Megavirales Composing a Fourth Domain of Life: Mimiviridae and Marseilleviridae

Abstract

The 2003 discovery of Acanthamoeba polyphaga Mimivirus led to several breakthroughs and subsequent discussions related to the evolution, origin and definition of viruses and dramatically boosted scientific interest in giant viruses. Mimivirus was the largest virus with respect to particle size and genome length, and its analysis blurred the paradigms of the viral world. Since 2008, several new viruses have been recovered from a variety of phagocytic protists and water samples. All of the protist-associated giant viruses have been proposed to share an ancestral origin and to constitute a new domain of life distinct from Bacteria, Archaea, and Eukarya, and they differ in many respects from other viruses and strongly challenge the canonical virus paradigm. Mimiviridae have a capsid diameter of approximately 500 nm and large genomes that encode more than 1,000 predicted proteins. Mimivirus and Marseillevirus were shown to harbor mRNA. Moreover, the Mimiviridae and Marseilleviridae encode proteins involved in translation and the Mimiviridae are themselves susceptible to infection by other viruses. In addition, the genomes of these viruses are mosaics composed of genes related to eukaryotes, bacteria and archaea, and they harbor signs of considerable modifications resulting from horizontal gene transfer, and gene duplication. Importantly, a growing body of evidence indicates that Mimiviridae and Marseill­eviridae are widely distributed in the biosphere and there are several clues suggesting the pathogenicity of these giant viruses that infect phagocytic protists. In this chapter, we will summarize the current knowledge on the Mimiviridae and Marseilleviridae families.