Molecular genetic analysis and ecological evidence reveals multiple cryptic species among thynnine wasp pollinators of sexually deceptive orchids

DNA barcoding and hypopygium shape support delimitation of sympatric Dissomphalus species (Hymenoptera, Bethylidae) from the Atlantic rainforest

Dissomphalus is a cosmopolitan genus of Bethylidae and has 269 Neotropical species divided into 32 species-groups, mostly defined by the genital and the tergal process structures. Dissomphalusrectilineus and D.concavatus are sympatric species in the ulceratus species-group. Members of the species-group share many similarities in the morphology of the head, hypopygium, tergal process and genitalia, but may be distinguished by the structure of the hypopygium. Previous studies have found intermediate structures of the hypopygium in the sympatric areas and raised questions about the distinctiveness of these two species. We sequenced 340 bp of the mitochondrial gene cytochrome oxidase I of 29 specimens from Brazil and Paraguay, calculated the genetic divergence among specimens, and recovered the phylogenetic relationships between taxa. In addition, we compared the morphology of the hypopygium to evaluate its use as a species-specific diagnostic character using the genetic divergence values. We recovered three well-supported monophyletic groups (intraclade divergence from 1.3 to 13.4%) and three hypopygium morphologies associated with each clade, two of them associated with D.rectilineus and D.concavatus (as described in the literature); the third one is new, not associated with any known species. The divergence between the D.rectilineus and D.concavatus clades was 19%, while the third clade is divergent from each species by 19–20%. If fully described, the hypopygium shape associated with the COI sequence will represent an extremely promising approach to the diagnosis of Dissomphalus species.

UV-B light contributes directly to the synthesis of chiloglottone floral volatiles

BACKGROUND AND AIMS

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.

METHODS

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.

KEY RESULTS

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. Spectrophotometry revealed that the sepals and petals blocked UV-B light from reaching the labellum inside the bud. Rates of chiloglottone production increased with developmental stage, increasing exposure time and increasing UV-B irradiance intensity. Cycloheximide did not inhibit the initial production of chiloglottone within 5 min of UV-B exposure. However, inhibition of chiloglottone production by cycloheximide occurred over 2 h of UV-B exposure, indicating a requirement for de novo protein synthesis to sustain chiloglottone production under UV-B.

CONCLUSIONS

The sepals and petals of Chiloglottis orchids strongly block UV-B wavelengths of light, preventing chiloglottone production inside the bud. While initiation of chiloglottone biosynthesis requires only UV-B light, sustained chiloglottone biosynthesis requires both UV-B and de novo protein biosynthesis. The internal amounts of chiloglottone in a flower reflect the interplay between developmental stage, duration and intensity of UV-B exposure, de novo protein synthesis, and feedback loops linked to the starting amount of chiloglottone. It is concluded that UV-B light contributes directly to chiloglottone biosynthesis. These findings suggest an entirely new and unexpected biochemical reaction that might also occur in taxa other than these orchids.

Mismatch in the distribution of floral ecotypes and pollinators: insights into the evolution of sexually deceptive orchids

Plants are predicted to show floral adaptation to geographic variation in the most effective pollinator, potentially leading to reproductive isolation and genetic divergence. Many sexually deceptive orchids attract just a single pollinator species, limiting opportunities to experimentally investigate pollinator switching. Here, we investigate Drakaea concolor, which attracts two pollinator species. Using pollinator choice tests, we detected two morphologically similar ecotypes within D. concolor. The common ecotype only attracted Zaspilothynnus gilesi, whereas the rare ecotype also attracted an undescribed species of Pogonothynnus. The rare ecotype occurred at populations nested within the distribution of the common ecotype, with no evidence of ecotypes occurring sympatrically. Surveying for pollinators at over 100 sites revealed that ecotype identity was not correlated with wasp availability, with most orchid populations only attracting the rare Z. gilesi. Using microsatellite markers, genetic differentiation among populations was very low (GST = 0.011) regardless of ecotype, suggestive of frequent gene flow. Taken together, these results may indicate that the ability to attract Pogonothynnus has evolved recently, but this ecotype is yet to spread. The nested distribution of ecotypes, rather than the more typical formation of ecotypes in allopatry, illustrates that in sexually deceptive orchids, pollinator switching could occur throughout a species' range, resulting from multiple potentially suitable but unexploited pollinators occurring in sympatry. This unusual case of sympatric pollinators highlights D. concolor as a promising study system for further understanding the process of pollinator switching from ecological, chemical and genetic perspectives.

Discovery of pyrazines as pollinator sex pheromones and orchid semiochemicals: implications for the evolution of sexual deception

Sexually deceptive orchids employ floral volatiles to sexually lure their specific pollinators. How and why this pollination system has evolved independently on multiple continents remains unknown, although preadaptation is considered to have been important. Understanding the chemistry of sexual deception is a crucial first step towards solving this mystery. The combination of gas chromatography-electroantennographic detection (GC-EAD), GC-MS, synthesis and field bioassays allowed us to identify the volatiles involved in the interaction between the orchid Drakaea glyptodon and its sexually attracted male thynnine wasp pollinator, Zaspilothynnus trilobatus. Three alkylpyrazines and one novel hydroxymethyl pyrazine were identified as the sex pheromone of Z. trilobatus and are also used by D. glyptodon for pollinator attraction. Given that our findings revealed a new chemical system for plants, we surveyed widely across representative orchid taxa for the presence of these compounds. With one exception, our chemical survey failed to detect pyrazines in related genera. Collectively, no evidence for preadaptation was found. The chemistry of sexual deception is more diverse than previously known. Our results suggest that evolutionary novelty may have played a key role in the evolution of sexual deception and highlight the value of investigating unusual pollination systems for advancing our understanding of the role of chemistry in evolution.

Pyrazines Attract Catocheilus Thynnine Wasps

Five previously identified semiochemicals from the sexually deceptive Western Australian hammer orchid Drakaea livida, all showing electrophysiological activity in gas chromatography-electroantennogram detection (EAD) studies, were tested in field bioassays as attractants for a Catocheilus thynnine wasp. Two of these compounds, (3,5,6-trimethylpyrazin-2-yl)methyl 3-methylbutanoate and 2-(3-methylbutyl)-3,5,6-trimethylpyrazine, were attractive to male wasps. Additionally, the semiochemical 3-(3-methylbutyl)-2,5-dimethylpyrazine, a close analogue to 2-(3-methylbutyl)-3,5,6-trimethylpyrazine, identified in five other species of thynnine wasps, was equally active. The three remaining compounds from D. livida, which were EAD-active against Catocheilus, did not attract the insects in field trials. It is interesting that two structurally similar compounds induce similar behaviours in field experiments, yet only one of these compounds is present in the orchid flower. Our findings suggest the possibility that despite the high specificity normally characterising sex pheromone systems, the evolution of sexual deception may not be entirely constrained by the need to precisely match the sex pheromone constituents and blends. Such evolutionary flexibility may be particularly important during the early stages of speciation.

Caught in the act: pollination of sexually deceptive trap-flowers by fungus gnats in Pterostylis (Orchidaceae)

BACKGROUND AND AIMS

Pterostylis is an Australasian terrestrial orchid genus of more than 400 species, most of which use a motile, touch-sensitive labellum to trap dipteran pollinators. Despite studies dating back to 1872, the mechanism of pollinator attraction has remained elusive. This study tested whether the fungus gnat-pollinated Pterostylis sanguinea secures pollination by sexual deception.

METHODS

The literature was used to establish criteria for confirming sexual deception as a pollination strategy. Observations and video recordings allowed quantification of each step of the pollination process. Each floral visitor was sexed and DNA barcoding was used to evaluate the degree of pollinator specificity. Following observations that attraction to the flowers is by chemical cues, experimental dissection of flowers was used to determine the source of the sexual attractant and the effect of labellum orientation on sexual attraction. Fruit set was quantified for 19 populations to test for a relationship with plant density and population size.

KEY RESULTS

A single species of male gnat (Mycetophilidae) visited and pollinated the rewardless flowers. The gnats often showed probing copulatory behaviour on the labellum, leading to its triggering and the temporary entrapment of the gnat in the flower. Pollen deposition and removal occurred as the gnat escaped from the flower via the reproductive structures. The labellum was the sole source of the chemical attractant. Gnats always alighted on the labellum facing upwards, but when it was rotated 180 ° they attempted copulation less frequently. Pollination rate showed no relationship with orchid population size or plant density.

CONCLUSIONS

This study confirms for the first time that highly specific pollination by fungus gnats is achieved by sexual deception in Pterostylis. It is predicted that sexual deception will be widespread in the genus, although the diversity of floral forms suggests that other mechanisms may also operate.

Floral odour chemistry defines species boundaries and underpins strong reproductive isolation in sexually deceptive orchids

BACKGROUND AND AIMS

The events leading to speciation are best investigated in systems where speciation is ongoing or incomplete, such as incipient species. By examining reproductive barriers among incipient sister taxa and their congeners we can gain valuable insights into the relative timing and importance of the various barriers involved in the speciation process. The aim of this study was to identify the reproductive barriers among sexually deceptive orchid taxa in the genus Chiloglottis.

METHODS

The study targeted four closely related taxa with varying degrees of geographic overlap. Chemical, morphological and genetic evidence was combined to explore the basis of reproductive isolation. Of primary interest was the degree of genetic differentiation among taxa at both nuclear and chloroplast DNA markers. To objectively test whether or not species boundaries are defined by the chemistry that controls pollinator specificity, genetic analysis was restricted to samples of known odour chemistry.

KEY RESULTS

Floral odour chemical analysis was performed for 600+ flowers. The three sympatric taxa were defined by their specific chiloglottones, the semiochemicals responsible for pollinator attraction, and were found to be fully cross-compatible. Multivariate morphometric analysis could not reliably distinguish among the four taxa. Although varying from very low to moderate, significant levels of genetic differentiation were detected among all pairwise combinations of taxa at both nuclear and chloroplast loci. However, the levels of genetic differentiation were lower than expected for mature species. Critically, a lack of chloroplast DNA haplotype sharing among the morphologically indistinguishable and most closely related taxon pair confirmed that chemistry alone can define taxon boundaries.

CONCLUSIONS

The results confirmed that pollinator isolation, mediated by specific pollinator attraction, underpins strong reproductive isolation in these taxa. A combination of large effective population sizes, initial neutral mutations in the genes controlling floral scent, and a pool of available pollinators likely drives diversity in this system.

Mate-searching behaviour of common and rare wasps and the implications for pollen movement of the sexually deceptive orchids they pollinate

Pollinator behaviour directly affects patterns of pollen movement and outcrossing rates in plants. In orchids pollinated by sexual deception of insects, patterns of pollen movement are primarily determined by the mate-searching behaviour of the deceived males. Here, using a capture-mark-recapture study (CMR) and dietary analysis, we compare mate-searching behaviour in relation to local abundance of two pollinator species and explore the implications for pollen movement in sexually deceptive Drakaea (Orchidaceae). Drakaea are pollinated solely by the sexual deception of male thynnine wasps. The rare Drakaea elastica and widespread D. livida occur sympatrically and are pollinated by the rare but locally common Zaspilothynnus gilesi, and the widespread and abundant Z. nigripes, respectively. Local abundance was significantly different with Z. nigripes twice as abundant as Z. gilesi. For the 653 marked wasps, there was no significant difference in median movement distance between Z. gilesi and Z. nigripes. However, the maximum movement distance was twice as high for Z. gilesi (556 m) compared with Z. nigripes (267 m). This is up to three times greater than previously reported for thynnines in CMR studies. Recapture rates were six times higher in Z. gilesi (57%) compared to Z. nigripes (9%). Pollen loads and wasp longevity were similar, suggesting that this difference in recapture rate arises due to differences in the number of males moving at a scale >500 m rather than through diet or mortality. Differences in the frequency of longer movements may arise due to variation in the spatial distribution of the wingless females. We predict that pollen movement will largely be restricted to within populations of Drakaea (<500 m), with few movements between populations (>500 m).

The production of a key floral volatile is dependent on UV light in a sexually deceptive orchid

BACKGROUND AND AIMS

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.

METHODS

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.

KEY RESULTS

Chiloglottone production only occurs in specific floral tissues (the labellum calli and sepals) of open flowers. Upon flower opening chiloglottone production is rapid and levels remain more or less stable both day and night, and over the 2- to 3-week lifetime of the flower. Furthermore, it was determined that chiloglottone production requires continuous sunlight, and determined the optimal wavelengths of sunlight in the UV-B range (with peak of 300 nm).

CONCLUSIONS

UV-B light is required for the synthesis of chiloglottones - the semiochemicals used by Chiloglottis orchids to sexually lure their male pollinators. This discovery appears to be the first case to our knowledge where plant floral odour production depends on UV-B radiation at normal levels of sunlight. In the future, identification of the genes and enzymes involved, will allow us to understand better the role of UV-B light in the biosynthesis of chiloglottones.

Phylogenetics of tribe Orchideae (Orchidaceae: Orchidoideae) based on combined DNA matrices: inferences regarding timing of diversification and evolution of pollination syndromes

BACKGROUND AND AIMS

Tribe Orchideae (Orchidaceae: Orchidoideae) comprises around 62 mostly terrestrial genera, which are well represented in the Northern Temperate Zone and less frequently in tropical areas of both the Old and New Worlds. Phylogenetic relationships within this tribe have been studied previously using only nuclear ribosomal DNA (nuclear ribosomal internal transcribed spacer, nrITS). However, different parts of the phylogenetic tree in these analyses were weakly supported, and integrating information from different plant genomes is clearly necessary in orchids, where reticulate evolution events are putatively common. The aims of this study were to: (1) obtain a well-supported and dated phylogenetic hypothesis for tribe Orchideae, (ii) assess appropriateness of recent nomenclatural changes in this tribe in the last decade, (3) detect possible examples of reticulate evolution and (4) analyse in a temporal context evolutionary trends for subtribe Orchidinae with special emphasis on pollination systems.

METHODS

The analyses included 118 samples, belonging to 103 species and 25 genera, for three DNA regions (nrITS, mitochondrial cox1 intron and plastid rpl16 intron). Bayesian and maximum-parsimony methods were used to construct a well-supported and dated tree. Evolutionary trends in the subtribe were analysed using Bayesian and maximum-likelihood methods of character evolution.

KEY RESULTS

The dated phylogenetic tree strongly supported the recently recircumscribed generic concepts of Bateman and collaborators. Moreover, it was found that Orchidinae have diversified in the Mediterranean basin during the last 15 million years, and one potential example of reticulate evolution in the subtribe was identified. In Orchidinae, pollination systems have shifted on numerous occasions during the last 23 million years.

CONCLUSIONS

The results indicate that ancestral Orchidinae were hymenopteran-pollinated, food-deceptive plants and that these traits have been dominant throughout the evolutionary history of the subtribe in the Mediterranean. Evidence was also obtained that the onset of sexual deception might be linked to an increase in labellum size, and the possibility is discussed that diversification in Orchidinae developed in parallel with diversification of bees and wasps from the Miocene onwards.

The discovery of 2-hydroxymethyl-3-(3-methylbutyl)-5-methylpyrazine: a semiochemical in orchid pollination

Drakaea livida (Orchidaceae) is pollinated by sexual deception of the wasp Zaspilothynnus nigripes (Thynnidae). It is shown that the orchid emits the same compound, 2-hydroxymethyl-3-(3-methylbutyl)-5-methylpyrazine, that females emit when calling for mates. This novel pyrazine was isolated and identified by GC-EAD and GC-MS and confirmed by synthesis. This compound may represent the first known case of pyrazines as sex pheromones in Hymenoptera.

Host tracking or cryptic adaptation? Phylogeography of Pediobius saulius (Hymenoptera, Eulophidae), a parasitoid of the highly invasive horse-chestnut leafminer

Classical biological control is often advocated as a tool for managing invasive species. However, accurate evaluations of parasitoid species complexes and assessment of host specificity are impeded by the lack of morphological variation. Here, we study the possibility of host races/species within the eulophid wasp Pediobius saulius, a pupal generalist parasitoid that parasitize the highly invasive horse-chestnut leaf-mining moth Cameraria ohridella. We analysed the population genetic structure, host associations and phylogeographic patterns of P. saulius in Europe using the COI mitochondrial gene. This marker strongly supports a division into at least five highly differentiated parasitoid complexes, within two of which clades with differing degrees of host specialization were found: a Balkan clade that mainly (but not only) attacks C. ohridella and a more generalist European group that attacks many hosts, including C. ohridella. The divergence in COI (up to 7.6%) suggests the existence of cryptic species, although this is neither confirmed by nuclear divergence nor morphology. We do not find evidence of host tracking. The higher parasitism rates observed in the Balkans and the scarcity of the Balkan–Cameraria haplotypes out of the Balkans open the possibility of using these Balkan haplotypes as biological control agents of C. ohridella elsewhere in Europe.