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Modern mutant fish replicate creatures from ancient seas

A 'Devonian' aquarium: Modern mutant fish replicate creatures from ancient seas

Skeleton of a mutant (top) and normal zebrafish, showing the structural differences with the jaw. Credit: Tetsuto Miyashita

Zebrafish is a common aquarium species that is valuable to both hobbyists and researchers. Researchers have now created an unusual change in those who have echoes of Jurassic Park ̵

1; but see lone fool.

A research team that studied jaw development in the earliest known vertebrates – fish from about 400 million years ago – has found that a mutation with a gene in the toe-sized zebrafish produced a surprising lookalike of long-extinct species.

The phenomenon revealed in their experiments, called developmental plasticity, explains why a real Jurassic Park will continue to be a distant concept, best suited for cinematic narrative rather than scientific reality.

“The plasticity we found in the mutants is a key to the great mystery,” said Dr. Tetsuto Miyashita, now a paleontologist at the Canadian Museum of Nature and a former postdoctoral fellow at the University of Chicago. He led the research team, which also included researchers at universities in Alberta and Southern California. The results are published today in Journal of Experimental Biology.

Miyashita is interested in the development of important characteristics in vertebrates, of which the jaws were one of the first critical developments. “We generally believe that evolution adds new things. For example, the first jaws evolved in fish 450 million years ago, giving them the ultimate competitive advantage over other living things. Today we would starve and suffocate without jaws. But sometimes evolution goes “The opposite is true. Delete something that has been around for millions of years, and it suddenly opens up new evolutionary directions. My idea is to see it in action.”

A 'Devonian' aquarium: Modern mutant fish replicate creatures from ancient seas

Shell of an anaspid, a 430 million year old fossil jawless fish resembling mutant zebrafish. This sample comes from Cornwallis Island, Nunavut, Canada. Credit: Tetsuto Miyashita

To do this, Miyashita bred zebrafish that had a mutation in a gene that instructs the cells to form a hinge joint in the jaw. The mutant zebrafish is born without a jaw joint so that the upper and lower jaws melt into one – which creates a gaping mouth that cannot be closed. With her wide eyes and gaping mouth, Miyashita reckons that they resemble the figure in Edward Munch’s artwork “The Scream”.

Surprisingly, Miyashita observed that these mutants swimming in the aquarium could survive and thrive – despite the lack of a hinge with a jaw to bite. “Instead of digesting for food, the fish chase it until the food ends up in their large gape,” says Miyashita. “They do not seem to be able to move their lips or close their mouths – these fish literally have their jaws dropped, fixed in that position.”

The researchers observed that the skulls of the jawless mutants were rebuilt with a shortened face, enlarged cheeks and massive neck muscles – similar features that first appeared in some of the earliest known jawless fish during the Silurian and Devonian periods almost half a billion years ago.

Known as anaspids and thelodonts, these are long extinct fish that swam in ancient seas far away from the zebrafish’s line of origin. But just like the mutant zebrafish, these creatures’ fossils show that they also lacked biting jaws and probably had a similar feeding strategy.

It seems like a Jurassic Park moment, as if Miyashita created Devonian jawless fish of modern. But, he explains, there is a more nuanced answer. “The resemblance is more a coincidence than by design. Anaspiders and thelodonts are distant cousins ​​half a billion away,” he says. “Zebrafish did not come from them, so can not ‘go back’ to them.”

A 'Devonian' aquarium: Modern mutant fish replicate creatures from ancient seas

A normal zebrafish (left) and mutant zebrafish with gaping mouth (right). Credit: Tetsuto Miyashita

Essentially, the mutant zebrafish experiments suggest that genetic engineering does not allow for the restoration of an ancestor. It allows only superficial convergence of traits that mutants develop out of necessity. “The discovery sets an unrealistically high bar for a Jurassic Park-like scenario. Constructed similarities are profound, but origin and content are completely different.”

Miyashita notes that the scientific phenomenon of evolutionary plasticity is the reason why features of extinct organisms sometimes occur in lab-made mutants, as in the highly publicized case of ‘dino chickens’. Although these types of mutants are sometimes referred to as a rewinding of the evolutionary clock, plasticity means that they are no more than occasions.

That said, these initial experiments provide insights for future scientific research – especially for how extinct jawless fish (the forerunners of today’s fish) may have been feeding, breathing and swimming. The mutant zebrafish can be studied by biologists to further investigate why evolution makes a temporary “leap”. Understanding how jaw fusion occurs in mutants can also pave the way for a new treatment for certain joint diseases.

Miyashita began her new position at the Canadian Museum of Nature on April 1, 2020 during the COVID pandemic. This followed postdoctoral work at the University of Chicago where he completed the zebrafish experiments. Researchers at the University of Alberta contributed CT scans and analysis of skull morphology. Miyashita plans to continue his work with the mutant zebrafish by establishing a new functioning “Devonian” aquarium at the museum.

How the zebrafish got its stripes

More information:
Tetsuto Miyashita et al, nkx3.2 mutant zebrafish contain jaw loss through a phenocopy of the main forms of Paleozoic jawless fish, Journal of Experimental Biology (2020). DOI: 10.1242 / jeb.216945

Provided by the Canadian Museum of Nature

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