The feathers, fur, and scales that adorn different members of the animal kingdom may look very different, but they are all made of the same basic stuff. And, in the end, all it takes is a relatively simple genetic adjustment to produce one instead of the other.
By targeting the Sonic Hedgehog Gene (Shh)geneticists Michel Milinkovitch and Rory Cooper of the University of Geneva in Switzerland have modified embryonic chickens to grow feathers instead of scales on their usually scaly legs.
And the alteration is permanent. Once hatched, the chicken’s unusually feathery legs will remain so throughout its life.
“We demonstrate that transient stage-specific agonism of Sonic Hedgehog (Shh) pathway signaling in chicken triggers a complete and permanent transition from reticulate scales to feathers on the ventral surfaces of the foot and toes,” researchers write.
“The resulting ectopic feathers are developmentally comparable to the feathers adorning the body, with down-like feathers developing into regenerative outline feathers with bilateral symmetry in adult chickens.”
Previously, Milinkovitch’s lab demonstrated that the various keratinized skin appendages (hairs, feathers, etc.) have the same evolutionary origin in ancestral reptilians hundreds of millions of years ago who then diversified into the incredible variety of animals living in the world today. .
And in all of these animals, the development of these structures begins as structures called placodes in the skin, which thicken and then begin to develop into scales, spines, or whatever appendages the animal tends to have.
Shh plays a key role in the development of these appendages. The signaling pathway is important along with its fingers in many embryonic pies, mediating the development of body shapes and structures, patterning, differentiation, and growth.
For example, Shh is an important factor in feather shape and diversification, flight feather positioning in birds, and hair follicle development in mice.
We know that some chickens, such as Brahmas, Sablepoots and Silkies, have feathered feet, but the genetic mechanism behind this was unknown. So Cooper and Milinkovitch decided to investigate by targeting Shh in embryonic chickens.
The target of the study was broiler chickens from the Ross chicken breeding brand, which have very scaly, featherless legs. The researchers injected into the veins of embryonic chickens still in the egg a substance which promotes Shh pathway signaling to trigger feather growth in areas normally covered in scaly skin.
“We used the classic ‘egg candling’ technique, in which a powerful torch illuminates the blood vessels inside the eggshell,” Cooper explains.
“This allowed us to precisely treat chicken embryos with a molecule that specifically activates the Shh pathway, injected directly into the bloodstream.”
It worked. When the chickens hatched, their normally bald feet were covered in fluffy juvenile feathers, similar to those covering their bodies. And, as the chickens grew, so did the feathers.
They lost their babies and developed adult feathers, not only on their bodies but also on their feet. A single treatment in the bud was enough to induce this change from scales to feathers.
Some eggs were treated with a control solution, which did not contain the active molecule that stimulated Shh pathway signaling in the experimental group.
The hens hatched from these eggs had normal legs; RNA sequencing comparing the two groups showed that the changes made to Shh in the experimental group were permanent.
The researchers say this has implications for our understanding of how animals evolved and diversified. Natural variations in Shh signaling are likely an evolutionary driver of skin appendage diversity, and this diversity has not been particularly difficult to achieve.
“Our results indicate that an evolutionary leap – from scales to feathers – does not require large changes in genome composition or expression,” says Milinkovitch.
“Instead, a transient change in the expression of one gene, Shh, can produce a cascade of developmental events leading to the formation of feathers instead of scales.”
The research has been published in Scientists progress.