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No Parasite, No Fertility

It's an offer a wasp can't refuse: If the wasp doesn't host a bacterial parasite, it isn't able to produce healthy eggs. Researchers are puzzled about how the bacteria exert such power. They might be providing some compound the wasps need, or possibly setting a time bomb that only the bacteria can disarm.

Bacteria known as Wolbachia infect an assortment of invertebrates including insects and crustaceans. The bacteria, which are passed from generation to generation through the mother's eggs, toy with their host's reproductive systems to ensure their survival in the host population. For instance, infected males (who cannot pass the bacteria through sperm) and uninfected females do not produce offspring, while uninfected males and infected females can mate just fine. But unlike symbiotic bacteria, Wolbachia didn't seem necessary for the host's survival.

Now, researchers have discovered that wasps called Asobara tabida do need the bacteria; without them, they can't produce eggs. A team led by Franck Dedeine of Claude Bernard University in Villeurbanne, France, demonstrated this by treating wasp larvae with antibiotics to kill Wolbachia. They were surprised to discover that when the female larvae grew up, they didn't produce any of the cells that go on to become eggs. The researchers conclude that it's the first case of Wolbachia and a host evolving a so-called mutualistic relationship, in which the bacteria have evolved to benefit the host.

The finding, reported in the 15 May issue of the Proceedings of the National Academy of Sciences, is "an exciting discovery," says biologist John Werren of the University of Rochester, New York. "It shows that Wolbachia are the stars of manipulating reproduction in hosts." But Werren isn't sure the relationship is mutualistic. Other more likely possibilities, he says, are that the wasp has evolved so dramatically to cope with Wolbachia that the eggs will die if the bacteria aren't there; or perhaps the bacteria set a time bomb toxin that kills developing eggs unless the bacteria are still around to defuse it.

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