Do Viruses Exchange Genes across Superkingdoms of Life?

Malik, Shahana S.; Azem-e-Zahra, Syeda; Kim, Kyung Mo; Caetano-Anollés, Gustavo; Nasir, Arshan

Depending on the nature of the infected host, viruses can be broadly classified into three major groups, archaeoviruses, bacterioviruses (Krupovic et al., 2016) , and eukaryoviruses, in addition to the lesser-known virophages that parasitize giant viruses (La Scola et al., 2003 Scola et al., , 2008 . While host jumps are common (Longdon et al., 2014; Geoghegan et al., 2017) , such as HIV from chimps (Sharp and Hahn, 2010) , SARS Coronavirus from bats (Li et al., 2005) , H1N1 from birds (Webby and Webster, 2001) , and arboviruses that replicate in mammalian cells and insect vectors, viruses are not known to infect cellular organisms separated by superkingdom (domain of life) boundaries (Nasir et al., 2014 . This has been confirmed by recent studies revealing strong biases in the distribution of viral replicon types in superkingdoms such as the paucity of discovered RNA and retrotranscribing viruses in prokaryotes and their abundance and diversity in eukaryotic species such as mammals and vertebrates (Nasir et al., 2014; Koonin et al., 2015) . The highly specific nature of virus-host interactions logically constrains genetic exchange to occur more frequently between the interacting partners. For example, bacterioviruses are known to capture bacterial genes involved in toxins and photosynthesis Lindell et al., 2004) . Similarly, eukaryoviruses often capture genes involved in antiviral immunity from eukaryotic cells (Elde and Malik, 2009; Rappoport and Linial, 2012) . Thus, host-constrained evolution of viral lineages has led to favoring either the "escape" or "reduction" models for the origin of modern viruses, both attributing viral origins from modern or ancient host cells (reviewed in Hendrix et al., 2000; Forterre and Krupovic, 2012; Nasir et al., 2012b) .