- Canberra, Australian Capital Territory, Australia
Rod Peakall
The Australian National University, Research School of Biology, Faculty Member
The ``sexually deceptive'' orchid Chiloglottis trapeziformis attracts males of its pollinator species, the thynnine wasp Neozeleboria cryptoides, by emitting a unique volatile compound, 2-ethyl-5-propylcyclohexan-1,3-dione,... more
The ``sexually deceptive'' orchid Chiloglottis trapeziformis attracts males of its pollinator species, the thynnine wasp Neozeleboria cryptoides, by emitting a unique volatile compound, 2-ethyl-5-propylcyclohexan-1,3-dione, which is also produced by female wasps as a male-attracting sex pheromone.
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It has recently been discovered that novel di-, tri- and tetra- substituted pyrazines are semiochemicals in Drakaea, an orchid genus that secures pollination by the sexual deception of male thynnine wasps. We examined if similar pyrazines... more
It has recently been discovered that novel di-, tri- and tetra- substituted pyrazines are semiochemicals in Drakaea, an orchid genus that secures pollination by the sexual deception of male thynnine wasps. We examined if similar pyrazines were also present in the distantly related Caladenia barbarossa, a sexually deceptive orchid that is also pollinated by a thynnine wasp. Here we report for the first time the occurrence of two pyrazines, (3,5,6-trimethylpyrazin-2-yl)methyl 3-methylbutanoate (1) and 3-(3-methylbutyl)-2,5-dimethylpyrazine (2) in the orchid genus Caladenia. The former is known as a semiochemical involved in pollinator attraction in Drakaea livida. This convergence of floral odour between distantly related plants provides an exciting opportunity to understand the evolution and molecular basis of this sophisticated chemical mimicry.
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Australia is a major centre of diversity for pollination by sexual deception, a pollination syndrome unique to orchids and characterised by highly specific pollinator relationships. Chiloglottis pescottiana is a rare natural hybrid... more
Australia is a major centre of diversity for pollination by sexual deception, a pollination syndrome unique to orchids and characterised by highly specific pollinator relationships. Chiloglottis pescottiana is a rare natural hybrid between sexually deceptive C. trapeziformis and C. valida. We utilised bioclimatic models to predict the potential range of the parental orchid species, the hybrid and their pollinators. The predicted ranges of the parental orchid species rarely overlapped (only 2% of the core range), with the geographic separation of the species reflecting the occupation of largely distinct climatic niches and limiting opportunities for hybridisation. Comparison of the predictions with independent distributions of the orchid taxa revealed a close match. Unexpectedly, our results revealed that several related and morphologically similar orchid species are, nevertheless, ecologically distinct from C. valida. Our study demonstrates that bioclimatic modelling provides...
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An increasing number of genetic studies of rare plants are motivated by the potential contribution they can make to the conservation of the species concerned. However, while these studies frequently identify conservation implications, few... more
An increasing number of genetic studies of rare plants are motivated by the potential contribution they can make to the conservation of the species concerned. However, while these studies frequently identify conservation implications, few have demonstrated practical conservation outcomes. In this paper, we criticallyevaluate the practical outcomes of our genetic study of three endangered plant species: Haloragodendron lucasii, Zieria prostrata and Wollemia nobilis. Each genetic study provided a number of conservation implications or management recommendations for the species concerned. However, for three reasons, not all of the implications actually led to practical outcomes. First, similar recommendations were already in place. Second, management recommendations, while sound, were not relevant to the current management priorities. Third, irrespective of the genetic outcome, a given recommendation prevailed as the preferred management option. In order to achieve effective and cost-e...
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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 orchids is... more
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 orchids 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, while 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, though the floral odour of D. livida induced copulation more frequently than that of C. pectinata. The role of colour in pollinator sexua...
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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,... more
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)pheno...
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Research Interests: Plant Biology and Ecology
The endangered plant Zieria 'prostrata' ms is known from only four headlands along three kilometres of the north coast of New South Wales, Australia. Given its restricted range and small population sizes, Zieria... more
The endangered plant Zieria 'prostrata' ms is known from only four headlands along three kilometres of the north coast of New South Wales, Australia. Given its restricted range and small population sizes, Zieria 'prostrata' has been the subject of extensive management and research for almost a decade. In this paper, we review the history of management and research actions undertaken on Z. prostrata and evaluate their practical outcomes. By revisiting past management and research actions, and assessing their outcomes, we can learn much about the effective management and recovery of threatened flora. This review highlights five valuable lessons. First, effective survey is a priority for many rare plants. Second, accurate information and documentation are essential for effective conservation. Third, critical evaluation of the need for ex-situ conservation is necessary. Fourth, critical evaluation of the need for population enhancement and the most effective methods by w...
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One of the most intriguing natural observations is the pollination of orchids by sexual deception. Chemicals underpin this interaction between the orchid and its sexually attracted male insect pollinator, with the signaling compounds... more
One of the most intriguing natural observations is the pollination of orchids by sexual deception. Chemicals underpin this interaction between the orchid and its sexually attracted male insect pollinator, with the signaling compounds involved, called semiochemicals, predicted to mimic the chemical composition of the sex pheromone. We identified floral semiochemicals from Caladenia (spider orchids) for the first time. We further demonstrate that C. crebra attracts its single pollinator species with a unique system of (methylthio)phenols, three of which are new natural products. Furthermore, as predicted, the same compounds constitute the sex pheromone of the pollinator, the thynnine wasp Campylothynnus flavopictus, representing the first occurrence of sulfurous sex pheromones in Hymenoptera.
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Pollination by sexual deception in orchids is characterised by a high degree of pollinator specificity, which may account for the rarity of natural hybrids within the group. Only one such hybrid has been formally recognised in Australia,... more
Pollination by sexual deception in orchids is characterised by a high degree of pollinator specificity, which may account for the rarity of natural hybrids within the group. Only one such hybrid has been formally recognised in Australia, Chiloglottis × pescottiana R.S.Rogers, which has intermediate floral morphology between its putative parents, C. valida D.L.Jones and C. trapeziformis Fitzg. In this paper, genetic and morphometric analyses confirm the hybrid origin of this taxon. Allozyme analysis of C. trapeziformis and C. valida revealed fixed allelic differences at four ‘diagnostic’ loci and significant frequency differences at three other loci. In all cases, C. × pescottiana exhibited fixed heterozygosity at the diagnostic loci. Multidimensional scaling of both the genetic data and seven morphometric traits revealed distinct clusters of C. trapeziformis and C. valida while C. × pescottiana formed an intermediate cluster between the two parents. To test for genetic compatibility between C. trapeziformis, C. valida and C. × pescottiana, a series of reciprocal artificial crosses were performed. In all cases, the percentage of capsules formed was at least as great for between-species crosses as for within-species selfs and crosses (range 75–100%). No significant differences in the percentage of seed with normal embryos was detected between self- and cross-pollinations within C. trapeziformis (range 77–81%), C. valida (range 59–74%) and C. × pescottiana (range 30–51%), but the percentage of normal embryos was notably lower in C. × pescottiana. The cross C. trapeziformis female by C. valida male produced significantly more normal embryos (90%) compared with the reverse cross (46%). Artificial backcrosses of C. × pescottiana to C. valida and C. trapeziformis had lower percentages of normal embryos when C. × pescottiana was the pollen donor (39–43%) rather than recipient (62–68%), suggesting reduced pollen viability in the latter taxon. The size of F2 embryos in C. × pescottiana seed capsules was smaller than the embryos of both C. valida and C. trapeziformis. Despite confirmation of hybridisation, little evidence for backcrossing was found. Thus, while the specific pollinator relationships may occasionally break down in these sexually deceptive orchids, reduced viability of hybrid pollen and F2 seed, and inefficient pollination of the hybrid, may minimise introgression. It is concluded from the available evidence that hybridisation has not been a major evolutionary factor in the diversification of sexual deception worldwide.
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Chiloglottone, a wasp pheromone and attractant of sexually deceptive Chiloglottis orchids, and several structural analogs were synthesized. The synthetic approach is facile, high yielding and versatile, enabling rapid divergence to... more
Chiloglottone, a wasp pheromone and attractant of sexually deceptive Chiloglottis orchids, and several structural analogs were synthesized. The synthetic approach is facile, high yielding and versatile, enabling rapid divergence to generate dialkylated analogs of chiloglottone. The key transformation was an organocadmium-mediated desymmetrization of glutaric anhydride derivatives. This library of synthetic 2,5-dialkylated 1,3-cyclohexanediones may assist in future identification of natural products
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The outcome of the arms race between hosts and pathogens depends heavily on the interactions between their genetic diversity, population size and transmission ability. Theory predicts that genetically diverse hosts will select for higher... more
The outcome of the arms race between hosts and pathogens depends heavily on the interactions between their genetic diversity, population size and transmission ability. Theory predicts that genetically diverse hosts will select for higher virulence and more diverse pathogens than hosts with low genetic diversity. Cultivated hosts typically have lower genetic diversity and thus small effective population sizes, but can potentially harbour large pathogen population sizes. On the other hand, hosts, such as weeds, which are genetically more diverse and thus have larger effective population sizes, usually harbour smaller pathogen population sizes. Large pathogen population sizes may lead to more opportunities for mutation and hence more diverse pathogens. Here we test the predictions that pathogen neutral genetic diversity will increase with large pathogen population sizes and host diversity, whereas diversity under selection will increase with host diversity. We assessed and compared the...
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Sexually deceptive orchids produce floral volatiles that attract male insect pollinators. This interaction between flower and pollinator normally is highly specific. In the few cases where the chemical composition of the volatiles is... more
Sexually deceptive orchids produce floral volatiles that attract male insect pollinators. This interaction between flower and pollinator normally is highly specific. In the few cases where the chemical composition of the volatiles is known, the compounds have been found to be identical to those that comprise the sex pheromone of the female wasp. In this study, we investigated whether there is potential for flexibility in the molecular structure of the chemical cues used to mediate these specific interactions. Specifically, we asked whether strong sexual attraction can be maintained with structural modifications of sex pheromone components. Our study focused on the orchid, Drakaea glyptodon, which is pollinated by males of the thynnine wasp, Zaspilothynnus trilobatus. Three alkylpyrazines and a unique hydroxymethylpyrazine are components of the female produced sex pheromone of Z. trilobatus, and also the semiochemicals produced by the orchid that lures the males as pollinators. A blend of 2-butyl-3,5-dimethylpyrazine and 2-hydroxymethyl-3,5-diethyl-6-methylpyrazine (3:1) is as attractive as the full blend of four compounds. Therefore, in this study we substituted 2-hydroxymethyl-3,5-diethyl-6-methylpyrazine with one of five structurally related parapheromones in a blend with 2-butyl-3,5-dimethylpyrazine. All blends tested stimulated approaches by male wasps, with some also eliciting landing and attempted copulation. High-level calculations (G4(MP2)) showed the energy differences between the structural isomers were small, although the degree of sexual attraction varied, indicating the importance of structural factors for activity. One of the parapheromones, 2-hydroxymethyl-3,5-dimethyl-6-ethylpyrazine, elicited similar proportions of approaches, landings, and attempted copulations as the sex pheromone at the ratio and dose tested. The findings suggest that there is potential for chemical flexibility in the evolution of sexual deception.
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ABSTRACT
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Reconstructing evolutionary history for emerging species complexes is notoriously difficult, with newly isolated taxa often morphologically cryptic and the signature of reproductive isolation often restricted to a few genes. Evidence from... more
Reconstructing evolutionary history for emerging species complexes is notoriously difficult, with newly isolated taxa often morphologically cryptic and the signature of reproductive isolation often restricted to a few genes. Evidence from multiple loci and genomes is highly desirable, but multiple inputs require 'common currency' translation. Here we deploy a Shannon information framework, converting into diversity analogue, which provides a common currency analysis for maternally inherited haploid and bi-parentally inherited diploid nuclear markers, and then extend that analysis to construction of minimum-spanning networks for both genomes. The new approach is illustrated with a quartet of cryptic congeners from the sexually deceptive Australian orchid genus Chiloglottis, still in the early stages of speciation. Divergence is more rapid for haploid plastids than for nuclear markers, consistent with the effective population size differential (Nep < Nen ), but divergence ...
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ABSTRACT Species with specialized ecological interactions present significant conservation challenges. In plants that attract pollinators with pollinator-specific chemical signals, geographical variation in pollinator species may indicate... more
ABSTRACT Species with specialized ecological interactions present significant conservation challenges. In plants that attract pollinators with pollinator-specific chemical signals, geographical variation in pollinator species may indicate the presence of cryptic plant taxa. We investigated this phenomenon in the rare sexually deceptive orchid Drakaea elastica using a molecular phylogenetic analysis to resolve pollinator species boundaries, pollinator choice experiments and a population genetic study of the orchid. Pollinator choice experiments demonstrated the existence of two ecotypes within D. elastica, each attracting their own related but phylogenetically distinct pollinator species. Despite the presence of ecotypes, population genetic differentiation was low across populations at six microsatellite loci (FST = 0.026). However, Bayesian STRUCTURE analysis revealed two genetic clusters, broadly congruent with the ecotype distributions. These ecotypes may represent adaptation to regional variation in pollinator availability and perhaps the early stages of speciation, with pronounced morphological and genetic differences yet to evolve. Resolution of the taxonomic status of the D. elastica ecotypes is required as this has implications for conservation efforts and allocation of management funding. Furthermore, any reintroduction programmes must incorporate knowledge of ecotype distribution and pollinator availability to ensure reproductive success in restored populations. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, ●●, ●●–●●.
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Research Interests: Plant Biology and Ecology
Chiloglottone, a wasp pheromone and attractant of sexually deceptive Chiloglottis orchids, and several structural analogs were synthesized. The synthetic approach is facile, high yielding and versatile, enabling rapid divergence to... more
Chiloglottone, a wasp pheromone and attractant of sexually deceptive Chiloglottis orchids, and several structural analogs were synthesized. The synthetic approach is facile, high yielding and versatile, enabling rapid divergence to generate dialkylated analogs of chiloglottone. The key transformation was an organocadmium-mediated desymmetrization of glutaric anhydride derivatives. This library of synthetic 2,5-dialkylated 1,3-cyclohexanediones may assist in future identification of natural products
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Australian sexually deceptive Chiloglottis orchids attract their specific male wasp pollinators by means of 2,5-dialkylcyclohexane-1,3-diones or 'chiloglottones', representing a newly discovered class of volatiles with unique... more
Australian sexually deceptive Chiloglottis orchids attract their specific male wasp pollinators by means of 2,5-dialkylcyclohexane-1,3-diones or 'chiloglottones', representing a newly discovered class of volatiles with unique structures. This study investigated the hypothesis that UV-B light at low intensities is directly required for chiloglottone biosynthesis in Chiloglottis trapeziformis. Chiloglottone production occurs only in specific tissue (the callus) of the labellum. Cut buds and flowers, and whole plants with buds and flowers, sourced from the field, were kept in a growth chamber and interactions between growth stage of the flowers and duration and intensity of UV-B exposure on chiloglottone production were studied. The effects of the protein synthesis inhibitor cycloheximide were also examined. Chiloglottone was not present in buds, but was detected in buds that were manually opened and then exposed to sunlight, or artificial UV-B light for ≥5 min. Spectrophotomet...
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Plants use a diverse range of visual and olfactory cues to advertize to pollinators. Australian Chiloglottis orchids employ one to three related chemical variants, all 2,5-dialkylcyclohexane-1,3-diones or 'chiloglottones' to... more
Plants use a diverse range of visual and olfactory cues to advertize to pollinators. Australian Chiloglottis orchids employ one to three related chemical variants, all 2,5-dialkylcyclohexane-1,3-diones or 'chiloglottones' to sexually attract their specific male pollinators. Here an investigation was made of the physiological aspects of chiloglottone synthesis and storage that have not previously been examined. The location of chiloglottone production was determined and developmental and diurnal changes by GC-MS analysis of floral tissue extracts was monitored in two distantly related Chiloglottis species. Light treatment experiments were also performed using depleted flowers to evaluate if sunlight is required for chiloglottone production; which specific wavelengths of light are required was also determined. Chiloglottone production only occurs in specific floral tissues (the labellum calli and sepals) of open flowers. Upon flower opening chiloglottone production is rapid an...
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Research Interests: Genetics and DNA marker
The hypothesis that pollinators have been important drivers of angiosperm diversity dates back to Darwin, and remains an important research topic today. Mounting evidence indicates that pollinators have the potential to drive... more
The hypothesis that pollinators have been important drivers of angiosperm diversity dates back to Darwin, and remains an important research topic today. Mounting evidence indicates that pollinators have the potential to drive diversification at several different stages of the evolutionary process. Microevolutionary studies have provided evidence for pollinator-mediated floral adaptation, while macroevolutionary evidence supports a general pattern of pollinator-driven diversification of angiosperms. However, the overarching issue of whether, and how, shifts in pollination system drive plant speciation represents a critical gap in knowledge. Bridging this gap is crucial to fully understand whether pollinator-driven microevolution accounts for the observed macroevolutionary patterns. Testable predictions about pollinator-driven speciation can be derived from the theory of ecological speciation, according to which adaptation (microevolution) and speciation (macroevolution) are directly ...
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The specific processes that facilitate pathogen transmission are poorly understood, particularly for wild animal populations. A major impediment for investigating transmission pathways is the need for simultaneous information on host... more
The specific processes that facilitate pathogen transmission are poorly understood, particularly for wild animal populations. A major impediment for investigating transmission pathways is the need for simultaneous information on host contacts and pathogen transfer. In this study, we used commensal Escherichia coli strains as a model system for gastrointestinal pathogens. We combined strain-sharing information with detailed host contact data to investigate transmission routes in mountain brushtail possums. Despite E. coli being transmitted via the faecal-oral route, we revealed that, strain-sharing among possums was better explained by host contacts than spatial proximity. Furthermore, and unexpectedly, strain-sharing was more strongly associated with the duration of brief nocturnal associations than day-long den-sharing. Thus, the most cryptic and difficult associations to measure were the most relevant connections for the transmission of this symbiont. We predict that future studies that employ similar approaches will reveal the importance of previously overlooked associations as key transmission pathways.
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• Sexually deceptive orchids are predicted to represent a special case of plant speciation where strong reproductive isolation may be achieved by differences in floral scent. • In this study of Australian sexually deceptive Chiloglottis... more
• Sexually deceptive orchids are predicted to represent a special case of plant speciation where strong reproductive isolation may be achieved by differences in floral scent. • In this study of Australian sexually deceptive Chiloglottis orchids, we performed choice experiments to test for wasp pollinator specificity in the field; identified the compounds involved in pollinator attraction by gas chromatography with electroantennographic detection (GC-EAD), gas chromatography with mass selective detection (GC-MS), chemical synthesis and behavioural bioassays; and mapped our chemical findings on to a phylogeny of the orchids. • Field experiments confirmed pollination is a highly specific interaction, but also revealed a pool of nonpollinating &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;minor responder&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; wasps. Six novel compounds, all 2,5-dialkylcyclohexan-1,3-diones, called &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;chiloglottones&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;, were discovered to be involved in pollinator attraction. Bioassays confirmed that pollinator specificity has a strong chemical basis, with specificity among sympatric orchids maintained by either different single compounds or a variation in a blend of two compounds. The phylogenetic overlay confirmed that speciation is always associated with pollinator switching and usually underpinned by chemical change. • If the chemical differences that control reproductive isolation in Chiloglottis have a strong genetic basis, and given the confirmed pool of potential pollinators, we conclude that pollinator-driven speciation appears highly plausible in this system.
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Sexually deceptive Chiloglottis orchids lure their male thynnine wasp pollinators to the flower by emitting semiochemicals that mimic the specific sex pheromone of the wasp. Sexual deception is possible because chemical rather than visual... more
Sexually deceptive Chiloglottis orchids lure their male thynnine wasp pollinators to the flower by emitting semiochemicals that mimic the specific sex pheromone of the wasp. Sexual deception is possible because chemical rather than visual cues play the key role in wasp mate search, suggesting that cryptic wasp species may be frequent. We investigated this prospect among Neozeleboria wasp pollinators of Chiloglottis orchids, drawing on evidence from molecular phylogenetic analysis at three genes (CO1, rhodopsin and wingless), population genetic and statistical parsimony analysis at CO1, orchid associations and their semiochemicals, and geographic ranges. We found a compelling relationship between genetically defined wasp groups, orchid associations, semiochemicals and geographic range, despite a frequent lack of detectable morphological differences. Our findings reveal multiple cryptic species among orchid pollinators and indicate that chemical changes are important for wasp reproduc...