It's a wasp! It's a bee! It's....an orchid?: How plants take advantage of horny pollinators

By Natalie Rossington

      Have you ever been tricked by a plant? Played by a less complex organism? Ha. No, of course not you are thinking, as you munch on your tasty apple or your juicy tomato. Wrong. We’ve been tricked by that little innocent apple tree or tomato plant for thousands of years. It seduced us by producing something tasty, so we took it out of the harsh wild environment where it faced competition for water, light, nutrients, space and lovingly planted it where we could irrigate it and give it fertilizer. It’s living the good life now. Soon enough, we were planting whole fields of these things, spreading their seeds around the world, and making reproduction a walk in the park. Sound familiar? If you lean real close you can hear these cultivated plants snickering or quietly (yet evilly) laughing at you.

Keanu just realized what's really going on.

Now, I know you feel a bit embarrassed because you never quite realized what’s really going on in the relationship between you and agriculture, but I guarantee you are not as embarrassed as the insect pollinators of the orchid genus Ophrys. They are duped in a completely different way. This little orchid flower produces a powerful concoction of chemicals that exactly replicate female pollinator sex pheromones. These pheromones secreted by the orchid combined with some modified floral hairs and blue-colored patch on the fused flower petals (or labellum) create a sex-driven craze among male pollinators. In their valiant attempts to mate with the flower (aptly called “pseudocopulation”), the males are doused in orchid pollen (Schiestl et al., 1999). And because the insect is not so smart, it flies off to have sex with another flower and has now effectively pollinated the orchid. Those sneaky devils.

       As the second largest plant family in the world with about 22,075 species, the diversity within Orchidaceae is astounding. This diversity is maintained by pollinator specificity among the orchids. This means each orchid species is pollinated by a different species of insect (Mant et al., 2005). Species in the genus Ophrys produce chemical compounds to attract male pollinators and each orchid produces a different combination of chemical compounds to attract a specific insect (Schiestl et al., 2000). This kind of relationship creates incredible floral diversity - take a look at the images below. If you squint your eyes or take off your glasses, even to us these flowers look like wasps or bees.

You don't even have to squint to see the "insects" in some of these flowers, amazing! From left to right: O. speculum, O. umbilicata, O. insectivfera, O. fuciflora, and O. apifera. 

    Why do these orchids do it? What kinds of chemicals do they secrete? How do the chemicals trick these horny pollinators? We will address and answer these three questions about these special little flowers. 

Whoa guys, wait your turn.

       Why do they do it? The fundamental reason these orchids (and plants in general) seem so sneaky (yet so crafty) is they can not move. How do you have sex if you can’t move? You must reward or trick someone to do it for you. Some plants provide a sugary nectar to reward pollinators such as birds, bats, and butterflies. Others, like Ophrys, found a more efficient way that secreting costly nectar to attract pollinators and simply swindle pollinators into doing their dirty work. 

     What kinds of chemicals do the orchids secrete? In order to elicit the "pseudocopulation" response from male pollinators, the orchids secrete electrophysiologically active chemical compounds from their flowers and, to a lesser extent, their leaves.  These active compounds, or pheromones, are volatile and change the behavior of the pollinators. To compare bouquet of compounds produced by female pollinators and orchids,  researchers used gas chromatography to create a kind of chemical fingerprint of compounds present. These two fingerprints are remarkably similar in both type and quantity of the compounds. The fingerprints also showed the compounds consist of a variety of hydrocarbons like docasene and farenosol (Schiestl et al., 2000). Some orchids produce up to 100 different compounds to attract male pollinators (Ayasse et al., 2001)!

Chemical fingerprints of pheromones present on female 

pollinators (a) and Ophrys (b) are remarkably similar. 

Adapted from Schiestl et al., 2000.

        How do the chemically active compounds trick these horny pollinators? Insects such as bees and wasps detect pheromones through pheromone receptors in their antennae. The detection of pheromones causes an electrical signal that is relayed to the insect’s brain. The electrical signal due to the reception of the pheromone elicits a sex-crazed response among male pollinators and pseudocopulation ensues (Ayasse et al. 2001; Schiestl et al., 2000).

I hope you have gained an appreciation for the amazing creativity and problem solving skills present in the plant world, especially amongst the orchids. I leave you, again, with a video narrated by the great Sir David Attenborough who has led us around the world (in the same light blue shirt and khakis) to explore the awesomeness and oddities present on our Earth. He, of course, tells the story of Ophrys better than me and uses awesome words like “bamboozle” to describe the relationship between orchid and male pollinator. 

References:

Ayasse, M., R.J. Paxton, and J. Tengo. 2001. Mating behavior and chemical communication in the order Hymenoptera. Annual Review of Entomology 46: 31-78.

Mant, J., R. Peakall, and F.P. Schiestl. 2005. Does selection on floral odor promote differentiation among populations and species of the sexually deceptive orchid genus Ophrys?. Evolution 59: 1449-1463.

Schiestl, F., M. Ayasse, H.F. Paulus, C. Lofstedt, B.S. Hansson, F. Ibarra, and W. Francke. 1999. Orchid pollination by sexual swindle. Nature 399: 421-422.

Schiestl, F., M. Ayasse, H.F. Paulus, C. Lofstedt, B.S. Hansson, F. Ibarra, and W. Francke. 1999. Sex pheromone mimicry in the early spider orchid (Ophrys sphegodes): Patterns of hydrocarbons as the key mechanism for pollination by sexual deception. Journal of Comparative Physiology 186: 567-574.

Image sources:

www.wikipedia.org/wikipedia/commons
www.opringtonfieldclub.org.uk