Maximum likelihood tree of 101 PolB sequences in the complete reference set. The phylogenetic tree was built using PhyML  (Jones-Taylor-Thornton substitution model , 100 bootstrap replicates) based on a multiple sequence alignment generated using M-Coffee . This tree is unrooted per se. The phage group was arbitrarily chosen as an outgroup for presentation purposes. The lengths of branches do not represent sequence divergence. Bootstrap values lower than 70% are not shown. The selected 51 representatives for the phylogenetic mapping and the associated branches are highlighted in bold face and black lines, respectively. Different colors correspond to different taxa: viruses (blue), eukaryotes (orange), bacteria (green) and archaea (pink).