The skin of terrestrial vertebrates can have a variety of keratinized appendages, including hair, feathers, or scales. Yet, despite the diversity of forms within and among species, the embryonic development of these appendages usually begins in a similar way – from cells that produce a localized thickening on the skin surface and express particular genes.
For instance, one of these genes, called Sonic hedgehog (Shh), controls a signaling pathway which allows the transmission of messages within and between cells, and is involved in the development of a variety of structures, such as the neural tube, limb buds, and skin appendages.
By manipulating the Shh pathway and modifying its expression, a team of researchers led by the University of Geneva (UNIGE) has recently discovered how to permanently transform the scales which usually cover the feet of chicken into feathers, thus opening novel perspectives for studying mechanisms that enabled radical revolutionary transitions among species.
In previous research, the scientists have shown that hair, feathers, and scales are homologous structures inherited from a common reptilian ancestor. Now, they used the feathers of the embryo chicken as a case study to understand the genetic pathways behind skin appendage development.
“We used the classic technique of ‘egg candling,’ in which a powerful torch illuminates blood vessels on the inside of the eggshell. This allowed us to precisely treat chicken embryos with a molecule that specifically activates the Shh pathway, injected directly into the bloodstream,” said study co-author Rory Cooper, a postdoctoral fellow in Artificial and Natural Evolution at UNIGE.
The experiment revealed that this single stage-specific treatment was sufficient to trigger the formation of abundant juvenile down-type feathers, in body regions which would normally be covered in scales. These artificially-induced feathers were comparable to those covering the rest of the chicken’s body, as they were regenerative and subsequently replaced by adult feathers.
“Our results indicate that an evolutionary leap – from scales to feathers – does not require large changes in genome composition or expression. Instead, a transient change in expression of one gene, Shh, can produce a cascade of developmental events leading to the formation of feathers instead of scale,” said senior author Michel Milinkovitch, a professor of Evolutionary and Developmental Biology at UNIGE.
These findings – published in the journal Science Advances – could help scientists better understand the evolutionary mechanisms generating the wide diversity of animal forms observed in nature.
Birds evolved from a group of meat-eating dinosaurs called theropods. One of the most significant changes during this evolution was the development of feathers from scales.
The ancestors of birds were theropod dinosaurs that had reptilian scales. These scales are simple, flattened structures made of keratin, a protein that also forms human hair and nails.
Some dinosaurs, like those from the group Coelurosauria, began to develop feather-like structures. These first “feathers” were actually more like thin, hair-like filaments, now sometimes called “dino fuzz.” This was likely a simple mutation that caused the scales to elongate.
Over time, these filaments became more complex. They began to branch out, leading to structures that are more similar to modern feathers. These primitive feathers could have been advantageous for multiple reasons, such as thermal regulation or display during mating rituals, which would encourage their continuation through natural selection.
Eventually, these branched structures developed into the feathers we see in birds today. They evolved into different forms for different purposes – some for flight, others for insulation, camouflage, or mating displays.
It’s important to note that this is a simplified explanation of a very complex evolutionary process that occurred over millions of years. Also, the understanding of this process is based on the current scientific consensus and could change as new discoveries are made.
We have direct evidence of this transition in the fossil record. Several species of non-avian dinosaurs have been found with impressions of feathers in the rock surrounding their fossils. Some of the most notable of these are in the Chinese Lagerstätte, which has provided beautifully preserved examples of feathered dinosaurs like Sinosauropteryx and Microraptor.