Living fossils have hot sex: study
CHICAGO (AFP) — It sounds like something out of "Star Trek" -- a tropical plant that uses toxic and come-hither odors and rising temperatures to ensure pollination.
But in fact, this scenario is straight out of nature.
In a paper released Thursday, US biologists report that the Australian cycad, a primitive tropical plant with large seed cones, uses a novel "pull-push" method to manipulate the tiny flying insects, or thrips, that it relies on for pollination.
The thrips tend to congregate in the male cones (which are much like pine cones) where they feed and make their homes -- but at a certain time of day, the plant will heat up and emit a toxic order, repelling the insects.
The pollen-laden insects then fly to the neighbouring or surrounding female cones which are emitting a more attractive odor, where they pollinate the female plant's eggs.
"The cycads are trading food for sex," said Robert Roemer, a co-author of the paper in the journal Science. "Pollen is the only thing these thrips eat, so they totally rely on the plants. And the thrips are the only animals that pollinate the plants."
The curious "mating ritual" only occurs during a short pollination period that occurs once a year to once every several years.
"Then they are done and the cones disintegrate," said Irene Terry, the University of Utah biologist who was lead author on the paper.
During the pollination period, the cycads can increase the temperature in their cones each day between 11:00 am and 3:00 pm. The male cones particularly tend to raise the temperature higher - up to 25 degrees Fahrenheit warmer than the ambient air.
The plants, use a metabolic process to turn up the heat -- burning sugars, starches, and fats that have been stockpiled to fuel routine cell functions.
This process is accompanied by a massive release of odors. One chemical in particular, named beta-myrcene, increases to toxic, lethal levels and drives the pollen-covered insects out of the male cones.
A similar process is occurring in the female cones, but because the female plants don't heat up so much, the odors generated aren't offensive. In fact at lower concentrations the beta-myrcene chemical odor is slightly attractive, so the insects end up taking refuge there and pollinating their eggs.
The novel push-pull pollination strategy that the cycads evolved may be an intermediate evolutionary stage on the path from plants using odors to repel herbivore predators to plants using attractive odors to attract pollinating insects, Terry said.
"It is thought that early on, these odors were used as defensive mechanisms to repel plant eaters, and that some early pollination systems evolved with insects that used the odors to find the plant," she said.
Cycads belong to the same group of plants as modern conifers such as pines or firs. They are known as "living fossils" because they date back 250- 290-million years to the Permian Period. This species can be found in the coastal mountains of Queensland and New South Wales in Australia.
