AsianScientist (Apr. 26, 2013) – Scientists have sequenced the genome of the coelacanth, an enigmatic prehistoric fish, providing valuable insights into how aquatic animals may have evolved limbs to live on land millions of years ago.
There is a reason why the coelacanth is called a “living fossil”: this extraordinary fish looks remarkably like its fossil ancestors that date back at least 300 million years.
Now, an international team of researchers have sequenced and analyzed the genome of an African coelacanth (Latimeria chalumnae).
In their study, published last week in Nature, the researchers found that the coelacanth’s genes evolved very slowly compared to other animals: the genes of lizards and mammals evolved at least twice as quickly. This may explain its primitive appearance as the fish has changed so little in 300 million years.
This ancient fish, which can be as long as two meters and as heavy as 90 kilograms, was thought to have gone extinct 70 million years ago until a fisherman caught one off the coast of South Africa in 1938. Since then, only 309 of this exceedingly rare fish that dwells in deep sea caves have been sighted off the east coast of sub-Saharan Africa and Indonesia.
The rarity of the fish is one reason why sequencing the coelacanth’s genome proved to be no easy task since the researchers needed fresh tissue and blood for their work.
Another reason was that coelacanths died immediately when caught because of the change in pressure and temperature. The DNA needed for genome analysis also degraded quickly in the hot tropical sun, becoming unusable.
The researchers managed to obtain genetic material for their work only by working with fishermen off the South African coast, teaching them how to collect and preserve coelacanth tissue when they accidentally caught one.
They located a fragment of DNA within the coelacanth’s genome that is also found in land animals but not in fish without lobed fins, such as tuna, tilapia, and sharks. Since they cannot study live coelacanths in the laboratory, the researchers inserted the fragment into a mouse embryo and found that the fragment activated a network of genes that forms bones in wrists, ankles, fingers and toes.
Although it is unclear what the DNA fragment’s function is within coelacanths, the researchers suggest that it may have been important in forming the ends of limbs. This fin-to-limb transition may have enabled a fish-like animal to crawl out of the water millions of years ago.
“The coelacanth with its distinctive fleshy fins represents an intermediary phase in the evolution of land animals from aquatic fishes,” says Prof. Byrappa Venkatesh, from the Institute of Molecular and Cellular Biology in Singapore, whose group was involved in the project.
“By comparing the genomes of coelacanth, human and other vertebrates our group has been able to discover gene regulatory elements that played a key role in the development of our limbs and fingers as well as our ability to detect air-borne odorants.”
The article can be found at: Amemiya et al. (2013) The African Coelacanth Genome Provides Insights Into Tetrapod Evolution.
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Source: A*STAR.
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