"Among the many early revelations from molecular phylogenetic studies of bacteria (Woese, 1987) was the recognition that the mycoplasmas represented an evolutionarily derived condition rather than a primitive one, as once believed. Now that phylogenetic relationships and genome sizes are determined for a broader array of organisms, it is clear that the mycoplasmas are just one example of genome shrinkage that has occurred in a variety of obligately host-associated bacteria. Other prominent examples are Rickettsia and related pathogens within the α-proteobacteria; insect symbionts within the γ-proteobacteria, as exemplified by Buchnera aphidicola in aphids; the chlamydiae; and the parasitic spirochetes, such as Borrelia burgdorferi (the agent of Lyme disease)."