Group II: SsDNA Viruses
Although bacteriophages were first described in 1927, it was only in 1959 that Sinshemer working with phage Phi X 174 showed that they could possess single stranded DNA genomes. Despite this discovery until relatively recently it was believed that the majority of DNA viruses belonged to the double stranded clade. Recent work suggests that this may not be the case with single stranded viruses forming the majority of viruses found in sea water, fresh water, sediment, terrestrial, extreme, metazoan-associated and marine microbial mats. Many of these "environmental" viruses belong to the family Microviridae. However, the vast majority has yet to be classified and assigned to genera and higher taxa. Because most of these viruses do not appear to be related or are only distantly related to known viruses additional taxa will be created for these.
All viruses in this group require formation of a replicative form - a double stranded DNA intermediate - for genome replication. This is normally created from the viral DNA with the assistance of the host's own DNA polymerase.
The evolutionary history of this group is currently poorly understood. The parvoviruses have frequently invaded the germ lines of diverse animal species including mammals, fishes, birds, tunicates, arthropods and flatworms. In particular they have been associated with the human genome for ~98 million years.
Read more about this topic: DNA Virus
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