Novel RNA viruses reveal a complex mycovirome in the smut fungus Thecaphora thlaspeos

This study reports the discovery of eight novel monosegmented, bicistronic RNA viruses belonging to the genera *Totivirus* and *Eimeriavirus* in the smut fungus *Thecaphora thlaspeos*, revealing a complex and variable mycovirome that expands the known diversity of viruses in Ustilaginomycotina.

The Gist

Imagine a tiny, invisible world living inside a fungus that attacks plants. For a long time, scientists thought this specific fungus, called Thecaphora thlaspeos, was just a lonely traveler. But in this new study, researchers discovered that this fungus is actually a bustling, crowded city filled with eight different types of viral tenants.

Here is the story of this discovery, broken down into simple concepts:

1. The Detective Work: Finding Ghosts in the Machine

The scientists didn't go out into a field to catch these viruses with nets. Instead, they acted like digital detectives. They went back to a massive library of computer data (RNA sequences) that had already been collected from this fungus.

Think of the fungus's DNA as a giant instruction manual. The researchers used a computer program to scan this manual, looking for "typos" or foreign words that didn't belong to the fungus. They found eight distinct sets of instructions that belonged to viruses.

How they knew it was real:
To make sure these weren't just glitches or plant viruses accidentally mixed in, they checked the fungus's "DNA hard drive" (DNA libraries). They found zero traces of these viruses there. This proved the viruses are made of RNA (like a temporary sticky note) and are actively living inside the fungus, not permanently written into its genetic code.

2. The Tenants: Two Different Neighborhoods

The eight viruses they found fall into two distinct "neighborhoods" or families:

• The "Totiviruses" (5 tenants): These are the classic fungal viruses. They have a specific way of operating: they write a long instruction manual that tells the cell to build a "capsid" (the virus's shell) and a "polymerase" (the engine that copies the virus). To save space, they overlap these instructions. It's like writing two stories on the same piece of paper, where the second story starts in the middle of the first one, but you have to shift your reading glasses slightly to understand it.
• The "Eimeriaviruses" (3 tenants): These are the new kids on the block. They are similar but have a different trick. Instead of overlapping instructions, they use a "stop-and-start" sign. They finish the first story, put up a "STOP" sign, and immediately put up a "START" sign for the next story right next to it.

3. The Host's Language: Speaking the Same Tongue

One of the most fascinating findings is how well these viruses speak the fungus's language.

Imagine the fungus is a factory that only understands a specific dialect. If a virus tries to speak a different dialect, the factory ignores it. But these viruses have evolved to speak the exact same dialect as the fungus. They use the same "words" (codons) to build their proteins. This suggests they have been living with this fungus for a very long time, slowly adapting until they fit in perfectly, like a long-term roommate who knows exactly where you keep the sugar.

4. The Mystery of the Two Strains: Why are they different?

The researchers looked at two different strains of the fungus (let's call them Strain A and Strain B).

• Strain A was hosting all 8 viruses.
• Strain B was missing 3 of them.

This is like finding two houses in the same neighborhood. One house has a full garage, a basement, and an attic full of stuff. The other house, which is built almost identically, is missing the attic and the basement.

This tells us that who you are (your genetics) determines who you live with. The fungus's own genetic makeup acts like a bouncer, deciding which viruses can move in and which ones get kicked out. It's a complex ecosystem where the host and the viruses are constantly negotiating.

5. Why Does This Matter?

You might ask, "So what? It's just a fungus with some viruses."

• It's a New World: Before this, we knew very little about viruses in this specific type of fungus (smut fungi). This study opens the door to a whole new world of discovery.
• The "Superpower" Potential: Some viruses make their fungal hosts weaker (a phenomenon called hypovirulence). If scientists can figure out how to use these viruses, they might be able to create a "biological weapon" to stop the fungus from attacking crops like broccoli or cabbage (which are in the same family as the fungus's host).
• Understanding Life: It shows us that even in the microscopic world, there is incredible diversity, complex social structures, and constant evolution happening right under our noses.

The Bottom Line

This paper is like discovering that a quiet, seemingly empty house is actually a busy apartment complex with eight different families living inside, each with their own unique rules, all speaking the same language as the building itself. It changes our understanding of how fungi and viruses interact and opens up new possibilities for protecting our food supply.

Technical Summary

1. Problem Statement

Mycoviruses (fungal viruses) are ubiquitous and can significantly influence fungal phenotypes, including virulence and metabolism. While high-throughput sequencing (HTS) has revealed complex viromes in some fungi, the diversity and host associations of mycoviruses in smut fungi (subphylum Ustilaginomycotina) remain poorly explored, particularly in the genus Thecaphora.

• Specific Gap: Although Ustilago maydis (corn smut) is a well-studied model, little is known about the virome of Thecaphora thlaspeos, a biotrophic pathogen infecting Brassicaceae hosts. Previous genomic analyses of related species (T. frezii) showed no viral signatures, prompting an investigation into whether T. thlaspeos harbors a hidden or complex viral community.

2. Methodology

The study employed a retrospective transcriptome mining approach using publicly available RNA-seq data.

• Data Source: Re-analysis of 13 RNA-seq libraries from BioProject PRJEB24478 (Courville et al., 2019):
  • Fungal Samples: 5 libraries from T. thlaspeos mycelium (Strain LF2: 4 biological replicates; Strain LF1: 1 replicate).
  • Controls: 8 libraries from the host plant Arabis hirsuta.
  • Validation: 8 DNA sequencing libraries from the same bioproject were analyzed to rule out endogenous viral elements (EVEs) or retrotransposons.
• Discovery Pipeline:
  1. Preprocessing: Reads were trimmed (Trimmomatic) and assembled de novo (rnaSPAdes).
  2. Screening: Assembled contigs were screened via BLASTX against NCBI RefSeq viral proteins.
  3. Curation: Tentative viral contigs were extended via iterative read mapping and polished using Geneious.
  4. Validation:
     • RNA Specificity: Confirmed by the complete absence of viral reads in DNA libraries.
     • Host Specificity: Viral reads were abundant in fungal libraries but negligible in plant controls (ruling out index hopping artifacts).
     • Codon Usage: Relative Synonymous Codon Usage (RSCU) analysis confirmed viral genomes closely matched the host's codon bias.
• Characterization:
  • Phylogenetics: RNA-dependent RNA polymerase (RdRp) sequences were aligned and used for Maximum Likelihood phylogenetic reconstruction (IQ-TREE).
  • Genomic Architecture: ORF prediction, domain analysis (CDD), and identification of translational recoding mechanisms (e.g., -1 PRF vs. stop-restart).
  • Structural Analysis: Prediction of RNA secondary structures (pseudoknots) downstream of slippery motifs using KnotInFrame, ProbKnot, and IPknot.

3. Key Contributions

• Discovery of Novel Virome: Identification of eight novel RNA viruses infecting T. thlaspeos, representing the first reported mycoviruses for this species.
• Taxonomic Expansion: Classification of these viruses into two genera:
  • Genus Totivirus (Family Orthototiviridae): 5 novel viruses (TtTV1–5).
  • Genus Eimeriavirus (Family Pseudototiviridae): 3 novel viruses (TtEV1–3).
• Strain-Level Variation: Documentation of significant intraspecific variation in virome composition between two mating types (LF1 and LF2) of the same fungal species.
• Mechanistic Insight: Detailed characterization of distinct translational recoding strategies used by these viruses to express their polymerase.

4. Key Results

A. Viral Abundance and Specificity

• Fungal Specificity: Viral reads constituted 0.022%–0.048% of fungal libraries but were virtually absent in plant controls (<0.0001%).
• RNA Nature: Zero reads mapped to viral assemblies in DNA libraries, confirming these are active RNA viruses and not integrated genomic elements.
• Abundance: TtEV3 was the most abundant virus in strain LF2 (Mean RPKM ~22.8), while TtTV1 showed the most stable accumulation across replicates.

B. Genomic Architecture and Translation

All eight viruses possess monosegmented, bicistronic genomes (4.4–5.3 kb) encoding a Capsid Protein (CP) and an RdRp, but they utilize different mechanisms for RdRp expression:

1. Totiviruses (TtTV1–5):
   • Use -1 Programmed Ribosomal Frameshifting (-1 PRF).
   • Contain a conserved slippery heptanucleotide motif (GG(A/G)TTTT) followed by a predicted pseudoknot structure.
   • The pseudoknots were found to be thermodynamically less stable than nested structures, suggesting a dynamic folding landscape that may regulate frameshifting efficiency.
2. Eimeriaviruses (TtEV1–3):
   • Use a termination–reinitiation (Stop/Start) mechanism.
   • Lack the canonical slippery motif; instead, they feature a conserved ATGA tetranucleotide junction where the 'A' serves as the stop codon for CP and the start of RdRp.
   • No stable pseudoknots were predicted downstream of the junction, implying a different regulatory mechanism.

C. Evolutionary Dynamics

• Strain Differences: Strain LF2 harbored all 8 viruses, whereas Strain LF1 (mating type a1b1) lacked TtEV2, TtEV3, and TtTV2.
• Competitive Release: In the absence of competing viruses, TtEV1 abundance in LF1 was significantly higher (RPKM ~28.3) compared to LF2 (RPKM ~13.3), suggesting potential resource competition or superinfection exclusion.
• Selection Pressure: High nucleotide divergence (6.4%–12.4%) between strains was observed, yet amino acid sequences remained highly conserved (>97%), indicating strong purifying selection. Transition-to-transversion ratios exceeded neutral expectations, consistent with RNA virus mutation patterns.

D. Phylogenetic Placement

• Phylogenetic analysis of RdRp sequences placed the new viruses in distinct clades with maximal statistical support (100% UFboot) within Totivirus and Eimeriavirus.
• Pairwise amino acid identities with known species were <55%, well below the ICTV 70% threshold for species demarcation, confirming they represent novel species.

5. Significance

• Expanding the Mycovirome: This study significantly broadens the known diversity of the order Ghabrivirales within the Ustilaginomycotina, a group previously underrepresented in viral studies.
• Host-Virus Co-evolution: The finding that viral codon usage is tightly adapted to the host (T. thlaspeos) and that virome composition varies by host strain/genotype suggests a long-term co-evolutionary relationship and highlights the role of host genetics in shaping viral communities.
• Biological Control Potential: Understanding the complex, multi-virus interactions (synergistic vs. antagonistic) in smut fungi provides a foundation for exploring mycoviruses as biocontrol agents against plant pathogens.
• Methodological Benchmark: The study demonstrates the efficacy of mining existing transcriptomic datasets for virus discovery and establishes a rigorous framework (including DNA validation and RSCU analysis) for distinguishing true mycoviruses from artifacts.

Future Directions: The authors recommend experimental validation (RT-PCR, RACE for full genome termini), functional assays to confirm translational mechanisms, and phenotypic studies using virus-cured isogenic lines to determine the impact of these viruses on fungal virulence and plant pathogenesis.