Maximum likelihood tree of PolB amino acid sequences from NCLDVs. Alignment was constructed with the use of T-Coffee. All the gap-containing amino acid residue sites were removed before tree construction. The phylogenetic tree was constructed using PhyML  available at Phylogeny.fr  using WAG matrix and gamma distribution. Branch labels indicate bootstrap percentages (≥ 50%) after 100 replicates. The tree is essentially an unrooted tree, albeit mid-point rooted only for presentation purpose. The same method was used for the phylogenetic trees in Fig. 2, Fig. 3 and in the additional file 3. HcDNAV and ASFV sequences are indicated by filled diamond marks. CeV: Chrysochromulina ericina virus; PoV: Pyramimonas orientalis virus; HaV: Heterosigma akashiwo virus.