Scientists have discovered remnants of ancient, giant viruses within the DNA of a single-celled organism that shares a common ancestor with complex life forms, including ourselves. This remarkable finding suggests that viruses may have played a far more significant role in evolution than previously understood, potentially contributing genes that gave early cells a survival advantage.
The research, led by evolutionary biologist Alex de Mendoza Soler at Queen Mary University of London, examined the DNA of a single-celled organism called *Amoebidium appalachense*. This protist, which was first discovered living within the exoskeletons of freshwater insects, is closely related to animals.
"It's like finding Trojan horses hiding inside the *Amoebidium's
DNA," explained de Mendoza Soler. "These viral insertions are potentially harmful, but *Amoebidium* seems to be keeping them in check by chemically silencing them."
The team discovered that *Amoebidium
uses a process called 5-methylcytosine (5mC) to modify one of the four building blocks of DNA, effectively silencing these foreign genetic sequences. This modification is achieved by an enzyme called DNMT1, which is found in all multicellular organisms.
"Here, we show how a unicellular eukaryote closely related to animals undergoes a recurrent process of mixing its genome with that of its giant virus predators," the researchers wrote in their study published in *Science Advances*.
While viral attacks are typically detrimental to their hosts, *Amoebidium
seems to have developed a unique way to cope with these invaders. By silencing the viral genes, the organism not only survives but also incorporates some of the foreign genetic material into its own lineage.
To investigate the prevalence of this phenomenon, the researchers compared the genomes of various isolated *Amoebidium
species. They found a high degree of diversity in the viral material, indicating that the process of incorporating viral DNA is ongoing and dynamic.
"Viral insertions may have played a role in the evolution of complex organisms by providing them with new genes," said de Mendoza Soler. "And this is allowed by the chemical taming of these intruders' DNA."
These findings have significant implications for understanding the intricate relationship between viruses and their hosts. While traditionally viewed as invaders, the study suggests that viruses may have contributed to the evolution of complex life by providing a source of novel genetic material.
This research also sheds light on the presence of endogenous retroviruses in humans and other mammals. These remnants of ancient viruses are believed to be the remnants of viral infections that did not lead to the host's demise. While once considered inactive relics, increasing evidence suggests that many endogenous retroviruses may have provided beneficial functions that contributed to their preservation in our DNA.
The discovery of ancient viruses in the DNA of *Amoebidium
provides valuable insights into the evolution of complex life and highlights the complex and often unexpected ways in which viruses have shaped the history of life on Earth.