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The Birds, the Bees, and the Mites

1 September 2006 (All day)
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Science

A kind of cupid.
Soil-dwelling arthropods, such as springtails, help fertilize mosses.

In the classic sex-ed story of the birds and the bees, insects flit from daisy to daisy, fertilizing girl blossoms with pollen rubbed off from boy buds. This activity has long been thought to have originated with plants that flower. But new research in today's issue of Science indicates that mites and other soil-dwelling arthropods, called springtails, ferry sperm from male to female mosses.

Ferns and mosses use swimming sperm to procreate, and thus biologists had assumed they didn't need a pollinator's services. Yet these sperm can only swim a couple of centimeters before tuckering out, and botanists have long wondered how female plants can produce their version of seeds--sporophytes--with the closest guy 10 to 20 centimeters away.

So botanist Nils Cronberg of Lund University in Sweden and colleagues embarked on a kind of moss Sex Ed 101 in the lab. They put male and female clusters of silver moss (Bryum argenteum Hedwig) on dishes coated with plaster of Paris to trap any sperm trying to making a run for it. The clusters were either allowed to touch or were placed 2 or 4 centimeters apart. Without mites or springtails, the females only made sporophytes when in contact with the males. When the animals had their run of the dishes for 20 hours, however, female plants produced offspring both 2 cm and 4 cm away.

To determine whether the mites and springtails were just poking around or whether they visited the plants for a reason, the researchers compared how many bugs camped out on fertile plants versus sterile plants. At least fives times as many animals hunkered on the fertile plants than the barren ones. The researchers don't yet know whether the creatures get a reward for their work, much as bees get nectar.

It's "a beautiful little experiment," says paleobotanist Peter Wilf of Pennsylvania State University in State College, who notes that the strategy gives mosses a way to propagate in dry places. Also, considering that mites, springtails, and mosses predate flowering plants by about 300 million years, the results extend terrestrial plant-animal interactions "quite a bit" back in time, says bryologist Jon Shaw of Duke University in Durham, North Carolina.

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