In 1938, a living remnant, presumably extinct for 65 million years, was accidentally caught in a trawl net off the coast of South Africa.
The 2 meter long coelacanth (6.5 feet)Latimeria chalumnae) appears to be one of our closest family members – and seems largely unchanged since its most recent appearance in the fossil record of the time of non-bird dinosaurs.
Now, new genetic evidence shows that this deep-sea predator has undergone hidden but widespread evolution at the genetic level – by hijacking genes from other species.
While the genetic databases searched 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 coelacanth strangely enough has many variations of this gene.
Even more unusually, these different variations of the CGGBP genes did not all share a common ancestor. This suggests that 62 of these genes were wiped out by the coelacanth of other, unrelated species about 10 million years ago – by means of horizontal gene transfer.
These genes, with their ability to ‘jump’ and even between genomes such as viruses, are known as transposons.
If they jump in the right place in the genome, cellular machinery will copy it, just like any other gene. But they can also jump in the wrong place, where it can be harmful and considered parasitic.
Sometimes, however, they may end up in a position that is useful to their host species and eventually lose their ability to jump around, but are preserved within their new place in the genome, which often appears to be in the coelacanth. .
“Horizontal gene transfer blurs the picture of where the transpons come from, but we know of other species that can occur via parasitism,” Yellan said. “The most likely explanation is that they have been introduced several times throughout evolutionary history.”
Although it is common to find transposers like these in many species, it is unusual to find so many.
Test tube experiments and computer modeling have shown that at least eight of the proteins encoding these genes bind to different repeating DNA sequences, indicating that they – like the human version – are involved in gene regulation. Some of them are expressed only in specific tissues.
“We do not 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,” explained molecular geneticist Tim Hughes of the University of Toronto .
Selacant has bony lobed fins and is more closely related to us and our closest relatives, the lungfish, than other species of fish. Our far-shared ancestor means that the genome of the coelacanth can help us unravel many mysteries about our own evolution.
Unfortunately, these fish are rarely seen and threatened, and the opportunities to study them are limited. But the information we have from them is already fruitful.
A recent study on their genes suggests that our bitter receptors may play a role rather than protecting us from toxic substances such as metabolic regulation and hormone perception. Now selakanthenes show that transposons may play a larger role than we realize in tetrapod evolution.
“Our findings are a rather striking example of this phenomenon of transposition that contributes to the genome of the host,” Hughe said.
This research was published in Molecular biology and evolution.