ABSTRACT
This chapter reviews information concerning the mechanisms of sources of new variation, returning at the end to ask if the genetic new information sheds any additional light on the theoretical question of how RNA-based genetic elements arose in the first place. It is thought that RNA is only used as the primary repository of genetic information by a limited and heterogeneous assortment of viruses. This characteristic sets the relevant groups of viruses apart from all other living forms, and raises the evolutionary question of how such a distinction arose. Intermolecular DI RNAs combining sequences from different genome segments of influenza virus have now been clearly demonstrated, besides bizarre Sindbis virus defective interfering (DI) RNAs which have covalently attached cellular tRNA sequences. DI RNAs are thought to be random variants which overgrow the population because they are truncated templates which are copied faster than their parental RNAs.
