Washington:
Ancient viruses that infected vertebrates hundreds of millions of years ago played a crucial role in the evolution of our advanced brains and large bodies, a study said Thursday.
The research, published in the journal Cell, examined the origins of myelin, an insulating layer of fatty tissue that forms around nerves and allows electrical impulses to travel faster.
According to the authors, a gene sequence obtained from retroviruses – viruses that invade their host's DNA – is crucial for myelin production, and that code is now found in modern mammals, amphibians and fish.
“What I find most remarkable is that all the diversity of modern vertebrates that we know of, and the sizes that they have reached: elephants, giraffes, anacondas, bullfrogs, condors, would not have happened,” says senior author and neuroscientist. Robin Franklin of Altos Labs-Cambridge Institute of Science told AFP.
In new research led by Tanay Ghosh, a computational biologist and geneticist in Franklin's lab, analysts searched genome databases to discover the genetics likely associated with the cells that produce myelin.
In particular, he was interested in investigating mysterious 'non-coding regions' of the genome that have no apparent function and were once dismissed as junk, but are now recognized as being of evolutionary importance.
Ghosh's search landed on a particular sequence derived from an endogenous retrovirus, which had long been lurking in our genes, which the team named 'RetroMyelin'.
To test their findings, researchers conducted experiments in which they destroyed the RetroMyelin sequence in rat cells and found that they no longer produced a basic protein necessary for myelin formation.
Faster reactions, bigger bodies
They then looked for RetroMyelin-like sequences in the genomes of other species, finding similar code in jawed vertebrates – fellow mammals, birds, fish, reptiles and amphibians – but not in jawless vertebrates or invertebrates.
This led them to believe that the series appeared on the tree of life around the same time as jaws, which first emerged about 360 million years ago in the Devonian period, called the Age of Fish.
“There has always been an evolutionary pressure to make nerve fibers conduct electrical impulses faster,” says Franklin. “If they do that faster, you can act faster,” he added, which is useful for both predators trying to catch things and prey trying to flee.
Myelin allows rapid impulse conduction without increasing the diameter of nerve cells, allowing them to be packed closer together.
It also provides structural support, meaning nerves can grow longer, allowing for longer limbs.
In the absence of myelin, invertebrates have found other ways to transmit signals more quickly; giant squids, for example, have developed broader nerve cells.
Finally, the team wanted to know whether the retroviral infection occurred once, in a single ancestral species, or if it happened more than once.
Are there more discoveries waiting?
To answer this, they used computational methods to analyze the RetroMyelin sequences from 22 jawed vertebrate species, finding that the sequences were more similar within than between species.
The finding suggested that multiple waves of infection led to the diversity of vertebrate species we see today, the team said.
“You tend to think of viruses as pathogens or disease-causing agents,” says Franklin.
But the reality is more complicated, he said: At various points in history, retroviruses have invaded the genome and integrated themselves into a species' reproductive cells, allowing them to be passed on to future generations.
One of the best-known examples is the placenta – one of the defining features of most mammals – which we acquired in the deep past from a pathogen embedded in our genome.
Ghosh said the myelin finding could simply be the next step in an emerging field. “There are still many things to understand in terms of biology about how these sequences drive different evolutionary processes,” he said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)