In contrast, the coelacanth seems to have changed very little--physically or genetically--since one wayward branch of the fish family headed for land roughly 360 million years ago. Because it has changed so little the coelacanth is ideal for genetic comparisons. Any genetic feature found in all land animals but lacking in the coelacanth could represent a change that makes living on land possible.
Noonan said that coelacanth's close relative, the lungfish, could also fill in the genetic gap between land animals and fish, but the coelacanth has one practical advantage: "The lungfish genome is enormous," said Noonan, who is now a postdoctoral fellow at the Lawrence Berkeley National Laboratory. At 35 times the size of the human genome, sequencing the lungfish is an unlikely proposition. In contrast, the coelacanth genome is smaller than that of either humans or mice.
To make his case for the coelacanth, Noonan sequenced a group of coelacanth genes called the protocadherin gene cluster. He chose this region because it is extremely variable between different species, making it easy to see differences and similarities. This region has 54 genes in humans and 97 genes in the zebrafish, whose genome has been sequenced. He found that the coelacanth had 49 genes in the cluster, much like humans and other land animals. What's more, humans and coelacanths both have subgroups of these genes that zebrafish lack. "The coelacanth is evolving very slowly, that's what makes them interesting," Noonan said.
Although it isn't known why coelacanths evolve so slowly, Noonan suggested that their lifespan might
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Contact: Amy Adams
amyadams@stanford.edu
650-723-3900
Stanford University Medical Center
16-Nov-2004