Deadly bacteria pop up in fruit flies.

Talk about a killer headache.

Bacteria that are normally content just to meddle in the sex lives of insects may also erupt in deadly population explosions in the brain and other tissues of the fruit fly, two scientists now report.

Seymour Benzer and Kyung-Tai Min, geneticists at the California Institute of Technology in Pasadena, came across this unexpected discovery when they noticed that some fruit flies they had bought recently weren't living as long as other members of the species, Drosophila melanogaster.

The progeny of the insects also died young, suggesting an inherited reduction in life span. Yet when male short-lived flies bred with normal fruit flies, this premature death sentence did not pass on, indicating that it can be inherited only from female short-lived flies.

The investigators, who were originally looking for genetic mutations that trigger neurodegeneration, used an electron microscope to study the brains of the short-lived flies. They saw strange particles in the cells of the brain, as well as the retina, ovary, and other tissues.

The number of particles increased as the insects grew older. "With advancing age, the cell, including the nucleus, becomes packed with them, akin to a bag filled with popcorn," the two scientists report in the Sept. 30 Proceedings of the National Academy of Sciences (PNAS).

Benzer and Min suspected that the particles might be intracellular bacteria, particularly those known as Wolbachia, which are inherited from the mother and populate the ovaries.

The researchers confirmed their hunch by isolating Wolbachia DNA from the fruit flies and showing that an antibody that binds to the bacterium marks the particles. The investigators observed that treating the flies with an antibiotic gave the insects a normal life span.

"The evidence is pretty convincing that it's Wolbachia," says John H. Werren of the University of Rochester (N.Y.), who has written a commentary for the next issue of PNAS.

The new finding startled Wolbachia scientists, who had previously regarded the bacterium as a "prudent parasite," says Werren. Since the bacteria live in an insect's eggs, they spread only if the female reproduces. This strategy should drive the bacteria to evolve into forms that do not harm their host, he explains.

Wolbachia has gained fame for the unusual ways it manipulates the reproduction of insects and other arthropods without hurting them (SN: 11/16/96, p. 318). In some insects, for example, infected males can reproduce only with infected females, ensuring the bacterium's spread.

Beyond these reproductive machinations, "no one had found any negative repercussions of significance to having a Wolbachia infection. This [finding] suggests there can be some," says Scott L. O'Neill of Yale University Medical School.

The investigators do not yet know whether this lethal strain of Wolbachia, dubbed popcorn by Benzer and Min, stems from a single genetic alteration in the normal form or represents a variant that has undergone more complex changes to produce a different life cycle. In the latter case, popcorn may have evolved the ability to spread directly between fruit flies, rather than depending solely on the flies' reproduction.

"What we want to know is how closely related the popcorn strain is to the strain normally found," Werren says.

Benzer and Min plan to study why popcorn begins its population explosion only in adult flies--it seems to live peacefully inside earlier stages of the insect life cycle. The researchers suggest that further studies of Wolbachia, including the popcorn strain, may provide insight into related bacteria that cause such human diseases as typhus.

Finally, Benzer notes that many researchers have trouble keeping their fruit flies alive in the laboratory and that Wolbachia may sometimes be responsible for these so-called weak populations.