In 1938, a living relic, believed to be 65 million years old, was accidentally caught in a trawl net off the coast of South Africa.
2 meter long coelacanth (6.5 feet)Latimeria chalumnae) proved to be one of the closest relatives of our fish – looking largely unchanged since its most recent appearance in the fossil record from the time of non-avian dinosaurs.
Now, new genetic evidence shows that this deep predator has undergone a hidden evolution, but on a large scale genetically – by diverting genes from other species.
While searching the genetic databases for the ancestral version of a human gene involved in gene regulation, CGGBP1, University of Toronto molecular geneticist Isaac Yellan unexpectedly discovered that coelacanth strangely has many variations of this gene.
Even more unusual, these different variations of CGGBP genes did not all share a common ancestor. This suggests that at some point, about 10 million years ago, 62 of these genes were passed from coelacanth to other unrelated species – through horizontal gene transfer.
These genes, with their ability to “jump” around and even between genomes a bit like viruses, are known as transposons.
If they happen to jump to the right place in the genome, the cellular machines will copy them just like any other gene. But they can also jump to the wrong place, where they can be harmful and are considered parasites.
However, occasionally they may reach a useful position for their host species and may lose the ability to jump, but are preserved in their new place in the genome, which seems to have happened in the coelacanth, since many times .
“Horizontal gene transfer manages the image of where transposons come from, but we know from other species that it can occur through parasitism,” Yellan said. “The most likely explanation is that they have been introduced several times throughout the history of evolution.”
Although it is common to find transposons like these in many species, it is unusual to find so many.
Experiments with test tubes and computer modeling have shown at least eight of the proteins that these genes encode to bind to distinct distinct DNA sequences, suggesting that – like the human version – they are involved in gene regulation. Some of them are expressed only in specific tissues.
“We don’t know what these 62 genes do, but many of them encode DNA-binding proteins and probably have a role in gene regulation, where even subtle changes are important in evolution,” said University of Toronto molecular geneticist Tim Hughes.
The coelacanth has lobed fins similar to the legs and is more closely related to us and its closest fish relatives, lungfish, than other types of fish. Our very distant ancestor means that the coelacanth’s genome has the potential to help us discover many mysteries about our own evolution.
Unfortunately, these fish are rarely seen and on the verge of extinction, so opportunities to study them are limited. But the information we have from them is already proving fruitful.
A recent study of their genes suggests that our bitter receptors may have roles beyond protecting us from toxic substances, such as metabolic regulation and hormone detection. Now, coelacanth genes have shown that transposons play a bigger role than we realize in the evolution of tetrapods.
“Our findings provide a striking example of this transposon phenomenon that contributes to the host genome,” Hughe said.
This research was published in Molecular biology and evolution.