Ancient viruses played a role in the development of our brains

A new study going back hundreds of millions of years has come to this discovery. Ancient viruses that infected vertebrates played a crucial role in the evolution of our advanced brains and large bodies. The work, published Thursday in the journal Cell, explores the origins of myelin, an insulating fatty membrane that forms around nerves and allows electrical impulses to travel faster.

According to the authors, genetic sequence derived from retroviruses—viruses that invade host DNA—is critical for myelin production. And this code is found today in modern mammals, amphibians and fish. “What I find most remarkable is that all this diversity of known living vertebrates and the sizes they have reached – elephants, giraffes, anacondas … – would not exist” without infection by these retroviruses, neurobiologist Robin Franklin, a co-author of the study, said AFP.

The researchers searched genomic databases to try to uncover genetic factors associated with myelin production. Tanay Ghosh, a biologist and geneticist who worked with Robin Franklin, was particularly interested in the mysterious “non-coding” regions of the genome that have no obvious function and were once thought to be useless but are now recognized as important. in evolution.

Jaw factor

His research led to a retrovirus sequence that has long been present in our genes, which researchers named “Retromyelin.” To test their discovery, they performed experiments deleting this sequence in rats and noticed that they no longer produced the protein needed to form myelin.

The scientists then began looking for similar sequences in the genomes of other species and found a similar code in jawed vertebrates—mammals, birds, fish, reptiles and amphibians—but not in jawless vertebrates or invertebrates. They concluded that the sequence appeared on the tree of life at about the same time as jaws, about 360 million years ago.

Brad Zuchero of Stanford University, who was not involved in the work, called the study a “fascinating insight” into the history of our jawed ancestors. “There has always been selection pressure for nerve fibers to conduct electrical impulses faster,” said Robin Franklin. “By doing it faster, you can act faster,” he explained, which is useful for predators chasing prey or prey trying to escape.

Myelin allows these signals to be transmitted quickly without increasing the diameter of the nerve cells, allowing them to move closer together. It also provides structural support, meaning the nerves can grow further, allowing larger limbs to develop. In the absence of myelin, invertebrates have found other ways to quickly transmit electrical signals: for example, giant squids are equipped with larger nerve cells.

Useful viruses

Finally, the team wanted to understand whether the viral infection occurred once, in one ancestral species, or multiple times. To answer this question, they analyzed RetroMyelin sequences from 22 species of jawed vertebrates. These sequences were more similar within a species than between different species. According to the researchers, this suggests that several waves of infection occurred, contributing to the diversity of vertebrate species known today.

“We tend to think of viruses as pathogens, agents that cause disease,” observed Robin Franklin. But the reality is more complex, he says: At different points in history, retroviruses have entered the genome and integrated into the reproductive cells of species, allowing them to be passed on to subsequent generations. One of the best known examples is the placenta, common to most mammals, derived from a pathogen long integrated into the genome.

For Tanay Ghosh, the discovery of myelin may be just the first step in a developing field. “There is still a lot to be understood about how these sequences influence various evolutionary processes,” he said.