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 Nucleocytoviricota, characterized by large particles and genomes. Previous studies have suggested that these viruses may have evolved from a small ancestor, but the underlying mechanisms are not fully understood. In this study, we isolated one of the largest giant viruses, pandoravirus, which belongs to a known viral species but has a genome 200 kbp larger than that of other strains in the same species. Comparative genomics identified a 180-kbp genomic fragment containing 168 genes in the newly isolated virus that is absent from other strains of the same species. Further comparative analysis indicated that this 180-kbp region has been horizontally transferred from a distantly related pandoravirus. Our findings suggest that giant viruses can exchange a massive number of genes by a horizontal transfer of a large genomic fraction, which may have contributed to their gigantism.