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Dorsal fin marked the start of vertebrates growing arms and legs

Researchers are describing how ancient creatures adapted arms and legs. The study demonstrates that fish, mice, and modern-day vertebrates share a genetic advantage that was used to build limbs.

Experts at the University of Chicago Medical Center are reporting that the unpaired dorsal fin in ancient fish was duplicated to produce paired appendages, like pelvic fins, arms, and legs.

“The unpaired dorsal fin is the first one you see in the fossil record,” said study co-author Neil Shubin. “Here we show that the genetic mechanisms that pattern all the fins and other paired appendages originally arose there and were redeployed to others.”

The research team performed genetic testing in mice and fish to examine the expression of Sonic hedgehog (Shh), which is a gene that is critical in the formation of limbs.

Mice have a type of on-and-off switch called ZRS that controls the expression of Sonic hedgehog. If ZRS is inhibited in a mouse, the limbs cannot properly develop.

The team used gene editing tools to knock out ZRS in a well-known aquarium fish that is known as a Japanese rice fish. Instead of influencing the development of the paired fins, the fish did not grow unpaired dorsal fins. Meanwhile, the paired pelvic and pectoral fins developed as expected.

The researchers examined potential other genetic enhancers and found a related “shadow enhancer” called sZRS that was a critical component working alongside the ZRS switch.

When ZRS and sZRS were suppressed in the medaka, both the dorsal fin and paired fins were lost. The researchers have determined that it is likely that ZRS was first used to develop dorsal fins, and later copied and reused as sZRS when paired fins first emerged about 475 million years ago.

“It’s very ancient, and the sequence and function are conserved across all vertebrates,” said Shubin. “It turns out the primitive role for the ZRS was involved with the dorsal fin. It’s only later that its activity in the paired fins required this other shadow enhancer.”

Shubin said that understanding the activity of these enhancers helps to identify the traces of evolutionary ancestors present in all vertebrates, including humans.

“A number of human maladies are based on mistakes in the ZRS that can lead extra or missing fingers, or changes in the shape of hands,” said Shubin.

“Humans probably have this shadow enhancer too, so if we want to study the dynamics of how this affects limb patterning, what we see in these fish models is a great place to start.”

The study is published in Nature Genetics.

By Chrissy Sexton, Staff Writer

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