Blind cavefish cast light on the dark days of mammalian evolution | Science

Pale, shrunken and blind, the Somali Stonefish leads a peaceful life in the world's leanest waters. With their horrible paleness and lack of eyes, these pasty fish do not seem to have much in common with mammals, but the humble grotto has much more to offer than it sees in the eye.

Scientists reported yesterday in the newspaper Current biology this caveman could enlighten us on a mysterious chapter of the evolution of mammals: the loss of DNA repair by solar energy. Most organisms have mechanisms for repairing their own sunlight-activated DNA molecules, but mammals have lost the trait somewhere along their path, as well as Somali rockfish.

As an instruction manual for life, DNA is a valuable product. Persistent damage to this vital code contributes to both aging and increased susceptibility to cancer. Unfortunately, the process of copying and reading DNA can be tainted by errors and the environment around us is fraught with pitfalls, ranging from harmful chemicals to ultraviolet light rays. able to modify genetic sequences.

But thanks to a suite of cell machines capable of repairing compromised DNA, most of these genetic accidents are corrected without consequence. The photoreactivation system, which uses an enzyme called photolyase, powered by solar energy, is one of those crucial repair tools that corrects errors in DNA caused by exposure. UV light. This intelligent defense mechanism means that the same danger that damages the DNA, sunlight, also triggers a genetic code repair system.

Photoreactivation is widespread in the tree of life, but it is completely absent in mammals. And for a long time, we believed ourselves alone. But scientists started to discowith a handful of species of fungi and nematodes (and some selected populations of crustaceans related to caves) that also had lost their DNA-powered DNA repair capabilities. The lastborn of the dark group, the Somali cavaliers, could be the first non-mammalian vertebrate to haveWe have proceeded to a similar stage in the history of evolution.

"[Photoreactivation] is a system so well preserved, bacteria with plants and many animals, "says Nicholas Foulkes, a biologist at the Karlsruhe Institute of Technology in Germany. "When you see the loss of function, it's deep."

So, how could a cave-fish look like a mammal? It turns out that the answer literally keeps us in the dark. Our mammal ancestors enjoyed a very nocturnal lifestyle, says evolutionary biologist King Maor of University College London. Hundreds of millions of years ago, it is possible that our warm-blooded ancestors hid during the day to avoid being eaten by dinosaurs in love with the sun.

This nocturnal nature may have activated the principle "use it or lose it" in our evolution. The darker traits (such as photoreactivation with solar energy) could have been ruled out due to about 100 million years of obsolescence, says Maor. These genetic losses then persisted into modern times, even after mammals had begun to come into the open.

Foulkes' research group, whose lead author on the new Haiyu Zhao study, aims to study DNA repair in other nocturnal animals to learn more about the loss of photoreactivation mechanisms . The Somali caveman (Phreatichthys andruzzii), with his aversion to the sun, was a perfect creature to examine.

First, the researchers needed a point of comparison. For this, they chose another freshwater fish as a leaf: the zebrafish, a staple food well studied in many biology laboratories. Like most other animals, zebrafish genomes encode the sunlight-activated photoreactivation system, allowing them to survive exposure to high doses of UV light in well-lit environments. But zebrafish treated with ultraviolet light and trapped in total darkness is more sensitive to the repercussions of damage to DNA.

In addition, when the researchers conducted these same experiments on Somali rock fish, the fish was hypersensitive to UV rays. In the wild, the species lives away from the sun and exposing fish to conditions that mimic sunlight did not help them survive UV radiation.

By searching through the fish genome, the researchers found that the zebrafish made three restorative photolyases that ate in the presence of sunlight, while the Somali cavalfish only coded a defective system. After further examination, the researchers were able to determine the differences in How zebrafish and cavefish control the expression of photolyase.

In the presence of light, a molecular "key" in zebrafish cells is guided to a genetic "lock", which is released to activate the mechanisms of DNA repair. Curiously, the fish-cavities seemed to have intact locks, ready to release the expression of photolyase – but the keys seem to have been lost over time. The Foulkes team is currently searching for damaged or missing keys in the cave fish genome.

"It's like evolution is caught red-handed," says Foulkes. "You can see the process by which the repair system is being lost."

More than 200 species of fish-cavities populate the Earth, but this Somali specimen is the first to have lost the photoreactivation system. Even among cavernicolous fish, however, P. andruzzii is an extremist who has spent 3 million years away from the sun. In the eternal darkness of the underwater caves, it is in the interest of this swimmer to conserve energy for the long way to go – according to Foulkes, these fish can live more than fifty years – which means getting rid any unnecessary genetic baggage.

Although mammals do not share the lifestyles of cave fish, these genetic losses may reveal the obscure evolutionary trajectories shared by divergent species. Rather than developing a useful trait under the pressure of the environment, the creatures seem to have abandoned a system that was no longer useful, explains Silvia Fuselli, expert in fish-cavities at the University of Ferrara in Italy.

"Maybe these fish reproduce something that happened to our ancestors millions of years ago," says Foulkes.

Since some sun species will probably still successfully evade human discovery in the cavernous caverns of the Earth and the trenches of the deep seabed, we probably have not found the last creature that has lost photoreactivation. "That's what happens in these fish, in the mushrooms, in [crustaceans]… It will always be something that people will find regularly, "says David Carlini, a biologist at the American University, who studies freshwater crustaceans living in caves.

And as far as we know, P. andruzzii is still quite unique among most of his brothers who hate light. Until more species that prefer darkness can be studied, the Somali cavern fish could be the beacon to solve the mystery of the loss of our ability to heal in the sun.