A phylogenomic data-driven exploration of viral origins and evolution

Nasir, Arshan; Caetano-Anollés, Gustavo

The very basal viral groups in the uToL exemplified by ToP, ToV, and evoPCO reconstructions (Figs. 7A and 8 and fig. S4 ) included minus-ssRNA viruses and families of dsRNA viruses that harbor segmented genomes. This is remarkable. The influenza virus genome typically contains six to eight RNA segments and evolves by random genetic drift or by the reassortment of genome segments with other coinfecting influenza viruses. Thus, it is likely that proto-virocells had segmented RNA genomes that often "mated" by combining with other RNA segments. This is compatible with the proposal of Woese (107) that the earliest cells stored genes in the form of segmented RNAs. These findings support the general idea that RNA came before DNA. The ubiquity of the use of RNA primers in the synthesis of DNA and its deoxyribonucleotide precursors from ribonucleotide precursors of RNA (108) is additional support for this argument. The principle of continuity dictates that a possible shift from RNA to DNA was gradual and was likely mediated by retrotranscribing viruses (for example, Fig. 7A ).