Wolbachia bacteria effects after experimental interspecific transfers in terrestrial isopods

Wolbachia bacteria are intracellular parasites, vertically transmitted from mothers to offspring through the cytoplasm of the eggs. They manipulate the reproduction of their hosts to increase in frequency in host populations. In terrestrial isopods for example, Wolbachia are responsible for the full feminization of putative males, therefore increasing the proportion of females, the sex by which they are transmitted. Vertical transmission, however, is not the only means for Wolbachia propagation. Infectious (i.e., horizontal) transmission between different host species or taxa is required to explain the fact that the phylogeny of Wolbachia does not parallel that of their hosts. The aim of this study was to investigate, by experimental transinfections, whether Wolbachia strains could be successfully transferred to a different, previously uninfected isopod host. While Wolbachia survived in all the studied recipient species, vertical transmission was efficient only in cases where donor and recipient species were closely related. Even in this case, Wolbachia strains did not always keep their ability to entirely feminize their host, a deficiency that can be link to a low bacterial density in the host tissues. In addition, Wolbachia infection was associated with a decrease in host fertility, except when the bacterial strain came from the same host population as the recipient animals. This suggest that Wolbachia could be adapted to local host populations. It therefore seems that isopod Wolbachia are highly adapted to their host and can hardly infect another species of hosts. The successful infection of a given Wolbachia strain into a new isopod host species therefore probably requires a strong selection on bacterial variants.