Evolution of Tailed Phages: Insights from Comparative Phage Genomics

Abstract

Abstract Many phage researchers believe that phages are as old as their bacterial hosts. If this hypothesis is true, then we have to postulate elements of vertical evolution for phages. In view of the postulated antiquity of these relationships we might not expect sequence similarities between more distantly related phages. Comparative phage genomics can reveal DNA sequence, protein sequence, or gene map similarities in the absence of sequence similarities between increasingly diverged phages. For even more distant relationships, data from structural biology can be informative. Recent genomics-based ideas on phage evolution are dominated by two interpretations. In one view all doublestranded DNA tailed phage genomes are mosaics with access, by horizontal exchange, to a large common genetic pool, but in which access to the gene pool is not uniform for all phages (22). In this hypothesis horizontal gene transfer dominates over vertical evolution. In fact, it is in some way an updated version of the classical modular theory of phage evolution developed 30 years ago on the basis of heteroduplex mapping with lambdoid coliphages (2, 7). Other investigators studying phages from dairy bacteria observed strong elements of vertical evolution in the structural gene cluster of phages that were not erased by horizontal gene transfer events (5).