Pollination, evolution and an orchid’s seductive ruse.

In a PR coup for dumpy little green orchids everywhere, research from my PhD recently landed on the cover of the journal Evolution. But what is it about?

Spring. The Blue Mountains, west of Sydney. Altitude 1000m. Frosty winds whip a swaying eucalypt canopy infiltrated by billowing cloud. Down below, amongst snowgrass tufts, rotting logs and bracken dwell the diminutive bird orchids. Genus: Chiloglottis. They huddle in tight colonies, sporadically sprayed by the high country squall.

Each plant holds two leaves pressed flat to the damp ground. Between the leaves a stem rises, holding aloft a single intricate flower in dusky shades of green and burgundy. When banks of cloud give way to azure sky and the shrike-thrushes resume their piping, these small blooms become irresistible lures.

Their target are the gracile flower wasps. Slim glossy black insects, zooming silently on shimmering wings. They are helplessly drawn to the flower. The bird orchid is emitting a scent, detectable only to wasps, which signals the promise of a mate. Known as ‘sexual deception’, the elaborate ruse uses a precise mimicry of female wasp pheromones to fool male wasps into pollinating the orchid.

However, here on the forest floor there is not only one species of orchid outwitting wasps for its own reproductive ends. Look closer and slight differences in the characteristics of flowers and visiting wasps betray something more complex and interesting. There are actually two species here, looking largely the same, growing in the same places, both deceiving their wasp pollinators through the false promise of sex.

By emitting subtle variations of their chemical trickery, these orchids have “tuned in” to two different pollinator species. This research paper explores this phenomenon as a way of separating the gene pools of closely related organisms. At the heart of it, the story here is about the forces that keep species apart once they split, or reproductive isolation.

First, we show that the different pheromones emitted by the two orchids are responsible for attracting different pollinators. Through arcane powers of chemical synthesis that I do not understand, chemists created synthetic orchid pheromones for us. We took these into the landscape and showed that the two chemicals attract two different wasps. The only perceivable difference between the wasps involved is yellow spangles on the carapace of one of the varieties. What’s more, this specific attraction is exclusive. Chemical A only attracts wasp A, and chemical B only appeals to wasp B.

Next, we take real flowers of both kinds and place them in a row and watch the hapless wasps roll in. We see that wasp A is only attracted to flower A, even when flower B is present just centimetres away. The results are identical to the results of the synthetic pheromone experiment.

On the basis of scent, we therefore expect that orchid A may never mate with orchid B. Exclusive attraction ensures that despite living amongst one another, some orchids may never exchange genes. Despite looking almost the same to us, they may as well exist on separate islands. They distinct separate species.

In order to back this up we then looked at the genetics of the species. By using the same kind of genes used in human DNA fingerprinting we were able to show that the two kinds of orchid exhibit differences in their gene pools of a degree expected if they were different species. Furthermore, analysis showed not a single individual displaying the genetics of a hybrid. Our last tests were to make hand-pollinated hybrids to check that hybrids could indeed form. These crosses showed hybrid offspring germinated and grew faster than pure crosses.

The potential for animals to drive the formation of plant species has long been recognized. This study gives us a strong case study of how that process might look. Our orchids are spectacular examples of the power of pollinators to create and maintain plant species. Through selective pollinator attraction, the orchids have been set upon unique and separate evolutionary journeys.

Further reading:

Whitehead, M. R. and Peakall, R. (2014) Pollinator specificity drives strong prepollination reproductive isolation in sympatric sexually deceptive orchids. Evolution 68: 1561–1575. doi: 10.1111/evo.12382

Rod Peakall and Michael R. Whitehead (2014) Floral odour chemistry defines species boundaries and underpins strong reproductive isolation in sexually deceptive orchids Annals of Botany 113 (2): 341-355 first published online September 19, 2013 doi:10.1093/aob/mct199

Plant pollinator interactions in the South African flora

The slides from my recent departmental seminar at the ANU are below.

The first half of the talk concentrates on plant-pollinator interactions, floral guilds and floral evolution. The second half is a slideshow of vistas, creatures and plants I encountered in my work.

Roses reflect greatest above 620 nm, Violets reflect at 420 – 480 nm…

Roses are red,  Violets are blue,  Botany is sexy, But less so than you.

Roses are red,
Violets are blue,
Botany is sexy,
But less so than you.

Along with odour, flower colour is perhaps the most important cue plants use to advertise to pollinators. Change the colour of a flower and that change can have large consequences on which pollinating animals are likely to visit[1]. Bees, for example, are attracted to purple flowers with UV highlights. If that plant were to mutate to white, it could very well find itself being visited by nocturnal moths[2].

In studying plant-pollinator evolution and ecology, it is very important then that we have some objective quantification of the colour of a flower. Human eyes are famously fallible and many insects and birds can see outside the range of our colour vision (400 – 700 nm).

The instrument we use is a spectrometer[3]. It uses optic fibres to bounce an initially white-light beam off the surface you want to measure. The wavelengths of light that are reflected (as opposed to absorbed) determine the colour of the surface you are looking at. The spectrometer collects the reflected light, separates the wavelengths through diffraction and digitises the signal. The result is a graph such as the one above.

In the graph, the wavelength is given on the horizontal axis, while the proportion of reflectance is on the vertical. The rainbow bar above provides an approximation of how the human eye perceives a given wavelength of light. The rose therefore will reflect greatest at wavelengths above 620 nm, the red part of the spectrum. A violet most strongly reflects around 420 – 480 nm. A pure white surface would show high reflectance across the range of the visible light spectrum.

Dedicated to my sweetheart, who for the second year in a row has been alone on Valentine’s.

Kniphofia are red, Agapanthus are blue.

Fieldwork is fun, But I do miss you.