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Limbs can be incredibly useful. Whether it’s the wing of a bat, the elongated leg of a hopping frog or our own grasping arms, limbs have been adapted to all sorts of ecosystems and functions through the course of evolutionary time.
The earliest limbs date back to over 375 million years ago. The fossil record has beautifully documented how the fleshy fins of ancient fish became more and more limb-like and allowed our amphibious ancestors to come ashore. These creatures, like us, are known as tetrapods—or “four limbs.” Now a study on a modern fish familiar to aquarium enthusiasts has provided new insight into the genetic underpinnings of this transcendent change. Boston Children's Hospital biologist M. Brent Hawkins and colleagues published a study today in Cell that demonstrates mutations to either of two zebrafish genes can create a very limb-like fin in these fish. By using gene-editing techniques to replay the mutation in the lab, the researchers were able to pinpoint how some zebrafish grow fins that have more of a resemblance to our arms. |
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Finding the relevant genes started with looking for fish with particular mutations. The Harris Lab, of which Hawkins is a part, screened over 10,000 mutated animals for particular skeletal defects. Among those that stuck out were zebrafish that had extra bones in their fins. Much like lab mice and fruit flies, zebrafish are classic study animals for understanding genetics and development. They’re classified as teleosts—bony fish that support their fins on pointed rays. Only, some of the mutant zebrafish had fins that had extra bones. Not only that, but the new bones were attached to muscles and even formed joints, just like a limb. “Finding a fish with extra fin bones that should never be there was quite the ‘Eureka!’ moment,” Hawkins says.
Most striking of all was that the new bones required other changes to the fish’s anatomy. “Because development is an integrated process, this one mutation creates a new bone, but also creates a joint and brings along changes in musculature,” Hawkins says. With a single mutation, fins became something much more like arms. And so Hawkins and colleagues set about finding what could have been responsible for such a change. |
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