Final words

Viral genomes are small, can't cover the same range of G+C content 
and Nc as large cellular organisms

More prone to fluctuations in genome composition and codon bias, 
but exact causes are not known

Differences may reflect different replication strategies
eg. plants have no immune response to evade

Phylogeny probably plays an important role, must be accounted for

Viral codon usage may adapt to host in some viruses, but in different 
directions and in ways that may be difficult to detect

What causes what?