Pollination by deceit in Paphiopedilum barbigerum (Orchidaceae): a staminode exploits the innate colour preferences of hoverflies (Syrphidae)

The lady's 'slippery' orchid: functions of the floral trap and aphid mimicry in a hoverfly-pollinated Phragmipedium species in Brazil

BACKGROUND AND AIMS

Trap flowers are fascinating cases of adaptation, often linked to oviposition-site mimicry systems. Some trap flowers do not imprison pollinators for a pre-determined period, but rather force them to move through a specific path, manipulating their movements in a way that culminates in pollen transfer, often as they leave through a secondary opening.

METHODS

We investigated the previously unknown pollination system of the lady's slipper orchid Phragmipedium vittatum and assessed the function of micro-morphological traits of its trap flowers.

KEY RESULTS

Our observations revealed that P. vittatum is pollinated by females of two hoverfly species (Syrphidae). Eggs laid by flies on or near raised black spots on the flowers indicate that the orchid mimics aphids which serve as food for their aphidophagous larvae. Dark, elevated aphid-like spots appear to attract the attention of hoverflies to a slipping zone. This region has downward projecting papillate cells and mucilage secretion that promote slipperiness, causing potential pollinators to fall into the labellum. They then follow a specific upward route towards inner aphid-like spots by holding onto upward oriented hairs that aid their grip. As hoverflies are funnelled by the lateral constriction of the labellum, they pass the stigma, depositing pollen they may be carrying. Later, they squeeze under one of the articulated anthers which places pollen smears onto their upper thorax. Then, they depart through one of the narrow lateral holes by holding onto hairs projecting from the petals.

CONCLUSIONS

This study confirms the system of aphid mimicry in Phragmipedium and highlights the sophisticated micro-morphological traits used by trap flowers in pollinator attraction, trapping, guidance and release, thus promoting precise pollen transfer.

Isolation and identification of beneficial orchid mycorrhizal fungi in Paphiopedilum barbigerum (Orchidaceae)

Seed germination and seedling development in nearly all orchid species rely on a symbiotic relationship with mycorrhizal fungi; however, this is not the case with all mycorrhizal fungi. This study aims to provide an understanding about the important role of mycorrhiza in seed germination and growth of Paphiopedilum barbigerum. Therefore, we isolated and identified endophytic fungi from the roots of wild P. barbigerum. The beneficial mycorrhizal fungi Epulorhiza sp. FQXY019 and Tulasnella calospora FQXY017 were screened by seed symbiotic germination tests and found to promote seed germination. However, only the seeds inoculated with FQXY019 progressed from the seed germination to rooting stage. This shows that mycorrhizal fungi and P. barbigerum have a specific relation at different growth phases. In addition, we selected FQXY019 and inoculated it into MS medium, B5 medium, OMA medium, and PDA medium. The results showed that FQXY019 co-cultured on PDA significantly promoted the increase in seedling fresh weight, leaf length, and root length (p < .01). Furthermore, it significantly promoted the root number and leaf number of seedlings compared with those co-cultured on MS, B5, and OMA media and control (p < .05). Thus, this study demonstrated the promoting effect of Epulorhiza sp. FQXY019 on seed germination and seedling development, making it an alternative method for the artificial propagation of P. barbigerum.

A Decrease in the Staminode-Mediated Visitor Screening Mechanism in Response to Nectar Robbers Positively Affects Reproduction in Delphinium caeruleum Jacq. ex Camb. (Ranunculaceae)

Simple Summary

Nectar robbers frequently have direct or indirect negative effects on plant reproductive success. However, nectar robbers can also indirectly contribute to the reproductive success of plants in some cases. The negative effects of nectar robbing on plant reproductive success have been widely reported, but the reasons for possible positive effects demand further investigation. Hence, our study was designed to assess the effects of nectar robbers on the reproductive success of Delphinium caeruleum. This will facilitate an understanding of the mutualism between plants and their visitors.

Abstract

Nectar-robbing insects, which are frequently described as cheaters in plant–pollinator mutualisms, may affect plant reproductive fitness by obtaining nectar rewards without providing pollination services. The negative effects of nectar robbing on plant reproductive success have been widely reported, but the reasons for possible positive effects demand further investigation. The goal of the study was to evaluate the effects of nectar robbing on the reproductive success of Delphinium caeruleum. Two staminodes cover the stamens and pistils in the flowers of D. caeruleum, forming a “double door” type of structure that compels pollinators to physically manipulate the staminodes to access the sex organs. In order to explore whether the operative strength required to open the staminodes is affected by actions associated with nectar robbing, we set up five different treatment groups: no nectar robbing, natural nectar robbing, artificial nectar robbing, hole making, and nectar removal. A biological tension sensor was used to measure the operative strength required to open the staminodes in the flowers. We also assessed the effect of nectar robbing on the flower-visiting behavior of pollinators and the effect of nectar robbing on reproductive fitness by the flower. The results showed that the operative strength needed to open staminodes was reduced by nectar robbers but not by artificial nectar robbing, hole making, or nectar removal. The flowers’ continuous visitation rate and visitation frequency by pollinators decreased significantly in robbed flowers. Both the pollen export and pollen deposition in naturally robbed flowers were significantly higher than those in nonrobbed flowers. Our results demonstrate that nectar robbers play an indirect positive role in the reproductive fitness of D. caeruleum flowers by reducing the operative strength of staminodes to promote pollen transfer. The reduction in operative strength of staminodes might be an adaptive mechanism that responds to nectar robbing.

Cypripedium lichiangense (Orchidaceae) mimics a humus-rich oviposition site to attract its female pollinator, Ferdinandea cuprea (Syrphidae)

Most species in the genus Cypripedium (Cypripedioideae) produce trap flowers, making it a model lineage to study deceptive pollination. Floral attractants in most species studied appear to target bee species of different sizes. However, more recent publications report fly pollination in some subalpine species, suggesting novel suites of adaptive floral traits. Cypripedium lichiangense (section Trigonopedia) is an endangered subalpine species endemic to the Hengduan Mountains, China. We observed and analysed its floral traits, pollinators and breeding systems over 2 years in situ and in the lab. Cypripedium lichiangense was visited by females of Ferdinandea cuprea (Syrphidae). The pollinia were carried dorsally on the fly thoraces. The eggs of this fly were frequently found in the saccate labellum and on other floral organs, suggesting brood-site mimesis. The orchid is self-compatible, but cross-pollination produces more viable embryos. We propose a new mode of floral mimesis, humus-rich oviposition site mimicry, for C. lichiangense. Compared with the mimesis of aphid colonies attracting syrphid pollinators (subfamily Syrphinae), whose larvae are entomophagic, as reported in some Paphiopedilum species (Cypripedioideae), pollination by deceit in C. lichiangense represents a distinct and separate mode of exploitation of another saprophagic (or phytophagic) larvae syrphid lineage in the subfamily Eristalinae and appears to indicate diversity of pollination strategies in Section Trigonopedia of Cypripedium. However, this new brood-site mimesis seems to be less attractive to pollinators. As a possible adaptation to the weak attracted pollination strategy, this plant species has a long flowering period and extended lifespan of individual flowers to ensure reproductive success.

Orchid conservation in China from 2000 to 2020: Achievements and perspectives

We review achievements in the conservation of orchid diversity in China over the last 21 years. We provide updated information on orchid biodiversity and suggestions for orchid conservation in China. We outline national policies of biodiversity conservation, especially of orchid conservation, which provide general guidelines for orchid conservation in China. There are now approximately 1708 known species of Orchidaceae in 181 genera in China, including five new genera and 365 new species described over the last 21 years. The assessment of risk of extinction of all 1502 known native orchid species in China in 2013 indicated that 653 species were identified as threatened, 132 species were treated as data-deficient, and four species endemic to China were classified as extinct. Approximately 1100 species (ca. 65%) are protected in national nature reserves, and another ~66 species in provincial nature reserves. About 800 native orchid species have living collections in major botanical gardens. The pollination biology of 74 native orchid species and the genetic diversity and spatial genetic structure of 29 orchid species have been investigated at a local scale and/or across species distributions. The mycorrhizal fungal community composition has been investigated in many genera, such as Bletilla, Coelogyne, Cymbidium, Cypripedium, and Dendrobium. Approximately 292 species will be included in the list of national key protected wild plants this year. Two major tasks for near future include in situ conservation and monitoring population dynamics of endangered species.

Cypripedium subtropicum (Orchidaceae) employs aphid colony mimicry to attract hoverfly (Syrphidae) pollinators

In Orchidaceae, pollination is mostly animal-mediated, and one-third of species have evolved a deceptive pollination mechanism without rewards. Cypripedium is a representative lineage of nonrewarding orchids restricted to temperate regions. Cypripedium subtropicum flowers are pollinated by hoverflies and have hairy tufts that visually resemble an aphid colony covered with honey dew. We recorded the behavior of hoverflies on the flowers, determined the breeding system of the species and the structure of hairy tufts, and investigated the roles of hairy tufts and floral volatiles in this specialized pollination by using pollination experiments, scanning electron microscopy, bioassays and chemical analyses. The white hairy tufts covering the sidelobes of the labellum provide edible rewards and serve as crucial visual lures for hoverflies. The flowers emit primarily (E)-β-farnesene and a smaller amount of β-pinene that were found to attract hoverflies. Our results suggest that C. subtropicum uses both visual mimicry of an aphid-colonized labellum with a reward and chemical mimicry of aphid alarm pheromones to attract hoverflies for pollination. This is the first described example of a rewarding mimicry system in plants, where the models are animals with their secretions and the reward is similar in nutrients to that of the model mimicked.

Pollination by hoverflies in the Anthropocene

Pollinator declines, changes in land use and climate-induced shifts in phenology have the potential to seriously affect ecosystem function and food security by disrupting pollination services provided by insects. Much of the current research focuses on bees, or groups other insects together as 'non-bee pollinators', obscuring the relative contribution of this diverse group of organisms. Prominent among the 'non-bee pollinators' are the hoverflies, known to visit at least 72% of global food crops, which we estimate to be worth around US$300 billion per year, together with over 70% of animal pollinated wildflowers. In addition, hoverflies provide ecosystem functions not seen in bees, such as crop protection from pests, recycling of organic matter and long-distance pollen transfer. Migratory species, in particular, can be hugely abundant and unlike many insect pollinators, do not yet appear to be in serious decline. In this review, we contrast the roles of hoverflies and bees as pollinators, discuss the need for research and monitoring of different pollinator responses to anthropogenic change and examine emerging research into large populations of migratory hoverflies, the threats they face and how they might be used to improve sustainable agriculture.

The complete chloroplast genome sequences of a highly Endangered orchid species Paphiopedilum barbigerum (Orchidaceae)

Paphiopedilum barbigerum (Orchidaceae) is an endangered species with highly ornamental and horticultural value. The chloroplast genome of the species was assembled using next-generation sequencing method. The complete cp genome sequence is 156,329 bp in length, comprising a pair of inverted repeat regions (IRs) of 34,214 bp each, separated by a large single-copy (LSC) region of 86056 bp, and a small single-copy (SSC) region of 1845 bp. The chloroplast genome contains 126 functional genes, including 80 protein-coding genes (PCGs), 38 tRNA genes, and 8 rRNA genes. The phylogenetic position of the species based on the complete cp genome was also inferred in this study.

Competition between anthocyanin and flavonol biosynthesis produces spatial pattern variation of floral pigments between Mimulus species

Flower color patterns have long served as a model for developmental genetics because pigment phenotypes are visually striking, yet generally not required for plant viability, facilitating the genetic analysis of color and pattern mutants. The evolution of novel flower colors and patterns has played a key role in the adaptive radiation of flowering plants via their specialized interactions with different pollinator guilds (e.g., bees, butterflies, birds), motivating the search for allelic differences affecting flower color pattern in closely related plant species with different pollinators. We have identified LIGHT AREAS1 (LAR1), encoding an R2R3-MYB transcription factor, as the causal gene underlying the spatial pattern variation of floral anthocyanin pigmentation between two sister species of monkeyflower: the bumblebee-pollinated Mimulus lewisii and the hummingbird-pollinated Mimulus cardinalis. We demonstrated that LAR1 positively regulates FLAVONOL SYNTHASE (FLS), essentially eliminating anthocyanin biosynthesis in the white region (i.e., light areas) around the corolla throat of M. lewisii flowers by diverting dihydroflavonol into flavonol biosynthesis from the anthocyanin pigment pathway. FLS is preferentially expressed in the light areas of the M. lewisii flower, thus prepatterning the corolla. LAR1 expression in M. cardinalis flowers is much lower than in M. lewisii, explaining the unpatterned phenotype and recessive inheritance of the M. cardinalis allele. Furthermore, our gene-expression analysis and genetic mapping results suggest that cis-regulatory change at the LAR1 gene played a critical role in the evolution of different pigmentation patterns between the two species.

Trait evolution in the slipper orchid paphiopedilum (Orchidaceae) in China

The well-known orchid genus Paphiopedilum has attracted much attention from biologists because of its diverse floral traits. Although these traits have been thoroughly described, little is known about their evolutionary trajectory. In this study, we explored their evolutionary patterns and trajectory through phylogenetic analyses and close observations, and 10 characters in 21 Chinese species mapped onto an existing phylogenetic tree. Lip shape, staminode shape, petal shape, and petal width are relatively congruent with molecular phylogenies, thereby validating the existing traditional classification system. All four of those characters, along with flower number, are strongly conserved, and are significantly affected by phylogeny. By contrast, flower color (including that of the dorsal sepal, lip, and petal) is significantly convergent among those examined species and less affected by phylogeny. Therefore, this character is independent of evolution and mainly influenced by environmental factors. All of these characters are key, classical indicators when distinguishing among species within the subgenera Brachypetalum and Paphiopedilum.

Reticulate evolution and sea-level fluctuations together drove species diversification of slipper orchids (Paphiopedilum) in South-East Asia

South-East Asia covers four of the world's biodiversity hotspots, showing high species diversity and endemism. Owing to the successive expansion and contraction of distribution and the fragmentation by geographical barriers, the tropical flora greatly diversified in this region during the Tertiary, but the evolutionary tempo and mode of species diversity remain poorly investigated. Paphiopedilum, the largest genus of slipper orchids comprising nearly 100 species, is mainly distributed in South-East Asia, providing an ideal system for exploring how plant species diversity was shaped in this region. Here, we investigated the evolutionary history of this genus with eight cpDNA regions and four low-copy nuclear genes. Discordance between gene trees and network analysis indicates that reticulate evolution occurred in the genus. Ancestral area reconstruction suggests that vicariance and long-distance dispersal together led to its current distribution. Diversification rate variation was detected and strongly correlated with the species diversity in subg. Paphiopedilum (~80 species). The shift of speciation rate in subg. Paphiopedilum was coincident with sea-level fluctuations in the late Cenozoic, which could have provided ecological opportunities for speciation and created bridges or barriers for gene flow. Moreover, some other factors (e.g. sympatric distribution, incomplete reproductive barriers and clonal propagation) might also be advantageous for the formation and reproduction of hybrid species. In conclusion, our study suggests that the interplay of reticulate evolution and sea-level fluctuations has promoted the diversification of the genus Paphiopedilum and sheds light into the evolution of Orchidaceae and the historical processes of plant species diversification in South-East Asia.

Pollination by brood-site deception

Pollination is often regarded as a mutualistic relationship between flowering plants and insects. In such a relationship, both partners gain a fitness benefit as a result of their interaction. The flower gets pollinated and the insect typically gets a food-related reward. However, flower-insect communication is not always a mutualistic system, as some flowers emit deceitful signals. Insects are thus fooled by irresistible stimuli and pollination is accomplished. Such deception requires very fine tuning, as insects in their typically short life span, try to find mating/feeding breeding sites as efficiently as possible, and following deceitful signals thus is both costly and time-consuming. Deceptive flowers have thus evolved the ability to emit signals that trigger obligate innate or learned responses in the targeted insects. The behavior, and thus the signals, exploited are typically involved in reproduction, from attracting pheromones to brood/food-site cues. Chemical mimicry is one of the main modalities through which flowers trick their pollen vectors, as olfaction plays a pivotal role in insect-insect and insect-plant interactions. Here we focus on floral odors that specifically mimic an oviposition substrate, i.e., brood-site mimicry. The phenomenon is wide spread across unrelated plant lineages of Angiosperm, Splachnaceae and Phallaceae. Targeted insects are mainly beetles and flies, and flowers accordingly often emit, to the human nose, highly powerful and fetid smells that are conversely extremely attractive to the duped insects. Brood-site deceptive plants often display highly elaborate flowers and have evolved a trap-release mechanism. Chemical cues often act in unison with other sensory cues to refine the imitation.

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.

The structure and roles of sterile flowers in Viburnum macrocephalum f. keteleeri (Adoxaceae)

The formation and ecological roles of sterile flowers in flowering plants are interesting issues in floral biology and evolution. Here, we investigated the morphological and anatomical characteristics of both fertile and sterile flowers of Viburnum macrocephalum f. keteleeri, a self-incompatible and insect-pollinated shrub, during different developmental stages of flowers. In addition, pollinator visitation rates and fruit set were determined in intact inflorescences and those with sterile flowers removed. The results indicate that sterile and fertile flowers were developmentally similar during early developmental stages, and that development of the flower types diverged about 15 days before flowering. In addition, pollinator visitation rates, number of pollen grains on stigmas and fruit set were significantly higher in inflorescences with sterile flowers than those without sterile flowers. The results suggest that sterile flowers of this species evolved from fertile flowers under long-term selective pressure, and play a crucial role in enhancing reproductive success through effectively attracting pollinators to the plant and thus enhancing fruit set.

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.

A syrphid fly uses olfactory cues to find a non-yellow flower

Syrphid flies are frequent flower visitors, but little is known about the cues they use to find flowers. We determined the importance of visual and olfactory cues in a flight cage bioassay using Cirsium arvense (Asteraceae) flower heads and experienced Episyrphus balteatus (Diptera, Syrphidae). We tested the response of antennae of the flies to headspace inflorescence scent samples by using gas chromatography coupled to electroantennography (GC-EAD). The bioassays revealed that both sexes of experienced flies rely on olfactory, not visual, cues to find C. arvense flower heads. The GC-EAD measurements demonstrated that male and female flies have olfactory receptors for several of the compounds emitted by the inflorescences. These electroantennographic-active compounds may be responsible for the attraction of flies to the C. arvense flower heads. Among the compounds eliciting an antennal response are methyl salicylate and 2-phenylethanol, which were previously described as syrphid attractants. Overall, our study demonstrates for the first time that a syrphid fly uses olfactory and not visual cues to find a pollen/nectar host-plant.

How plants cope with biotic interactions

In their natural environment, plants interact with many different organisms. The nature of these interactions may range from positive, for example interactions with pollinators, to negative, such as interactions with pathogens and herbivores. In this special issue, the contributors provide several examples of how plants manage both positive and negative biotic interactions. This review aims to relate their findings to what we know about the complex natural environments in which plants have evolved. Molecular analyses of plant genomes and expression profiles have shown how intricately plants may regulate responses to single or multiple biotic interactions. Plant responses are fine-tuned by signalling hormone interactions. When multiple organisms interact with a single plant this may result in antagonistic or synergistic effects. The emerging fields of ecogenomics and metabolomics undoubtedly will refine our understanding of the multilayered regulation that plants use to manage relationships with their biotic environment. However, we can only understand why plants have such an intricate regulatory apparatus if we consider the ecological context of plant biotic interactions.