Maximum likelihood tree of PolB sequences belonging to NCLDVs. The phylogenetic tree was built using PhyML  (Jones-Taylor-Thornton substitution model , 100 bootstrap replicates) based on a multiple sequence alignment generated using MUSCLE . Bootstrap values lower than 50% are not shown. GOS sequences are marked with filled circles and displayed in purple. The tree was mid-point rooted. The DNA polymerase gene from the recently released Ostreococcus virus OtV5 (GenBank: EU304328) was included in this tree. The OtV5 PolB was not included in our reference set as it was not available at the time of our phylogenetic mapping study. The length of the scale bar corresponds to 0.5 substitutions per site.