Primitive fish may shed light on evolution of limbs

The genome of a primitive fish that was once thought to have died when the dinosaurs did has now been sequenced by scientists — and when put into mice, some of the fish DNA caused mice to sprout limbs.

The new analysis, described today (April 17) in the journal Nature, could help to reveal how primitive fish swapped their fins for limbs when they moved from land to sea.

The fish, called a coelacanth, seems to carry snippets of DNA that can turn on genes that code for forelimbs and hind limbs in mice. The new discovery could shed light on how four-legged creatures, called tetrapods, evolved. [Image Gallery: The Freakiest Fish]

"It really is a cornerstone from which we can view tetrapod evolution," said study co-author Chris Amemiya, a geneticist at the Benaroya Research Institute in Seattle, Wash.

Living fossil

The coelacanth was once thought to have gone extinct about 70 million years ago, roughly around the time dinosaurs vanished. But in 1938, a fish trawler brought a bluish-purple, 3.3-foot-long (1 meter) fish with fleshy fins to the South African naturalist Marjorie Courtenay-Latimer. It turned out to be an African coelacanth.

Over the next several decades, scientists unearthed a few hundred of the elusive creatures living around the Comoros Islands in the Indian Ocean, as well as off parts of Indonesia.

The coelacanth intrigued scientists because it was a kind of "living fossil": It had changed so little over the last 400 million years that it might reveal how fish first grew limbs and walked on land.

Deepening the mystery, other research showed that fish, mice and other animals carry many of the same genes. But in fish, those genes code for fins, whereas in land-based animals, they create limbs.

Mysterious genes

Because the fish were so endangered, it was difficult to study their body plan in detail. But Amemiya and his colleagues managed to get tissue samples from a coelacanth from the Comoros Islands.

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