Tree pollination needs male-only rot

From the region that gave us pollination by cockroaches and dung beetles, here’s another of Nature’s peculiarities: a plant that relies on a fungus as well as a pollinating insect.

In Malaysia, the pale fuzz of the Choanephora fungus attacks the male flowers dangling from the chempedak fruit tree. That fungus provides a food reward for the pollinating insects, reports a team of researchers based at Kyoto University in Japan.

“This is the first report on a pollination mutualism in which a fungus plays an indispensable role,” say Shoko Sakai and her colleagues in the March American Journal or Botany. “[W]e should be more aware of the roles fungi can play in pollination.”

The chempedak tree, Artocarpus integer, is a close relative of jackfruits in the mulberry family and produces edible fruits about 30 centimeters long. “They taste good,” notes Sakai, now at the Smithsonian Tropical Research Institute in Ancón, Panama.

In the wild, the trees bloom irregularly. Sakai monitored one for 5 years before it blossomed. Male and female flowers hang on the same tree in separate clusters, giving off the scent of ripe watermelon. Sakai cautions that she was not able to perform all the standard experiments that nail down pollination details, such as excluding insects from blossoms to check for self-fertilization. Still, she monitored flowers on six trees, including one reachable only by a walkway 20 meters above ground.

The main flower visitors were two species of gall midges. Sakai collected adults dusted with pollen but found none in their stomachs. Nor do the blooms offer the insects nectar. Both adults and larvae fed on the fungus, however.

“It’s a sneaky little system,” Olle Pellmyr of Vanderbilt University in Nashville comments appreciatively. He says he can’t think of another plant that depends on a fungus to reward pollinators. A fungus makes a dangerous partner though, and Pellmyr muses about how the tree could limit fungus virulence. The concern echoes one from his own research on yucca flowers, which get pollinated by moths that lay eggs in the flowers (SN: 7/3/99, p. 11). There, evolution of floral abortion keeps the moths in line. If too many moths attack a flower, it dies, starving the larvae.

Fungi do play a role, though not as dramatic a one, in pollination of the arrow arum, Peltandra virginica, notes Joseph M. Patt of The Nature Conservancy’s Delaware Bayshores Office in Delmont, N.J. He found that a particular species of tiny fly breeds in and pollinates these swamp plants and seems to transmit the rust fungus that attacks them. The fungus, which smells like the floral scent that Patt nicknamed dumpsterone, attracts the flies to feed on it. Instead of the main reward as in the Choanephora scenario, this fungus seems to provide just a snack before the flower opens for the main course.

The idea of a plant depending entirely on fungal reward strikes him as extreme but plausible, especially in Malaysia. As he puts it, “In the tropics, you get such fantastic interactions.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.