Genus demarcation criteria
A picornavirus species is a class of phylogenetically
related types or strains which would normally be expected to share (i) a
significant degree of amino acid identity of P1, 2C, 3C and 3D proteins,
(ii) monophyly in phylogenetic trees, (iii) essentially identical genome
maps, and (iv) a significant degree of compatibility in proteolytic
processing, replication, encapsidation and genetic recombination.
Members of a genus would normally be expected to have
homologous IRES structures and proteins; their sequences cluster on the
same branch in phylogenetic trees (monophyly). All known picornavirus
genera differ by (i) genome maps that exhibit distinctive features in
comparison to their closest relatives, (ii) significant divergence
(number of differences per site between sequences) of the orthologous
proteins exceeding 66% of P1cap and 64% of 2Chel,
3Cpro and 3Dpol [these values are based on current
sequence data and may vary with additional data available in future],
(iii) lack of detectable homology of proteins L (if present), 2B, 3A,
3B. If these rules do not apply, a novel species or genus may be
To distinguish virus names from species names where
they have been the same, the Picornaviridae
Study Group recommends that viruses be assigned a type number, e.g.
encephalomyocarditis virus will now be known as encephalomyocarditis
virus 1 (EMCV-1); this currently affects some virus names in the
following genera: Aphthovirus and
Sapelovirus. It is recommended that
strain designations should include information on host species,
lab-specific ID number, country of sampling, year of sampling, e.g.
Creation of a novel genus is justified if several
deviant features are observed. Authors are advised to demonstrate that a
novel sequence exhibits high divergence plus lack of homology of non-orthologous
proteins (L and 2A) when compared to the closest picornavirus relatives.
The experts of the Picornaviridae
Study Group will review and comment on proposals submitted to the ICTV.