Researchers from The Harris Lab used CRISPR gene editing techniques to make zebrafish with limb-like fins.

via Smithsonian

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.

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