An international team of researchers has deciphered the genome of a creature whose evolutionary history is both enigmatic and enlightening : the African coelacanth . This sea cave inhabitant, which is 1.5 meters long, was believed to be extinct until a living specimen was found off the coast of Africa in 1938.
Since then, scientists have been wondering about these primitive-looking fish, popularly dubbed “living fossils.” Especially because it is striking that today’s coelacanths look so much like the fossilized skeletons of their ancestors more than 300 million years ago. Its genome, which has just been sequenced by the Broad Institute of MIT and Harvard (USA), and analyzed by an international consortium of experts., Confirms that genes evolve more slowly in this animal than in other organisms , and that explains why his appearance has barely changed, as published this week in the journal Nature .
The researchers hypothesize that this slow rate of change may be because coelacanths have barely had to adapt: they live mainly off the coast of East Africa (and a second species of coelacanth lives off the coast of Indonesia), at depths that undergone few modifications in the last millennia.
“There are still a few places on our planet where organisms do not have to change to survive, and the deep ocean is one of them, ” says Kerstin Lindblad-Toh, co-author of the work. “Coelacanths are probably very specialized for an extreme and very specific environment, but that doesn’t change,” he adds.
The jump from the ocean to the mainland
On the other hand, coelacanths possess some characteristics that are strangely similar to those observed in terrestrial fauna, including lobed fins , which resemble the extremities of four-legged terrestrial animals, known as tetrapods. Scientists suspect that an ancestral lobe-fin fish species likely gave rise to the first four-legged amphibian creatures that left the water to occupy the mainland, but so far it has not been possible to determine whether the coelacanth is the candidate. most likely.
In any case, the nearly 3 billion DNA “letters” of the coelacanth genome, comparable in length to that of humans, provide invaluable information on the genetic changes that allowed tetrapods to flourish on earth. Thus, for example, many of the changes identified in the genome affected genes involved in the perception of odor and the detection of odors in the air . There were also significant changes in the immune system , possibly to respond to new pathogens found on land, and new key genetic regions emerged that may have been “evolutionarily recruited” to form tetrapod limbs, fingers and toes, and the mammalian placenta.
“This is just the first of many analyzes about what the coelacanth can teach us about the emergence of terrestrial vertebrates, including humans,” the authors conclude.
