The cytoplasmic symbiotic bacteria Wolbachia could induce host speciation in insects.
Wolbachia are symbiotic bacteria that live in the cytoplasm of an estimated 15—20% of all insect species, including wasps of the genus Nasonia. When two different species of Nasonia mate, hybrid offspring are suppressed. The presence of the bacteria causes an incompatibility between the sperm and egg of the two Nasonia species, resulting in the loss of the sperm's chromosomes upon fertilization.
Seth Bordenstein and colleagues at the University of Rochester, New York, report in the 8 February Nature, that Nasonia species treated with antibiotics produced large numbers of hybrid offspring (Nature 2001, 409:707-710). Furthermore, the hybrids were viable and fertile. Thus, the incompatibility caused by these bacteria is the principal mechanism for the reproductive isolation between Nasonia species, implicating Wolbachia in the early stages of speciation in this genus of wasps.
Given that Wolbachia also infect arachnids, isopods and nematodes, their role in promoting speciation could be quite common.