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Part I. Evolution of ChlamydialesThe last common ancestor of the four families of Chlamydiales.At some time in the distant past, the Chlamydiales lineage separated into different pathways that led eventually to the present families. What was their last common ancestor (LCA) like and how have the families become different from each other? The common properties of any group are usually taken as representing the most ancient properties of that group, bearing in mind the chance of convergent evolution in similar habitats. What characters are common to all Chlamydiales families? This question can be answered only incompletely. The problem is that so much is known about one family, Chlamydiaceae. Information on the other families is sparse and often limited to a single publication. As knowledge of the other families approaches the level already reached with Chlamydiaceae, order-wide comparison of phenotype and genotype will become more evolutionarily informative. Five characters are shared by all or nearly all members of the 4 families of Chlamydiales. In listing them I will note the few exceptions. 1. All have 80 to 90% 16S rRNA relatedness to each other (Everett et al., 1999). 2. With one exception, all are obligate intracellular parasites. The exceptions are some as yet unclassified intra-amoebic endosymbionts of Parachlamydiaceae Fritsche et al., (2000). 3. All stain as Gram-negative, with one exception. Elementary bodies of Parachlamydia acanthamoeba retain the Gram stain. This Gram-positive reaction is probably due to their unusually thick cell envelopes and not to possession of canonical Gram-positive walls. 4. Their cells are distinctly pleomorphic, with extreme variants labelled EBs [see: elementary bodies] and RBs [see: reticulate bodies]. In thin-section electron micrographs, only EBs are caught in the act of attaching to and entering host cells [see: entry], and only RBs are seen to divide [see: developmental cycle in pictures]. The evolutionary beginnings of this division of labor into infectious and reproductive roles may have been brought into the first host cell by the Chlamydiales ancestor in the form of the nearly universal tendency of microorganisms to exhibit different phenotypes in multiplying and quiescent states (Huisman et al., 1996; Kaprelyants et al., 1993) The appearance in the Chlamydiales lineage of precursors of modern EBs and RBs may have coincided with the shift from endosymbiosis to intracellular parasitism. An extracellularly-stable EB that readily attached to and entered new host cells would have been of obvious advantage in a horizontally transmitted intracellular parasite, less so in a vertically passed endosymbiont. Perhaps the beginnings of the developmental cycle lay in appearance of genes for the histone-like proteins involved in condensation of the DNA of Chlamydiaceae RBs (Hackstadt, 1999). Do the other families also have these genes? 5. Multiplication occurs in membrane-bound cytoplasmic vacuoles (inclusions), each of which eventually contains many chlamydial cells. Some Parachlamydiaceae fit this description (Amman et al., 1997). Others do not. Some appear to multiply in the amoebic cytoplasm unprotected by any enclosing membrane (Horn et al., 1999). Others are found throughout the cytoplasm as single chlamydial cells, each surrounded by a membrane (Fritsche et al., 2000) More about this later. A sixth character that may be shared by all families is a drastically reduced genome size, but order-wide data are lacking [see: Part II. Genome degradation]. There emerges a tentative picture of the LCA of the four Chlamydiales
families. It was a Gram-negative obligate intracellular parasite that multiplied in membrane-bound cytoplasmic inclusions containing many chlamydial cells. It was pleomorphic, with division of labor between infectious and reproductive cell types. One can only speculate on the identity of the host for the LCA. It was probably a single-celled eukaryote. What to do about the Parachlamydiaceae that do not fit into this picture? Keeping in mind that some members of the family do fit, the simple explanation is that the deviations are the result of divergence after the families separated. This concept of the LCA of the Chlamydiales families suggests that the fundamental, order-defining attributes of Chlamydiales were established a very long time ago. It is likely that divergence in Chlamydiales has been restrained by their obligate intracellular way of life. Organisms that can live extracellularly may radiate into a much wider range of habitats that offer opportunities for more radical divergence from the ancestral type. After all, at a first approximation the inside of any one eukaryotic cell is pretty much like that of any other. NEXT: Part I. Evolution of Chlamydiales: divergence |
