Abstract
Giant viruses (GVs) form a diverse group of virus that all belong to the Nucleo Cytoplasmic Large DNA virus (NCLDV) family. They infects a wide range of eukaryotic hosts (for example, vertebrates, insects, protists,…) and also show a huge range in genome size (between 100 kb and 1.2 Mb). Here we review some recent results that shed light on the origin and genome evolution of these viruses with a specific emphasis on the nature of their relationships with cellular organisms. We show that genome gigantism is explained by gene transfers and gene duplication and do not result from genome reduction from a cellular ancestors. We discuss the importance of mobile genetic elements, the role of ORFans during GV evolution and propose that the evolutionary success of GV is intimately link to the extreme plasticity of their genomes. Finally we speculate about the different scenario that explain GV origins and argue that GVs probably emerge from simple genetic elements followed by multiple waves of genomic expansion/simplification.
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