Interspecific horizontal Wolbachia transmission in action

Many animals harbor symbiotic microorganisms inside their body. They are especially abundant in insects. The most common endosymbiont is Wolbachia. This bacterium can influence the reproduction of its host to its own advantage facilitating its vertical transmission. Unlike other endosymbionts, Wolbachia usually does not show coevolution with its host suggesting that the bacteria can switch horizontally between insect species. However, examples of the occurrence of horizontal transmission and subsequent spread in field populations are scarce. The eastern cherry fruit fly Rhagoletis cingulata is an important economic pest species infesting cherries species in its native range in North America. It was recently introduced to Europe were it co- infests cherries with the native European cherry fruit fly Rhagoletis cerasi. The shared habitat of the two insect species resulted in the horizontal transmission of a Wolbachia strain from R. cerasi to R. cingulata. Additionally specific R. cerasi populations are infected by a Wolbachia strain that is present in R. cingulata. Thus, the two fruit fly species provide a unique opportunity to resolve the dynamics of a horizontal transfer of Wolbachia in nature. Our research objective will be accomplished by five-component strategy. First, we will sequence and compare the whole genome of different Wolbachia strains to precisely characterize the identities and relatedness of different Wolbachia strains. Second, we will specifically survey Wolbachia in invasive R. cingulata populations in Europe and trace the spatial and temporal distribution of the newly acquired endosymbiont in natural populations. Third, we will test for reproductive effects on their hosts through laboratory crosses. Forth, we will determine the introduction routes of R. cingulata performing extensive genomic characterization of native and invasive fly populations. Fifth, we will investigate the interaction of different Wolbachia strains within the host fly by comparing the density of strains in single and double infected flies. Taken together, our joint study of Wolbachia and its native and invasive Rhagoletis hosts will provide novel insights into the early stages of Wolbachia transmission, the spread of the endosymbiont in nature and the consequences of the new infection for the fly.