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.

My bruised human ego

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This is the best photo I got of a group of baboons who gave me quite an experience the other day.

On a sandy fynbos trail, I rounded a corner obscured by vegetation and came abruptly face to face with a troupe of seven of these creatures. The closest member was only 3 metres from me. All of them were stopped, standing or sitting,  looking at me as I did the same. My first reaction was one of awe, these creatures are impressively muscular and intimidating up close. One of them, a very large male, was wearing a radio collar. My second instinct was to take advantage of the photo opportunity, but my camera was in my backpack.

My only close experience with monkeys comes from Indonesian macaques, and extrapolating from the damage these ones wreak on tourists’ belongings I was not keen to get the baboons interested in anything I owned. I was also aware that some baboon troupes in the Cape have a reputation for raiding. Bins, bags, picnics, cars, houses are all fair game. They have overcome their fear of humans and are now a famous nuisance requiring full time management.

My bag therefore remained zipped and in place on my back. I raised my arms and hissed, to try and persuade them off the trail. One of the leaders began to advance on me, and the others stood up to follow suit.

Finally, I was forced to concede the path to the baboons. I back-stepped into the bush beside the track, allowing them a 2 metre thoroughfare which they calmly took in an orderly and nonchalant fashion. Only after passing me, when their backs were exposed, did they pick up speed into a quick trot for a dozen metres to put some distance between us.

 

Red Hill fynbos track

Kleinplaas Dam fynbos track

 

A most engaging mantid

Recently, I was fortunate enough to spend eight days in Ndumo Game Reserve, where for several hours a day I remained perched above clusters of large flowers smelling rather like a long-drop toilet. Tagging along as help on a study of Stapelia gigantea promised to be a chance to see a new South African biome and the wonderful creatures that come along with it.

The carrion flower (Stapelia gigantea, bottom right) in rural Zululand aloe country.

Driving to and from the field site every day we would encounter giraffe, wildebeest, nyala, impala and warthogs going about their daily activities. At dusk we’d sit in a bird hide, count waterfowl and watch crocodiles cruise on by. Our nights were serenaded by the wailing bush baby, the guttural grunting of wildebeest, the booming-bass of hippos and occasionally the manic whinny of a hyena, while the porch light drew in a bewildering buffet of invertebrate curiosity.

Croc on dusk, silently sweeping past the bird hide.

But perhaps the most endearing animal found all trip was one of the most captivating mantids I have ever seen. She is a cryptically coloured Hymenopodidae, belonging to the same subfamily as the spectacular orchid mantis. Unlike other mantids I have encountered she is very easy to handle and shows no desire to flee the hand or captivity. She is a voracious feeder and any moth or fly introduced to her enclosure scarcely lasts 5 minutes before straying too close to her raptorial forelimbs. On her second night in our field accommodation she had already laid a small ootheca.

She also displays a charming and unusual shadow boxing routine complete with weaving, jabs and feints.

Edit: I have since learned that she belongs to genus Oxypilus, a group of mantids called “Boxer mantis”, for reasons made obvious in this video. (Thanks Mantidboy for the ID).

The above video was shot with a Canon 500D, Canon 100mm f/2.8 macro in an improvised stove-top studio. A piece of white paper provided the background, the camera was stabilized on a bag of rice. This left my hands free to experiment with the lighting, provided by a cheap head torch.