University of Utah scientists say that they have discovered a strange method of reproduction in primitive plants called cycads.
The researchers have revealed that some species of the primeval plants, which are also known as "living fossils", heat up and emit a toxic odour to drive pollen-covered insects out of male cycad cones, and then use a milder odour to draw the bugs into female cones so the plants are pollinated.
They believe that the unusual form of sexual reproduction used by cycads, reported in the journal Science, may represent an intermediate step in the evolution of plant pollination.
"People think of plants as just sitting there and looking pretty and sending out some odours to attract pollinators, but these cycads have a specific sexual behaviour tuned to repel, attract and deceive the thrips (small flying insects) that pollinate them," says Irene Terry, research associate professor of biology at the University of Utah and principal author of the study.
The researchers have revealed that the thrips enter male cycad cones to eat the pollen, and get covered by it in the process. According to them, the "push-pull pollination" method used by some cycads makes the adult thrips fly away, and then lures them back so that some pollen-laden thrips enter female cycad cones and pollinate them.
"They (cycads) are trading food for sex. 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," says study co-author Robert Roemer, who is Terry's husband and a professor of mechanical engineering at the University of Utah.
Cycads are a group of plants that the fossil record suggests have been around since the Permian Period roughly 290 million to 250 million years ago. It once was thought that pollination of cycads occurred randomly by wind blowing the pollen from male cones to nearby female cones.
Terry, however, insists that the eggs contained in the female cones of some species of cycads, including Macrozamia lucida that was studied by her, cannot be reached by wind-blown pollen because the cone scales are too tightly packed.
She has also revealed that an individual cycad plant has a pollination period, known as coning season, once a year to once every several years. According to her, this period lasts only four weeks or less, and "then they're done and the cones disintegrate."