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

AcNV presents the largest RNA virus genome ever recorded (Supp . Fig 1.A-C) . At ca. 35.9 kb, AcNV is over 2.4 kb longer that the second largest: Ball phyton nidovirus 1 (BPNV-1) RNA genome (Stenglein et al., 2014) . When comparing the 84 refseq genomes of nidovirus available at NCBI and their taxonomic assignment to specific Nidovirales families or sub-families, three major groups became evident: (i) Arteriviridae members, "small nidoviruses"; (ii) Mesoniviridae, "intermediate nidoviruses", and (iii) Coronaviridae and Ronaviridae "large nidoviruses" (Supp. Fig 1.A-B) . Within the Coronaviridae group, particular dynamics in size may be observed such as a wider pattern of longitudes of subfamily Torovirinae members presenting a either ca. 20 kb genomes or larger ca. 32-33 kb ones, the latter being the snakehosted BPNV-1 and Morelia viridis nidovirus. On the other hand, sub-family Coronavirinae members present a more restricted pattern of genome length, which may be grouped by genus, from Deltacoronavirus to Betacoronavirus members (Supp . Fig 1.A) . It is possible that the discovery of new Ronaviridae like viruses would help complement this emergent grouping by RNA genome size. The placement of AcNV in this context is intriguing; the similar and only other mollusk nido-like virus BNLV-1 was reported to have a much shorter 20.2 kb RNA genome. Nevertheless, there are some hints that may suggest that BNLV-1 could be longer than reported: The described BNLV-1 sequence NC_032496 does not present any stop codon interrupting the ongoing ORF1a, thus the suggested translation start site and the reported 5'UTR could correspond to a truncated coding region of ORF1aPP. In this direction, while nidoviruses ORF1a is typically much longer than ORF1b, ORF1a of BNLV-1 is about the same size as