Giant Viruses and their mobile genetic elements: the molecular symbiotic hypothesis

Abstract

SummaryAmong the virus world, Giant viruses (GVs) compose one of the most successful eukaryovirus families. In contrast with other eukaryoviruses, GV genomes encode a wide array of mobile genetic elements (MGEs) that encompass diverse, mostly prokaryotic-like, transposable element families, introns, inteins, restriction-modification systems and enigmatic classes of mobile elements having little similarities with known families. Interestingly, several of these MGEs may be beneficial to the GVs, fulfilling two kinds of functions: 1) degrading host or competing virus/ virophages DNA and 2) promoting viral genome integration, dissemination and excision into the host genomes.By providing fitness advantages to the virus in which they reside, these MGES compose a kind of molecular symbiotic association in which both partners should be regarded as grantees. Thus, protective effects provided by some of these MGEs may have generated an arms race between competing GVs in order to encode the most diverse arsenal of anti-viral weapons, explaining the unusual abundance of MGEs in GV genomes by a kind of ratchet effect.Highlight-Giant Virus (GV) genomes are loaded with diverse classes of mobile genetic elements (MGEs)-MGEs cooperate with GV genes in order to fulfill viral functions.-Site-specific endonucleases encoded by MGEs are used as anti-host or anti-competing viral compounds-Integrase/transposase genes derived from MGEs have been recruited to generate integrative proviral forms.-MGEs and GVs may thus compose a mutualistic symbiosis