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

In tandem, a growing multifaceted RNA virosphere is emerging, uncovering the viral dark matter associated with every single living organism assessed (Greninger, 2018) . Despite the exponential growth of data generation during the last years and the parallel discovery of thousands of new emergent viruses, one crucial characteristic of RNA virus genomes, which has maintained constant in the metagenomics revolution (perhaps the only constant), is an upperlimit on genome length of <34 kb in RNA viruses (Shi et al., 2018) . Ball python nidovirus (Nidovirales; Coronaviridae) presents a 33.4 kb RNA virus genome (Stenglein et al., 2014) which still remains to be the largest reported to date and has been associated with respiratory disease in experimental infections of ball pythons (Bodewes et al., 2014; Hoon-Hanks et al, 2018) . Nidoviruses are positive-stranded RNA enveloped viruses with a monosegmented large genome. The linear nidoviruses genome is infectious, and its RNA molecule is capped and polyadenylated. The order Nidovirales is comprises four different families: Arteriviridae, Mesoniviridae, Coronaviridae and Roniviridae (Adams et al., 2017) . While genome size of nidoviruses differs significantly amid ca 12.7-33.4 kb, a general conserved genome organization, their replication strategy and sequence similarity in the replicase protein suggest that nidoviruses share a common ancestor (Gorbalenya et al., 2006) . Nidoviruses are widespread in humans and numerous animal species, and have been detected in terrestrial and marine mammals, fish, birds, reptiles, insects and crustaceans (De Groot et al., 2012; Nga et al., 2011; Lauber et al., 2012) . Interestingly, despite their evident broad host range, only one nidovirus has been associated with mollusks (Phylum Mollusca), more specifically with a Turritella sea snail (Gastropoda) (Shi et al., 2016) . The California sea hare (Aplysia californica) is a sea slug marine opisthobranch gastropod mollusk in the family Aplysiidae. This gigantic gastropod (up to 75 cm and 7 kg of weight) and valuable laboratory animal presents a "simple" nervous system of about 20 thousand neurons, distributed in 10 ganglia of ca. two thousand cells each.