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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?

The basic details of sexual deception seem simple enough. The obvious example is, does the insect attempt to mate with the flower? but maybe that would say more about the insect than the flower, which could be getting unexpected attention. Likewise emitting pheromones of an insect is consistent with sexual deception, but it attraction could be a result of chance, not something the flowers have optimised for. Also it takes two to tango, and even if the plant is doing everything to sexually deceive, the insects might not display sexual behaviour if they’re not in the mood. So Phillips et al. have identified more things to look for.

First they look to see who the flower is attracting. So far sexual deception relies on male insects. Phillips et al. say that female attraction is a theoretical possibility, but so far it seems to target males. They do this with chemical signals. Phillips et al. note all sexually deceptive flowers use odour to attract visitors. They also note it is a very specific odour. Sexually-deceptive flowers attract one sex of one specific type of pollinator. Close-up, they have some form of insect-shaped structure, to mimic the sight of a sexual partner.

Finally Phillips et al. note there is no nectar reward. If this is about sexual deception, then it’s clear the insect has to be deceived. It’s not engaging in opportunistic behaviour on the off-chance the flower might be a female insect, before departing for nectar.

These extra criteria increase the amount of evidence you can look for, for sexual deception. They also help make it clear exactly what Phillips et al. mean when they talk about sexual deception. This is particularly helpful in this case, as Phillips et al. talk about fungus gnats. To need to be able to make a useful comparison with fly behaviour, as flies and gnats might not sexually behave in the same way, regardless of interference from deceptive flowers.

The team conducted a few experiments, like picking flowers and moving them behind screens, to see if the insects could find the hidden flowers. They could, showing that the insects were responding, at a distance, to the scent of the flowers.

They also cut off and reversed the labellum, the landing pad, of the orchid. They then watched to see what happened. The gnats flew to the labellum and, when it was upside-down, landed on it upside-down. They saw the that the normal labellum would get more sexual activity than an inverted labellum, but both received sexual attention from the gnats.

The gnats themselves were all male, and some could be tracked through the pollination process. It’s clear that P. sanguinea is sexually deceptive, and that Phillips et al. have identified an entirely new pollinator for sexually deceptive species.

As well as expanding the range of pollinators botanists should look for, Phillips et al. have accessibly catalogued what kind of behaviours anyone should look for in sexual deception. The relationship between P. sanguinea and the fungus gnat might be very specific, but the way it was discovered has much wider significance.

Phillips R.D., B. A. Retter, C. Hayes, G. R. Brown, K. W. Dixon & R. Peakall (2013). Caught in the act: pollination of sexually deceptive trap-flowers by fungus gnats in Pterostylis (Orchidaceae), Annals of Botany, 113 (4) 629-641. DOI: http://dx.doi.org/10.1093/aob/mct295

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