Abstract
Giant viruses are ecologically important players in aquatic ecosystems that have challenged concepts of what constitutes a virus. Herein, we present the giant Bodo saltans virus (BsV), the first characterized representative of the most abundant group of giant viruses in ocean metagenomes, and the first isolate of a klosneuvirus, a subgroup of the Mimiviridae proposed from metagenomic data. BsV infects an ecologically important microzooplankton, the kinetoplastid Bodo saltans. Its 1.39 Mb genome encodes 1227 predicted ORFs, including a complex replication machinery. Yet, much of its translational apparatus has been lost, including all tRNAs. Essential genes are invaded by homing endonuclease-encoding self-splicing introns that may defend against competing viruses. Putative anti-host factors show extensive gene duplication via a genomic accordion indicating an ongoing evolutionary arms race and highlighting the rapid evolution and genomic plasticity that has led to genome gigantism and the enigma that is giant viruses.
Highlights
Viruses are the most abundant biological entities on the planet and there are typically millions of virus particles in each milliliter of marine or fresh waters that are estimated to kill about 20% of the living biomass each day in surface marine waters (Suttle, 2007)
The vast majority of aquatic viruses are less than 100 nm in diameter and primarily infect prokaryotes, it is increasingly clear that a subset of the viruses in aquatic ecosystems are comparative Leviathans that have been colloquially classified as giant viruses
The first isolated giant virus in the family that later became known as the Mimiviridae, infects a marine heterotrophic flagellate that was initially identified as Bodo sp. (Garza and Suttle, 1995), and later shown to be Cafeteria roenbergensis (Fischer et al, 2010)
Summary
Viruses are the most abundant biological entities on the planet and there are typically millions of virus particles in each milliliter of marine or fresh waters that are estimated to kill about 20% of the living biomass each day in surface marine waters (Suttle, 2007). The isolation and sequencing of mimivirus, a giant virus infecting Acanthamoeba polyphaga (La Scola et al, 2003; Raoult et al, 2004), transformed our appreciation of the biological and evolutionary novelty of giant viruses This led to an explosion in the isolation of different groups of giant viruses infecting Acanthamoeba spp. including members of the genera Pandoravirus, Pithovirus, Mollivirus, Mimivirus and Marseillevirus (Boughalmi et al, 2013; Colson et al, 2013; Legendre et al, 2014; Legendre et al, 2015; Philippe et al, 2013). Each of these isolates expanded our understanding of the evolutionary history and biological complexity of giant viruses, all are pathogens of Acanthamoeba spp., a widespread taxon that is representative of a single evolutionary branch of
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