Comparative Genomics Unveils Regionalized Evolution of the Faustovirus Genomes.

Faustovirus is a recently discovered genus of large DNA virus infecting the amoeba Vermamoeba vermiformis, which is phylogenetically related to Asfarviridae. To better understand the diversity and evolution of this viral group, we sequenced six novel Faustovirus strains, mined published metagenomic datasets and performed a comparative genomic analysis. Genomic sequences revealed three consistent phylogenetic groups, within which genetic diversity was moderate. The comparison of the major capsid protein (MCP) genes unveiled between 13 and 18 type-I introns that likely evolved through a still-active birth and death process mediated by intron-encoded homing endonucleases that began before the Faustovirus radiation. Genome-wide alignments indicated that despite genomes retaining high levels of gene collinearity, the central region containing the MCP gene together with the extremities of the chromosomes evolved at a faster rate due to increased indel accumulation and local rearrangements. The fluctuation of the nucleotide composition along the Faustovirus (FV) genomes is mostly imprinted by the consistent nucleotide bias of coding sequences and provided no evidence for a single DNA replication origin like in circular bacterial genomes.