A change has taken this fish from its wings to its limbs

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How exactly that it could have happened hundreds of millions of years ago, however, it has long been an evolutionary mystery that has puzzled scientists.

Now, American scientists say they may have come across a possible answer.

By modifying a single gene, researchers at Harvard University and Boston Children’s Hospital designed zebrafish that showed the beginning of limbl-type appendages.

To answer the way animals made the transition from sea to land, scientists have traditionally looked at fossil documents. But for the past 30 years, scientists have been looking for changes in genes that could explain the shift from wings to modern limbs.

“The most surprising thing to me is that such a dramatic change in the skeleton and wing muscles is possible with just one mutation,” said Brent Hawkins, a postdoctoral researcher at Harvard University and Boston Children’s Hospital and the first author of the study. Thursday in Cell magazine.

“Prior to this discovery, the transition from wing to limb was thought to involve multiple changes in a multitude of genes,” he said in an email.

Of course, this transformation was still a very gradual and complex process, but our mutants show that it may have involved fast jumps and that developing animals are able to incorporate new bones quite easily.

This illustration, produced by the study's researchers, describes (from the left) how a human limb has multiple long bones that allow a wide range of movements; the skeleton of the wings of a normal zebrafish that has no joint; and that of a mutant zebrafish, with new bones moving away from the body in a pattern similar to the limbs.

Innate ability

While previous research has identified genes that are needed to make limb and wing bones, no one has previously found a genetic change that could move a wing to a more similar pattern, Hawkins said in an email.

The mutation discovered by the researchers causes a change in the pectoral fin bones of zebrafish, which attaches to the shoulder joint of the fish just like the way the human arm attaches to the shoulder.

A new set of long bones – called intermediate radii – develops, forming a joint similar to a human elbow. Genetic change includes new muscles and joints that are found in the limbs, but not in the simple fins.

The discovery of scientists showed that fish, which were thought to have lost the machines needed to evolve similar body parts, actually retain an innate ability to form these structures.

Miniature zebrafish have become a mainstay of genetic research. They are easy to keep in large numbers and reproduce easily, with a single pair of pairs producing hundreds of eggs each week. These fish are also gentle and easy to handle, plus their eggs and embryos are translucent and easy to examine.

The researchers moved the genes of the zebrafish at random and then systematically examined the fish to find those that had undergone interesting changes in their shape – in this case similar structures.

They then sequenced the DNA to find out which genes were affected before using the CRISPR gene editing tool – a method of modifying genes that won the Nobel Prize in Chemistry last year.

This genetic link between the wings and the limbs could, with more research, shed light on how some animals have made the transition from sea to land and what genetic mechanics are needed for this to happen.

One question the team hopes to further examine is whether the new bones change the way zebra fins work and how the fish move.

“That such complex and coordinated changes can result from changing a single letter of DNA was quite shocking and shows that our fish ancestors had the raw genetic material and the latent ability to create limbs,” Hawkins said.

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