Evolutionary relationships among pollinators and repeated pollinator sharing in sexually deceptive orchids

Pollinator diversity of the food-deceptive orchids in southern Italy

The orchid family is renowned for its enormous diversity in pollination biology. Many orchid species use deception to attract pollinators, and the main strategy in terrestrial orchids is food deception. Food-deceptive orchids usually show a low number of pollinator visitations, making field observations of pollinators difficult. In this study visual census, pollinator capture and molecular analysis of pollinaria found on caught insects allowed us to obtain information on species composition of orchid pollinators. A total of 321 insects were caught; most specimens were hymenopterans (Apis mellifera, Bombus ruderatus and Eucera rufa) and coleopterans (Tropinota hirta and T. squalida). The identity of species to which pollinaria found on the insect's body belonged was confirmed by molecular analysis. Moreover, some individuals of Billaea lata (Tachinidae, Diptera) were captured and photographed with the pollinaria on their head. Two new and important results emerged clearly in this work: a dipteran, Billaea lata, pollinator of Anacamptis pyramidalis, and two beetles in the genus Tropinota are pollinators of Orchis italica. Our results confirm that generalized food-deceptive orchids of the genera Orchis and Anacamptis show weak pollinator specificity.

Sexual attraction with pollination during feeding behaviour: implications for transitions between specialized strategies

BACKGROUND AND AIMS

Understanding the origin of pollination by sexual deception has proven challenging, as sexually deceptive flowers are often highly modified, making it hard to resolve how any intermediate forms between sexual deception and an ancestral strategy might have functioned. Here, we report the discovery in Caladenia (Orchidaceae) of sexual attraction with pollination during feeding behaviour, which may offer important clues for understanding shifts in pollination strategy.

METHODS

For Caladenia robinsonii, we observed the behaviour of its male wasp pollinator, Phymatothynnus aff. nitidus (Thynnidae), determined the site of release of the sexual attractant, and experimentally evaluated if the position of the attractant influences rates of attempted copulation and feeding behaviour. We applied GC-MS to test for surface sugar on the labellum. To establish if this pollination strategy is widespread in Caladenia, we conducted similar observations and experiments for four other Caladenia species.

KEY RESULTS

In C. robinsonii, long-range sexual attraction of the pollinator is via semiochemicals emitted from the glandular sepal tips. Of the wasps landing on the flower, 57 % attempted copulation with the sepal tips, while 27 % attempted to feed from the base of the labellum, the behaviour associated with pollen transfer. A similar proportion of wasps exhibited feeding behaviour when the site of odour release was manipulated. A comparable pollination strategy occurs in another phylogenetically distinct clade of Caladenia.

CONCLUSIONS

We document a previously overlooked type of sexual deception for orchids involving long-distance sexual attraction, but with pollination occurring during feeding behaviour at the labellum. We show this type of sexual deception operates in other Caladenia species and predict that it is widespread across the genus. Our findings may offer clues about how an intermediate transitional strategy from a food-rewarding or food-deceptive ancestor operated during the evolution of sexual deception.

Conservation assessment of the Drakaea livida (Orchidaceae) ecotypes and an evaluation of methods for their identification

Morphologically cryptic taxa must be accounted for when quantifying biodiversity and implementing effective conservation measures. Some orchids pollinated by sexual deception of male insects contain morphologically cryptic ecotypes, such as the warty hammer orchid Drakaea livida (Orchidaceae). This species is comprised of three cryptic pollination ecotypes, which can be distinguished based on differences in pollinator species and floral volatiles. The present study aims were: (a) to investigate the geographic range of the three D. livida ecotypes, enabling assessment of their conservation status; and (b) to test the efficacy of different methods of identifying the D. livida ecotypes. Three methods of ecotype identification were assessed: morphometric analysis, genome size comparison, and analysis of chemical volatile composition of labellum extracts from pollinated flowers. MaxEnt species distribution models revealed that each ecotype has a different predicted geographic range, with small areas of overlap at the range margins. One ecotype is known from just ten populations over a limited geographic area, the majority of which has been cleared for agriculture, and urban development. While there was broad overlap between the ecotypes in individual morphological traits, multivariate analysis of morphological traits provided correct assignment to ecotype in 87% of individuals. Using the labellum of pollinated flowers, screening for volatile chemical compounds associated with particular ecotypes returned an even higher correct assignment rate, of 96.5%. As such, we advocate that the use of volatiles from the labellum of recently pollinated flowers is an effective way to determine the ecotype of unknown individuals of D. livida, with minimal impact on the flowering plant.

Pollinator-Mediated Selection on Floral Traits of Primula tibetica Differs Between Sites With Different Soil Water Contents and Among Different Levels of Nutrient Availability

Abiotic environmental factors are predicted to affect plant traits and the intensity of plant-pollinator interactions. However, knowledge of their potential effects on pollinator-mediated selection on floral traits is still limited. We separately estimated the effects of soil water (two sites with different soil water contents) and N-P-K nutrient availability (different levels of nutrient addition) on pollinator-mediated selection on floral traits of Primula tibetica (an insect-pollinated perennial herbaceous species). Our results demonstrated that floral traits, plant reproductive success and pollinator-mediated selection on floral traits varied between sites with different soil water contents and among different levels of nutrient addition. The strength of pollinator-mediated selection was stronger at the site with low soil water content than at the site with high soil water content, and first decreased and then increased with increasing N-P-K nutrient addition. Our results support the hypothesis that abiotic environmental factors influence the importance of pollinators in shaping floral evolution.

Three Chemically Distinct Floral Ecotypes in Drakaea livida, an Orchid Pollinated by Sexual Deception of Thynnine Wasps

Sexually deceptive orchids are unusual among plants in that closely related species typically attract different pollinator species using contrasting blends of floral volatiles. Therefore, intraspecific variation in pollinator attraction may also be underpinned by differences in floral volatiles. Here, we tested for the presence of floral ecotypes in the sexually deceptive orchid Drakaea livida and investigated if the geographic range of floral ecotypes corresponded to variation in pollinator availability. Pollinator choice trials revealed the presence of three floral ecotypes within D. livida that each attracts a different species of thynnine wasp as a pollinator. Surveys of pollinator distribution revealed that the distribution of one of the ecotypes was strongly correlated with that of its pollinator, while another pollinator species was present throughout the range of all three ecotypes, demonstrating that pollinator availability does not always correlate with ecotype distribution. Floral ecotypes differed in chemical volatile composition, with a high degree of separation evident in principal coordinate analysis. Some compounds that differed between ecotypes, including pyrazines and (methylthio)phenols, are known to be electrophysiologically active in thynnine wasp antennae. Based on differences in pollinator response and floral volatile profile, the ecotypes represent distinct entities and should be treated as such in conservation management.

The impact of global warming on the niches and pollinator availability of sexually deceptive orchid with a single pollen vector

Orchidaceae are among the most endangered plants in the world. Considering the sensitive nature of pollinator-plant relationship the most vulnerable are species which are dependent on a single pollen vector. In this paper the future distribution of suitable niches of Australian sexually deceptive orchid Leporella fimbriata and its pollinator (Myrmecia urens) was estimated using three machine learning algorithms. While the potential range of fringed hare orchid depending on modelling method will be larger or slightly reduced than currently observed, the ant will face significant loss of suitable niches. As a result of global warming the overlap of orchid and its only pollen vector will most probably decrease. The unavailability of pollen vector will lead to decreased reproductive success and as a result it will be a great threat for L. fimbriata existence.

Pollination by nectar-foraging pompilid wasps: a new specialized pollination strategy for the Australian flora

The Pompilidae is a cosmopolitan and diverse group of wasps, which commonly feed on nectar. However, pollination systems specialized on pompilids have not been documented in detail outside of southern Africa. Here, we studied Caladenia drummondii (Orchidaceae) where, based on floral traits and preliminary field observations, we predicted pollination by sexual deception of male pompilid wasps. Detailed pollinator observations were undertaken using floral baiting experiments at sites spanning 375 km. Following evidence for nectar on some flowers of C. drummondii, the sugar content on the labellum was analysed by GC-MS. Floral spectral reflectance was measured and compared with Caladenia using other pollination strategies. Males of a single species of pompilid wasp (Calopompilus sp.) were the only visitors capable of pollinating C. drummondii. Attempts to feed from the surface of the labellum were frequent and were associated with removal and deposition of pollinia. GC-MS analysis revealed larger quantities of sugar on the labellum than reported in other Caladenia species. While no sexual or courtship behaviour was observed, the zig-zag and circling flight on approach to the flower is suggestive of odour-based attraction. Floral spectral reflectance was similar to sexually deceptive Caladenia. This study represents the first confirmation of a specialized pompilid pollination system outside of Africa. Although pollination occurs during nectar-foraging, long-distance sexual attraction cannot be ruled out as an explanation for the exclusive male visitation. The similarity in floral spectral reflectance to other Caladenia indicates colour may not impose a constraint on the evolution of pollination by pompilids.

Evolution of reproductive structures for in-flight mating in thynnine wasps (Hymenoptera: Thynnidae: Thynninae)

Thynnine wasps have an unusual mating system that involves concurrent in-flight copulation and nuptial feeding of wingless females by alate males. Consequently, thynnine genitalia play a multifunctional role and have likely been subject to various different selective pressures for both reproductive success and food provisioning. Here, we present a new molecular phylogeny for the Australian Thynninae and use 3D-geometric morphometrics and comparative methods to investigate the morphological evolution of select genital structures across the group. We found significant morphological integration between all male and female structures analysed, which is likely influenced by sexual selection, but also reproductive isolation requirements and mechanical constraints. The morphology of the primary male and female coupling structures was correlated with female body size, and female genitalia exhibited strong negative size allometry. Those male and female coupling structures have evolved at similar evolutionary rates, whereas female structures appear to have evolved a higher degree of morphological novelty over time. We conclude that the unique reproductive strategies of thynnine wasps have resulted in complex evolutionary patterns in their genital morphology, which has likely played a central role in the extensive diversification of the subfamily across Australasia and South America. Our study reinforces the need to treat composite characters such as genitalia by their component parts, and to consider the roles of both male and female reproductive structures in evolutionary studies.

Museum records indicate male bias in pollinators of sexually deceptive orchids

Deception has evolved in a range of taxa. When deception imposes costs, yet persists over generations, exploited species typically have traits to help them bear or minimise costs. The sexually deceptive orchids, Cryptostylis spp., are pollinated by tricking male haplodiploid wasps (Lissopimpla excelsa) into mating with flowers, which offer no reward and often elicit sperm wastage. We hypothesise that by attracting haplodiploid species, orchids have a pollinator ideally suited to withstand the costs of sexual deception-and a selective advantage compared to other orchids. Haplodiploid females can reproduce with or without sperm-albeit when spermless, females can only have sons. Through orchid deception and sperm wastage, deceived haplodiploid populations could become male biased, providing enough males to share between orchids and females. In this way, pollinator populations can persist despite high densities of sexually deceptive orchids. Here, we aim to broadly test this prediction using museum and digital records of the pollinator, L. excelsa, from sites with or without orchids. For robustness, we also analyse the sex ratio of a sister ichneumonid species that occurs in the same areas but is not deceived by orchids. We found that at sites with orchids, L. excelsa was significantly more male biased than at sites without orchids and significantly more male biased than the sister ichneumonid. This survey is the first to test the population-level effects of sexually deceptive orchids on their pollinator. It supports our prediction that orchid deception can drive male-biased sex ratios in exploited pollinators.

Ecosystem services provided by aculeate wasps

The aculeate wasps are one of the most diverse and speciose insect taxa; they are omnipresent across ecosystems and exhibit diverse co-evolutionary and exploitative associations with other organisms. There is widespread conjecture that aculeate wasps are likely to perform essential ecological and economic services of importance to the health, well-being and nutritional needs of our planet. However, the scope and nature of the ecosystem services they provide are not well understood relative to other insect groups (e.g. bees, butterflies, beetles); an appreciation of their value is further tarnished by their public reputation as pointless pests. Here, we conduct the first comprehensive review of how aculeate wasps contribute to the four main areas of ecosystem services: regulatory, provisioning, supporting and cultural services. Uniting data from a large but previously disconnected literature on solitary and social aculeate wasps, we provide a synthesis on how these insects perform important ecosystem services as parasites, predators, biological indicators, pollinators, decomposers and seed dispersers; and their additional services as a sustainable alternative to meat for human consumption, and medicinal potential as sources of research leads for anti-microbials and cancer treatments. We highlight how aculeate wasps offer substantial, but largely overlooked, economic benefits through their roles in natural pest management and biological control programs. Accordingly, we provide data-driven arguments for reasons to consider the ecosystem service value of aculeate wasps on a par with other 'useful' insects (e.g. bees). Finally, we provide a research roadmap identifying the key areas of research required to capitalise better on the services provided by these important insects.

Why Variation in Flower Color May Help Reproductive Success in the Endangered Australian Orchid Caladenia fulva

Caladenia fulva G.W. Carr (Tawny Spider-orchid) is a terrestrial Australian endangered orchid confined to contiguous reserves in open woodland in Victoria, Australia. Natural recruitment is poor and no confirmed pollinator has been observed in the last 30 years. Polymorphic variation in flower color complicates plans for artificial pollination, seed collection and ex situ propagation for augmentation or re-introduction. DNA sequencing showed that there was no distinction among color variants in the nuclear ribosomal internal transcribed spacer (ITS) region and the chloroplast trnT-trnF and matK regions. Also, authentic specimens of both C. fulva and Caladenia reticulata from the reserves clustered along with these variants, suggesting free interbreeding. Artificial cross-pollination in situ and assessment of seed viability further suggested that no fertility barriers existed among color variants. Natural fruit set was 15% of the population and was proportional to numbers of the different flower colors but varied with orchid patch within the population. Color modeling on spectral data suggested that a hymenopteran pollinator could discriminate visually among color variants. The similarity in fruiting success, however, suggests that flower color polymorphism may avoid pollinator habituation to specific non-rewarding flower colors. The retention of large brightly colored flowers suggests that C. fulva has maintained attractiveness to foraging insects rather than evolving to match a scarce unreliable hymenopteran sexual pollinator. These results suggest that C. fulva should be recognized as encompassing plants with these multiple flower colors, and artificial pollination should use all variants to conserve the biodiversity of the extant population.

Orchid conservation: from theory to practice

BACKGROUND

Given the exceptional diversity of orchids (26 000+ species), improving strategies for the conservation of orchids will benefit a vast number of taxa. Furthermore, with rapidly increasing numbers of endangered orchids and low success rates in orchid conservation translocation programmes worldwide, it is evident that our progress in understanding the biology of orchids is not yet translating into widespread effective conservation.

SCOPE

We highlight unusual aspects of the reproductive biology of orchids that can have important consequences for conservation programmes, such as specialization of pollination systems, low fruit set but high seed production, and the potential for long-distance seed dispersal. Further, we discuss the importance of their reliance on mycorrhizal fungi for germination, including quantifying the incidence of specialized versus generalized mycorrhizal associations in orchids. In light of leading conservation theory and the biology of orchids, we provide recommendations for improving population management and translocation programmes.

CONCLUSIONS

Major gains in orchid conservation can be achieved by incorporating knowledge of ecological interactions, for both generalist and specialist species. For example, habitat management can be tailored to maintain pollinator populations and conservation translocation sites selected based on confirmed availability of pollinators. Similarly, use of efficacious mycorrhizal fungi in propagation will increase the value of ex situ collections and likely increase the success of conservation translocations. Given the low genetic differentiation between populations of many orchids, experimental genetic mixing is an option to increase fitness of small populations, although caution is needed where cytotypes or floral ecotypes are present. Combining demographic data and field experiments will provide knowledge to enhance management and translocation success. Finally, high per-fruit fecundity means that orchids offer powerful but overlooked opportunities to propagate plants for experiments aimed at improving conservation outcomes. Given the predictions of ongoing environmental change, experimental approaches also offer effective ways to build more resilient populations.

Niche Perspectives on Plant-Pollinator Interactions

Ecological niches are crucial for species coexistence and diversification, but the niche concept has been underutilized in studying the roles of pollinators in plant evolution and reproduction. Pollination niches can be objectively characterized using pollinator traits, abundance, and distributions, as well as network topology. We review evidence that floral traits represent adaptations to pollination niches, where tradeoffs in trait deployment reinforce niche specialization. In turn, specialized pollination niches potentially increase speciation rates, foster species coexistence, and constrain species range limits. By linking studies of adaptation with those on speciation and coexistence, the pollination niche provides an organizing principle for research on plant reproduction, and conceptually unites these studies with fields of biology where the niche perspective is already firmly established.

A specialised pollination system using nectar-seeking thynnine wasps in Caladenia nobilis (Orchidaceae)

Caladenia is a diverse Australian genus that is exceptional among orchids in having both species pollinated by food-seeking and sexually deceived insects. Here, we investigated the pollination of Caladenia nobilis, a species predicted to be food-deceptive due to its large, cream-coloured and apparently nectarless flowers. Pollinator observations were made using experimental clumps of flowers. Measurements of floral colour were undertaken with a spectrometer, nectar was tested using GC-MS, and reproductive success was quantified for 2 years. While C. nobilis attracted nine species of insect, only males of the thynnine wasp Rhagigaster discrepans exhibited the correct size and behaviour to remove and deposit pollen. Male R. discrepans attempted to feed from the surface of the labellum, often crawling to multiple flowers, but showed no evidence of sexual attraction. Most flowers produced little or no nectar, although some may provide enough sucrose to act as a meagre reward to pollinators. Floral colouration was similar to a related Caladenia species pollinated by sexual deception, although the sexually deceptive species had a dull-red labellum. Reproductive success was generally low and highly variable between sites and years. In addition to most visitors being of inappropriate size for pollinia removal, the lack of response to the orchid by several co-occurring species of thynnine wasp suggests filtering of potential pollinators at the attraction phase. Our discovery of a pollination strategy that may be intermediate between food deception and food reward raises the question, how many putatively rewardless orchids actually produce meagre amounts of nectar?

Does size really matter? A comparative study on floral traits in orchids with two different pollination strategies

The lock and key hypothesis assumes that male and female genitalia match in a unique system to prevent interspecific crosses. This hypothesis is largely investigated in animals, while there is a distinct lack of studies on plants. Nevertheless, we expect the lock and key hypothesis could apply to plants with complex floral morphologies, such as orchids. Here we apply a comparative approach to examine the variation of floral functional traits in food- and sex-deceptive orchids. To understand if a specific deception strategy is related to a specific variation in floral traits evaluated the variation in sterile and fertile traits among species and subsequently examined the correlations between male and female reproductive organs of the same species with the aim of investigating the role of the lock and key hypothesis in deceptive orchids. Our results show that the functional morphology of fertile traits plays a pivotal role in limiting gene flow in species that grow in sympatry. In particular, it was observed that the Reproductive Standardisation Index (RSI) is significantly different in the two pollination strategies and that the correlation between pollinarium length and stigmatic cavity length is stronger in food-deceptive species when compared to the sex-deceptive species. These results reveal that the lock and key hypothesis contributes to maintain boundaries in plants with very complex floral morphology.

Experimental examination of pollinator-mediated selection in a sexually deceptive orchid

BACKGROUND AND AIMS

Selection exerted by pollinators on flowers is predicted to occur along two distinct axes. While pollinator attraction to flowers is governed by pollinator preferences, pollen transfer efficiency is mediated by the mechanical fit of pollinators to flower morphology. Although pollinator attraction in sexually deceptive orchids is typically underpinned by floral odour, morphological traits are expected to play a vital role in mechanical fit during floral contact with pollinators.

METHODS

Here we utilize a comprehensive and novel procedure to test for pollinator-mediated selection through mechanical fit with the flower labellum in the orchid Chiloglottis trapeziformis. This approach combines detailed pollinator observations related to plant reproductive fitness with complementary experimental manipulation and phenotypic selection analysis.

KEY RESULTS

Experiments with virgin flowers revealed that pollen removal occurs only during vigorous pseudocopulation. This behaviour involves male wasps that grasp the insectiform callus structure on the labellum while probing the labellum tip in a forward orientation. Both orientation and duration of pseudocopulation were significant predictors of pollen removal, confirming a direct relationship between pollinator behaviour and plant fitness. Controlled floral manipulation that either shortened or elongated the distance between the callus and the labellum tip detected no change in pollinator attraction. The duration of pseudocopulation, however, was significantly reduced on flowers with shortened or elongated callus-tip distances, consistent with stabilizing selection. Phenotypic selection analysis confirmed this prediction in natural populations by uncovering evidence for stabilizing selection on the distance between the callus and the labellum tip.

CONCLUSIONS

Our experimental manipulations and selection analysis in natural populations thus demonstrate stabilizing selection on the distance from the callus to the labellum tip, and illustrate the utility of employing multiple approaches to confirm selection exerted by pollinators on floral form.

An experimental evaluation of traits that influence the sexual behaviour of pollinators in sexually deceptive orchids

Pollination by sexual deception of male insects is perhaps one of the most remarkable cases of mimicry in the plant kingdom. However, understanding the influence of floral traits on pollinator behaviour in sexually deceptive plants is challenging, due to the risk of confounding changes in floral odour when manipulating morphology. Here, we investigated the floral traits influencing the sexual response of male Zaspilothynnus nigripes (Tiphiidae) wasps, a pollinator of two distantly related sexually deceptive orchids with contrasting floral architecture, Caladenia pectinata and Drakaea livida. In D. livida, the chemical sexual attractant is emitted from the labellum, whereas in C. pectinata, it is produced from the distal sepal tips, allowing manipulative experiments. When controlling for visual cues, there was no difference in long-distance attraction, although the floral odour of D. livida induced copulation more frequently than that of C. pectinata. The role of colour in pollinator sexual attraction was equivocal, indicating that colour may not be a strong constraint on the initial evolution of sexual deception. The frequency of wasp visitors landing on C. pectinata decreased when the amount of floral odour was reduced, but attempted copulation rates were enhanced when the source of floral odour was associated with the labellum. These latter variables may represent axes of selection that operate across many sexually deceptive species. Nonetheless, the observed variation in floral traits suggests flexibility among species in how sexual deception can be achieved.

(Methylthio)phenol semiochemicals are exploited by deceptive orchids as sexual attractants for Campylothynnus thynnine wasps

Until recently, (methylthio)phenols as natural products had only been reported from bacteria. Now, four representatives of this class of sulfurous aromatic compounds have been discovered as semiochemicals in the orchid Caladenia crebra, which secures pollination by sexual deception. In this case, field bioassays confirmed that a 10:1 blend of 2-(methylthio)benzene-1,4-diol (1) and 4-hydroxy-3-(methylthio)benzaldehyde (2) sexually attracts the male thynnine wasp Campylothynnus flavopictus (Tiphiidae:Thynnineae), the exclusive pollinator of C. crebra. Here we show with field bioassays that another undescribed species of Campylothynnus (sp. A) is strongly sexually attracted to a 1:1 blend of compounds 1 and 2, which elicits very high attempted copulation rates (88%). We also confirm that this Campylothynnus species is a pollinator of Caladenia attingens subsp. attingens. Chemical analysis of the flowers of this orchid revealed two (methylthio)phenols, compound 2 and 2-(methylthio)phenol (3), as candidate semiochemicals involved in pollinator attraction. Thus, (methylthio)phenols are likely to be more widely used than presently known. The confirmation of this Campylothynnus as a pollinator of C. attingens subsp. attingens at our study sites was unexpected, since elsewhere this orchid is pollinated by a different thynnine wasp (Thynnoides sp). In general, sexually deceptive Caladenia only use a single species of pollinator, and as such, this unusual case may offer a tractable study system for understanding the chemical basis of pollinator switching in sexually deceptive orchids.