Long a symbol of life and fertility in Eastern cultures, the fig tree has now shown that its sexual prowess is tops in the plant kingdom, at least in one respect: It appears to hold the distance record for how far its pollen routinely travels to fertilize other fig trees. The finding, reported in today's Nature, suggests that although fig trees may require a relatively large area to reproduce successfully, they also may be better than other trees at coping with isolation or habitat fragmentation.
Playing the role of Cupid in this long-distance love affair is the tiny fig wasp--no larger than a flea--that's born inside a fig fruit, eats its way out, and buzzes on to a new tree to lay its eggs inside a flower and die. Because the wasp lives only 2 to 3 days, scientists had assumed it could not travel very far. But circumstantial evidence had suggested otherwise: The wasps somehow traveled hundreds of kilometers to colonize a formerly lifeless volcanic island in the Pacific Ocean called Anak Krakatoa, and they have been snarled in nets attached to a research airplane flying over open water in the Pacific.
To probe how far the wasps wander in their moment on Earth, John Nason, an ecological geneticist at the University of Iowa, Iowa City, interrogated the fig trees. His team analyzed the genetic makeup of seven species of these trees on Barro Colorado Island, located in a Panama lake. "The method is a lot like DNA typing in humans," Nason says. By comparing the DNA of the mother tree to that of the fruit, researchers could infer roughly the father's genetic makeup. Rather than genotype every last fig on the island to find the fathers, the researchers tallied the number of different fathers and estimated their distribution across the island, based on the known fig census. For one species, they found, the fathers were spread over an area 40 times the island's size, suggesting that the wasps had carried pollen across the lake. Indeed, they found, pollen appeared routinely to travel as much as 14 kilometers, which the researchers claim is a new distance record for plants.
Because figs are the only trees to bear fruit during the dry season, many animals on Barro Colorado would die if the fig trees weren't there, Nason maintains. Therefore, he and others say, nature-preserve planners must set aside a larger area than previously thought to protect species that rely on figs for food. "Even though this is a very well-maintained preserve," Nason says, "the only reason the figs have survived is that [Panamanians] haven't deforested the area around it." Indeed, adds Nickolas Waser, a biologist at the University of California, Riverside, "this kind of result is a valuable first step in determining ... how tropical forest reserves might be designed that would maintain biodiversity." However, he says, "we simply need a lot more information of this kind for other species."