Molecular evolution and phylogenetic utility of Wolbachia ftsZ and wsp gene sequences with special reference to the origin of male-killing

A detailed assessment of the evolution and phylogenetic utility of two gene s, ftsZ and wsp, was used to investigate the origin of male-killing Wolbach ia, previously isolated from the ladybird Adalia bipunctata and the butterf ly Acraea encedon. The analysis included almost all available sequences of B-group Wolbachia and two outgroup ta,:a and showed that (1) the two gene r egions differ in phylogenetic utility, (2) sequence variation is hen correl ated with phylogenetic information content, (3) both genes show significant rate heterogeneity between lineages, (4) increased substitution rates are associated with homoplasy in the data, (5) wsp sequences of some taxa appea r to be subject to positive selection, and (6) only a limited number of cla des can be inferred with confidence due to either lack of phylogenetic info rmation or the presence of homoplasy. With respect to the evolution of male -killing, the two genes nevertheless seemed to provide unbiased information . However, they consistently produce contradictory results. Current data th erefore do not permit clarification of the origin of this behavior. In addi tion, A. bipunctata was found to be a host to two recently diverged strains of male-killing Wolbachia that showed increased substitution rates for bot h genes. Moreover, the wsp gene, which codes for an outer membrane protein, was found to be subject to positive selection in these taxa. These finding s were postulated to be the product of high selection pressures due to anta gonistic host-symbiont interactions in this ladybird species. In conclusion , our study demonstrates that the results of a detailed phylogenetic analys is, including characterization of the limitations of such an approach, can serve as a valuable basis for an understanding of the evolution of Wolbachi a bacteria. Moreover, particular features of gene evolution, such as elevat ed substitution rates or the presence of positive selection, may provide in formation about the dynamics of Wolbachia-host associations.