<p>Maximum-likelihood phylogenetic tree constructed on concatenated protein sequences of 285 orthologs shared among <i>w</i>Mel, R. prowazekii, R. conorii, <i>C. crescentus,</i> and E. coli. The location of the most recent common ancestor of the α-Proteobacteria (<i>Caulobacter</i>, <i>Rickettsia</i>, <i>Wolbachia</i>) is defined by the outgroup <i>E. coli.</i> The unit of branch length is the number of changes per amino acid. Overall, the amino acid substitution rate in the <i>w</i>Mel lineage is about 63% higher than that of <i>C. crescentus</i>, a free-living α-Proteobacteria. <i>w</i>Mel has evolved at a slightly higher rate than the <i>Rickettssia</i> spp., close relatives that are also obligate intracellular bacteria that have undergone accelerated evolution themselves. This higher rate is likely in part to be due to an increase in the rate of slightly deleterious mutations, although we have not ruled out the possibility of G+C content effects on the branch lengths.</p
