Human and simian adenoviruses contain large genomes that undergo evolution, including recombination, as noted for HAdV-D53, -B55 and D-56 recently; it is also recognized in the literature that many of the original serotypes are also recombinants, for example, “18 of the 39 prototypes of group II” (Wigand and Fliedner, Archiv fur die gesamte Virusforschung (Arch Virol.) 24:245 (68) (PMID: 5698889). Inspection of the HAdV genome data shows it is not possible to have strict cut-off numbers, which are arbitrary, artificial and not reflective of Nature’s rules.

Specifically, for HAdV-B11 and -B35, the genome percent difference is 1.8%; for HAdV-B14 and -B55 it is 1.1%, with the latter two types differing by the recombination of 2.6% of genome representing the epitope from HAdV-B11 into the genome of HAdV-B14 (97.4% of the genome).  This results in a significant change in its pathogenic effects, e.g., the host response where the body ‘sees’ HAdV-B55 as HAdV-B11, a renal pathogen, but the recombinant virus is a respiratory pathogen, with a different tissue tropism.

Additionally, a small genome percent difference between established prototypes is not unusual: the difference between HAdV-B21 and –B50 is 1.16%.  A comparison of the difference between HAdV-B3 prototype and a current field strain circulating fifty years later is 1.83%.  Two isolates of HAdV-D37 differ by 1.64%:  the “GW” prototype versus a 1991 isolate.

A new type number will be assigned where whole genome sequence data reveal that either :

(1) the virus isolate encodes novel hexon loops 1 and/or 2, corresponding to the antigen epsilon epitope

or (2) is a recombinant with one or two of these regions derived from previously designated genotypes

or (3) is a recombinant that has a unique combination of these three regions (penton base; hexon loops 1 and 2; fiber knob) derived from previously recognized types.

Unlike papillomavirus taxonomy, in which the differences between 2% and 10% homology define a subtype and less than 2% a variant, the exact cut-offs for recognizing types of adenovirus are not absolute and change according to the data.

Criteria for determining whether hexon, penton and fiber are different are multiple sequence alignments and phylogenetic analysis showing a subclade with a different cohort at a bootstrap value of 80 or greater.

Whether the hexon loops are novel depends on nt sequence divergence, i.e.  >2.5% from loop 2 prototype sequence or >2.4% from loop 1 sequence.  ( Madisch, et al. (2005) J. Virol. 79:15265).

In general, the taxonomy of adenovirus is different from other viruses.

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