A giant virus in a primordial soup, surrounded by smaller life forms.

Giant Viruses: Are They Redefining the Tree of Life?

Lena Kashyap in Science & Nature September 2025 • 3 min read.

"Explore how the discovery of giant viruses is challenging traditional classifications and reshaping our understanding of microbial evolution."

For decades, biology textbooks presented a neat division of life into three domains: Bacteria, Archaea, and Eukarya. However, the discovery of giant viruses is throwing this classification into question. These viruses, much larger and more complex than traditional viruses, possess characteristics that blur the lines between viruses and cellular life.

Giant viruses, discovered in 2003, infect amoebae and boast genomes far exceeding those of many bacteria. Their unique features, including genes involved in translation and energy production, challenge the conventional view of viruses as simple, inert particles.

This article explores how the ancestrality and mosaicism of giant viruses support the definition of a fourth 'TRUC' (Things Resisting Uncompleted Classifications) of microbes, potentially revolutionizing our understanding of the tree of life.

What Makes Giant Viruses So Different?

A giant virus in a primordial soup, surrounded by smaller life forms.

Classical viruses are typically small, with limited genetic material and an inability to be seen under a light microscope. Giant viruses, on the other hand, are significantly larger, with some exceeding 0.2 micrometers in diameter—making them visible under a standard light microscope. Furthermore, their genomes can surpass 350,000 base pairs, dwarfing those of many bacteria.

The complexity of giant viruses extends beyond their size. Unlike traditional viruses that rely almost entirely on the host cell's machinery for replication, giant viruses possess a remarkable array of genes, including some involved in:

DNA repair
Protein folding
Even, in some cases, translation – the process of creating proteins from mRNA.

This level of genetic autonomy suggests a closer relationship to cellular life than previously imagined for viruses. It challenges the long-held view of viruses as mere hijacking agents.

The Future of Microbial Classification

The ongoing exploration of giant viruses promises to further blur the lines between viruses and cellular organisms. As we uncover more about their origins, evolution, and interactions with other microbes, the traditional three-domain system may need to be revised to accommodate these biological enigmas. The fourth TRUC concept offers a framework for understanding these complex entities, paving the way for a more nuanced and comprehensive understanding of life on Earth.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.3389/fmicb.2018.02668, Alternate LINK

Title: Ancestrality And Mosaicism Of Giant Viruses Supporting The Definition Of The Fourth Truc Of Microbes

Subject: Microbiology (medical)

Journal: Frontiers in Microbiology

Publisher: Frontiers Media SA

Authors: Philippe Colson, Anthony Levasseur, Bernard La Scola, Vikas Sharma, Arshan Nasir, Pierre Pontarotti, Gustavo Caetano-Anollés, Didier Raoult

Published: 2018-11-27

Everything You Need To Know

What are the key differences between giant viruses and classical viruses?

Giant viruses differ significantly from classical viruses in size, complexity, and genetic autonomy. Classical viruses are small and possess limited genetic material, whereas giant viruses are much larger, sometimes visible under a standard light microscope, and contain genomes that can surpass those of many bacteria. Furthermore, giant viruses possess genes involved in DNA repair, protein folding, and even translation, granting them a degree of independence from the host cell's machinery that is not observed in classical viruses. This suggests a closer relationship to cellular life than previously believed.

How did the discovery of giant viruses challenge the traditional classification of life into three domains?

The discovery of giant viruses challenged the long-standing three-domain system (Bacteria, Archaea, and Eukarya) by blurring the lines between viruses and cellular life. Their large size, complex genomes, and the presence of genes typically associated with cellular organisms—such as those involved in translation and energy production—questioned the conventional view of viruses as simple, inert particles. This led to discussions about whether giant viruses might represent a fourth domain of life or 'TRUC' (Things Resisting Uncompleted Classifications), requiring a reevaluation of the tree of life.

In what ways do giant viruses revolutionize our understanding of microbial evolution?

Giant viruses revolutionize our understanding of microbial evolution by introducing the idea of a fourth 'TRUC' of microbes. They possess unique features such as genes involved in translation and energy production, that challenge the conventional view of viruses. By exploring the ancestrality and mosaicism of giant viruses, the traditional three-domain system may need to be revised to accommodate these biological enigmas.

What is the significance of giant viruses possessing genes related to translation and energy production?

The presence of genes related to translation and energy production in giant viruses is highly significant because these functions were previously considered exclusive to cellular organisms. Translation, the process of creating proteins from mRNA, is a fundamental aspect of cellular life. The presence of these genes in giant viruses suggests a level of genetic autonomy and a closer evolutionary relationship to cells than previously imagined. This discovery challenges the traditional view of viruses as mere hijacking agents dependent on the host cell for all functions.

What are the implications of classifying giant viruses as 'TRUC' (Things Resisting Uncompleted Classifications)?

Classifying giant viruses as 'TRUC' implies acknowledging their unique and ambiguous position in the tree of life. It recognizes that these viruses don't fit neatly into the existing categories of Bacteria, Archaea, or Eukarya, nor do they align perfectly with the traditional definition of viruses. This classification prompts further research into their origins, evolution, and interactions with other microbes, paving the way for a more nuanced and comprehensive understanding of life on Earth. It also encourages a reevaluation of existing classification systems to accommodate biological entities that challenge conventional boundaries.