Evolution and phylogeny of Wolbachia: reproductive parasites of arthropods.

Wolbachia are cytoplasmically inherited bacteria found in reproductive tissues of many arthropod species. These bacteria are associated with reproductive alterations in their hosts, including parthenogenesis, reproductive incompatibility and feminization. A fine-scale phylogenetic analysis was done using DNA sequences from ftsZ, a rapidly evolving bacterial cell-cycle gene. ftsZ sequences were determined for 38 different Wolbachia strains from 31 different species of insects and one isopod. The following results were found: (i) there are two major division of Wolbachia (A and B) which diverged 58-67 millions years before present based upon synonymous substitution rates; (ii) a general concordance is found between the ftsZ and 16S rDNA phylogenies, indicating that these represent bacterial strain (rather than simply gene) phylogenies; however, a possible example of recombination between A and B division bacteria may have occurred in the feminizing Wolbachia present in an isopod; (iii) extensive horizontal transmission of Wolbachia has occurred between insect taxa, including different insect orders; one strain in particular (designated Adm) shows extensive recent horizontal transmission; (iv) there is an association between the Wolbachia found in a parasitic wasp (Nasonia) and its fly host (Protocalliphora), suggesting exchange of bacteria between these species; (v) parthenogenesis induction has evolved several times among the Wolbachia; and (vi) some insects harbour infections with more than one Wolbachia strain, even within individual insects.