200507 sonic hedgehog

Feathers, digits and the Sonic hedgehog

Bird feathers and human digits share a remarkably similar developmental history, according to a new study.

Grafts of Sonic hedgehog-producing cells produce a duplication of the wing, including its feathers. See full caption below story. Credit: Matthew Towers, University of Sheffield

And it all comes down to what is quite genuinely known as the Sonic hedgehog (SHH). [You can read one take on the story of its name here.]

The SHH gene is known to give our digits their identity, such that the thumb has quite a different mechanism from the finger next to it.

Now a research team led by Matthew Towers from the University of Sheffield, UK, and Marian Ros from the University of Cantabria, Spain, is suggesting that flight feather identity evolved in much the same way. Their paper is published in the journal Development.

In the bird embryo, feathers begin as buds – thickenings of the epidermis – that then develop into follicles, from which the keratin-based feathers are produced. 

Not all feathers are the same, however, even on the same bird; breast feathers and wing feathers vary greatly, for example.

Until now, the researchers say, it hasn’t been clear which signals regulate feather identity.

In their study using chicken embryos, they found that SHH is required in the earliest stages of wing development for the mature birds to develop flight feathers. They also defined a set of genes that are likely to be involved in this process. 

They report that SHH works in a defined temporal sequence to specify the different flight feather identities, mirroring how it specifies different digit identities. This similarity suggests that the digit identity network was co-opted for flight feather development during evolution.

“Flight feathers are one of the most important evolutionary adaptations that allowed birds to take to the air,” Towers says. “Our unexpected findings, showing that the digits and flight feathers share remarkably similar developmental programs, provide important insights into how the bird wing evolved to permit flight.”

The researchers say they now hope to discover how the early exposure of embryonic chick wing bud cells to SHH is “memorised” to allow flight feather formation at a much later stage of development.

Full caption

SHH is normally produced at the posterior margin of the embryonic chicken wing bud. In the study, grafts of SHH-producing cells were made to the anterior side of a wing bud of another chicken embryo. 

This operation duplicates the tissues of the mature wing, including the black-pigmented feather buds as shown in the image. 

The flight feathers buds are the ones protruding from the left and right margins of the wing. Duplicated tissues are on the anterior side of the image (left), and normal tissues are on the posterior side (right).

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