Testing cannibalism as a mechanism for horizontal transmission of Wolbachia in Drosophila

Wolbachia are intracellular symbionts of many species of animals, mostly arthropods. Vertical transmission of Wolbachia is exclusively maternal and this endobacterium promotes reproductive manipulations of its hosts, increasing the fitness of infected females. Moreover, Wolbachia provides its hosts with a wide range of adaptive features ranging from protection against viral infections to dietary niche occupancy. Therefore, Wolbachia can potentially contribute to the evolutionary processes of sexual selection and speciation. The horizontal transmission of Wolbachia is strongly suggested by the non-concordant phylogeny of this endosymbiont and that of its hosts. However, the ecological mechanism(s) responsible for endosymbiont transmission between different hosts is still largely unknown. In the present study, we look at ingestion as a possible natural form of Wolbachia horizontal transmission. To this aim, we tested cannibalism between infected and uninfected Drosophila hosts, under different conditions of nutrition and gut integrity. Although ingestion represents a general and incontestable portal of entry for microorganisms, we did not find infection by Wolbachia in the progeny of cannibal individuals fed on infected flies. Our study suggests that if ingestion is a vehicle for horizontal transmission of Wolbachia in nature, either it happens very rarely or it requires other factors or conditions to be effective. We discuss the likeliness of this mechanism with respect to the likelihood of each step necessary for horizontal transmission.