Editing Viroid (section)

==RNA world hypothesis==
Diener's 1989 hypothesis<ref>Diener, T O. "Circular RNAs: relics of precellular evolution?."Proc.Natl.Acad.Sci.USA, 1989;86(23):9370-9374</ref> had proposed that the unique properties of viroids make them more plausible macromolecules than [[intron]]s, or other RNAs considered in the past as possible "living relics" of a hypothetical, pre-cellular [[RNA world]]. If so, viroids have assumed significance beyond plant virology for evolutionary theory, because their properties make them more plausible candidates than other RNAs to perform crucial steps in the evolution of life from inanimate matter (abiogenesis).  Diener's hypothesis was mostly forgotten until 2014, when it was resurrected in a review article by Flores et al.,<ref name="Flores-2014">{{cite journal | vauthors = Flores R, Gago-Zachert S, Serra P, Sanjuán R, Elena SF | title = Viroids: survivors from the RNA world? | journal = Annual Review of Microbiology | volume = 68 | pages = 395–414 | date = June 18, 2014 | pmid = 25002087 | doi = 10.1146/annurev-micro-091313-103416 | url = https://digital.csic.es/bitstream/10261/107724/1/Annu.%20Rev.%20Microbiol.%20Flores%20et%20al%202014.pdf | hdl = 10261/107724 | hdl-access = free }}</ref> in which the authors summarized Diener's evidence supporting his hypothesis as:
# Viroids' small size, imposed by error-prone replication.
# Their high [[guanine]] and [[cytosine]] content, which increases stability and replication fidelity.
# Their circular structure, which assures complete replication without genomic tags.
# Existence of structural periodicity, which permits modular assembly into enlarged genomes.
# Their lack of protein-coding ability, consistent with a [[ribosome]]-free habitat.
# Replication mediated in some by [[ribozymes]]—the fingerprint of the RNA world.

The presence, in extant cells, of RNAs with molecular properties predicted for RNAs of the RNA world constitutes another powerful argument supporting the RNA world hypothesis. However, the origins of viroids themselves from this RNA world has been cast into doubt by several factors, including the discovery of [[retrozyme]]s (a family of [[retrotransposon]] likely representing their ancestors) and their complete absence from organisms outside of the [[plant]]s (especially their complete absence from [[prokaryote]]s including [[bacteria]] and [[archaea]]).<ref name=":3" /><ref name=":0" /><ref name=":1" /> However, recent studies suggest that the diversity of viroids and  others viroid-like elements is broader than previously thought and that it would not be limited to plants, encompassing even the [[prokaryotes]]. Matches between viroid cccRNAs and [[CRISPR]] spacers suggest that some of them might replicate in prokaryotes.<ref name=Lee/>