‘Living fossil’ genome unlocked : Nature News & Comment

‘Living fossil’ genome unlocked : Nature News & Comment.

– extracts from the article –

Like lungfish, the other surviving lineage of lobe-finned fishes, coelacanths are actually more closely related to humans and other mammals than to ray-finned fishes such as tuna and trout.

Ending one long-standing argument, analysis of the coelacanth genome clearly shows that it is not the closest living fishy relative to tetrapods, Amemiya says: that honour belongs to the lungfish. However, he adds, the lungfish genome is unlikely to be sequenced any time soon because it is much larger and more complicated than that of the coelacanth.

Comparison of protein-coding genes in coelacanths with those of cartilaginous fishes shows that the coelacanths have been steadily accruing DNA changes.

But the rate of change has been remarkably slow.

Scientists already had hints of the coelacanth’s sluggish evolution. In a 2012 study, researchers in Japan and Tanzania compared the DNA of the African and Indonesian coelacanths. Specifically, they looked at HOX genes, which help to guide embryonic development (K. Higasa et al. Gene 505, 324–332; 2012). Even though the two species separated, by one estimate, perhaps 6 million years ago, their genes are remarkably similar. For these particular genes, the difference between the two species of coelacanth was about 11 times smaller than that between the HOX genes of humans and chimps, two species that parted ways perhaps 6 million to 8 million years ago.

As expected, the genome holds clues to the genetic changes behind the transition from a lobed fin to a tetrapod limb, Amemiya says. For example, the analysis found that coelacanths and tetrapods share a regulatory gene sequence that helps to promote limb development. But other findings came as a complete surprise. The fish is the first vertebrate found to lack genes for immunoglobulin-M, an almost universal immune-system protein. Instead, it has two genes for a distantly related immune protein that evidently “pick up the slack”, he says.

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