Phylogenetic Analysis of Adenovirus Sequences: Proof of the Necessity of Establishing a Third Genus in the Adenoviridae Family

Bovine adenovirus (BAV) serotypes 4 through 8 have been found clearly distingushable from BAVs 1,2,3, and 9 and from all other mastadenoviruses, and were therefore classified as subgroup 2 BAVs (1 ) and were considered as candidate members of a new taxon (2 ). The distinction was originally based on biological properties, such as the requirement of primary cell culture for propagation, the special appearance of the caused inclusion bodies, and the lack of antigenic crossreaction with other mastadenoviruses (1 ). The separation was later strengthened by DNA studies revealing special restriction-enzyme pattern, smaller genome size (3 ) and lack of cross-DNA hybridization with subgroup 1 BAVs (4 ). Similarly, the egg-drop syndrome (EDS) virus (5 ) seemed to be an atypical aviadenovirus (6 –9 ) and was described as a candidate member of a new genus (2 ). The official classification of subgroup 2 BAVs into a new genus and the EDS virus into a new Aviadenovirus genus was, however postponed, until further evidence is gathered (2 ,10 ). Recently, a new ovine adenovirus isolate (OAV287) emerged (11 ) that differed from the offcially accepted OAV serotypes (12 ,13 ). The genome of OAV287 has been completely sequenced and was found to have a genomtc organization different from that of HAV-2 and most of other mastadenoviruses (14 ,15 ). Because of the close genetic relationship found (based on comparative study of a single gene sequence), we have informally proposed at different adenovirus meetings that subgroup 2 BAVs, OAV287, and EDS virus should all be classified into a common taxon. This could be a new (third) genus, with a proposed name of A Tadenovirus describing the characteristic high AT content found in their genomes (16 ). The idea evoked considerable oppositton, perhaps mainly because all the 51 types of human adenovnuses (HAVs) (17 ,18 ) are very simtlar to each other compared to the differences between the two subgroups of BAVs containing only nine offictally accepted serotypes (10 ). Furthermore, the sequencing and genetic study of BAVs (and generally of all animal adenoviruses) were missing, but were required before a decision could have been made. The situatton changed significantly recently, since two animal adenovirus genomes have been completely sequenced (15 ,19 ), and our group also have sequenced and analyzed characteristic genome parts of different animal adenoviruses.