo, and manipulative experiments were used to study the pollination biology and breeding system of E. alta at three sites near Manaus, Brazil. To gain a better understanding of the underlying mechanisms of pollinator attraction, nectar composition and secretion patterns were also studied, floral scent composition was analysed and a bioassay was conducted.

Key Results

Flower visitors, pollinator composition, pollinia transfer efficiency of particular pollinator species and natural fruit set differed among the investigated populations of E. alta. Flowers were self-compatible, partially autogamous and effectively pollinated by five bee species (four Centris species and Xylocopa muscaria). Visiting insects appeared to imbibe small amounts of hexose-rich nectar. Nectar sugar content was highest on the third day after flower opening. Floral fragrance analyses revealed 42 compounds, of which monoterpenes and benzenoids predominated. A bioassay using floral parts revealed that only floral tissue from the labellum chamber and labellum tip was attractive to flower visitors.

Conclusions

The data suggest that observed differences in reproductive success in the three populations cannot be explained by absolute abundance of pollinators alone. Due to behavioural patterns such as disturbance of effective pollinators on flowers by male Centris varia bees defending territory, pollinia transfer efficiencies of particular pollinator species also vary between study sites and result in differing reproductive success.     

Local extinction of a rare plant pollinator in Southern Utah (USA) associated with invasion by Africanized honey bees

Twenty-five years ago, Arctomecon humilis, a pollinator-dependent, endangered poppy globally restricted to the extreme northeastern Mojave Desert in southwestern Utah, was pollinated by native bee species and the European honey bee. Follow-up studies beginning in 2012 failed to find the two most important native bee pollinator species, one of which, Perdita meconis, is a strict poppy specialist. We had four objectives: (1) confirm the status of formerly important native bee pollinators; (2) determine the role of the Africanized honey bee which reportedly invaded southern Utah in 2008; (3) examine the effect of the ostensible change in pollinator fauna on fruit set in four populations; (4) describe the pollination proficiency of species that presently visit poppy flowers. For the fourth consecutive survey, P. meconis was absent; its local extinction in Utah now seems certain. Another previously important native pollinator, Eucera quadricincta, was very rare. Also uncommon was the European honey bee, having been largely replaced by Africanized honey bees which have become, in most populations, the prevalent pollinator. Africanized bees forage early in the day and quickly strip flowers of their copious pollen leaving little for native bees. We argue that the invasion of southern Utah by Africanized bees is the most likely cause of the severe disruption of the A. humilis pollination system. The ascension of the Africanized bee is also associated with reduced fruit set in all poppy populations, especially those where plants are sparse. Arctomecon humilis now appears to depend mostly on an invasive species for pollination.     

Reproductive isolation and pollination success of rewarding Galearis diantha and non-rewarding Ponerorchis chusua (Orchidaceae)

Background and Aims

Increasing evidence challenges the conventional perception that orchids are the most distinct example of floral diversification due to floral or prezygotic isolation. Regarding the relationship between co-flowering plants, rewarding and non-rewarding orchids in particular, few studies have investigated whether non-rewarding plants affect the pollination success of rewarding plants. Here, floral isolation and mutual effects between the rewarding orchid Galearis diantha and the non-rewarding orchid Ponerorchis chusua were investigated.

Methods

Flowering phenological traits were monitored by noting the opening and wilting dates of the chosen individual plants. The pollinator pool and pollinator behaviour were assessed from field observations. Key morphological traits of the flowers and pollinators were measured directly in the field. Pollinator limitation and interspecific compatibility were evaluated by hand-pollination experiments. Fruit set was surveyed in monospecific and heterospecific plots.

Key Results

The species had overlapping peak flowering periods. Pollinators of both species displayed a certain degree of constancy in visiting each species, but they also visited other flowers before landing on the focal orchids. A substantial difference in spur size between the species resulted in the deposition of pollen on different regions of the body of the shared pollinator. Hand-pollination experiments revealed that fruit set was strongly pollinator-limited in both species. No significant difference in fruit set was found between monospecific plots and heterospecific plots.

Conclusions

A combination of mechanical isolation and incomplete ethological isolation eliminates the possibility of pollen transfer between the species. These results do not support either the facilitation or competition hypothesis regarding the effect of nearby rewarding flowers on non-rewarding plants. The absence of a significant effect of non-rewarding P. chusua on rewarding G. diantha can be ascribed to low levels of overlap between the pollinator pools of two species.     

Orchid pollination by sexual deception: pollinator perspectives

The extraordinary taxonomic and morphological diversity of orchids is accompanied by a remarkable range of pollinators and pollination systems. Sexually deceptive orchids are adapted to attract specific male insects that are fooled into attempting to mate with orchid flowers and inadvertently acting as pollinators. This review summarises current knowledge, explores new hypotheses in the literature, and introduces some new approaches to understanding sexual deception from the perspective of the duped pollinator. Four main topics are addressed: (1) global patterns in sexual deception, (2) pollinator identities, mating systems and behaviours, (3) pollinator perception of orchid deceptive signals, and (4) the evolutionary implications of pollinator responses to orchid deception, including potential costs imposed on pollinators by orchids. A global list of known and putative sexually deceptive orchids and their pollinators is provided and methods for incorporating pollinator perspectives into sexual deception research are provided and reviewed. At present, almost all known sexually deceptive orchid taxa are from Australia or Europe. A few sexually deceptive species and genera are reported for New Zealand and South Africa. In Central and Southern America, Asia, and the Pacific many more species are likely to be identified in the future. Despite the great diversity of sexually deceptive orchid genera in Australia, pollination rates reported in the literature are similar between Australian and European species. The typical pollinator of a sexually deceptive orchid is a male insect of a species that is polygynous, monandrous, haplodiploid, and solitary rather than social. Insect behaviours involved in the pollination of sexually deceptive orchids include pre‐copulatory gripping of flowers, brief entrapment, mating, and very rarely, ejaculation. Pollinator behaviour varies within and among pollinator species. Deception involving orchid mimicry of insect scent signals is becoming well understood for some species, but visual and tactile signals such as colour, shape, and texture remain neglected. Experimental manipulations that test for function, multi‐signal interactions, and pollinator perception of these signals are required. Furthermore, other forms of deception such as exploitation of pollinator sensory biases or mating preferences merit more comprehensive investigation. Application of molecular techniques adapted from model plants and animals is likely to deliver new insights into orchid signalling, and pollinator perception and behaviour. There is little current evidence that sexual deception drives any species‐level selection on pollinators. Pollinators do learn to avoid deceptive orchids and their locations, but this is not necessarily a response specific to orchids. Even in systems where evidence suggests that orchids do interfere with pollinator mating opportunities, considerable further research is required to determine whether this is sufficient to impose selection on pollinators or generate antagonistic coevolution or an arms race between orchids and their pollinators. Botanists, taxonomists and chemical ecologists have made remarkable progress in the study of deceptive orchid pollination. Further complementary investigations from entomology and behavioural ecology perspectives should prove fascinating and engender a more complete understanding of the evolution and maintenance of such enigmatic plant‐animal interactions.     

Reproductive biology of an alpine orchid Phaius delavayi

The genus Phaius is distributed mainly in pantropical areas, and most species reported are autogamous. In contrast, Phaius delavayi, an alpine plant endemic to temperate zones of China, is strictly pollinator dependent. Only female workers and males of the bumblebee species B. hypnorum and B. lepidus are effective pollinators. The pollinaria are deposited on top of the head or the prothorax of the pollinators. The different depositions of pollinaria are regarded as an adaptation of the chamber dimensions of the flower to the size of the pollinator. The natural fruit set reported here (24.6–37.1%) suggests that pollination is successful in the deceptive P. delavayi. The yellow perianth and sweet floral scent of P. delavayi are important cues in attracting pollinators, while flowering time and mass population are associated with the process of “learning and avoidance” of the rewardless flowers. This orchid, therefore, employs a deceptive pollination strategy with diverse pollinators and pollinaria depositions rather than autogamy as do its congeners.     

Does floral trait variability enhance reproductive success in deceptive orchids?

Unusually high intra-specific floral trait variability has often been described within deceptive orchid populations, as opposed to rewarding ones. Such variability is traditionally thought to have consequences on reproduction in this orchid group, i.e. phenotypically variable deceptive species may have a reproductive success advantage compared to those with a constant floral display. The proposed reason for this hypothetic pattern is that floral trait variability decreases pollinator avoidance learning in dealing with nectarless flowers, hence increasing their visitation rate. However, despite an intuitive and appealing hypothesis and a possible mechanism to explain it, the often-cited higher reproductive success induced by floral trait variability still remains unsupported.Here, we review the literature and consider eight studies that have experimentally or correlatively tested this hypothesis in deceptive orchids. In all these experiments, we have found no difference in average reproductive success between populations with high versus low flower trait variability, either in scent variable or colour polymorphic species. We discuss possible explanations for the lack of this pattern including the incapability of pollinators in perceiving the variability, the scarce relevance of polymorphic traits in the choice of species to forage on, or a different pollinator behaviour than the one proposed. We suggest that the high phenotypic variability is not likely to enhance deceptive orchids’ reproductive success, but is more likely to be a consequence of relaxed selection by pollinators. Nonetheless, information regarding orchid pollination strategy or pollinator cognitive abilities is often superficial, hence calling researchers for additional investigations that can contribute to a better understanding of this debated and yet unsupported hypothesis.     

Change of floral orientation affects pollinator diversity and their relative importance in an alpine plant with generalized pollination system,Geranium refractum (Geraniaceae)

Floral orientation may affect pollinator attraction and pollination effectiveness, and its influences may differ among pollinator species. We, therefore, hypothesized that, for plant species with a generalized pollination system, changes in floral orientation would affect the composition of pollinators and their relative contribution to pollination. Geranium refractum, an alpine plant with downward floral orientation was used in this study. We created upward-facing flowers by altering the flower angle. We compared the pollinator diversity, pollination effectiveness, and pollinator importance, as well as female reproductive success between flowers with downward- and upward-facing orientation. Results indicated that the upward-facing flowers were visited by a wider spectrum of pollinators (classified into functional groups), with higher pollinator diversity than natural flowers. Moreover, due to influences on visitation number and pollen removal, the pollinator importance exhibited by the main pollinator groups differed between flower types. Compared with natural flowers, the pollination contribution of principal pollinators (i.e., bumblebees) decreased in upward-facing flowers and other infrequent pollinators, such as solitary bees and muscoid flies, removed more pollen. Consequently, stigmatic pollen loads were lower in upward- than in downward-facing flowers. These findings reveal that floral orientation may affect the level of generalization of a pollination system and the relative importance of diverse pollinators. In this species, the natural downward-facing floral orientation may increase pollen transfer by effective pollinators and reduce interference by inferior pollinators.