In 1938, a living relic, thought to be extinct 65 million years ago, was accidentally caught in a trawl on the coast of South Africa.
The 2-meter-long (6.5-foot) coelacanthLatimeria chalumnae) turned out to be one of our closest fish relatives; it seemed virtually unchanged from its most recent appearance in the fossil record of the time of non-avian dinosaurs.
Now, new genetic evidence shows that this deep-water predator has undergone a hidden but broad evolution at the genetic level, hijacking genes from other species.
While searching the genetic databases for the ancestral version of a human gene involved in gene regulation, CGGBP1, the molecular geneticist at the University of Toronto Isaac Yellan, unexpectedly found that the coelacanth has strangely many variations of this gene. .
Even more unusually, these different variations of CGGBP genes did not all share a common ancestor. This suggests that at some points about 10 million years ago, 62 of these genes were dragged by the coelacanth of 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 jump to the right place in the genome, the cellular machinery will copy them like any other gene. But they can also jump to the wrong place, where they can be harmful and therefore considered parasites.
Sometimes, however, they may end up in a useful position for their host species and end up losing their ability to jump, but they are kept within their new place in the genome, which is what seems to have happened to the coelacanth several times.
“Horizontal gene transfer blurs the image of where the transposons came from, but we know from other species that it can be produced by parasitism,” Yellan said. “The most likely explanation is that they were introduced several times throughout evolutionary history.”
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 showed that at least eight of the proteins that these genes encode bind to repeated DNA sequences, suggesting that, like the human version, they are involved in gene regulation. . Some of them are only expressed in specific tissues.
“We don’t know what these 62 genes do, but many of them encode DNA-binding proteins and probably play a role in gene regulation, where even subtle changes are important in evolution,” he explained. Tim Hughes, molecular geneticist at the University of Toronto.
Coelacanths have lobed fins similar to their legs and are more related to us and our closest fish relatives, the lung fish, than other types of fish. Our very distant shared ancestor means that the coelacanth genome has the potential to help us unravel many mysteries about our own evolution.
Unfortunately, these fish are rarely seen and endangered, so opportunities to study them are limited. But the information we receive is already fruitful.
A recent study of its genes suggests that our bitter receptors may have functions beyond protecting us from toxic substances, such as metabolic regulation and hormonal detection. Now the coelacanth genes have shown that transposons play a more important role than we realize in the evolution of tetrapods.
“Our findings provide a rather striking example of this phenomenon of transposons that contribute to the host genome,” Hughe said.
This research was published in Molecular biology and evolution.