Evolution of a defective virus from a cellular defense mechanism

Adeno-associated virus is a defective DNA virus, requiring the presence of a helper virus in order to replicate. In this paper we consider its origin in light of several observations, most notably the following: its own replication inhibits that of the helper virus; its DNA structure resembles that of transposable (moveable) elements; and extrachromosomal circles of DNA, about the size of adeno-associated virus DNA, have been found recently in eukaryotic cells. We have arrived at a hypothesis consisting of two main ideas: (1) that cells may use transposable DNA as a mechanism of defense against viral attack, and (2) that adeno-associated virus may have evolved directly from this cellular defense mechanism.