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N S, Sano T, Naoi T, R JG. Molecular phylogeny and secondary structure analysis of hop stunt viroid (HSVd) associated with Mulberry (Morus alba) in India. Arch Microbiol 2024; 206:240. [PMID: 38698140 DOI: 10.1007/s00203-024-03966-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2024] [Accepted: 04/17/2024] [Indexed: 05/05/2024]
Abstract
Hop stunt viroid (HSVd), a small, single stranded, circular, non-coding infectious RNA known to cause infection in various economically important crop plants. In the present investigation, a study was conducted in the southern part of Karnataka districts of India to detect the possible association of HSVd infection in mulberry plants. A total of 41 mulberry plants showing typical viroid-like symptoms along with asymptomatic samples were collected and screened using conventional Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) using a specific set of HSVd-Fw/ HSVd-Re primers. Out of 41 samples, the study confirmed the presence of HSVd in six samples of mulberry collected from Ramanagara (1 sample), Chikkaballapur (3 samples) and Doddaballapura (2 samples) regions with an expected HSVd amplicon size of ∼ 290-300 nucleotides. The mechanical transmission of HSVd was also confirmed on cucumber (cv. Suyo) seedlings through bioassay, which was reconfirmed by RT-PCR. The amplicons were cloned, sequenced, and the representative nucleotide sequences were deposited in the NCBI GenBank. Subsequently, molecular phylogenetic analysis showed that HSVd mulberry isolates from this study were most closely related to grapevine isolates, indicating a common origin. On the other hand, it was shown to belong to a different group from mulberry isolates so far reported from Iran, Italy, Lebanon, and China. The secondary structure analysis of HSVd mulberry Indian isolates exhibited substitutions in the terminal left, pathogenicity, and variable regions compared to those of the Indian grapevine isolates. As far as this study is concerned, HSVd was detected exclusively in some mulberry plants with viral-like symptoms, but the pathogenesis and symptom expression needs to be further investigated to establish the relationship between HSVd and the disease symptoms in the mulberry plants.
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Affiliation(s)
- Shilpa N
- Molecular Phytodiagnostic Laboratory, Department of Studies in Botany, University of Mysore, Manasagangotri, Mysuru, Karnataka, 570 006, India
- Department of Studies in Microbiology, University of Mysore, Manasagangotri, Mysuru, Karnataka, 570 006, India
| | - Teruo Sano
- Faculty of Agriculture and Life Science, Hirosaki University, Bunkyo-cho, 3, Hirosaki, 036-8561, Japan
| | - Takashi Naoi
- Faculty of Agriculture and Life Science, Hirosaki University, Bunkyo-cho, 3, Hirosaki, 036-8561, Japan
| | - Janardhana G R
- Molecular Phytodiagnostic Laboratory, Department of Studies in Botany, University of Mysore, Manasagangotri, Mysuru, Karnataka, 570 006, India.
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2
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Koonin EV, Kuhn JH, Dolja VV, Krupovic M. Megataxonomy and global ecology of the virosphere. THE ISME JOURNAL 2024; 18:wrad042. [PMID: 38365236 PMCID: PMC10848233 DOI: 10.1093/ismejo/wrad042] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/22/2023] [Revised: 12/22/2023] [Accepted: 12/28/2023] [Indexed: 02/18/2024]
Abstract
Nearly all organisms are hosts to multiple viruses that collectively appear to be the most abundant biological entities in the biosphere. With recent advances in metagenomics and metatranscriptomics, the known diversity of viruses substantially expanded. Comparative analysis of these viruses using advanced computational methods culminated in the reconstruction of the evolution of major groups of viruses and enabled the construction of a virus megataxonomy, which has been formally adopted by the International Committee on Taxonomy of Viruses. This comprehensive taxonomy consists of six virus realms, which are aspired to be monophyletic and assembled based on the conservation of hallmark proteins involved in capsid structure formation or genome replication. The viruses in different major taxa substantially differ in host range and accordingly in ecological niches. In this review article, we outline the latest developments in virus megataxonomy and the recent discoveries that will likely lead to reassessment of some major taxa, in particular, split of three of the current six realms into two or more independent realms. We then discuss the correspondence between virus taxonomy and the distribution of viruses among hosts and ecological niches, as well as the abundance of viruses versus cells in different habitats. The distribution of viruses across environments appears to be primarily determined by the host ranges, i.e. the virome is shaped by the composition of the biome in a given habitat, which itself is affected by abiotic factors.
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Affiliation(s)
- Eugene V Koonin
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, United States
| | - Jens H Kuhn
- Integrated Research Facility at Fort Detrick, Division of Clinical Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Fort Detrick, Frederick, MD 21702, United States
| | - Valerian V Dolja
- Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331, United States
| | - Mart Krupovic
- Institut Pasteur, Université Paris Cité, Archaeal Virology Unit, 75015 Paris, France
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3
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Nie Y, Zhang Y, Wu J. The Secondary Structure of Potato Spindle Tuber Viroid Determines Its Infectivity in Nicotiana benthamiana. Viruses 2023; 15:2307. [PMID: 38140547 PMCID: PMC10748084 DOI: 10.3390/v15122307] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2023] [Revised: 11/20/2023] [Accepted: 11/23/2023] [Indexed: 12/24/2023] Open
Abstract
The function of RNAs is determined by their structure. However, studying the relationship between RNA structure and function often requires altering RNA sequences to modify the structures, which leads to the neglect of the importance of RNA sequences themselves. In our research, we utilized potato spindle tuber viroid (PSTVd), a circular-form non-coding infectious RNA, as a model with which to investigate the role of a specific rod-like structure in RNA function. By generating linear RNA transcripts with different start sites, we established 12 PSTVd forms with different secondary structures while maintaining the same sequence. The RNA secondary structures were predicted using the mfold tool and validated through native PAGE gel electrophoresis after in vitro RNA folding. Analysis using plant infection assays revealed that the formation of a correct rod-like structure is crucial for the successful infection of PSTVd. Interestingly, the inability of PSTVd forms with non-rod-like structures to infect plants could be partially compensated by increasing the amount of linear viroid RNA transcripts, suggesting the existence of additional RNA secondary structures, such as the correct rod-like structure, alongside the dominant structure in the RNA inoculum of these forms. Our study demonstrates the critical role of RNA secondary structures in determining the function of infectious RNAs.
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Affiliation(s)
| | | | - Jian Wu
- State Key Laboratory for Managing Biotic and Chemical Threats to the Quality and Safety of Agroproducts, Key Laboratory of Biotechnology in Plant Protection of MARA and Zhejiang Province, Institute of Plant Virology, Ningbo University, Ningbo 315211, China; (Y.N.); (Y.Z.)
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4
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Gardes J, Maldivi C, Boisset D, Aubourg T, Demongeot J. An Unsupervised Classifier for Whole-Genome Phylogenies, the Maxwell© Tool. Int J Mol Sci 2023; 24:16278. [PMID: 38003468 PMCID: PMC10671764 DOI: 10.3390/ijms242216278] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2023] [Revised: 10/20/2023] [Accepted: 11/02/2023] [Indexed: 11/26/2023] Open
Abstract
The development of phylogenetic trees based on RNA or DNA sequences generally requires a precise and limited choice of important RNAs, e.g., messenger RNAs of essential proteins or ribosomal RNAs (like 16S), but rarely complete genomes, making it possible to explain evolution and speciation. In this article, we propose revisiting a classic phylogeny of archaea from only the information on the succession of nucleotides of their entire genome. For this purpose, we use a new tool, the unsupervised classifier Maxwell, whose principle lies in the Burrows-Wheeler compression transform, and we show its efficiency in clustering whole archaeal genomes.
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Affiliation(s)
- Joël Gardes
- Orange Labs, 38229 Meylan, France; (J.G.); (C.M.); (D.B.)
| | | | - Denis Boisset
- Orange Labs, 38229 Meylan, France; (J.G.); (C.M.); (D.B.)
| | - Timothée Aubourg
- Faculty of Medicine, Université Grenoble Alpes, AGEIS EA 7407 Tools for e-Gnosis Medical, 38700 La Tronche, France;
| | - Jacques Demongeot
- Faculty of Medicine, Université Grenoble Alpes, AGEIS EA 7407 Tools for e-Gnosis Medical, 38700 La Tronche, France;
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Fontdevila Pareta N, Khalili M, Maachi A, Rivarez MPS, Rollin J, Salavert F, Temple C, Aranda MA, Boonham N, Botermans M, Candresse T, Fox A, Hernando Y, Kutnjak D, Marais A, Petter F, Ravnikar M, Selmi I, Tahzima R, Trontin C, Wetzel T, Massart S. Managing the deluge of newly discovered plant viruses and viroids: an optimized scientific and regulatory framework for their characterization and risk analysis. Front Microbiol 2023; 14:1181562. [PMID: 37323908 PMCID: PMC10265641 DOI: 10.3389/fmicb.2023.1181562] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2023] [Accepted: 04/25/2023] [Indexed: 06/17/2023] Open
Abstract
The advances in high-throughput sequencing (HTS) technologies and bioinformatic tools have provided new opportunities for virus and viroid discovery and diagnostics. Hence, new sequences of viral origin are being discovered and published at a previously unseen rate. Therefore, a collective effort was undertaken to write and propose a framework for prioritizing the biological characterization steps needed after discovering a new plant virus to evaluate its impact at different levels. Even though the proposed approach was widely used, a revision of these guidelines was prepared to consider virus discovery and characterization trends and integrate novel approaches and tools recently published or under development. This updated framework is more adapted to the current rate of virus discovery and provides an improved prioritization for filling knowledge and data gaps. It consists of four distinct steps adapted to include a multi-stakeholder feedback loop. Key improvements include better prioritization and organization of the various steps, earlier data sharing among researchers and involved stakeholders, public database screening, and exploitation of genomic information to predict biological properties.
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Affiliation(s)
| | - Maryam Khalili
- Univ. Bordeaux, INRAE, UMR BFP, Villenave d'Ornon, France
- EGFV, Univ. Bordeaux, INRAE, ISVV, Villenave d’Ornon, France
| | | | - Mark Paul S. Rivarez
- Department of Biotechnology and Systems Biology, National Institute of Biology, Ljubljana, Slovenia
- College of Agriculture and Agri-Industries, Caraga State University, Butuan, Philippines
| | - Johan Rollin
- Plant Pathology Laboratory, Gembloux Agro-Bio Tech, University of Liège, Gembloux, Belgium
- DNAVision (Belgium), Charleroi, Belgium
| | - Ferran Salavert
- School of Natural and Environmental Sciences, Faculty of Science, Agriculture and Engineering, Newcastle University, Newcastle upon Tyne, United Kingdom
| | - Coline Temple
- Plant Pathology Laboratory, Gembloux Agro-Bio Tech, University of Liège, Gembloux, Belgium
| | - Miguel A. Aranda
- Department of Stress Biology and Plant Pathology, Center for Edaphology and Applied Biology of Segura, Spanish National Research Council (CSIC), Murcia, Spain
| | - Neil Boonham
- School of Natural and Environmental Sciences, Faculty of Science, Agriculture and Engineering, Newcastle University, Newcastle upon Tyne, United Kingdom
| | - Marleen Botermans
- Netherlands Institute for Vectors, Invasive Plants and Plant Health (NIVIP), Wageningen, Netherlands
| | | | - Adrian Fox
- School of Natural and Environmental Sciences, Faculty of Science, Agriculture and Engineering, Newcastle University, Newcastle upon Tyne, United Kingdom
- Fera Science Ltd, York Biotech Campus, York, United Kingdom
| | | | - Denis Kutnjak
- Department of Biotechnology and Systems Biology, National Institute of Biology, Ljubljana, Slovenia
| | - Armelle Marais
- Univ. Bordeaux, INRAE, UMR BFP, Villenave d'Ornon, France
| | | | - Maja Ravnikar
- Department of Biotechnology and Systems Biology, National Institute of Biology, Ljubljana, Slovenia
| | - Ilhem Selmi
- Plant Pathology Laboratory, Gembloux Agro-Bio Tech, University of Liège, Gembloux, Belgium
| | - Rachid Tahzima
- Plant Pathology Laboratory, Gembloux Agro-Bio Tech, University of Liège, Gembloux, Belgium
- Plant Sciences Unit, Institute for Agricultural, Fisheries and Food Research (ILVO), Merelbeke, Belgium
| | - Charlotte Trontin
- European and Mediterranean Plant Protection Organization, Paris, France
| | - Thierry Wetzel
- DLR Rheinpfalz, Institute of Plant Protection, Neustadt an der Weinstrasse, Germany
| | - Sebastien Massart
- Plant Pathology Laboratory, Gembloux Agro-Bio Tech, University of Liège, Gembloux, Belgium
- Bioversity International, Montpellier, France
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6
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Lee BD, Neri U, Roux S, Wolf YI, Camargo AP, Krupovic M, Simmonds P, Kyrpides N, Gophna U, Dolja VV, Koonin EV. Mining metatranscriptomes reveals a vast world of viroid-like circular RNAs. Cell 2023; 186:646-661.e4. [PMID: 36696902 PMCID: PMC9911046 DOI: 10.1016/j.cell.2022.12.039] [Citation(s) in RCA: 28] [Impact Index Per Article: 28.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2022] [Revised: 11/11/2022] [Accepted: 12/20/2022] [Indexed: 01/26/2023]
Abstract
Viroids and viroid-like covalently closed circular (ccc) RNAs are minimal replicators that typically encode no proteins and hijack cellular enzymes for replication. The extent and diversity of viroid-like agents are poorly understood. We developed a computational pipeline to identify viroid-like cccRNAs and applied it to 5,131 metatranscriptomes and 1,344 plant transcriptomes. The search yielded 11,378 viroid-like cccRNAs spanning 4,409 species-level clusters, a 5-fold increase compared to the previously identified viroid-like elements. Within this diverse collection, we discovered numerous putative viroids, satellite RNAs, retrozymes, and ribozy-like viruses. Diverse ribozyme combinations and unusual ribozymes within the cccRNAs were identified. Self-cleaving ribozymes were identified in ambiviruses, some mito-like viruses and capsid-encoding satellite virus-like cccRNAs. The broad presence of viroid-like cccRNAs in diverse transcriptomes and ecosystems implies that their host range is far broader than currently known, and matches to CRISPR spacers suggest that some cccRNAs replicate in prokaryotes.
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Affiliation(s)
- Benjamin D Lee
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USA; Nuffield Department of Medicine, University of Oxford, Oxford OX3 7BN, UK
| | - Uri Neri
- The Shmunis School of Biomedicine and Cancer Research, Tel Aviv University, Tel Aviv 6997801, Israel
| | - Simon Roux
- Department of Energy Joint Genome Institute, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
| | - Yuri I Wolf
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USA
| | - Antonio Pedro Camargo
- Department of Energy Joint Genome Institute, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
| | - Mart Krupovic
- Institut Pasteur, Université Paris Cité, CNRS UMR6047, Archaeal Virology Unit, 75015 Paris, France
| | - Peter Simmonds
- Nuffield Department of Medicine, University of Oxford, Oxford OX3 7BN, UK
| | - Nikos Kyrpides
- Department of Energy Joint Genome Institute, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
| | - Uri Gophna
- The Shmunis School of Biomedicine and Cancer Research, Tel Aviv University, Tel Aviv 6997801, Israel
| | - Valerian V Dolja
- Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331, USA
| | - Eugene V Koonin
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USA.
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7
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Juhas M. The World of Microorganisms. BRIEF LESSONS IN MICROBIOLOGY 2023:1-16. [DOI: 10.1007/978-3-031-29544-7_1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/02/2023]
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8
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Lai X, Wang H, Wu C, Zheng W, Leng J, Zhang Y, Yan L. Comparison of Potato Viromes Between Introduced and Indigenous Varieties. Front Microbiol 2022; 13:809780. [PMID: 35602024 PMCID: PMC9114672 DOI: 10.3389/fmicb.2022.809780] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2021] [Accepted: 04/12/2022] [Indexed: 11/16/2022] Open
Abstract
Viral disease in potatoes has been a major problem in potato production worldwide. In addition to the potential risk of introducing new diseases in new areas, viral-disease epidemics/pandemics can be initiated by “spillover” of indigenous viruses from infected alternative hosts into introduced cultivars. To investigate the tendency of potential viral infection/resistance, we analyzed the viromes of introduced and indigenous varieties of potatoes among different tissues using RNA-seq libraries. Bioinformatics analyses revealed that potato viruses PVM, PVY, and PVS were dominant and the most frequently identified viruses infecting potato virus-free plants in the field, and showed an infection bias between introduced and indigenous cultivars. PVY and PVS were the major viruses in introduced varieties, whereas PVM showed an extraordinarily high percentage in the indigenous variety. Other three common viruses, PVH, potato mop-top virus, and potato leafroll virus were identified specifically in the indigenous variety. There was a tendency for tissue-specific infection and sequence variation in viruses: underground parts (tubers, roots) harbored more unusual viruses, and tubers harbored relatively more variation with a high frequency of single nucleotide polymorphisms than other tissues. Taken together, our study provides a comprehensive overview of the composition, distribution, and sequence variation of viruses between introduced and indigenous varieties of potatoes.
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Affiliation(s)
- Xianjun Lai
- Panxi Crops Research and Utilization Key Laboratory of Sichuan Province, College of Agricultural Science, Xichang University, Xichang, China
| | - Haiyan Wang
- Sichuan Key Laboratory of Molecular Biology and Biotechnology, College of Life Sciences, Sichuan University, Chengdu, China
| | - Caiyun Wu
- Panxi Crops Research and Utilization Key Laboratory of Sichuan Province, College of Agricultural Science, Xichang University, Xichang, China
| | - Wen Zheng
- Panxi Crops Research and Utilization Key Laboratory of Sichuan Province, College of Agricultural Science, Xichang University, Xichang, China
| | - Jing Leng
- Panxi Crops Research and Utilization Key Laboratory of Sichuan Province, College of Agricultural Science, Xichang University, Xichang, China
| | - Yizheng Zhang
- Sichuan Key Laboratory of Molecular Biology and Biotechnology, College of Life Sciences, Sichuan University, Chengdu, China
| | - Lang Yan
- Panxi Crops Research and Utilization Key Laboratory of Sichuan Province, College of Agricultural Science, Xichang University, Xichang, China
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9
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Zhang Y, Zhang X, Xu Y, Fang S, Ji Y, Lu L, Xu W, Qian H, Liang ZF. Circular RNA and Its Roles in the Occurrence, Development, Diagnosis of Cancer. Front Oncol 2022; 12:845703. [PMID: 35463362 PMCID: PMC9021756 DOI: 10.3389/fonc.2022.845703] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2022] [Accepted: 03/14/2022] [Indexed: 12/19/2022] Open
Abstract
Circular RNAs (circRNAs) are non-coding single-stranded covalently closed circular RNA, mainly produced by reverse splicing of exons of precursor mRNAs (pre-mRNAs). The characteristics of high abundance, strong specificity, and good stability of circRNAs have been discovered. A large number of studies have reported its various functions and mechanisms in biological events, such as the occurrence and development of cancer. In this review, we focus on the classification, characterization, biogenesis, functions of circRNAs, and the latest advances in cancer research. The development of circRNAs as biomarkers in cancer diagnosis and treatment also provides new ideas for studying circRNAs research.
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Affiliation(s)
- Yue Zhang
- Jiangsu Key Laboratory of Medical Science and Laboratory Medicine, School of Medicine, Jiangsu University, Zhenjiang, China
| | - Xinyi Zhang
- Jiangsu Key Laboratory of Medical Science and Laboratory Medicine, School of Medicine, Jiangsu University, Zhenjiang, China
| | - Yumeng Xu
- Jiangsu Key Laboratory of Medical Science and Laboratory Medicine, School of Medicine, Jiangsu University, Zhenjiang, China
| | - Shikun Fang
- Jiangsu Key Laboratory of Medical Science and Laboratory Medicine, School of Medicine, Jiangsu University, Zhenjiang, China
| | - Ying Ji
- Jiangsu Key Laboratory of Medical Science and Laboratory Medicine, School of Medicine, Jiangsu University, Zhenjiang, China
| | - Ling Lu
- Child Healthcare Department, The Fourth Affiliated Hospital of Jiangsu University, Zhenjiang, China
| | - Wenrong Xu
- Jiangsu Key Laboratory of Medical Science and Laboratory Medicine, School of Medicine, Jiangsu University, Zhenjiang, China
| | - Hui Qian
- Jiangsu Key Laboratory of Medical Science and Laboratory Medicine, School of Medicine, Jiangsu University, Zhenjiang, China
| | - Zhao Feng Liang
- Jiangsu Key Laboratory of Medical Science and Laboratory Medicine, School of Medicine, Jiangsu University, Zhenjiang, China
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Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators? LIFE (BASEL, SWITZERLAND) 2022; 12:life12010103. [PMID: 35054497 PMCID: PMC8781251 DOI: 10.3390/life12010103] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/21/2021] [Revised: 01/10/2022] [Accepted: 01/11/2022] [Indexed: 01/09/2023]
Abstract
Viroids are a unique class of plant pathogens that consist of small circular RNA molecules, between 220 and 450 nucleotides in size. Viroids encode no proteins and are the smallest known infectious agents. Viroids replicate via the rolling circle mechanism, producing multimeric intermediates which are cleaved to unit length either by ribozymes formed from both polarities of the viroid genomic RNA or by coopted host RNAses. Many viroid-like small circular RNAs are satellites of plant RNA viruses. Ribozyviruses, represented by human hepatitis delta virus, are larger viroid-like circular RNAs that additionally encode the viral nucleocapsid protein. It has been proposed that viroids are direct descendants of primordial RNA replicons that were present in the hypothetical RNA world. We argue, however, that much later origin of viroids, possibly, from recently discovered mobile genetic elements known as retrozymes, is a far more parsimonious evolutionary scenario. Nevertheless, viroids and viroid-like circular RNAs are minimal replicators that are likely to be close to the theoretical lower limit of replicator size and arguably comprise the paradigm for replicator emergence. Thus, although viroid-like replicators are unlikely to be direct descendants of primordial RNA replicators, the study of the diversity and evolution of these ultimate genetic parasites can yield insights into the earliest stages of the evolution of life.
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