TUCSON, ARIZONA--Many insects teem with bacteria that perform various tasks for their hosts. Now add self-defense to the list of accomplishments. Last week here at the Ecological Society of America meeting, ecologists described how a little-studied type of bacteria protects pea aphids against parasitic wasps.
Over the past dozen years, scientists using molecular techniques have identified all sorts of bacteria that dwell inside insects' cells. Some of these bacteria manipulate their hosts' sex lives to help themselves (ScienceNOW, 10 May). In other cases, there's a tangible benefit to the host. Aphids, for example, depend on a bacterium called Buchnera that makes nutrients lacking in their plant foods. Pea aphids can also host another poorly understood kind of bacteria (members of the gamma proteobacteria clade) that they get from their mothers. Graduate student Kerry Oliver and co-workers at the University of Arizona in Tucson wondered what keeps this relationship going in aphid populations.
The researchers knew that aphids vary in their resistance to a major enemy, a parasitic wasp that lays an egg in a young aphid. The larva converts the aphid to a mummylike cocoon. To find out if the symbionts help protect the aphids, Oliver's group injected bacteria-free aphids with three varieties of the bacteria--T-type, R-type, and U-type. The types protected the aphids to various degrees. While U-type offered no protection against the larvae, the wasp larvae were more likely to shrivel and die in aphids infected with T-type or R-type. (Oliver doesn't yet know how the bacteria protect aphids. They could prime the aphids' immune systems, for example, or secrete a toxin, he says.)
Infected aphids that rejected a wasp larvae also produced more offspring as adults than uninfected aphids that rejected a wasp larvae. Other researchers have found that the infected aphids infected with R-type die younger, however, which means there may be a tradeoff, Oliver says.
The study is the first to show conclusively that a bacterial symbiont can protect aphids against parasites, says Richard Stouthamer, an ecologist at the University of California, Riverside. "This is one more example of how important these bacteria are in insects' lives." Oliver says the bacterial defenders also might be one reason why using the wasps to control pea aphid pests in U.S. farmer's fields doesn't always work.
Background on animal symbiosis