Genomes on the shrink

摘要

Because genetic events do not fossilize, we are forced to deduce the evolution of bacterial genomes by comparing the features of contemporary organisms. This comparative approach has exposed many of the alterations endured by bacterial genomes (bouts of expansion and contraction or changes in base compositions), but such reconstructions are often imprecise, and sometimes incorrect, because they are limited by the spectrum and relationships of the se-quenced genomes that are available. Determining whether changes in gene repertoires and genome size were gradual or episodic is not feasible when the genomes being considered diverged several hundred million years ago. Fortunately, the results reported by Nilsson et al. (1) in this issue of PNAS indicate how such transformations proceed and help to explain one of the most interesting and pervasive trends in the evolution of bacterial genomes.