Host-dependent evolution of three papova viruses

Abstract

A marked similarity has been observed in the organization of genomic information between two of the papova viruses, SV40 and polyoma virus (Py)¹, suggesting that these viruses may have diverged from a common ancestor during evolution. However, distinct differences between them and also between them and the human papova virus, BKV, are observed, for example, in their interactions with several animal cells², and the immunolo-gical cross-reaction between viral proteins^3,4. Results from DNA–DNA hybridization experiments^5–8 suggest that homologies between the viral DNAs are small, although the search for homology has depended largely on the stringency of the experimental conditions. The difficulties of using hybridization (or heteroduplex) analyses for determining the extent of homology between two DNA species has been considered in detail by Yang and Wu⁹. It has also recently been pointed out that in stringent hybridization conditions, greater than 85% homology is required for the observation of double-stranded-ness, and that the use of less stringent conditions reveals more extensive homology between Py and SV40 than had earlier been reported¹⁰. Nonetheless, the above data did not seem to allow detailed discussion of the relationships between papova viruses. We therefore turned to a comparison of the nucleotide sequences of the viral genomes and found striking homologies around the DNA replication origins of these viral species^11–18. Recently, the complete sequences of SV40^14,15 Py^19,20 and BKV DNAs^21,22 have been elucidated, providing insight into the evolutionary relationships of these viruses. We have now compared the nucleotide sequences of these viral genomes gene by gene and conclude that each of these viral species has evolved from a common ancestor and diverged with its host organism.