Phylogenetic evidence for horizontal transmission of Wolbachia in host-parasitoid associations

Endosymbiotic Wolbachia infect a number of arthropod species in which they can affect the reproductive system. While maternally transmitted, unlike mi tochondria their molecular phylogeny does nor parallel that of their hosts. This strongly suggests horizontal transmission among species, the mechanis ms of which remain unknown. Such transfers require intimate between-species relationships, and thus host-parasite associations are outstandingly appro priate for study. Here, we demonstrate that hymenopteran parasitoids of fru givorous Drosophila species are especially susceptible to Wolbachia infecti on. Of the five common European species, four proved to be infected; furthe rmore, multiple infections are common, with one species being doubly infect ed and two triply infected (first report). Phylogenetic statuses of the Wol bachia infecting the different species of the community have been studied u sing the gene wsp, a highly variable gene recently described. This study re veals exciting similarities between the Wolbachia variants found in parasit oids and their hosts. These arguments strongly support the hypothesis of fr equent natural Wolbachia transfers into other species and open a new field for genetic exchanges among species, especially in host-parasitoid associat ions.