Evolution of the bacterial genome : (bacterial DNA evolution, vertical vs. horizontal evolution)
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Not all parts of bacterial genomes have evolved at the same rate. Among related genomes, some gene segments are highly conserved, some are less conserved and some are highly variable. We have studied the relatedness of selected gene segments in the genomes of a group of E. coli strains and other enteric bacteria, using E. coli K12 DNA as a reference standard. We found variation in the degree of relatedness among the gene regions sampled, implying that evolutionary rates varied in different parts of enteric genomes and/or that different mechanisms of evolution have predominated in different gene regions, Mechanisms of evolution of the bacterial genome are discussed. These can be grouped as those mechanisms that are vertical in character, and those that are horizontal in character. Criteria are suggested that may help to identify genes whose inheritance has been primarily horizontal rather than vertical. Arguments are given that neither internal genome rearrangement nor promiscuous exchange of genes among the chromosomes of dissimilar bacteria have played dominant roles during evolution of the bacterial genome.