A Guide to the Major dsRNA Mycovirus Families: Structure and Diversity

dsRNA mycovirus families

A Guide to the Major dsRNA Mycovirus Families: Structure and Diversity

Last Updated: July 29, 2025

Author Bio: This research summary is based on the doctoral thesis of Wajeeha Shamsi, a researcher from the Atta-ur-Rahman School of Applied Biosciences at the National University of Sciences & Technology (NUST), Islamabad. Her work focuses on the identification and characterization of novel mycoviruses from environmental sources.

Introduction

In the intricate world of virology, classification is key. Just as we group animals into families based on shared traits, viruses are categorized to help us understand their structure, replication, and evolutionary history. Among the viruses that infect fungi, the most frequently encountered have genomes made of double-stranded RNA (dsRNA). These dsRNA viruses are incredibly diverse, grouped into distinct families with unique characteristics. Understanding these classifications is fundamental to the study of mycoviruses. This guide, based on the comprehensive literature review in Wajeeha Shamsi’s 2020 thesis, provides a clear overview of the six recognized dsRNA mycovirus families.

The General Replication Strategy of dsRNA Mycoviruses

Before diving into the specific families, it’s important to understand their shared replication strategy. The site for viral dsRNA replication is the cytoplasm of the fungal cell. The process occurs entirely within the protective protein shell of the virus, known as the capsid.

All known dsRNA viruses follow the same pattern of replication cycles. A polymerase inside the virion transcribes the dsRNA. The newly synthesized positive-strand RNA acts as a messenger RNA (mRNA) for producing viral proteins. It is also used as a template for synthesizing the complementary negative-strand RNA, creating a new dsRNA genome. In fungal dsRNA viruses, this entire process of transcription and replication occurs safely inside the complete virion.

A Tour of the Six Major dsRNA Mycovirus Families

There are six officially recognized families of dsRNA mycoviruses, each with a distinct genomic structure and set of characteristics.

1. Family Totiviridae

Members of this family are among the simplest dsRNA viruses.

  • Genome: Monosegmented (one piece) and bicistronic (two genes), ranging from 4.6 to 7.0 kbp.
  • Key Feature: The two genes, one for the capsid protein (CP) and one for the RNA-dependent RNA polymerase (RdRp), partially overlap.
  • Prototype Member: Saccharomyces cerevisiae virus L-A (ScV-L-A), found in yeast.

2. Family Partitiviridae

This family is defined by its segmented genome, where each segment is packaged separately.

  • Genome: Bi-segmented (two pieces), ranging in size from 1.4 to 2.4 kbp.
  • Key Feature: The larger dsRNA segment encodes the RdRp, while the smaller segment encodes the capsid protein.
  • Virion: Isometric particles measuring 25-43 nm in diameter.
  • Prototype Member: Atkinsonella hypoxylon virus (AhV).

3. Family Chrysoviridae

Chrysoviruses have a more complex, multi-segmented genome.

  • Genome: Typically four genome segments, each encoding a single gene. Segments range from 2.4 to 3.6 kbp.
  • Key Feature: dsRNA1 encodes the RdRp, and dsRNA2 encodes the major capsid protein. The functions of the proteins encoded by dsRNA3 and 4 are unknown.
  • Virion: Isometric particles measuring 35-40 nm in diameter.
  • Prototype Member: Penicillium chrysogenum virus.

4. Family Megabirnaviridae

As their name suggests, these viruses are large.

  • Genome: Bipartite (two segments), with a very large total genome size. dsRNA1 is around 8.9 kbp, and dsRNA2 is 7.2 kbp.
  • Key Feature: The RdRp is expressed as a fusion protein with the capsid protein (CP-RdRp). dsRNA2 is not directly involved in replication but helps make the process more efficient.
  • Virion: Independent isometric particles, 50 nm in diameter.
  • Prototype Member: Rosellinia necatrix megabirnavirus 1 (RnMBV1).

5. Family Quadriviridae

This family also has a multi-segmented genome, with four distinct parts.

  • Genome: Comprised of four dsRNA segments that are monocistronic and separately encapsidated. The genome size ranges from 3.7 to 4.9 kbp.
  • Key Feature: The RdRp is encoded by dsRNA3, while dsRNA2 and dsRNA4 encode capsid proteins that assemble together to form the virion. The function of the dsRNA1 product is not known.
  • Virion: Isometric particles, 45 nm in diameter.
  • Prototype Member: Rosellinia necatrix quadrivirus 1.

6. Family Reoviridae

This family contains the most complex of the mycoviruses, with highly segmented genomes.

  • Genome: Composed of more than ten segments (typically 11 or 12), each encoding a single gene. Segments range from 0.7 to 4.1 kbp.
  • Key Feature: All three known mycoreoviruses confer hypovirulence to their hosts.
  • Prototype Member: Mycoreovirus 1, isolated from the chestnut blight fungus Cryphonectria parasitica.

Conclusion

The diversity seen across the dsRNA mycovirus families is a testament to the complex evolutionary history of these fascinating entities. From the simple, single-genome Totiviridae to the highly segmented and complex Reoviridae, each family has a unique molecular architecture. A clear understanding of these classifications is essential for any researcher in the field, providing a critical framework for identifying new viruses and predicting their biological roles within their fungal hosts.

Source & Citations

Disclaimer: Some sentences have been lightly edited for SEO and readability. For the full, original research, please refer to the complete thesis PDF linked in the section above.

Join the Discussion!

With so much diversity, which of the dsRNA mycovirus families do you find the most interesting from an evolutionary perspective? Let us know your thoughts in the comments!

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