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Frog builds toes, then legs

Watching gene activity of growing coquí frogs reveals surprising sequence

A small frog appears to jump-start its skeletal development, turning on genes for building feet and toes before bothering to build its legs.

While researchers are still trying to figure out how a clump of cells becomes a wing or flipper or arm, the order of events has been established: The upper arm bone forms first, then the forearm, then the wrist bones, and finally fingers or toes.

But the new research, reported in the July–August Evolution & Development, hints that limb formation may not be so clear-cut.

“This is a very interesting idea,” says developmental geneticist Francesca Mariani, who was not involved with the research. “Maybe limb development has different ways of occurring.”

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