Expanding the size limit of RNA viruses: Evidence of a novel divergent nidovirus in California sea hare, with a ~35.9 kb virus genome

Humberto J Debat

ORF1b. Perhaps future studies should revisit BNLV-1 genome sequence. The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/307678 doi: bioRxiv preprint Given the estimated large size of AcNV genome, and the apparent constrains in terms of longer RNA viruses, the underlying basis of this discernible bar is under ongoing assessment. Probable causes of this size limit are under discussion, and the most plausible explanation could be associated with the high rate of RNA virus mutation associated with limited mechanisms of proof reading and repair, concomitant with high rates of replication (Holland, 1982; Denison et al., 2011) . Supporting these assumptions, in parallel, there appears to be a correlation between small genome DNA viruses and high rate of mutation. In evolutionary terms, mutation rates could be associated not only to polymerase fidelity, but also to viral biology, genome architecture and to replication speed (Duffy et al., 2008; Campillo-Balderas et al., 2015) . The size limit of RNA viruses is not specifically associated to monosegmented viruses. As a matter of fact, the largest members of the Reoviridae family, encompassing 9 to 12 dsRNA segments which added together grasp a total genome of ca. 32 kb, mirror the apparent size limit of Nidovirales and RNA viruses in general (Dolja & Koonin, 2018) . From and evolutionary perspective, it was recently suggested that the nido-like clade of animal viruses emerged in early invertebrates and through horizontal virus transmission then colonized vertebrates. This proposition was based on the relative small genome size of deep-rooted arthropod nidoviruses, and the observation that the largest genomes correspond to vertebrate coronaviruses, postulating that gene incorporation and layering (and concomitant genome length growth) succeeded during the co-evolution of nidoviruses and vertebrates (Dolja & Koonin, 2018) . The discovery here of the tentative AcNV associated to a mollusk, with the largest RNA genome of nidoviruses yet, with more affinity to intermediate-long Roniviridae than to larger coronaviruses, implies that the evolutionary history of Nidovirales could be more complex than expected.