Natural Wolbachia infections in the Drosophila yakuba species complex do not induce cytoplasmic incompatibility but fully rescue the wRi modification.

In this study, we report data about the presence of Wolbachia in Drosophila yakuba, D. teissieri, and D. santomea. Wolbachia strains were characterized using their wsp gene sequence and cytoplasmic incompatibility assays. All three species were found infected with Wolbachia bacteria closely related to the wAu strain, found so far in D. simulans natural populations, and were unable to induce cytoplasmic incompatibility. We injected wRi, a CI-inducing strain naturally infecting D. simulans, into the three species and the established transinfected lines exhibited high levels of CI, suggesting that absence of CI expression is a property of the Wolbachia strain naturally present or that CI is specifically repressed by the host. We also tested the relationship between the natural infection and wRi and found that it fully rescues the wRi modification. This result was unexpected, considering the significant evolutionary divergence between the two Wolbachia strains.

Wolbachia infections of the whitefly Bemisia tabaci.

We report the first systematic survey for the presence of Wolbachia endosymbionts in aphids and whiteflies, particularly different populations and biotypes of Bemisia tabaci. Additional agriculturally important species included were predator species, leafhoppers, and lepidopterans. We used a polymerase chain reaction (PCR)-based detection assay with ribosomal 16S rDNA and Wolbachia cell surface protein (wsp) gene primers. Wolbachia were detected in a number of whitefly populations and species, whitefly predators, and one leafhopper species; however, none of the aphid species tested were found infected. Single, double, and triple infections were detected in some of the B. tabaci populations. PCR and phylogenetic analysis of wsp gene sequences indicated that all Wolbachia strains found belong to group B. Topologies of the optimal tree derived by maximum likelihood (ML) and a ML tree in which Wolbachia sequences from B. tabaci are constrained to be monophyletic are significantly different. Our results indicate that there have been at least four independent Wolbachia infection events in B. tabaci. The importance of the presence of Wolbachia infections in B. tabaci is discussed.

Evolution of Wolbachia-induced cytoplasmic incompatibility in Drosophila simulans and D. sechellia.

The intracellular bacterium Wolbachia invades arthropod host populations through various mechanisms, the most common of which being cytoplasmic incompatibility (CI). CI involves elevated embryo mortality when infected males mate with uninfected females or females infected with different, incompatible Wolbachia strains. The present study focuses on this phenomenon in two Drosophila species: D. simulans and D. sechellia. Drosophila simulans populations are infected by several Wolbachia strains, including wHa and wNo. Drosophila sechellia is infected by only two Wolbachia: wSh and wSn. In both Drosophila species, double infections with Wolbachia are found. As indicated by several molecular markers, wHa is closely related to wSh, and wNo to wSn. Furthermore, the double infections in the two host species are associated with closely related mitochondrial haplotypes, namely siI (associated with wHa and wNo in D. simulans) and se (associated with wSh and wSn in D. sechellia). To test the theoretical prediction that Wolbachia compatibility types can diverge rapidly, we injected wSh and wSn into D. simulans, to compare their CI properties to those of their sister strains wHa and wNo, respectively, in the same host genetic background. We found that within each pair of sister strains CI levels were similar and that sister strains were fully compatible. We conclude that the short period for which the Wolbachia sister strains have been evolving separated from each other was not sufficient for their CI properties to diverge significantly.