The UNIGE team demonstrates how a particular change in gene expression causes feathers to replace scales in chickens. Scales, spines, feathers, and hair are examples of vertebrate skin appendages that make up an extremely diverse group of microorganisms. Despite the natural diversity of forms, these appendages share common early developmental processes at the embryonic stage. Two researchers from the University of Geneva (UNIGE) have discovered how the scales that normally cover chicken legs can be permanently transformed into feathers by specifically changing the expression of certain genes. These results, published in the journal Science Advances, Open new perspectives for the study of mechanisms that produce radical evolutionary changes in forms between species.

The skin of terrestrial vertebrates is adorned with various keratinized appendages such as hair, feathers, and scales. Despite the various forms within and between species, the embryonic development of skin appendages usually begins in a very similar way. Indeed, all these structures develop from cells that form local thickening of the skin surface and express certain genes. One of these genes, called sonic hedgehog (Shhh) controls the signaling pathway, the communication system that allows messages to pass within and between cells. Shh signaling is involved in the development of various structures, including the neural tube, limb buds, and skin appendages.

common ancestor

The lab of Michel Milinkovich, professor in the Department of Genetics and Evolution at the UNIGE Faculty of Natural Sciences, deals with the physical and biological processes that lead to the diversity of skin appendages in vertebrates. In particular, his group had previously shown that hair, feathers, and scales are homologous structures inherited from a common reptilian ancestor.

Chicken embryo feathers are used by scientists as a model system for understanding the development of skin appendages. Although it is known that some chicken breeds, such as Brahma and Sableput varieties, show feathering on the legs and backs of the feet, the genetic determinism of this trait is not fully understood.

Migration change for permanent changes

Because the signaling pathways responsible for this conversion have not been fully identified, Michel Milinković’s group explored the potential role of the pathway. Shhh. “We used the classic ‘egg illumination’ technique, in which a powerful torch illuminates the blood vessels inside the eggshell. This allowed us to precisely treat chicken embryos with a molecule that specifically activates the Tss pathway by injecting it directly into the blood,” explains Michel Milinkovich, PhD, in his lab. post researcher and study co-author Rory Cooper.

The two scientists observed that this one-step specific treatment was sufficient to produce large numbers of young plumage in areas normally covered with scales. It is noteworthy that these experimentally generated hairs can be compared with the hairs covering the rest of the body, since they are regenerative and are then autonomously replaced by adult hairs.

After comparison with embryos injected with a “control” solution (without the active molecule), RNA sequencing analysis showed that the Shh pathway was activated simultaneously and continuously after injection of the molecule. This confirms that activation of the Shh pathway underlies the conversion of scales to feathers.

“Our results show that the evolutionary leap from scales to feathers did not require major changes in genome composition or expression. Instead, a temporary change in the expression of a gene, Shhhcan cause a series of developmental events that lead to the formation of feathers instead of scales,” says Michel Milinkovich. Therefore, this research, which initially focused on the study of the development of scales and feathers, is important for understanding the evolutionary mechanisms that led to the enormous variety of animal forms observed in nature.