Video: Thrips desert cycad cone
Even among the most ancient plants, sex could be a hot, smelly affair.
The evidence comes from an Australian cycad that uses a nasty odour not only to lure tiny insects into collecting pollen, but also to repel them into visiting potential mates.
“It takes your breath away,” says Irene Terry from the University of Utah, Salt Lake City, US, of the smell of the palm-like cycad. “It’s a harsh, overwhelming odour like nothing you ever smelled before.”
Cycads predate flowering plants. They are thought to have appeared on Earth at least 280 million years ago. They resemble palm trees – although they are not related – with a crown of large leaves.
Birds and bees
At the centre of the crown, perched above the trunk, stands a single, large male or female cone. To reproduce, pollen from male cones must reach the interior of female cones.
Australia is home to a species of cycad, called Macrozamia lucida which has cone scales that are too close together for pollen to be blown in by the wind.
Instead, in 2004, Terry and her colleagues showed that tiny flying insects called thrips carry the pollen from male to female cones. The thrips eat the male pollen and get covered in it. Occasionally they end up in female cones as well.
Terry and her collaborators have since found out why. The key is a pungent odour emitted by the male cycad cones during the mating season.
Hot and smelly
For approximately four weeks a year, cycads go through a pollination period. During this time, male cones use energy stored within them to raise their temperature every morning. For four hours, their temperatures can rise by as much as 12 °C.
As they heat up, the odour increases. The tiny thrips desert the cones en masse, returning only after the smell-fest has ended (see video, right).
To see if the stench is what drives the thrips out of cycad cones, the researchers ran them through an odour maze. At one opening of a Y-shaped tube, they placed samples of smelly male cone scales, and at another an empty dish. The thrips entered the tube through the third arm and chose between the scales and the empty dish.
The insects’ choice depended on the time of day. At midday, when the scales were smelliest, the thrips avoided them. But early in the day and in the late afternoon, they appeared to be attracted by the smell.
The researchers found that the smells mostly came from a single chemical called beta-myrcene – a substance that is widely used in the cosmetic fragrance industry. Further experiment showed that low levels of beta-myrcene attract thrips, whereas high levels repel and even kill them.
Female cones smell similar to male cones – although they never reach the same pungent levels. The researchers think that by driving pollen-laden thrips out of male cones the cycads fool them into crawling inside female cones. There, they pollinate the cycad eggs.
The whole cycle starts again the following day “until the males wear out and the females are happily pollinated,” says Terry.
She and her colleagues believe that the unique way in which the Australian cycad uses smells to repel and attract pollinators may represent the evolutionary transition between plants using smells to repel pests and others using smells to attract pollinators.
Journal reference: Science (vol 318 p 70)