Impact of Horizontal Genetic Transfer on the Evolution of Bacterial Genomes

Analyses of bacterial genomes suggest that horizontal genetic transfer was an important process in the acquisition of new functions. Models of bacterial diversification are explored in detail in this chapter. Although the rate of horizontal transfer in Escherichia coli is substantial, and potentially useful information has been delivered in the form of selfish operons, it is clear that bacterial genomes are not growing ever larger in size; it is also clear that particular bacterial strains exhibit a definable set of metabolic capabilities and certainly do not perform every biochemical function possible. The acquired sequences—when they are maintained by natural selection and show signs of amelioration—must provide a selectable function that contributes to cellular fitness, few point mutation changes are likely to improve cellular fitness. For this reason, acquired genes are more likely to contribute to the long-term evolution of bacterial species than are point mutations. Both mutational processes and horizontal genetic transfer have allowed diversity of bacterial species in soils to arise from the common ancestor of all known life. If the roles of horizontal genetic transfer and chromosomal deletion are considered in shaping the composition of bacterial chromosomes, we can see that the mode of genome evolution has strong implications for how bacteria diversify. More recent work examining the role played by laterally transferred genes in lineage diversification concluded that the acquisition of laterally acquired genes (like the cob operons in Salmonella) was the initiating event leading to genetic isolation between bacterial species.