Origin and Evolution of Viroids

With their small, circular, nonprotein-coding RNA genome, which may be endowed with catalytic activity, viroids have been proposed to be "fossils" of an RNA world preceding the cellular world based on DNA and proteins. This chapter summarizes the arguments sustaining this proposal and discusses the possible evolutionary scenarios for the adaptation of ancestor viroids to a cellular environment. The implications of the quasispecies nature of viroid populations and the constraints governing their evolution are also addressed, highlighting how a deeper understanding of viroid evolution is closely linked to advances in the molecular mechanisms mediating plant-viroid interactions