Bacteriophages: evolution of the majority.

The dsDNA-tailed bacteriophages are probably the largest evolving group in the Biosphere and they are arguably very ancient. Comparative examination of genomes indicates that the hallmark of phage evolution is horizontal exchange of sequences. This is accomplished, first, by rampant non-homologous recombination between different genomes and, second, by reassortment of the variant sequences so created through homologous recombination. The comparative analysis suggests mechanisms by which new genes can be added to phage genomes and by which genes with novel functions may be assembled from parts. Horizontal exchange of sequences occurs most frequently among closely related phages, but it also extends across the entire global population at lower frequency. Bacteriophages also have probable ancestral connections with viruses of eukaryotes and archaea.