Insects known as thrips are shown leaving the cone of a male cycad
plant as the cone heats up and emits an odor to drive the insects
away. The thrips feed on cycad pollen, which is contained in whitish
sacs visible under the cone scales. As the odor dissipates, thrips are
attracted back to the cones, but some mistakenly go to female cones,
thereby pollinating the plants. University of Utah researchers believe
this "push-pull" pollination method may represent an intermediate
stage in the evolution of pollination. Photo: Irene Terry, University
University of Utah scientists discovered a strange method of
reproduction in primitive plants named cycads: The plants heat up and
emit a toxic odor to drive pollen-covered insects out of male cycad
cones, and then use a milder odor to draw the bugs into female cones
so the plants are pollinated.
The unusual form of sexual reproduction used by some species of cycads
- primeval plants known as "living fossils" - may represent an
intermediate step in the evolution of plant pollination, the
researchers report in the Friday, Oct. 5 issue of the journal Science.
"People think of plants as just sitting there and looking pretty and
sending out some odors to attract pollinators, but these cycads have a
specific sexual behavior 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 thrips enter male cycad cones to eat the pollen, and get covered
by it in the process. 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
"They [cycads] are trading food for sex," says study co-author Robert
Roemer, who is Terry's husband and a professor of mechanical
engineering at the University of Utah. "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."
As a mechanical engineer, Roemer studies heat transfer within the
Terry and Roemer conducted the study of tropical and subtropical
cycads and thrips in Australia with three scientists from the
University of Queensland: evolutionary biologist Gimme Walter, organic
chemist Chris Moore and insect physiologist Craig Hull.