In order for one species to diverge into two, a population must be divided into groups that cannot interbreed. Usually this reproductive isolation stems from genetic incompatibility, which can arise when a geographical barrier separates the groups, allowing them to drift apart genetically. A report published today in the journal Nature, however, describes the case of two closely related wasp species for which that barrier is not a river, nor a mountain range, but rather a microbial infection. The new research suggests that a bacteria known as Wolbachia pipiens spurred the evolution of these wasps.

Wolbachia is a particularly wily bacteria. In order to assure that as many wasps as possible pass the microbe on to the next generation, Wolbachia, which is passed down through the maternal line, alters the sperm of its male host such that it can only breed with an infected female. Any offspring from that pairing will thus inherit the microbe. Uninfected offspring can result only from matings between uninfected wasps. (The two wasp species can, in fact, interbreed if given an antibiotic to kill the Wolbachia.)

After studying the many genetic barriers that can keep the species apart, the researchers found that the presence of Wolbachia was the most formidable barrier to interbreeding and thus likely the oldest. "We're not claiming that Wolbachia are solely keeping the two wasp species reproductively separated in nature, but we have shown that the ability of the bacterium to alter that insect's reproduction predates other genetic reproductive barriers," says team member John Werren of the University of Rochester. "That supports the argument that Wolbachia can promote host speciation."