Wolbachia and plant pathogen mutual exclusion: a case study

The maternally transmitted α-proteobacterium Wolbachia pipientis is commonly known as a “reproductive parasite” of arthropods and mainly insects where it has been estimated that about 76% of the species can harbour the symbiont. Although the infection is usually pervasive in populations, it was argued that vertical transmission alone does not explain the large distribution of the bacterium among arthropods. Hence, horizontal transmission was assumed as a possible mechanism promoting the spread of the symbiont among taxa of related organisms, as well as among those showing close relationships, like prey-predator, parasite-host, and parasitoid-hosts, or among species of herbivorous insects that acquire Wolbachia by ingesting tissues of the host plants contaminated with the bacteria. In this case the transmission route of the bacterium resembles that of plant pathogens. In insects, Wolbachia is able to induce feminization of the host genotypic males, parthenogenesis, male killing and cytoplasmic incompatibility: these modifications of the host breed impart a selective advantage which allows Wolbachia to spread efficiently into the host populations. On the other side, several cases of positive effects induced by Wolbachia in the host fitness have been described, including protection against viruses, thus conferring evolutionary/adaptive advantages to the host. If generalized, the antiviral protection associated with Wolbachia infection might be exploited in future strategies to reduce insect-transmitted diseases. Besides viruses, this protection has been well documented for protozoans, filarial nematodes, as well as some bacterial species. In addition, mutual exclusion or competition between Wolbachia and other endocellular symbionts has been suggested. Here we provide evidences on a natural Wolbachia infection in a phytophagous insect species where the symbiont induces feminization of genetical males and mutual exclusion of endosymbiotic plant pathogens that have been detected in the uninfected populations. The exploitation of Wolbachia as a biocontrol agent preventing plant pathogens spreading is discussed.