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

Susan Milius

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.

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ROOM IN BLOOM. Inside the developing green fruit of a Panamanian fig, a wasp generation hatches and mates.

H. Stadelmann and Molbo

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.

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FAMILY PORTRAIT. Picture above shows male (wingless, left) and female fig wasps of one species studied.

H. Stadelmann and Molbo

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).

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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References:

Molbo, D., C.A. Machado … A. Herre. In press. Cryptic species of fig-pollinating wasps: Implications for the evolution of the fig-wasp mutualism, sex allocation, and precision of adaptation. Proceedings of the National Academy of Sciences. Abstract.

Further Readings:

Milius, S. 2002. Making scents of flowers. Science News 162(July 27):56–57. Available at Science News.

______. 2001. Alarming butterflies and go-getter fish. Science News 160(July 21):42–44. Available at Science News.

Pellmyr, O. 2003. Yuccas, yucca moths, and coevolution: A review. Annals of the Missouri Botanical Garden 90(No. 1):35–55. Available at UIdaho.edu.

For information on how farmers use fig wasps, go to http://waynesword.palomar.edu/pljune99.htm.

For pictures of yuccas and their moths, visit http://waynesword.palomar.edu/ww0902a.htm.

Sources:

Allen Herre
Smithsonian Tropical Research Institute
Apartado 2072
Balboa
Republic of Panama

Carlos A. Machado
Department of Ecology and Evolutionary Bioology
University of Arizona
Tucson, AZ 85721

Drude Molbo
Smithsonian Tropical Research Institute
Apartado 2072
Balboa
Republic of Panama

Olle Pellmyr
Department of Biological Sciences
University of Idaho
P.O. Box 443051
Moscow, ID 83844-3051


From Science News, Volume 163, No. 17, April 26, 2003, p. 259.