Coelacanth genome reveals clues to evolution of terrestrial animals
New research results published in the scientific journal Nature today show that the Lungfish, not the Coelacanth, is the closest relative of land-living animals. Kerstin Lindblad-Toh at Uppsala University and Broad Institute has led the study where an international group of researchers have mapped the Coelacanth’s genome and compared it to genomes of land-living animals.
Many genes related to smell, vision, immune system, urine production and development of extremities (legs and arms, fingers and toes) have changed when land-living animals have adapted to their new environment. The Coelacanth on the other hand has hardly changed at all in hundreds of millions of years, perhaps because it has always lived in a very stable environment at the bottom of the deep oceans.
In 1938 the first living Coelacanth was discovered off the coast of Africa. It was a very surprising discovery, since it had previously only been studied as 70 million year old fossils and was considered extinct. The Coelacanth lives in deep sea caves and normally does not come up to the surface. It has large round fins which long have been thought to be the origin of land-living animals’ extremities.
A large international group of almost 90 scientists from many continents has now mapped and analysed the Coelacanth genome. The researchers also mapped genes in the Lungfish, a fish that also has been an interesting candidate for the role as the ancestor of land-living animals. By comparing hundreds of genes in the Coelacanth and the Lungfish with many land-living animals, the scientists could work out which of the two is their closest ancestor.
“Now we can confidently say that the Lungfish is a closer relative to land-living animals than the Coelacanth. It seems logical, since the Lungfish has both rudimentary legs and a kind of lungs”, says Kerstin Lindblad-Toh, Professor of Comparative Genomics at Uppsala University, Director of Vertebrate Genome Biology at Broad Institute and Director of SciLifeLab Uppsala.
Since the Coelacanth looks very old and closely resembles the 70 million year old fossils that have been found, it would be natural to think that its genome has developed very slowly. However, the current study shows that the genome in general has changed at a similar pace to other fish and amphibians, but that parts of the genome coding for proteins have changed much slower in the Coelacanth than in any other vertebrate animal studied.
“We often talk about how species have changed over time,” says Kerstin Lindblad-Toh. “But there are still a few places on Earth where organisms don’t have to change, and this is one of them. Coelacanths are likely very specialized to such a specific, non-changing, extreme environment – it is ideally suited to the deep sea just the way it is.”
By comparing the Coelacanth to land-living animals the researchers could also study changes to the genome that have adapted animals to a life on land. Some genes have disappeared, for instance genes related to developing fins, and other genes have arisen or been improved, and in many cases the signals controlling the activity of different genes have changed.
“Not surprisingly, genes involved in smelling, hearing, urine production, placenta growth, immune system and the development of our specialised extremities have changed. Based on these findings, developmental biologists will be able to continue learning about the land-living animals’ development and adaptation to their new environment”, says Kerstin Lindblad-Toh.
Linda Koffmar