Food-deceptive Orchids Research Articles

Editorial and Retrospective 2008

Editorial and Retrospective 2008

Floral Scent in Food‐Deceptive Orchids: Species Specificity and Sources of Variability

One third of all orchid species are deceptive and do not reward their pollinators. Such deceptive orchids are often characterised by unusually high variation in floral signals such as colour and scent. In this study, we investigated the scent composition of two Mediterranean food-deceptive orchids Orchis mascula, Orchis pauciflora, and their hybrid, O. x colemanii. Scent was collected IN SITU by headspace sorption and was subsequently analysed with gas chromatography and gas chromatography-mass spectrometry. We compared variation of odour compounds within and between populations as well as species. We identified 35 floral scent compounds, mainly monoterpenes, which were shared by both species. Both quantitative and qualitative variability within and among populations was high. Many individuals within species could be classified to different "odour-types". In spite of high qualitative and quantitative intra- and inter-population variability, the species were clearly differentiated in their scent bouquets, whereas most hybrid individuals emitted an intermediate scent.

PATTERNS OF REPRODUCTIVE ISOLATION IN MEDITERRANEAN DECEPTIVE ORCHIDS

The evolution of reproductive isolation is of central interest in evolutionary biology. In plants, this is typically achieved by a combination of pre- and postpollination mechanisms that prevent, or limit, the amount of interspecific gene flow. Here, we investigated and compared two ecologically defined groups of Mediterranean orchids that differ in pollination biology and pollinator specificity: sexually deceptive orchids versus food-deceptive orchids. We used experimental crosses to assess the strength of postmating prezygotic, and postzygotic reproductive isolation, and a phylogenetic framework to determine their relative rates of evolution. We found quantitative and qualitative differences between the two groups. Food-deceptive orchids have weak premating isolation but strong postmating isolation, whereas the converse situation characterizes sexually deceptive orchids. Only postzygotic reproductive isolation among food-deceptive orchids was found to evolve in a clock-like manner. Comparison of evolutionary rates, within a common interval of genetic distance, showed that the contribution of postmating barriers was more relevant in the food-deceptive species than in the sexually deceptive species. Asymmetry in prezygotic isolation was found among food-deceptive species. Our results indicate that postmating barriers are most important for reproductive isolation in food-deceptive orchids, whereas premating barriers are most important in sexually deceptive orchids. The different rate of evolution of reproductive isolation and the relative strength of pre- and postmating barriers may have implication for speciation processes in the two orchid groups.

Fitness variation and genetic diversity in small, remnant populations of the food deceptive orchid Orchis purpurea

In much of Western Europe, orchid species have suffered dramatic declines in abundance, whereas the remaining populations often tend to be small and isolated, occur in ecologically marginal habitats and may show decreased reproductive output due to altered pollinator interactions or the absence of specialized pollinators. Furthermore, small and isolated populations are expected to suffer from genetic erosion and increasing genetic divergence among populations, through the effects of random genetic drift, increased levels of inbreeding and reduced gene flow, potentially leading to reduced possibilities of population recovery in the future. In this study, genetic diversity and fitness variation were studied in nine fragmented populations of the food deceptive orchid Orchis purpurea. Within-population genetic diversity and among-population differentiation were investigated using dominant AFLP markers. All studied populations were relatively small (range 12–302; mean: 90 individuals/population). Despite their small size, genetic diversity within populations was rather high (mean gene diversity H j: 0.21, range: 0.15–0.27) and genetic differentiation among populations was not higher than that typically observed in deceptive orchids ( F ST = 0.09). Nevertheless, Mantel tests showed a positive correlation between geographical and genetic distances indicating limited gene flow among populations. Fruit set, on the other hand, was very low (average fruit set: 5.5%), suggesting strong pollinator limitation, which, in turn, resulted in very low recruitment rates (average seedling recruitment per flowering individual: 0.12). Both measures significantly increased with increasing population size, indicating that a certain threshold value (>50 flowering individuals) has to be reached to produce a sizeable number of fruits and seedlings. These results suggest that although deceptive pollination clearly results in a fitness cost, it may serve as an effective means to maintain high gene diversity within and to counter high genetic differentiation among small orchid populations. From a genetic point of view, our results further suggest that small populations of orchid species should not be neglected as they may harbour as much genetic diversity as large populations.

Odour and colour polymorphism in the food-deceptive orchid Dactylorhiza romana

The food deceptive orchid, Dactylorhiza romana (Sebastiani) Soo exhibits a colour polymorphism with yellow, red, and intermediate orange morphs. In this study we tested if floral odour differed among the three distinct colour morphs. We identified 23 odour compounds in D. romana, and all of them occurred in the three colour morphs. Monoterpenes dominated the floral scent. On the basis of Euclidean distances of relative amounts of compounds, yellow morphs were closer to each other than to orange or red morphs. Differentiation of the morphs was mainly due to linalool and benzaldehyde. Linalool occurred in low relative amounts in the yellow morphs, but in high amounts in some of the red individuals, whereas benzaldehyde occurred in higher relative amounts in yellow morphs. Linalool and benzaldehyde are known to be important signal-substances in plant-insect communication, however, it remains to be shown whether insects can discriminate between flower morphs on the basis of the here shown odour differences.

Speciation in the Orchidaceae: confronting the challenges

The Orchidaceae is renowned for its large number of species (19,500) and its many diverse, even bizarre, specialized pollination systems. One unusual feature of orchids is the high frequency of food deception whereby animal pollination is achieved without providing nectar, pollen or other food rewards. Food-deceptive pollination is estimated to occur in approximately one-third of all orchids. Equally intriguing is pollination by sexual deception whereby pollination is achieved by the sexual attraction of male insects to the orchid flower. Sexual deception is found in several hundred species representing multiple lineages. Given their rich species diversity and extraordinary plant-animal interactions, orchids clearly offer exciting research opportunities in pollination biology, reproductive isolation and speciation, yet surprisingly they remain under-represented in scientific investigations both in these fields and more generally. In this special issue of Molecular Ecology, Moccia et al. provide an exemplar study that combine multiple lines of evidence to illuminate the mechanism of reproductive isolation between two closely related food-deceptive orchids. Their study demonstrates that many of the challenges that confront orchid researchers and impede progress in our understanding of speciation in the Orchidaceae can be overcome by the creative application and integration of both old and new tools in ecology and genetics.

The strength of reproductive isolation in two hybridizing food‐deceptive orchid species

Reproductive isolation is of fundamental importance for maintaining species boundaries in sympatry. In orchids, the wide variety of pollination systems and highly diverse floral traits have traditionally suggested a prominent role for pollinator isolation, and thus for prezygotic isolation, as an effective barrier to gene flow among species. Here, we examined the nature of reproductive isolation between Anacamptis morio and Anacamptis papilionacea, two sister species of Mediterranean food-deceptive orchids, in two natural hybrid zones. Comparative analyses of the two hybrid zones that are located on soils with volcanic origin and have different and well-dated ages consistently revealed that all hybrid individuals were morphologically and genetically intermediate between the parental species, but had strongly reduced fitness. Molecular analyses based on nuclear ITS1 and (amplified fragment length polymorphism) AFLP markers clearly showed that all examined hybrids were F1 hybrids, and that no introgression occurred between parental species. The maternally inherited plastid DNA markers indicated that hybridization between A. morio and A. papilionacea was bidirectional, as confirmed by the molecular analysis of seed families. The genetic architecture of the two hybrid zones suggests that the two parental species easily and frequently hybridize in sympatry as a consequence of partial pollinator overlap but that strong postzygotic barriers reduce hybrid fitness and prevent gene introgression. These results corroborate that chromosomal divergence is instrumental for reproductive isolation between these food-deceptive orchids and suggest that hybridization is of limited importance for their diversification.

Pollination of the European food-deceptive Traunsteinera globosa (Orchidaceae): the importance of nectar-producing neighbouring plants

European food-deceptive orchids generally flower early in spring and rely on naïve pollinators for their reproduction. Some species however, flower later in the summer, when many other rewarding plants species are also in bloom. In dense flowering communities, deceptive orchids may suffer from competition for pollinator resources, or might alternatively benefit from higher community attractiveness. We investigated the pollination strategy of the deceptive species Traunsteinera globosa, and more specifically whether it benefited from the presence of coflowering rewarding species. We carried out a population survey to quantify the density and reproductive success of the orchid as well as the density of all coflowering species. Our results suggest that the deceptive orchid not only benefited from the presence of coflowering species, but that interestingly the density of the species Trifolium pratense was significantly positively correlated with the orchid's reproductive success. This species might simply act as a magnet species attracting pollinators near T. globosa, or could influence the orchid reproductive fitness through a more species-specific interaction. We propose that morphological or colour similarities between the two species should be investigated in more detail to decipher this pollination facilitation effect.

Fine‐scale genetic structure of life history stages in the food‐deceptive orchid Orchis purpurea

In natural plant populations, fine-scale spatial genetic structure can result from limited gene flow, selection pressures or historical events, but the role of each factor is in general hard to discern. One way to investigate the origination of spatial genetic structure within a plant population consists of comparing spatial genetic structure among different life history stages. In this study, spatial genetic structure of the food-deceptive orchid Orchis purpurea was determined across life history stages in two populations that were regenerating after many years of population decline. Based on demographic analyses (2001-2004), we distinguished between recruits and adult plants. For both sites, there was no difference in the proportion of polymorphic loci and expected heterozygosity between life history stages. However, spatial autocorrelation analyses showed that spatial genetic structure increased in magnitude with life history stage. Weak or no spatial genetic structure was observed for recruits, whereas adult plants showed a pattern that is consistent with that found in other species with a predominantly outcrossing mating system. The observed differences between seedlings and adults are probably a consequence of changes in management of the two study sites and associated demographic changes in both populations. Our results illustrate that recurrent population crashes and recovery may strongly affect genetic diversity and fine-scale spatial genetic structure of plant populations.

Hybridization and conservation of Mediterranean orchids: Should we protect the orchid hybrids or the orchid hybrid zones?

Natural hybridization between plant species often occurs in disturbed habitats and it is generally considered a threat for rare and endangered species. A different situation occurs in Mediterranean food deceptive orchids, where hybridization is a common phenomenon, as a natural consequence of their unspecific pollination system. Here, we present molecular and ecological evidence from a hybrid zone between Orchis mascula and O. pauciflora in order to address the consequences of hybridization on local orchid evolution and to assess conservation priorities for hybrids and hybrid zones in orchid conservation programs. We find that, although hybrids among the two target taxa are formed relatively frequently, hybrid zones often consist of F1 individuals, whereas backcrosses and later generation hybrids appear to be either rare or absent, presumably as a consequence of post-zygotic reproductive barriers acting in these food-deceptive orchids. Experimental evidence further indicates that hybrids are less fit than the parental species in attracting pollinators and that parental taxa exhibit higher or similar fitness in sympatry compared to allopatric populations. Since introgression is not frequent as indicated by molecular analyses, fitness and phenotypic trait differences observed among sympatric and allopatric populations of the parental taxa may be a consequence of pollinator-mediated selection. Our experimental data confirm that hybridization is a natural phenomenon in food-deceptive orchids and that it does not pose a threat to their survival. Furthermore, sympatric zones provide the stage for evolutionary processes in orchids, and this peculiarity should be taken into account when devising orchid conservation strategies.

Evidence for pollinator sharing in Mediterranean nectar-mimic orchids: absence of premating barriers?

Pollinator specificity has traditionally been considered the main reproductive isolation mechanism in orchids. Among Mediterranean orchids, however, many species attract and deceive pollinators by mimicking nectar-rewarding plants. To test the extent to which deceptive orchid species share pollinators, we collected and identified hemipollinaria-carrying insects, and used ribosomal sequences to identify the orchid species from which hemipollinaria were removed. We found that social and solitary bees, and also flies, carried hemipollinaria belonging to nine orchid species with different degrees of specialization. In particular, Anacamptis morio, Dactylorhiza romana and Orchis mascula used a large set of pollinator species, whereas others such as Orchis quadripunctata seemed to be pollinated by one pollinator species only. Out of the insects with hemipollinaria, 19% were found to carry hemipollinaria from more than one orchid species, indicating that sympatric food-deceptive orchids can share pollinators. This sharing was apparent even among orchid sister-species, thus revealing an effective overlap in pollinator sets among closely related species. These results suggest varying degrees of pollinator specificity in these orchids, and indicate that pollinator specificity cannot always act as the main isolation mechanism in food-deceptive temperate orchids.

On the success of a swindle: pollination by deception in orchids

A standing enigma in pollination ecology is the evolution of pollinator attraction without offering reward in about one third of all orchid species. Here I review concepts of pollination by deception, and in particular recent findings in the pollination syndromes of food deception and sexual deception in orchids. Deceptive orchids mimic floral signals of rewarding plants (food deception) or mating signals of receptive females (sexual deception) to attract pollen vectors. In some food deceptive orchids, similarities in the spectral reflectance visible to the pollinator in a model plant and its mimic, and increased reproductive success of the mimic in the presence of the model have been demonstrated. Other species do not mimic specific model plants but attract pollinators with general attractive floral signals. In sexually deceptive orchids, floral odor is the key trait for pollinator attraction, and behaviorally active compounds in the orchids are identical to the sex pheromone of the pollinator species. Deceptive orchids often show high variability in floral signals, which may be maintained by negative frequency-dependent selection, since pollinators can learn and subsequently avoid common deceptive morphs more quickly than rare ones. The evolution of obligate deception in orchids seems paradoxical in the light of the typically lower fruit set than in rewarding species. Pollination by deception, however, can reduce self-pollination and encourage pollen flow over longer distances, thus promoting outbreeding. Although some food deceptive orchids are isolated through postzygotic reproductive barriers, sexually deceptive orchids lack post-mating barriers and species isolation is achieved via specific pollinator attraction. Recent population genetic and phylogenetic investigations suggest gene-flow within subgeneric clades, but pollinator-mediated selection may maintain species-specific floral traits.

Colour similarity to rewarding model plants affects pollination in a food deceptive orchid, Orchis boryi

We studied the pollination of Orchis boryi at five different locations on the Greek mainland. Orchis boryi is food deceptive and obligatorily insect pollinated. Primary pollinators were Apis mellifera and Bombus spp., which foraged on rewarding plant species nearby and visited O. boryi in between. To analyse floral colour similarity among rewarding plants and O. boryi as perceived by bees, a model of bee colour vision was employed. For each food plant an index was calculated that described the probability of a bee foraging on it to subsequently choose an orchid flower. This choice probability correlated to colour distance according to the model of bee colour vision, indicating that bees chose the deceptive orchid more frequently if they foraged on more similarly coloured species. At different sites different plant species served as models. Bees foraging on food plants from which a high choice rate to the orchid was observed visited the orchid less often after approaching it than other bees, which is likely to reflect avoidance learning. In general, the pollination syndrome appears to be a generalized form of Batesian mimicry, in which similarity to rewarding plants determines reproductive success. As expected by negative density-dependent selection, individual fruit set and pollinia export rate correlated negatively with orchid density, but were unaffected by food plant density, orchid frequency, individual variation of labellum colour, labellum size, or mouth width of the flowers.

Colour similarity to rewarding model plants affects pollination in a food deceptive orchid, Orchis boryi

We studied the pollination of Orchis boryi at five different locations on the Greek mainland. E2>Orchis boryi is food deceptive and obligatorily insect pollinated. Primary pollinators were Apis mellifera andBombus sppwhich foraged on rewarding plant species nearby and visited O. boryi in between. To analyse floral colour similarity among rewarding plants and O. boryi as perceived by bees, a model of bee colour vision was employed. For each food plant an index was calculated that described the probability of a bee foraging on it to subsequently choose an orchid flower. This choice probability correlated to colour distance according to the model of bee colour vision, indicating that bees chose the deceptive orchid more frequently if they foraged on more similarly coloured species. At different sites different plant species served as models. Bees foraging on food plants from which a high choice rate to the orchid was observed visited the orchid less often after approaching it than other bees, which is likely to reflect avoidance learning. In general, the pollination syndrome appears to be a generalized form of Batesian mimicry, in which similarity to rewarding plants determines reproductive success. As expected by negative density-dependent selection, individual fruit set and pollinia export rate correlated negatively with orchid density, but were unaffected by food plant density, orchid frequency, individual variation of labellum colour, labellum size, or mouth width of the flowers.