Figure 4

Putative evolution of large DNA viruses. A giant dsDNA virus or an endosymbiotic organism could be at the origin of the different classes of NCLDVs (A). This organism would have evolved mainly through genome regression as shown by the loss of the OBP ORFs (B, C, D, E). In addition, the giant microbe has probably transferred its genome to the host genome (F) and evolved to give rise to the phaeoviruses (G) and polydnaviruses (H). It is likely that horizontal gene transfer (HGT) occurred between the different organisms present in the primitive eukaryote (A, F). For example, the herpesvirus OBP might have been acquired from the NCLDV ancestor. In the case of the phaeoviruses, the viral integrated fragments could serve as templates for the production of new virions through rearrangement, recombination and mutation (I).