Genetics
Evidence Points to Genetic Expansion Behind Vertebrate Fingers, Toes
The evolutionary transition from life in the sea to life on land might have been nudged by a genetic expansion, according to an article appearing in the February Development. HMS researcher Susan Dymecki and her colleagues suggest that a gene previously expressed in the developing brain may also have come to be expressed in the tips of developing limbs, helping to bring about the development of toes and fingers in the first vertebrates.
"So the idea is you may get expansion of gene expression—not expression of a new gene—just expansion to a new area," says Dymecki, HMS assistant professor of genetics. She and colleagues Scott Baur and Jia J. Mai have recently identified the structure of a gene and also a genetic switch that could have brought about such a genetic expansion.
Until recently, the gene, which codes for a receptor found in the brains and skeletons of all vertebrates living today, was thought to be controlled by a single switch, or promoter. If that were true, a defect in the promoter should affect expression in the brain as well as the skeleton. But the researchers found that while mutant mice carrying such a defect lacked fingers and toes, their brains appeared, for the most part, normal.
On closer inspection, the researchers found that there was not one but two promoters, one controlling gene expression in the brain, the other, which carried the mutation, in the limbs. The defective promoter was farthest away from the gene. "This is the first time anyone has seen this distal promoter," says Dymecki.
She and Baur, who is a graduate student, propose that this distant promoter may have evolved more recently, perhaps as a result of a duplication of the one lying closer to the gene. Once formed, the new promoter may have accumulated mutations that enabled it to interact with transcription factors found in developing limb cells. As a consequence, the receptor previously expressed in the brain would have come to be expressed in the limb buds.
"This is speculation at this point—we could be wrong," Dymecki says. "But it could also lead to some exciting science." Baur is currently comparing the two promoters to see if he can find signs of a duplication. If so, the origin of life on land may have involved not just the invention of new genes but also putting old ones to new uses.
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