Paradoxical evolution of rickettsial genomes.
Rickettsia species are strictly intracellular bacteria that evolved approximately 150 million years ago from a presumably free-living common ancestor from the order Rickettsiales that followed a transition to an obligate intracellular lifestyle. Rickettsiae are best known as human pathogens vectored by various arthropods causing a range of mild to severe human diseases. As part of their obligate intracellular lifestyle, rickettsial genomes have undergone a convergent evolution that includes a strong genomic reduction resulting from progressive gene degradation, genomic rearrangements as well as a paradoxical expansion of various genetic elements, notably small RNAs and short palindromic elements whose role remains unknown. This reductive evolutionary process is not unique to members of the Rickettsia genus but is common to several human pathogenic bacteria. Gene loss, gene duplication, DNA repeat duplication and horizontal gene transfer all have shaped rickettsial genome evolution. Gene loss mostly involved amino-acid, ATP, LPS and cell wall component biosynthesis and transcriptional regulators, but with a high preservation of toxin-antitoxin (TA) modules, recombination and DNA repair proteins. Surprisingly the most virulent Rickettsia species were shown to have the most drastically reduced and degraded genomes compared to closely related species of milder pathogenesis. In contrast, the less pathogenic species harbored the greatest number of mobile genetic elements. Thus, this distinct evolutionary process observed in Rickettsia species may be correlated with the differences in virulence and pathogenicity observed in these obligate intracellular bacteria. However, future investigations are needed to provide novel insights into the evolution of genome sizes and content, for that a better understanding of the balance between proliferation and elimination of genetic material in these intracellular bacteria is required.