Horizontal Transfer, Genomic Diversity, and Genomic Differentiation

Abstract

The genome of an individual cellular organism or a DNA virus is best defined as a single example of a set of unique and uniquely placed DNA sequences. Two techniques that have provided detailed bases of comparison among Escherichia coli strains, as well as among those of many other bacterial species, are multilocus enzyme electrophoresis (MLEE) and restriction fragment length polymorphism (RFLP, also referred to as restriction analysis). Multilocus sequence typing (MLST) technique has been particularly useful in epidemiological studies of pathogenic bacteria such as Campylobacter, Helicobacter, Neisseria, and Streptococcus. A major difference between prokaryotic and eukaryotic recombination processes lies in the great phylogenetic distances crossed (very infrequently, to be sure, but with evolutionarily important effects) in prokaryotic horizontal transfer. Many types of antibiotic resistance are coded by plasmids, and conjugative plasmids facilitate horizontal gene transfer. The rates of horizontal transfer among the various bacteria in the same colon have not been investigated to any important extent; they would be worth knowing. The two sizeable hypervariable regions constitute genomic differentiation both in their site-specific dynamics of horizontal transfer and the resulting distinct local patterns of high variation. It summarizes Woese's view that horizontal transfer preceded vertical transfer and that horizontal transfer was the engine of evolution in supramolecular aggregates, which preceded cells, whose eventual achievement of sufficient complexity led to heredity and Darwinian evolution.