The fog is beginning to lift over the evolutionary landscape. Molecular phylogeny has identified three new families, Simkaniaceae, Parachlamydiaceae, and Waddliaceae. Like Chlamydiaceae, the new families are composed exclusively of obligate intracellular parasites or symbionts. Molecular phylogenetic comparisons have failed to uncover extracellular Chlamydiales or to reveal any but the most general relationship to the other great groups of Bacteria. So little is known about the families other than Chlamydiaceae that only superficial phenotypic comparison is possible, but even that is enough to suggest that all the Chlamydiales families descended from a common ancestor already having many of the characters found in extant species. Whole genome comparisons among three Chlamydiaceae species indicate that they are more alike than different and that phenotypic differences, virulence traits included, must depend on small genomic differences. All this leads to the tentative conclusion that early in the Chlamydiales lineage there arose a way of intracellular life so successful that it has been conserved ever since. Further elucidation of the pathways and mechanisms of Chlamydiales evolution will depend on future investigations, including determination of complete genome sequences for representatives of all four families. Many of the questions I asked will be answered, some of my conjectures will be supported, others refuted, and a host of new questions will be asked.
[JWM]