Cells responsible for producing myelin in the central nervous system, seen under a microscope (Altos Labs-Cambridge Institute of Science / Peggy Esink)
An ancient virus that infected vertebrates billions of years ago played a crucial role in the evolution of our advanced brains and large bodies, a new study suggests.
The work, published Thursday in the journal Cell, examines the origins of myelin, an insulating fatty membrane that forms around nerves and allows electrical impulses to be delivered more quickly.
According to the authors, the genetic sequence derived from retroviruses – viruses that invade their host’s DNA – is crucial for myelin production. And this code is found today in modern mammals, amphibians and fish.
“What’s most remarkable to me is that all this diversity of known modern vertebrates, and the sizes they’ve reached — elephants, giraffes, anacondas… — would not have happened” without this retrovirus infection, neuroscientist Robin Franklin, co-author of the study told AFP.
Researchers searched genome databases to try to uncover genetic factors associated with myelin production.
Tanay Ghosh, a biologist and geneticist who worked with Mr. Franklin, was particularly interested in the mysterious “non-coding” regions of the genome, which have no apparent function and were once considered useless, but which now have significance. In evolution.
Their research resulted in a retrovirus that has been in our genes for a long time, and which the researchers named “retromyelin.”
To test their findings, they conducted experiments involving the deletion of this sequence in mice, and observed that they then no longer produced the protein required for the formation of myelin.
– Quick reactions, big body –
The scientists then searched for similar sequences in the genomes of other species, and found the same code in jawed vertebrates — mammals, birds, fish, reptiles and amphibians — but not in jawless vertebrates or invertebrates.
They concluded that this sequence appeared around the jaws in the tree of life, around 360 million years ago.
The study was called a “fascinating insight” into the history of our jaw ancestors by Stanford University’s Brad Zucchero, who was not involved in the work.
Robin Franklin emphasized, “There has always been a selection pressure for nerve fibers to conduct electrical impulses more quickly.” “By doing it quickly, then you can act quickly,” he explained, which is useful for hunters chasing prey, or trying to escape prey.
Myelin allows the faster conduction of these signals without increasing the diameter of the nerve cells, allowing them to move closer together.
It also provides structural support, meaning nerves can grow further, allowing the growth of larger organs.
In the absence of myelin, invertebrates have found other ways to rapidly transmit electrical signals: giant squid, for example, are equipped with large nerve cells.
– Not just nasty viruses –
Finally, the team of researchers wanted to understand whether the viral infection occurred once, in a single 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.
This suggests that multiple waves of infection occurred, contributing to the diversity of vertebrate species known today, according to the researchers.
“We think of viruses as pathogens, disease-causing agents,” noted Robin Franklin.
But the reality is more complex, he says: At different times in history, retroviruses 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 — a feature of most mammals — long ago acquired from a pathogen integrated into the genome.
For Tanay Ghosh, this discovery on myelin may be just the first step in an emerging field. “There is still much to be understood about how these sequences influence different evolutionary processes,” he said.
