Origin and evolution of viral interleukin-10 and other DNA virus genes with vertebrate homologues

Phylogenies of gene families including members in both vertebrates and DNA viruses of the poxvirus and/or herpesvirus families showed that the viral genes originated at widely different times over the history of life. Certain of these viral genes (for example, the genes encoding the large and small subunits of ribonucleoside-diphosphate reductase) originated before animals diverged from fungi. while others originated much more recently. The most striking examples of recent origin involved viral genes encoding the cytokine interleukin-10 (IL-10), which originated independently in viruses at least three times since the divergence of the orders of eutherian mammals, presumably by viral capture of host genes. In certain domains, viral IL-10 genes showed significantly higher rates of nonsynonymous substitution than their nearest mammalian homologues. Though the mutation rate in these viral genes is up to 20 times that of the corresponding mammalian genes, a high mutation rate alone did not account for these differences because they were not seen in all domains. Rather. in certain domains it appears that functional constraints present in the case of mammalian IL-10 are relaxed in the case of the viral homologues. Furthermore. a nonrandom pattern of change with respect to amino acid residue charge in the N-terminal portion of the mature protein has occurred repeatedly in independently derived viral IL-10 genes, strongly suggesting that positive selection has led to divergence of this functionally important domain in viral IL-10.