Horizontal transfer of a 180-kbp genomic fraction among the largest viral genomes

Viruses are generally considered tiny biological entities with small genomes; however, some dsDNA viruses, known as giant viruses, have large genomes that are comparable to those of small bacteria. Previous studies indicated that the genomes of giant viruses expanded from a small ancestor by a combination of gene duplication, de novo gene creation, and horizontal gene transfer (HGT). Among them, virus-to-virus HGTs are recently recognized as an important mechanism for disseminating functional genes between giant viruses. In this study, we isolated a giant virus of a member of pandoraviruses, which have the largest genome sizes reaching 2.5 Mbp. Based on the average nucleotide identity of known pandoraviruses, this new pandoravirus belongs to an existing viral species. However, its genome was approximately 200 kbp larger than those of the other strains from the same species. A genome-wide comparison identified a 180-kbp region with 168 genes missing from the other strains but found in phylogenetically distant pandoraviruses. The GC ratio and homology of the deduced amino acid sequences in this 180-kbp region suggest that the new virus horizontally acquired this region from a distantly related pandoravirus. The gene composition in the 180-kbp region further indicates that this region was already large at the time of the horizontal transfer. Our findings suggest that pandoraviruses can horizontally exchange a large portion of their genomes. This event presumably represents one mechanism for accelerating genomic evolution and gigantism in giant viruses.