Scales, spines, feathers, and hair are examples of vertebrate integuments, which represent a remarkably diverse group of microorgans. Despite their natural variety of forms, these appendages share early developmental processes in the embryonic stage. Two researchers from the University of Geneva (UNIGE) have discovered how to permanently convert the scales that normally cover the legs of chickens into feathers by targeting the expression of specific genes. These findings, published in the journal Advances in Science, open new perspectives for studying the mechanisms that have enabled radical evolutionary shape changes between species.
Terrestrial vertebrate skin is adorned with various keratinized appendages such as hair, feathers, and scales. Despite the variety of forms within and between species, the embryonic development of the integuments generally begins very similarly. In fact, all of these structures develop from cells that create a local thickening on the skin’s surface and express specific genes. One of these genes, called Sonic Hedgehog (Hush), controls a signaling pathway — a communication system that allows messages to be transmitted within and between cells. Shh signaling is involved in the development of various structures including the neural tube, limb buds, and skin appendages.
A common ancestor
The laboratory of Michel Milinkovitch, professor in the Department of Genetics and Evolution at the UNIGE Faculty of Science, is interested in the physical and biological processes that generate limb diversity in vertebrates. In particular, his group has previously shown that hair, feathers and scales are homologous structures inherited from a common reptilian ancestor.
Chick embryo feathers are used by scientists as a model system to understand the development of integuments. Although some chicken breeds such as the Brahma and Sablepoot varieties are known to have feathered legs and dorsal footpads, the genetic determinism of this trait is not fully understood.
A temporary change for a permanent change
Since the signaling pathways responsible for this transformation are not yet fully understood, Michel Milinkovitch’s group investigated the possible role of the Shh signaling pathway. “We used the classic ‘egg candling’ technique, in which a powerful flashlight illuminates the blood vessels in the eggshell. This enabled us to precisely treat chick embryos with a molecule that specifically activates the Shh signaling pathway and injects it directly into the bloodstream. ‘ explains Rory Cooper, a postdoctoral researcher in Michel Milinkovitch’s lab and co-author of the study.
The two scientists found that this particular one-step treatment was enough to trigger the formation of lush juvenile down-like feathers in areas that would normally be covered with scales. Remarkably, these experimentally generated feathers are comparable to those covering the rest of the body in that they are regenerative and then autonomously replaced by adult feathers.
After comparison with embryos injected with a ‘control solution’ (without the active molecule), analysis using RNA sequencing showed that the Shh signaling pathway is activated immediately and persistently after injection of the molecule. This confirms that activation of the Shh signaling pathway underlies the conversion of scales to feathers.
“Our results show that an evolutionary leap – from scales to feathers – does not require major changes in the composition or expression of the genome. Instead, a transient change in the expression of a gene, Hush, can trigger a cascade of developmental events that lead to the formation of feathers instead of scales,” explains Michel Milinkovitch. This research, originally focused on studying the evolution of scales and feathers, therefore has important implications for understanding the evolutionary mechanisms underlying the vast diversity of animal forms observed in nature.