Genetic Switch for Limbs and Digits Found in Primitive Fish

From here:

Genetic instructions for developing limbs and digits were present in primitive fish millions of years before their descendants first crawled on to land, researchers have discovered.

Genetic switches control the timing and location of gene activity. When a particular switch taken from fish DNA is placed into mouse embryos, the segment can activate genes in the developing limb region of embryos, University of Chicago researchers report in Proceedings of the National Academy of Sciences. The successful swap suggests that the recipe for limb development is conserved in species separated by 400 million years of evolution.

“The genetic switches that drive the expression of genes in the digits of mice are not only present in fish, but the fish sequence can actually activate the expression in mice,” said Igor Schneider, PhD, postdoctoral researcher in the Department of Organismal Biology and Anatomy at the University of Chicago and lead author on the paper.

The results contradict a previous finding that a developmental switch from pufferfish DNA was not capable of gene expression in the limbs of mice, suggesting that tetrapods evolved a novel developmental system. But the new experiments suggest that the genetic switch controlling limb development was in fact present deep in Earth’s evolutionary tree.

“There previously was the idea that these switches had to be generated from scratch de novo, but no, they already existed, they were already there,” said Marcelo Nobrega, MD, PhD, assistant professor of human genetics at the University of Chicago Medical Center and another author of the study. “Maybe the key was expressing a gene earlier or later or in a specific territory, but it was just a modification of a program that was already encoded in the genomes of fish almost half a billion years ago and remains there to this day.”

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3 responses to “Genetic Switch for Limbs and Digits Found in Primitive Fish

  1. wm tanksley

    The statement “Genetic instructions for developing limbs and digits were present in primitive fish millions of years before their descendants first crawled on to land” is not precisely accurate. Rather, the genetic instructions that perform the growth stage of fins are very close to the genetic instructions that perform the growth stage of limbs and digits.
    It’s not that limbs were already designed and ready to grow, waiting only to be switched on; rather, they were switched on, but being used in a different pattern.
    This doesn’t contradict the hypothesis of this blog, but neither it quite as cool as it seems from the quote this article started with.

  2. From the article: “Future experiments will focus more closely on how the gene regulation system functions, examining the differences between the segments in fish and tetrapods that control development of either a fin or a limb. Subtle changes in the timing or location of gene expression may produce the dramatic differences in anatomy that first allowed animal life on Earth to explore land.

    “There is a whole universe of questions that are opened up by this discovery,” Shubin said.

    It sounds like there may be a lot more to discover.

  3. wm tanksley,

    Agreed. I was going to add a short editorial comment, cautioning people against interpreting this to mean fish have the information for making mouse paws, but I just didn’t have the time. So your comment is most welcome.

    I found it cool because its yet another example where what was assumed to be something that arise de novo turns out to be an example of deep homology.

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