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Mutant fish may explain how animals went from sea to land
zebrafish

The researchers were able to create extra bones in the fin by altering a single gene. This mutation marks a fundamental step in our understanding of the evolution of fins in limbs similar to ours.

Scientists at Harvard University have conceived zebrafish, as seen in the photo below, which show the initial formation of limb-like appendages after a single genetic change. According to the authors of the study, published in the journal Cell, the mutation marks a key step in understanding the evolution of fins to extremities similar to ours and shows how surprisingly simple genetic changes can create breakthroughs in the development of complex structures. Thus, these results can elucidate the transition from sea to land of vertebrates.

 

The image shows how the zebrafish transforms its extremities

“It was amazing that a single mutation could create entirely new bones and joints,” says M. Brent Hawkins, the study’s lead author. “In the 30,000 species of teleost fish, none have this type of variation, so the fact that we found a mutant like this really took our breath away”, recognizes the scientist. Teleosts form a diverse lineage that includes goldfish, salmon, eels, flounder, clownfish, balloonfish, catfish, and zebrafish.

The mutation that Hawkins and his colleagues discovered causes a change in the bones of the zebrafish’s pectoral fins called “proximal radiuses,” which attach to the fish’s shoulder joint, similar to the way the human arm attaches to our shoulder. But unlike humans and other tetrapods, zebrafish don’t have a series of these skeletal elements that articulate with joints, like the components of our arm and fingers. With this mutation, a new set of long bones called “intermediate radius” develops that are able to articulate with the existing proximal radius, forming a joint similar to our elbow.

“In this mutation, you get the new bone, you make the joint, and you put the muscles together all in one go,” says lead author Matthew Harris associate professor of genetics at Harvard Medical School and the Harvard Orthopedic School.

The zebrafish system is so coordinated that a simple mutation is enough to bring together the muscle, joint and bone gene in unison.

Limbs are considered a fundamental evolutionary innovation, allowing vertebrates to walk on land and, in the case of birds and bats, to fly. These results show that a group of fish thought to have lost or silenced the machinery needed to develop limb-like appendages actually retain an innate latency to form these structures. “With our work, we’ve found unexpected similarities between fins and limbs, and I think there are even more similarities that have yet to be discovered,” says Hawkins.

Land displacement

Despite these findings, questions remain as to whether these new bones alter the functionality of the zebrafish’s pectoral fins. The next steps will incorporate microscopy to determine whether the articulation of these new spines is sufficient to influence the way the fish moves. “It would be very interesting to see, for example, what happens if we put our mutant on a platform,” Harris said.

With the discovery of these mutants, the researchers open up a new line of questions about how vertebrates took their first steps towards land displacement and about the genetic and developmental mechanics necessary for this to happen.

Source: Harvard University

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