Horizontal transfer of Wolbachia between phylogenetically distant insect species by a naturally occurring mechanism

Wolbachia is a genus of alpha-proteobacteria found in obligate intracellula r association with a wide variety of arthropods, including an estimated 10- 20% of all insect species [1]. Wolbachia represents one of a number of rece ntly identified 'reproductive parasites' [2] which manipulate the reproduct ion of their hosts in ways that enhance their own transmission [3-9]. The i nfluence of Wolbachia infection on the dynamics of host populations has foc used considerable interest on its possible role in speciation through repro ductive isolation [3,10,11] and as an agent of biological control [2,12,13] . Although Wolbachia normally undergoes vertical transmission through the m aternal line of its host population [14], there is compelling evidence from molecular phylogenies that extensive horizontal (intertaxon) transmission must have occurred [1,9,15-17]. Some of the best candidate vectors for the horizontal transmission of Wolbachia are insect parasitoids [15], which com prise around 25% of all insect species and attack arthropods from an enormo us range of taxa [18]. In this study, we used both fluorescence microscopy and PCR amplification with Wolbachia-specific primers to show that Wolbachi a can be transmitted to a parasitic wasp (Leptopilina boulardi) from its in fected host (Drosophila simulans) and subsequently undergo diminishing vert ical transmission in this novel host species. These results are, to our kno wledge, the first to reveal a natural horizontal transfer route for Wolbach ia between phylogenetically distant insect species.