Scales, spines, feathers, and hairs are examples of vertebrate skin appendages, which make up a remarkably diverse group of minute organs. Despite their many natural forms, these appendages are involved in early developmental processes in the embryonic stage. Two researchers from the University of Geneva (UNIGE) have discovered how to permanently turn the scales that normally cover a chicken’s feet into feathers, by specifically modifying the expression of certain genes. These results were published in the journal Science advancesopening new avenues for studying the mechanisms that enabled the radical evolutionary shifts in morphology between species.
The skin of terrestrial vertebrates is decorated with various horny appendages, such as hair, feathers, and scales. Although forms vary within and among species, the embryonic development of skin appendages usually begins in a very similar fashion. In fact, all these structures develop from cells that produce localized thickenings on the surface of the skin and express specific genes. One of these genes, called the Sonic hedgehog (Hush), controls the signaling pathway – a communication system that allows messages to be transmitted within and between cells. Shh signaling is involved in the development of diverse structures, including the neural tube, limb buds, and skin appendages.
The lab of Michel Milinkovitch, a professor in the Department of Genetics and Evolution in the Faculty of Science at UNIGE, is interested in the physical and biological processes that generate the diversity of skin appendages in vertebrates. In particular, his group has previously demonstrated that hair, feathers, and scales are homologous structures inherited from a common ancestor of reptiles.
Scientists use the feathers of a chicken embryo as a model system to understand the development of the appendix. While certain breeds of chicken, such as the “Brahma” and “Sablepoot” varieties, are known to exhibit feathered legs and dorsal toe surfaces, the genetic determinism of this trait is not fully understood.
Transient modification for permanent change
Because the signaling pathways responsible for this transformation have not been fully identified, Michel Milinkovitch’s group investigated the possible role of the Shh pathway. “We used the classic technique of ‘egg candling’, where a strong flame ignites the blood vessels inside the eggshell. This allowed us to precisely treat chicken embryos with a molecule that specifically activates the Shh pathway, injected directly into the bloodstream,” explains Rory Cooper, MA researcher. Postdoctoral in the lab of Michel Milinkovic and co-author of the study.
The two scientists observed that this particular one-stage treatment was sufficient to stimulate the formation of small feathers of the down type, in areas normally covered with scales. Remarkably, these experimentally induced feathers are comparable to those covering the rest of the body, as they are regenerated and are subsequently and independently replaced by adult feathers.
After comparison with embryos injected with a ‘control’ solution (without the active molecule), RNA-sequencing analysis showed that the Shh pathway is activated immediately and continuously after injection of the molecule. This confirms that activation of the Shh pathway underlies the conversion of scales into feathers.
Our results indicate that the evolutionary jump – from scales to feathers – did not require major changes in genome composition or expression. Instead, a transient change in the expression of a single gene, Hush“It can produce a series of developmental events that lead to the formation of feathers instead of scales,” says Michel Milinkovitch. This research, which initially focused on studying the evolution of scales and feathers, has important implications for understanding the evolutionary mechanisms that generate the huge variety of animal forms observed in nature.