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Dinner for Three
6 June 2006 (All day)
An ugly little insect known as the glass-winged sharpshooter (Homalodisca coagulate) sucks the thin, nutrient-poor sap out of grapevines, while spreading an infection that causes the plants to wither and die. Now, evolutionary biologists have figured out how the pest thrives on such meager fare: It relies on a pair of bacteria that generate nutrients in a biochemical ménage a trio.
Sap lacks vitamins and the amino acids that combine to make proteins, yet many insects rely on it as their primary food source. Some, such as aphids, get help from bacteria living within specialized organs called bacteriomes. These bacteria extract meager nutrients from the sap and in turn provide amino acids and other nutrients that the aphids cannot make. Researchers suspected that the sharpshooter took advantage of similar helpers. But when Nancy Moran of the University of Arizona in Tucson, Jonathan Eisen, now at the University of California, Davis, and colleagues analyzed the insect's innards, they found more than they expected.
The scientists collected about 400 sharpshooters from a lemon orchard, dissected them, and analyzed the contents of their bacteriomes. The team then sequenced the entire genome of one resident bacterium, Baumannia cicadellinicola, and found 83 genes dedicated to making vitamins and related compounds. But to the researchers' surprise, Baumannia could not make the requisite amino acids from sap. A second bacteriome bacterium, Sulcia muelleri, could make the required amino acids but could not make the same vitamins as Baumannia.
This piecemeal approach to nutrition has never been seen before, says Eisen, whose team publishes its findings online today in PLoS Biology. Not only do these bacteria provide nutrition for the insect, but they also probably feed each other, he says.
The bacteria's coordinated effort raises a host of evolutionary puzzles, such as how the two microbes found their way into the insect and how they evolved to divide the workload, says Serap Aksoy, a molecular biologist at Yale University. Alexander Purcell, insect biologist at the University of California, Berkeley, says the finding might help farmers control sharpshooters by targeting the bacteria. But he cautions that the public is already wary of overusing antibiotics, so people may resist spraying such drugs on plants.