How giant viruses fuel the genetic evolution of organisms

Viruses are tiny invaders that cause a wide range of diseases, from rabies to tomato spotted wilt virus and, more recently, COVID-19 in humans. But viruses can do more than just cause disease, and not all viruses are small.

Large viruses, especially those in the large cytoplasmic nucleus DNA family of viruses, they can integrate their genome into that of their host, drastically changing the genetic makeup of that organism. This family of DNA viruses, otherwise known as “giant” viruses, has been known in scientific circles for some time, but the extent to which they affect eukaryotic organisms has been shrouded in mystery – until now.

“Viruses play a central role in the evolution of life on Earth. One way they shape the evolution of cell life is through a process called endogenization, in which they introduce new genomic material into their hosts. When a giant virus endogenizes itself in the genome of a host alga, it creates an enormous amount of raw material for evolution to work with, “said Frank Aylward, assistant professor in the Department of Life Sciences at Virginia Tech College of Science. and an affiliate of the Global Change Center housed in the Fralin Life Sciences Institute.

Mohammad “Monir” Moniruzzaman, a postdoctoral researcher in Aylward’s lab, studies endogenous viral elements, which are fragments or entire sequences of raw viral DNA that have been inserted into the genome of the infected host.

Together, Aylward and Moniruzzaman recently found that endogenous viral elements that come from giant viruses are much more common in chlorophyte green algae than previously thought.

Their results will be published today (November 18, 2020) in Nature.

Chlorophytes, a group of green algae, are an important group of photosynthetic organisms that underpin the food chain in many ecosystems and produce enormous quantities of food and oxygen around the planet. Chlorophytes thrive in our lakes and ponds, and their dynamics with giant viruses and their unique evolutionary history have been central to Aylward and Moniruzzaman’s research.

Chlorophyte algae are close relatives of terrestrial plants, and studying their interactions with giant viruses can shed light on the roles viruses played during early plant evolution.

“We now know that endogenous viral elements are common among chlorophytes, which suggests that plants could also interact with these giant viruses. Some data suggest that some primeval plants, such as mosses and ferns, experienced these endogenizing events over the course of the evolutionary timeline. But we’re not exactly sure the extent of this phenomenon in other primitive plants, ”said Moniruzzaman, the first author of this published paper.

To learn more about the prevalence of endogenous viral elements in algae, Moniruzzaman and Aylward performed a bioinformatic analysis on the sequenced genomes of different groups of algae.

They found that 24 of the 65 genomes analyzed had some types of viral signatures in their genomes, which originated from repeated endogenizations of distinct viruses. In an algal organism, Tetrabaena socialis, the researchers found that about 10% of its genes came from a virus in the large nuclear-cytoplasmic DNA virus family.

Although endogenization of viruses has been well studied, studies have mostly been limited to small ones RNA viruses, such as human immunodeficiency virus (HIV), the retrovirus responsible for acquired immunodeficiency syndrome (AIDS).

The study by Aylward and Moniruzzaman is one of the first to highlight large eukaryotic DNA viruses, which marks a major shift in the field.

Electron microscope image of an AaV, a giant virus that infects and kills single-celled algae that causes harmful algae blooms. Giant viruses that belong to the same group as AaVs can often insert their genomes into the genomes of their hosts. Image courtesy of Chuan Xiao and Yuejiao Xian, University of Texas at El Paso; Steven W. Wilhelm and Eric R. Gann, University of Tennessee, Knoxville.

“These large endogenous viral elements are much more common than previously thought. Now that we have a systematic analysis, other researchers will really start paying attention. This study shows that endogenous viral elements are quite common and therefore could be a common mechanism of genome evolution. I think these findings will broaden our view of the role of giant viruses as mere agents of host mortality to significant actors in the evolution of the host genome, ”Moniruzzaman said.

Now that Moniruzzaman and Aylward have confirmed that endogenization is taking place in larger viruses, they wonder what conditions are causing these viruses to inject EVE into green algae in the first place and why the hosts show no signs of rejection.

“We don’t know what the mechanism is or how DNA is maintained, but it is possible that endogenization is a random, almost accidental process. And once the viral DNA is endogenized, it can alter the host’s evolutionary dynamics and could further influence the evolution of that lineage, ”Aylward said.

The idea that there is a potentially beneficial relationship at play between the host and his virus is of particular interest to Moniruzzaman.

“There may be a reason why the host keeps these viral genomes in it. It’s not that these viral genes are causing the hosts to be unsuccessful or unable to survive in the environment. So that’s the point. : Are the endogenous viral elements useful for the host? And how are they getting in and staying there? “Moniruzzaman asked.

Reference: 18 November 2020, Nature.
DOI: 10.1038 / s41586-020-2924-2