Ancient viruses that infected vertebrates hundreds of millions of years ago played a crucial role in the development of our advanced brains and large bodies, according to a new study.
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This work, published Thursday in the journal Cell, examines the origin of myelin, an insulating fatty membrane that forms around nerves and allows electrical impulses to be distributed more quickly.
According to the authors, a genetic sequence acquired by retroviruses – viruses that invade their host's DNA – is crucial for myelin production. And this code can be found today in modern mammals, amphibians and fish.
“What I find most remarkable is that all this diversity of known modern vertebrates and the sizes they have reached – elephants, giraffes, anacondas… – would not have happened without the infection of these retroviruses,” said neuroscientist Robin Franklin. Co-author of the study told AFP.
Researchers searched genomic databases 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 obvious function and were once considered useless but are now recognized as important in evolution.
His research led to a sequence derived from a retrovirus that has long been present in our genes, which the researchers named “RetroMyelin.”
To verify their discovery, they conducted experiments in which this sequence was deleted in rats and observed that they then no longer produced the protein necessary for the formation of myelin.
Faster reactions, bigger bodies
The scientists then looked 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 in the tree of life around the same time as the pine tree, about 360 million years ago.
The study was called a “fascinating insight” into the history of our jawed ancestors by Brad Zuchero of Stanford University, who was not involved in the work.
“There has always been a selection pressure to get nerve fibers to transmit electrical impulses more quickly,” emphasizes Robin Franklin. “If you do that faster, you can act faster,” he explained, which is useful for predators that are chasing prey or trying to escape.
Myelin allows these signals to be transmitted quickly without increasing the diameter of the nerve cells, allowing them to be brought closer together.
It also provides structural support, meaning the nerves can continue to grow, allowing for the development of larger limbs.
In the absence of myelin, invertebrates have found other ways to transmit electrical signals quickly: giant squid, for example, are equipped with larger nerve cells.
Viruses aren't just bad
Ultimately, the research team wanted to understand whether the viral infection had 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 there were multiple waves of infection that contributed to the diversity of vertebrate species known today, the researchers said.
“We tend to think of viruses as pathogens, pathogens,” noted Robin Franklin.
But the reality is more complicated, he says: At various times in history, retroviruses have invaded 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, which is characteristic of most mammals and was created long ago by a pathogen integrated into the genome.
For Tanay Ghosh, this discovery about myelin could be just a first step in an emerging field. “There is still a lot to understand about how these sequences influence different evolutionary processes,” he said.