Genomic changes following host restriction in bacteria.

Many genomic sequences have been recently published for bacteria that can replicate only within eukaryotic hosts. Comparisons of genomic features with those of closely related bacteria retaining free-living stages indicate that rapid evolutionary change often occurs immediately after host restriction. Typical changes include a large increase in the frequency of mobile elements in the genome, chromosomal rearrangements mediated by recombination among these elements, pseudogene formation, and deletions of varying size. In anciently host-restricted lineages, the frequency of insertion sequence elements decreases as genomes become extremely small and strictly clonal. These changes represent a general syndrome of genome evolution, which is observed repeatedly in host-restricted lineages from numerous phylogenetic groups. Considerable variation also exists, however, in part reflecting unstudied aspects of the population structure and ecology of host-restricted bacterial lineages.