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. These viruses may have evolved from a small ancestor. During their evolution, virus-to-virus horizontal gene transfer has substantially contributed to the expansion of the genomic repertoire of giant viruses. In this study, we identified a horizontal transfer of a large fraction of the genome between viruses in pandoraviruses, a group of giant viruses with the largest genome sizes reaching 2.5 Mbp. We isolated a pandoravirus that belongs to a known viral species. However, its genome size was 200 kbp larger than that of other strains in the same species. Comparative genomics identified a 180-kbp genomic fraction with 168 genes in the newly isolated virus, which may have been horizontally transferred 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.IMPORTANCEGiant viruses are double-stranded DNA viruses belonging to the phylum