Getting infected may be the first step to becoming a new species, according to a new study. Some insects permanently harbor a bacterium that exerts bizarre effects on their reproduction. Now researchers have found that the bacteria may keep otherwise compatible wasps at species-length distance: If the researchers cure the wasps' infections, individuals from different species can mate and produce healthy offspring.
The bacterium Wolbachia permanently infects at least 20% of arthropod species. The germs live inside cells and are transmitted directly from mother to offspring in the egg's cytoplasm. Infection has reproductive consequences: Matings between infected males (who can't pass the infection along in their sperm) and uninfected females produce almost no offspring. But matings between infected females and uninfected males do succeed, and therefore Wolbachia spreads rapidly through populations (Science, 21 March 1997, p. 1743).
Because Wolbachia infection can make previously compatible mates incompatible, a team from John Werren's lab at the University of Rochester in New York investigated whether Wolbachia conspires to create new species. The team studied two closely related species of jewel wasps, Nasonia giraulti and Nasonia longicornis, that harbor different strains of Wolbachia. Although members of the species willingly interbreed, the team knew that the wasps' incompatible bacteria allow almost no offspring to survive.
To test whether the Wolbachia were enforcing the species barrier, the researchers fed the wasps antibiotics dissolved in sugar water. After three generations, the wasps were cured. When Wolbachia-free members of the two species mated, they produced many young. And these progeny could mate and produce offspring of their own, the team reports in the 8 February issue of Nature. "Wolbachia has the potential to play a causal role in speciation," says evolutionary biologist Seth Bordenstein, one of the study's authors.
The finding is "the first clear example that it's only a microbe that is holding the separation between these two species," says Michael Wade, an evolutionary geneticist at Indiana University, Bloomington. He adds that the work supports the controversial idea of infectious speciation--that microbes might cause new species to arise without any genetic differences.