Planet Earth

The Nocturnal Reptiles

By Kathy A SvitilJan 1, 2003 6:00 AM


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Belinda Chang and Thomas Sakmar, both molecular biologists at Rockefeller University, have re-created a gene from archosaurs, dinosaur ancestors that lived 250 million years ago. Chang says they're not exactly about to clone the whole creature—still a scientific impossibility—but the achievement offers a hint of how reptiles viewed their world during the Triassic Period. Sci-fi thrillers like Jurassic Park make the study of ancient DNA look like a snap, but it is extremely difficult to distinguish ancient molecules from modern ones. Moreover, DNA and proteins degrade into useless junk after just a few thousand years. So Chang and her colleagues opted to rebuild from scratch a bit of archosaur DNA—specifically the gene for rhodopsin, a light-sensing pigment in the eye. First the researchers use a computer program to analyze the rhodopsin gene sequence from 30 living vertebrates, including alligators and chickens, which are directly descended from archosaurs. Because the rhodopsin gene in each species evolved from a common ancestor, the program reveals how the gene has changed through the aeons and predicts the most likely archosaur version. Chang's team then synthesized that gene and plunked it into cultured monkey cells, which churned out fully functional rhodopsin.

The synthetic archosaur rhodopsin responds best to slightly redder colors than the light that most modern vertebrates sense. "This raises the possibility that archosaurs could see fairly well at relatively low light levels," says Chang. If so, archosaurs might have been active at night. Chang is now sorting through rhodopsin sequences of about 50 vertebrates to push even further back in time and to determine what the pigment looked like 400 million years ago. "This approach can not only reconstruct animals that no longer exist, but it also highlights major changes in gene function," she says.

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