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In 1938, a living relic, believed to have been extinct for 65 million years, was accidentally captured in a trawl net off the coast of South Africa.
The 2 m (6.5 ft) long coelacanth (Latimeria chalumnae) turned out to be one of our closest relatives to fish – seeming largely unchanged from its most recent appearance in the fossil record of the non-avian dinosaur era.
Now, new genetic evidence shows that this deep-sea predator has undergone a hidden but widespread evolution at the genetic level – by hijacking 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 the coelacanth strangely possesses many variants of this gene.
More unusually still, these different variations of the CGGBP genes did not all share a common ancestor between them. This suggests that at some point, around 10 million years ago, 62 of these genes were swept away by the coelacanth of other unrelated species – by horizontal gene transfer.
These genes, with their ability to “jump” around and even between genomes much like viruses, are called 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 in the wrong place, where they can be harmful and are therefore considered to be pests.
Sometimes, however, they can end up in a position useful to their host species and eventually lose their ability to jump but are retained in their new place in the genome, which appears to have happened in the coelacanth, several times. .
“Horizontal gene transfer blurs the picture of the origin of transposons, but we know from other species that it can occur via parasitism,” Yellan said. “The most likely explanation is that they have been introduced several times in the history of evolution.”
While it is common to find transposons 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 these genes code for bind to distinct repeat sequences of DNA, 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 code for DNA binding proteins and likely play a role in gene regulation, where even subtle changes are important in the genes. evolution, “said Tim Hughes, molecular geneticist at the University of Toronto.
Coelacanths have lobed, leg-like fins and are more closely related to us and our closest fish relatives, the lungfish, than to other types of fish. Our very distant common 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 the possibilities to study them are limited. But the information we have is already proving fruitful.
A recent study of their genes suggests that our bitter receptors may play a role beyond protecting us from toxic substances, such as metabolic regulation and hormone detection. Now, coelacanth genes have demonstrated that transposons potentially play a larger role than we think in the evolution of tetrapods.
“Our results provide a fairly striking example of this phenomenon of transposons contributing to the host genome,” Hughe said.
This research was published in Molecular biology and evolution.
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