Researchers have discovered that ancient viruses that infected vertebrates played a crucial role in the formation of myelin, and thus the development of our brains.
- Researchers report that ancient viruses played a role in myelin development.
- The team discovered that a genetic element derived from the retrovirus is essential for myelin production in mammals, amphibians and fish.
- The retrovirus would have modified part of the genome around 360 million years ago.
Millions of years ago, viruses infected vertebrates and promoted their brain development. It almost sounds like a movie script. But it is indeed a stage of evolution, according to work from the University of Cambridge, published on February 15, 2024 in the journal Cell.
Evolution of the brain: myelin linked to infection with an ancient retrovirus
In seeking to understand the origin of myelin, the membrane that insulates nerve fibers and allows the transmission of electrical impulses, scientists examined the genome databases of different living species, focusing on regions “non-coding” which seem to have played an important role in evolution.
The team thus noticed that a genetic sequence from retroviruses – viruses which invade the DNA of their host – participates in the production of myelin. Called RetroMyelin, it has been observed in the genome of mammals, amphibians and fish.
“What I find most remarkable is that all this diversity of known modern vertebrates, and the sizes they reached – elephants, giraffes, anacondas… – would not have happened,” without the infection of these retroviruses, explained to AFP neuroscientist Robin Franklin, co-author of the study.
To test their hypothesis about the link between myelin and this sequence derived from a retrovirus, the researchers deleted the part of the genome they were studying in rats. Results: The modified rodents did not produce a protein necessary for myelin formation.
Myelin: infection took place approximately 360 million years ago
The scientists then sought to identify all the species that had this sequence. They found it in jawed vertebrates such as mammals, but also birds, reptiles, fish and amphibians. In contrast, RetroMyelin has not been observed in the DNA of jawless vertebrates or invertebrates. This element allowed the researchers to determine that the sequence appeared in evolution around the same time as jaws, around 360 million years ago.
“There has always been selection pressure to make nerve fibers conduct electrical impulses more quickly”Robin Franklin told AFP, explaining the importance of myelin in the transmission of information between the brain and the body. “By doing this faster, then you can act faster.“An essential point for survival both for predators who hunt and their prey who need to flee them.
How did the retrovirus insert itself into the DNA?
This discovery still leaves one question: did the retrovirus infect a species which then evolved once, or did it take several contaminations? By analyzing RetroMyelin sequences from 22 species of jawed vertebrates, the team noticed that the greatest similarities between sequences were observed within the same species. Which leads them to believe that there have been several waves of infections.
“We tend to think of viruses as pathogens, agents causing disease”, said Robin Franklin. But these are not their only actions. By integrating the genome of living beings, retroviruses have also enabled the transmission of elements essential to our evolution to subsequent generations, as shown by work on myelin. His colleague Tanay Ghosh concludes: “There is still much to understand about how these sequences influence different evolutionary processes.”
