The glassy-winged sharpshooter, an insect pest threatening California's vineyards, depends on two bacterial passengers to supplement its thin diet -- and those bacteria rely on the insect, and each other, for vital nutrients, according to a new study from researchers at The Institute for Genomic Research, the University of Arizona and UC Davis.

Understanding what the symbiotic bacteria do for the host could lead to new methods for controlling the insect pests, said Jonathan Eisen, who recently joined UC Davis' Genome Center from The Institute for Genomic Research in Rockville, Md.

Sharpshooters feed by tapping into the xylem, the water transport system that runs through woody parts of plants. But xylem fluid is thin, watery stuff with few nutrients. So the sharpshooter must make up its diet in other ways.

The sharpshooter carries two types of bacteria that might make nutrients in exchange for living space: Baumannia cicadellinicola -- named for Paul Baumann, professor emeritus of microbiology at UC Davis and an authority on such symbiotic bacteria -- and Sulcia muelleri.

The researchers sequenced the entire genome of Baumannia after painstakingly separating the bacteria from the insects. They found pathways for making vitamins and other essentials, but no genes for making essential amino acids, which build proteins.

But the team still had a bunch of leftover sequences that did not fit into the Baumannia genome, Eisen said. Those leftovers turned out to contain the genes for making essential amino acids, but they belonged to the second, smaller symbiont, Sulcia.

Further sequencing showed that not only do the two bacteria complement each other, producing nutrients needed by the other and the host insect, but that some biochemical pathways actually start in one bacteria and continue in the other.

One side effect is that the genomes of the two symbionts have shrunk, compared to free-living bacteria. Eventually, they could lose their status as separate entities and simply be part of the insect.

Eisen also holds appointments as professor at the Department of Medical Microbiology and Immunology and the Section of Evolution and Ecology at UC Davis. In addition to Eisen, the members of the research team include Dongying Wu, Sean Daugherty, Kisha Watkins, Hoda Khouri, Luke Tallon and Jennifer Zaborsky at The Institute for Genomic Research; Susan Van Aken and Grace Pai at the J. Craig Venter Institute, Rockville, Md.; and Helen Dunbar, Phat Tran and Nancy Moran at the University of Arizona. The research is published in the June issue of the journal Public Library of Science and funded by a National Science Foundation grant to Moran.