Novel circoviruses identified in short-finned pilot whale and orca from the North Atlantic Ocean

The family Circoviridae comprises viruses with small single-stranded DNA genomes that are known to infect various animals, resulting in considerable morbidity and mortality in some hosts. Circoviruses have been recently identified through metagenomic sequencing in diverse terrestrial vertebrate species, but their distribution and diversity in marine vertebrates remains underexplored. Here, we use high-throughput sequencing (HTS) to identify circoviruses from archived tissue samples of delphinids (order Artiodactyla, infraorder Cetacea, family Delphinidae). Based on the HTS data, we designed specific abutting primer pairs to recover seven complete circovirus genomes from individual delphinid hosts, namely, the short-finned pilot whale (Globicephala macrorhynchus, n = 5) and the orca (Orcinus orca, n = 2). The circoviruses from the two delphinid species share <65.4 % genome-wide pairwise nucleotide identity with all classified circovirus representative sequences and 66 % amongst themselves. Accordingly, these viruses, which we have named shofin circovirus and orcin circovirus, respectively, represent two novel species. This report also marks the first detection of cetacean circoviruses in the North Atlantic Ocean (near St. Vincent, Caribbean). Notably, analysis of the capsid protein sequences and structures of the delphinid circoviruses revealed notable elaborations within the surface exposed loops that have been previously shown to be a major antigenic epitope in porcine circovirus 2. Collectively, the delphinid circovirus genomes expand the known diversity of circoviruses of marine vertebrates and suggest similar evolutionary pressures exerted by the immune systems of cetacean and suina hosts, both members of the order Artiodactyla.