Tag Archives: Pollination

Sexually deceptive orchids

Chiloglottis trapeziformis Plants use a diverse range of visual and olfactory cues to attract animal pollinators, with floral volatiles often particularly critical for securing insect pollination. More than 1700 different compounds have been documented in plant floral odours, however relatively few studies have determined the precise role of these compounds in pollinator attraction. There are even fewer cases where we have a comprehensive understanding of the physiology, biochemistry and biosynthesis of the floral volatile compounds involved in pollination.

The sexually deceptive orchids are a group of plants for which specific semiochemicals play a key role in pollination. These orchids lure specific male insect pollinators to their flower by emitting volatile semiochemicals that mimic the female sex pheromone. Remarkably, this pollination strategy has evolved independently in orchids on at least four continents (Africa, Australia, Europe and South America), and new discoveries continue to be made within this family.

A recent paper in Annals of Botany studies the sexually deceptive Chiloglottis trapeziformis orchid, which attracts its specific male wasp pollinators by means of chiloglottones, a newly discovered class of volatiles with unique structures. The authors find that while initiation of chiloglottone biosynthesis requires only UV-B light, sustained production requires both UV-B and de novo protein synthesis. The amount of chiloglottone in flowers reflects the interplay between developmental stage, time and intensity of UV-B exposure, de novo protein synthesis, and feedback loops that cap the internal amount.

 

Amarasinghe, R., Poldy, J., Matsuba, Y., Barrow, R. A., Hemmi, J. M., Pichersky, E., & Peakall, R. (2015) UV-B light contributes directly to the synthesis of chiloglottone floral volatiles. Annals of Botany, 115(4): 693-703
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. 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. 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.

Ecologists make their own flowers to study moths

Thanks to JSTOR Global Plants for pointing to an interesting paper in Functional Ecology, Shape matters: corolla curvature improves nectar discovery in the hawkmoth Manduca sexta.

Hawkmoth, Manduca sexta

Hawkmoth, Manduca sexta. Photo by Pondhawk/Flickr

The research looks at how the the 3D structure of a flower helps guide hawkmoths towards nectar. Scents and sight can guide a pollinator, but to what extent does the physical form aid a visitor? Campos et al. compared trumpet-shaped flowers against flatter flowers. There’s quite a few ideas of how the mechanical properties of a flower work with pollinators. There’s the shape, and also the internal structure such as grooves in the interior that can act as guides. What is significant and what is not?

The Campos paper is novel because they have found a way to control for these features. Actually finding flowers with all the relevant factors controlled for would either be near impossible or else very impossible. Instead the team printed them.

I hadn’t thought at all about 3D printing for Botany, until I listened to to documentary on 3D Bioprinting, a couple of weeks ago (BBC Radio, available worldwide). In fact flowers seem to be just one of many things that botanists could use 3D printing for.

The ‘flowers’ look very minimalist interpretations of flowers, so people who know more about the mechanics of flowers than me can debate to what extent the approximation is useful. In many ways the simplistic shapes give very clear parameters that can be tested. Another feature is that they’re monochrome. This is because of the limitations of 3D printing. The shapes are built from layers of plastic and each layer has its own colour, so it may well be a limitation for a while yet.

As well as demonstrating that botanists can make physical structures and vary them to test hypotheses, 3D printing also bodes well for replication. A factor in replicating experiments is the equipment. How trumpet-shaped is trumpet shape? Having a stored plan means that other scientists could download and run their own variations on published experiments.

I’m now wondering what could happen if you could combine TreeSketch with 3D Printing.


Campos E.O., Bradshaw Jr H.D. & Daniel T.L. (2015). Shape matters: corolla curvature improves nectar discovery in the hawkmoth Manduca sexta, Functional Ecology, 29 (4) 462-468. DOI: 10.1111/1365-2435.12378

Botanists uncover the secrets of sexual attraction

An interesting paper has moved into free access in Annals of Botany: Caught in the act: pollination of sexually deceptive trap-flowers by fungus gnats in Pterostylis (Orchidaceae). It sounds like a very specific paper, and in some ways it is, but it’s also a helpful starting point for looking at sexual deception and pollination.

Male fungus gnat (genus Mycomya) showing copulatory behaviour with the labellum of Pterostylis sanguinea. Photograph by R. D. Phillips.

A fungus gnat on the labellum of Pterostylis sanguinea, but is it true love? Photo: R D Phillips.

Typically we think of plants rewarding pollinators with nectar, but there’s no compelling reason that plants have to do this. All they need is to be pollinated. In fact attracting insects that are foraging through many plants for sugars could lead to valuable pollen being dumped on an incompatible plant, so if a plant could evolve a trick to attract insects to their own specific species, that could be a big advantage. Some orchids do this with sexual deception, but Phillips et al. point out that recent discoveries of deception in Asteraceae and Iridaceae mean that it could be a much more common method of pollination than realised.

The usual victims of sexual deception are Hymenoptera and Diptera. Phillips et al. found fungus gnats Mycetophilidae were pollinating Pterostylis sanguinea. They suspected that these orchids used sexual deception for pollination, so they looked closer. What gives their very specific question wider importance is that first they tackled the question: What exactly does pollination by sexual deception mean?
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Buzz pollination, floral traits and pollen release

Buzz pollination, floral traits and pollen release

Buzz pollination, floral traits and pollen release

The mechanism of pollen release by sonication (buzz pollination) remains poorly understood. In a comparative study of eight sympatric buzz-pollinated species of Pedicularis, Corbet and Huang find that workers of bumble-bees (Bombus friseanus) assort themselves among Pedicularis species according to body size, and adjust their buzzing behaviour (buzz/wingbeat frequency ratio) in relation to the floral traits (galea length, pollen-grain volume) of each species. A reconsideration of published experimental studies indicates that pollen release depends on the velocity component of the buzzing vibration, supporting a hypothesis that triboelectric charging of pollen grains may contribute to the mechanism.

Evolution of Old World Salvia in Africa

Evolution of Old World <i>Salvia</i> in Africa

Evolution of Old World Salvia in Africa

Salvia (Lamiaceae) is the largest genus in the mint family, and phenotypic diversity of the genus in Africa is largely the result of repeated colonizations of the continent from different sources. Will and Claßen-Bockhoff produce a phylogenetic reconstruction and suggest that parallel character evolution is the rule rather than the exception in Old World Salvia. Notable examples are given by papery, conspicuously coloured calyces and repeated switches from bee- to bird-pollination. Different staminal lever types also evolved in parallel and should not be used any longer for characterizing major clades.

Pollination by sexual deception in Pterostylis

Pollination by sexual deception in Pterostylis

Pollination by sexual deception in Pterostylis

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. The mode of attraction, however, is uncertain. Phillips et al. find that a single species of male gnat (Mycetophilidae) visits and pollinates the rewardless flowers of P. sanguinea, and that the gnats often show probing copulatory behaviour on the labellum, leading to its triggering and the temporary entrapment of the gnat in the flower. Pollen deposition and removal occurs as the gnat escapes from the flower via the reproductive structures. The labellum is the sole source of the chemical attractant involved. 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.

Effects of pollination limitation and seed predation on female reproductive success of a deceptive orchid

13101S1R1For many species of conservation significance, multiple factors limit reproduction. In a new study published in AoB PLANTS, Walsh et al. examined the contribution of plant height, number of flowers, number of stems, as well as the joint impacts of mutualists and antagonists on the pollination biology and seed production of the imperiled, deceptive orchid, Cypripedium candidum. They found flowering stem height to be the only morphological feature significant in reproduction, with taller flowering stems simultaneously receiving increased pollination and decreased seed predation. Furthermore they found decreased seed mass in individuals subjected to hand-self pollination treatments. Their results may help explain the factors limiting seed production in other Cypripedium and further emphasize the importance of management in orchid conservation.

Flow cytometric analysis of bee pollen loads

Flow cytometric analysis of bee pollen loads

Flow cytometric analysis of bee pollen loads

Understanding the species composition of pollen on pollinators has applications in agriculture, conservation and evolutionary biology, but current identification methods cannot always discriminate taxa at the species level. Kron et al. test the use of flow cytometry to characterize pollen loads from individual bees, using DNA content as a species marker, and find that they are able to quickly measure DNA contents for nuclei from hundreds to thousands of pollen grains per bee. They observe differences in pollen load diversity between bumble-bees and honey-bees and find evidence of between-cytotype pollinator movement in a population of Solidago. This technique provides a new tool to complement other methods for examining pollinator behaviour.

In the right place at the right time for pollination

A key innovation in the evolution of plants was the origin of the hermaphroditic flower, where both male and female sexual functions occur in the same complex structure. However, this innovation created a significant problem: sexual conflict, in which the function of one sex is compromised by the proximity and function of the other. This led to a further fundamental challenge in the function of animal-pollinated, hermaphroditic flowers: minimizing such sexual conflict while still enabling the male and female fertile parts to contact pollinators in the same place. Two solutions to sexual conflict have been explored evolutionarily by plants: (1) spatial separation of fertile parts (herkogamy) and (2) temporal separation of sexual functions (dichogamy).

To evaluate the effect of partial dichogamy and movement herkogamy on pollination accuracy in ‘generalist’ flowers (flowers pollinated by a variety of animal species), a recent paper in Annals of Botany investigates Parnassia epunctulata, a plant with open, white flowers, from subalpine meadows. The stamens of this species show a remarkable pattern of repositioning, and dehisce one by one over several days before the female phase. This feature permitted the authors to examine whether the anthers and stigma are positioned accurately, facilitating pollen removal and receipt.

The open flowers were visited by a variety of pollinators, most of which were flies. Seed set was pollinator-dependent (bagged flowers set almost no seeds) and pollen-limited (manual pollination increased seed set over open pollination). Analyses of adaptive accuracy showed that coordinated stamen movements and style elongation (movement herkogamy) dramatically increased pollination accuracy. Specifically, dehiscing anthers and receptive stigmas were positioned accurately in the vertical and horizontal planes in relation to the opposite sexual structure and pollinator position. In contrast, the spatial correspondence between anthers and stigma was dramatically lower before the anthers dehisced and after stamens bent outwards, as well as before and after the period of stigmatic receptivity. This shows for the first time that a combination of movement herkogamy and dichogamy can maintain high pollination accuracy in flowers with generalized pollination. Staggered pollen and stigma presentation with spatial correspondence can both reduce sexual interference and improve pollination accuracy.

Armbruster, W.S., Corbet, S.A., Vey, A.J., Liu, S.J., & Huang, S.Q. (2013) In the right place at the right time: Parnassia resolves the herkogamy dilemma by accurate repositioning of stamens and stigmas. Annals of Botany, 113 (1): 97-103.

How orchids feed specialized bee pollinators

A significant proportion of orchids in the subtribe Oncidiinae produce floral oil as a food reward that attracts specialized bee pollinators. This oil is produced either by glands (epithelial elaiophores) or by tufts of secretory hairs (trichomal elaiophores). Although the structure of epithelial elaiophores has been well documented, trichomal elaiophores are less common and have not received as much attention.

Variation in floral morphology in the genus Lockhartia

The flowers of Lockhartia are 5–30 mm in length and lack fragrance perceptible to humans. Oil secretion by flowers of Lockhartia was first reported by Silvera (2002), but the morphology and anatomy of their elaiophores have not previously been studied in detail. A recent paper in Annals of Botany surveys the flowers of 16 species of Lockhartia and shows that all have elaiophores (oil glands) of the trichomal type.

Specialized hairs on the legs or abdomen (but not the mouthparts) of oil-gathering bees are used to collect oils, and the latter are then used as food for larvae. Pollinaria of Lockhartia are small (typically 0·7–1·3 mm long) and their attachment to the bodies of bees has not been reported. This may be due to the fact that the thin stipe collapses upon drying and this obfuscates identification of the pollinarium to generic level. The situation is further exacerbated by the fast-flying and extremely timid nature of oil-collecting bees. As a result, they are much more difficult to capture or observe from short distances than male euglossine bees, for which an abundance of observational data exists.

Blanco, M. A., Davies, K. L., Stpiczyńska, M., Carlsward, B. S., Ionta, G. M., & Gerlach, G. (2013). Floral elaiophores in Lockhartia Hook. (Orchidaceae: Oncidiinae): their distribution, diversity and anatomy. Annals of Botany, 112(9), 1775-1791.