Molecular evolution of the Chlamydiaceae

Phylogenetic analyses of surface antigens and other chlamydial proteins wer e used to reconstruct the evolution of the Chlamydiaceae, Trees for all fiv e coding genes [the major outer-membrane protein (MOMP), GroEL chaperonin. KDO-transferase, small cysteine-rich lipoprotein and 60 kDa cysteine-rich p rotein] supported the current organization of the family Chlamydiaceae, whi ch is based on ribosomal, biochemical, serological, ecological and DNA-DNA hybridization data. Genetic distances between some species were quite large , so phylogenies were evaluated for robustness by comparing analyses of bot h nucleotide and protein sequences using a variety of algorithms (neighbour -joining, maximum-likelihood, maximum-parsimony with bootstrapping, and qua rtet puzzling). Saturation plots identified areas of the trees in which fac tors other than relatedness may have determined branch attachments. All nin e species were clearly differentiated by distinctness ratios calculated for each gene. The distribution of virulence traits such as host and tissue tr opism were mapped onto the consensus phylogeny. Closely related species wer e no more likely to share virulence characters than were more distantly rel ated species. This phylogenetically disjunct distribution of virulence trai ts could not be explained by lateral transfer of the genes we studied, sinc e we found no evidence for lateral gene transfer above the species level. O ne interpretation of this observation is that when chlamydiae gain access t o a new niche, such as a new host or tissue, significant adaptation ensues and the virulence phenotype of the new species reflects adaptation to its e nvironment more strongly than it reflects its ancestry.