Fig-Wasp Upset: Classic partnership isn’t so tidy after all

Textbooks that marvel over an extreme example of the buddy system–fig species that supposedly each pair up with a lone pollinating wasp species–may need rewriting, according to a new genetic analysis.

In four out of eight fig species tested in Panama, genetic markers reveal that the supposedly single type of wasp living in the flower turns out to be two species, reports Drude Molbo of the Smithsonian Tropical Research Institute (STRI) based in Balboa, Panama. Fig partnerships with multiple wasps may turn out to be “routine,” Molbo and her colleagues suggest in an upcoming Proceedings of the National Academy of Sciences. They also have evidence of a single wasp species teaming up with different figs.

Another pollination biologist, Olle Pellmyr of the University of Idaho in Moscow, welcomes the new study as “nice work.” The old idea that, except for a few oddballs, each of the world’s 800 fig species has an exclusive partnership with a wasp has been “dogma,” he says.

Pellmyr points out that biologists have long used fig wasps to study big questions, such as sex ratios, cheating in partnerships, and formation of new species. Molbo’s coauthor Allen Herre, also of STRI, says that the team’s findings will require some rethinking across a wide range of work, including his own.

The wasps, usually only a few millimeters long, make epic flights of up to 20 kilometers to find the right species of fig in bloom. The female wriggles into the flask-shaped flower, lays eggs, and dies there. Her offspring hatch and mate inside the fig flower. Each daughter then sets off to find a new fig plant in which to lay eggs. When she arrives inside a flower, she deposits her natal fig’s pollen.

Scientists have known that several female wasps can converge on the same flower. To sort out batches of offspring, Molbo identified DNA markers that distinguish the offspring of these females. As she analyzed populations in hundreds of fig flowers, some combinations of markers never showed up. The researchers began to suspect that the figs held pairs of wasp species.

To check their results, they turned to Carlos Machado, now of the University of Arizona in Tucson. He has identified DNA markers not from the cell nucleus, as Molbo does, but from mitochondria, the cell powerhouses. The mitochondrial markers displayed the same patterns.

“Drude was looking for one thing and found something very surprising and different,” says Herre.

One of the wasp pairs working the same fig species seems to have evolved from a shared ancestor within the past few million years, says Herre.

Pellmyr highlights this finding as a possible example of a species that split despite close quarters (SN: 7/21/01, p. 42: Available to subscribers at Alarming Butterflies and Go-Getter Fish).

Rethinking wasps and figs may rock some established ideas, but Herre says the finding does solve some puzzles in theories of resource allocation between sons and daughters. Now that Herre can tell the size of wasp broods that each mother provides, the male-female ratios better fit some earlier predictions.

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