Tracing The Origins Of Giant Viruses

Don’t be fooled by their size—giant viruses are more closely related to eukaryotes compared to bacteria or even small viruses, say researchers from India.

AsianScientist (Jun. 15, 2021) – According to researchers from India, giant viruses emerged from single-celled organisms that shed genes over time. Their findings were published in Molecular Biology and Evolution.

For virologists worldwide, the year 2003 holds special significance. Not only was the coronavirus-caused SARS spreading across Asia, but it was also the year that the first giant virus was discovered. As pathogens that straddle the line between living and non-living organisms, viruses had long been considered simple, small collections of genetic code.

But the newly-discovered Mimivirus was different. With nearly 1,000 protein-coding genes across a length of over one million base pairs, Mimivirus’ genome size and complexity rivalled that of small bacteria. While giant viruses defy people’s traditional notion of viruses, they are actually widespread in water bodies, infecting single-celled aquatic organisms to their detriment.

“Because these single-celled organisms greatly influence the carbon turnover in the ocean, giant viruses have an important role in our world’s ecology. It is just as important to study them and their evolution as it is to study disease-causing viruses,” explained lead author Dr. Kiran Kondabagil from the Indian Institute of Technology (IIT) Bombay.

To uncover how giant viruses came to be, Kondabagil and his IIT Bombay colleague Dr. Supriya Patil sought to prove one of two major theories surrounding Mimivirus evolution. The first, known as the reduction hypothesis, suggests that the giant viruses emerged from modern single-celled organisms that shed genes over the years. In contrast, the virus-first hypothesis proposes that giant viruses are relics of precellular forms of life that gained genes.

By creating phylogenetic trees of proteins involved in replication, the duo found that Mimivirus proteins were more closely related to eukaryotes compared to prokaryotes, or bacteria, and small viruses.

Through a technique called multidimensional scaling, Kondabagil and Patil also determined the similarity of Mimiviral proteins. The more similar the proteins are, the more likely it is that they co-evolved—suggesting that they are linked together in a larger protein complex for a specific function.

Indeed, their findings showed that the Mimiviral proteins were very much similar to each other. Finally, the researchers also showed that the genes related to DNA replication likely passed through purifying selection—a type of natural selection that purges harmful gene variants. Accordingly, purifying selection is often observed in genes involved in essential functions, such as DNA replication.

Taken together, their results map out the ancient origin of Mimivirus DNA replication machinery—supporting the reduction hypothesis that such machinery already existed in a single-celled ancestor. Over time, giant viruses were formed by getting rid of the ancestor’s unnecessary structures, leaving behind only the replication-related parts of the genome.

“Our findings are very exciting because they inform how life on earth has evolved. Because these giant viruses probably predate the diversification of the unicellular ancestor into bacteria, archaea and eukaryotes, they should have had major influence on the subsequent evolutionary trajectory of eukaryotes, which are their hosts,” said Kondabagil.

Beyond contributing to basic scientific knowledge, the team’s work could also open new doors in translational research like genetic engineering and nanotechnology.

“An increased understanding of the mechanisms by which viruses copy themselves and self-assemble means we could potentially modify these viruses to replicate genes we want or create nanobots based on how the viruses function. The possibilities are far-reaching!” concluded Kondabagil.

The article can be found at: Patil & Kondabagil. (2021) Coevolutionary and phylogenetic analysis of Mimiviral replication machinery suggest the cellular origin of Mimiviruses.


Source: Indian Institute of Technology Bombay; Photo: Shutterstock.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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