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The Secret Origin of Mycoviruses: Ancient Fungal Partners or Plant Invaders?
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
Every living thing has a history, an evolutionary story that explains how it came to be. For viruses, these stories are often shrouded in mystery, written in a genetic code that changes at a dizzying pace. Mycoviruses, the viruses that infect fungi, are no exception. Their existence raises a fundamental question that puzzles virologists: where did they come from? Did they arise alongside their fungal hosts millions of years ago, evolving in a slow, ancient dance? Or are they more recent invaders, opportunists that made a daring leap from plants into their fungal pathogens? The scientific community is actively exploring the origin of mycoviruses, and two major hypotheses have emerged. This article, guided by the literature review in Wajeeha Shamsi’s 2020 thesis, explores both compelling theories.
The Two Competing Theories on the Origin of Mycoviruses
The debate surrounding the origin of mycoviruses is centered on two primary, yet not mutually exclusive, ideas. As the thesis states, “There are two hypotheses regarding the origin of fungal viruses. The ‘ancient co-evolution hypothesis’ and the ‘plant virus hypothesis’. The former says that these viruses co-evolved with their hosts while the latter proposes that the viruses infecting the host plants of fungus migrated to them.” Let’s break down the evidence for each.
The Ancient Co-evolution Hypothesis: A Long-Term Partnership
The first theory, the ancient co-evolution hypothesis, proposes a deep and ancient connection. According to this hypothesis, prehistoric viral infections, arising from an unidentified source, began infecting the ancestors of modern fungi. From that point on, the viruses and their fungal hosts evolved in parallel, their histories forever intertwined.
If this is true, the mycoviruses would be expected to follow the same evolutionary pattern as their hosts. A correlation should exist between the evolutionary trees of the viruses and the fungi they infect. This hypothesis is supported by the primary mode of mycovirus transmission: vertical transmission. Most mycoviruses are passed down from a parent fungus to its offspring through its spores. This intracellular, generational transfer is a perfect mechanism for ensuring a long-term, co-evolutionary relationship.
A key piece of evidence supporting this model is the Helminthosporium victoriae virus 190S. This mycovirus assembles its capsid (the protective protein shell) using machinery that is encoded by the host fungus itself. This deep integration into the host’s cellular processes suggests a long history of adaptation and partnership, a hallmark of co-evolution. This theory paints a picture of mycoviruses as ancient, persistent residents of the fungal kingdom.
The Plant Virus Hypothesis: A Host Jump Scenario
The second theory, the plant virus hypothesis, offers a more dynamic and opportunistic explanation for the origin of mycoviruses. This hypothesis proposes that at least some fungal viruses didn’t evolve with fungi from the beginning but instead “jumped” from a plant host to a fungal host.
How could this happen? Picture a plant infected with a plant virus. Now, imagine a pathogenic fungus infecting that same plant. The fungus and the plant virus are now sharing the same cellular environment. This close proximity could provide the opportunity for the virus to cross the host kingdom barrier and establish a new infection in the fungus. This process is a form of horizontal virus transmission.
The strongest evidence for this hypothesis comes from the viruses of Cryphonectria parasitica, the fungus that causes chestnut blight. Four distinct hypoviruses found in this fungus (CHV1, CHV2, CHV3, and CHV4) show remarkable phylogenetic association and sequence homology to a group of plant viruses known as potyviruses. Furthermore, many hypovirulent mycoviruses with ssRNA genomes are more closely related to plant RNA viruses than they are to other dsRNA mycoviruses. This suggests a more recent common ancestor with plant viruses, strongly supporting the idea of a host jump.
Transmission Mechanisms as Clues to Origin
The way a virus spreads provides crucial clues to its evolutionary history. As mentioned, vertical transmission through spores supports the ancient co-evolution model. Mycoviruses lack an extracellular transmission route and any natural vectors, so being passed down intracellularly is their primary survival strategy.
However, horizontal transmission is also possible through a process called hyphal anastomosis, where the fungal filaments of two compatible individuals fuse and mix their cytoplasmic contents. While this is typically limited to closely related fungi, it represents a mechanism for viruses to move between individuals. The plant virus hypothesis is the ultimate example of horizontal transmission—not just between individuals, but across entire biological kingdoms.
Conclusion
So, what is the true origin of mycoviruses? The evidence suggests there isn’t a single, one-size-fits-all answer. It’s highly likely that both hypotheses are correct and that the diverse world of mycoviruses is the result of both ancient partnerships and more recent host-jumping events. Some mycoviruses may be the descendants of ancient viruses that have co-evolved with fungi for eons, while others may be the relatives of plant viruses that made the leap to a new kingdom. Understanding these dual origins is crucial for appreciating the complex evolutionary dynamics that shape the viral world around us.
Source & Citations
- Thesis Title: Molecular and Biological Characterization of Mycovirus Identified from Environmental Fungal Isolates
- Researcher: Wajeeha Shamsi
- Guide (Supervisor): Dr. Muhammad Faraz Bhatti
- University: Atta-ur-Rahman School of Applied Biosciences, National University of Sciences & Technology (NUST), Islamabad, Pakistan
- Year of Compilation: 2020
- Excerpt Page Numbers: 19, 20 (PDF pages 54, 55)
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.
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