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Part I. Evolution of ChlamydialesThe first intracellular Chlamydiales.The order Chlamydiales is unique among the major divisions of Bacteria in that it has no representatives known to be capable of extracellular living. This argues for an early restriction to intracellular existence. If the Chlamydiales lineage had evolved for a long time outside of host cells, I would expect some extant chlamydiae or their phylogenetically recognizable relatives to still be living extracellularly. The order Rickettsiales, of a diversity comparable to that of Chlamydiales, contains a few facultatively intracellular bacteria and many obligately intracellular ones (Weisberg et al., 1989; Dumler et al., 2001). The order itself falls within the alpha-division of the proteobacteria and has numerous extracellular relatives in that division. The figure below shows the phylogenetic relationships among Chlamydiales, Rickettsiales, and other Bacteria involved in the discussions to follow.
The obligate endosymbionts of protists and insects are generally alpha-, beta-, or gamma-proteobacteria (Weisberg et al., 1989; Du et al., 1994; Shigenobu et al., 2000; Akman et al., 2001; von Dohlen et al., 2001). Other well-characterized facultatively and obligately intracellular eubacteria are phylogenetically located in major bacterial groups with many extracellular representatives. Only Chlamydiales stands alone, strictly intracellular and phylogenetically isolated. Even Planctomycetales, once considered as a possible sister taxon, has now been shown to be no more closely related to Chlamydiales than to other groups of Bacteria (Ward, et al., 2000). This is an argument for early entry into host cells that would be falsified by discovery of a single member of Chlamydiales living extracellularly. Such a discovery would not necessarily be a bad thing---think of the experiments that could be done with extracellular or facultatively intracellular chlamydiae. Evolutionary time scales for appearance of the major forms of life have been estimated by paleontological and molecular methods. There is disagreement as to the exact time of some divergences, but the general pattern is clear. If life got started 3500 mya, and if the first Chlamydiales ancestor was not taken up by a host cell until at the earliest 2000 mya, it must have been the product of a long extracellular experience, long enough to evolve complex structure and behavior. Once the Chlamydiales ancestor had gone intracellular, the only hosts available for perhaps another 1500 my must have been invertebrates, first single-celled and then many-celled. Potential vertebrate hosts appeared only 500 mya, and only the last 200 my could have been spent in mammals and birds, the most important hosts from medical and veterinary points of view. Only yesterday, 0.5-0.7 mya, did hominids present themselves as potential hosts for chlamydiae, which by then were probably much like extant members of the order. The conclusion to be drawn from this trip down the pathway of Chlamydiales-host evolution is that much of the genetic makeup of Chlamydiales may be the product of selection and drift in hosts other than the ones that extant species now occupy. If I am wrong, and the Chlamydiales ancestor did not become intracellular until well after the first eukaryotes appeared, the argument changes, but only to a degree. I cannot believe that the first host for Chlamydiales was a vertebrate. NEXT: Part I. Evolution of Chlamydiales: In the beginning. Symbiosis or parasitism?
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