Coelacanth Genome Cracked: 'Living Fossil' Is Evolving At A Snail's Pace

  @rpalmerscience on April 17 2013 3:21 PM

Surprise, surprise: The DNA sequence of a “living fossil” reveals that it’s evolving at a snail’s pace compared to the rest of us.

In a paper published in the journal Nature on Wednesday, researchers from 40 institutions in 12 countries said they’d cracked the genetic code of the African coelacanth (Latimeria chalumnae). The fish was long thought to have gone extinct in the late Cretaceous period, until one was hooked off of the eastern South African coast in 1938.

Though the coelacanth has been around for millions of years, its DNA sequence indicates that it hasn’t changed much over the eons.

“We found that the genes overall are evolving significantly slower than in every other fish and land vertebrate that we looked at," Jessica Alfoldi, a researcher at the Broad Institute of MIT and Harvard and co-first author of the Nature paper, said in a statement Wednesday. "This is the first time that we've had a big enough gene set to really see that."

The coelacanth might not have changed over millions of years because it didn't need to. These fish live deep in the ocean, where conditions have largely remained the same for thousands of years.

“Coelacanths are likely very specialized to such a specific, nonchanging, extreme environment -- it is ideally suited to the deep sea just the way it is."

Ocean warming is a known consequence of climate change but it largely thought to occur in the upper reaches of the water. However, this may be due to a lack of data-- there are many more temperature probes floating closer to the surface than there are at lower depths. Recent studies suggest that ocean warming may go deeper than we think. The coelacanth’s relatively static evolutionary strategy might not be a good bet to place in the near future.

Scientists are also using the coelacanth’s DNA sequence to examine the larger question of how fish first made the transition to land. The coelacanth’s lobed fins look similar to the legs of a tetrapod, or four-legged land animal; researchers have long wondered if a coelacanth ancestor gave rise to the first creature to haul itself onto land. There are other candidates too, including the lungfish group. The lungfish’s genome is much longer and has yet to be fully sequenced, but comparing some lungfish genes to the coelacanth's genetic code seems to have clarified the evolutionary tree.

"We conclude that the lungfish, and not the coelacanth, is the closest living relative of tetrapods," the authors wrote in Nature.

Still, scientists can glean much from the coelacanth’s DNA about the transition from water to land.

When scientists compared genes from the coelacanth with 20 other vertebrates, they found that moving to dry ground wrought some detectable changes. Many genes associated with smell and immunity changed, the better to adjust both the senses and the body’s defenses to a new environment. There’s a particular gene of interest, called HoxD, which contains a sequence found in both coelacanths and four-legged land animals. This sequence might have been “recruited” to form hands, paws, or feet where fins previously resided.

"This is just the beginning of many analyses on what the coelacanth can teach us about the emergence of land vertebrates, including humans, and, combined with modern empirical approaches, can lend insights into the mechanisms that have contributed to major evolutionary innovations," co-lead author Chris Amemiya, University of Washington biologist and a director at the Benaroya Research Institute at Virginia Mason, said in a statement.

SOURCE: Amemiya et al. "The African coelacanth genome provides insights into tetrapod evolution." Nature, published online April 17, 2013.

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