Complete genome of Atlantic halibut reovirus (AHRV) associated with mortality in production of Atlantic halibut (Hippoglossus hippoglossus) fry

The Aquareovirus (AQRV) member Atlantic halibut reovirus (AHRV) is associated with severe liver pathology and high fry mortality and constitutes a significant problem for Atlantic halibut (Hippoglossus hippoglossus) production in Norway. Halibut is a batch spawner and it has been suspected that AHRV may be transmitted via eggs since outbreaks of disease arise in successive batches of fry originating from the same group of brood fish. In this study, we present the complete genome of AHRV representing the first complete AQRV genome sequence from a marine cold-water fish species in the North Atlantic. The terminal 5'- and 3'-ends of the segments have the canonical conserved nucleotides 5'-GUUUUAU-UCAUC-3'. The 13 putative proteins encoded in the 11 AHRV genome segments share the highest amino acid identity with members of species AQRV A and B. Phylogenetic analysis of the most conserved proteins (VP1, VP2, VP3 and VP5) groups AHRV in a major Glade together with the same two species. However, the differences in host and environment, and the amino acid sequence identity of the RdRp (similar to 80%) compared to either AQRV A or B, suggest that this virus could possibly represent a novel species within the genus. Furthermore, we show that even though AHRV RNA cannot be detected by real time RT PCR in the first egg batches from asymptomatic Atlantic halibut brood fish, the viral RNA load is not only detectable, but appears to increase in successive spawning batches.