On the trail of "mastermind genes": Researchers have shown that the scales on chicken feet can be transformed into feathers by influencing a gene with a signaling function. This result may shed light on the basis of physical differences between animal species and mechanisms of evolution, the scientists explain.
How does a living being with the typical characteristics of its species develop from a fertilized egg cell? Embryonic development still works like a miracle and the underlying processes still pose many mysteries to scientists. What seems clear is that regulatory systems, controlled by specific genes, are at work in shaping body shapes and structures. In the course of evolution, changes in these systems led to the formation of new species and new structural developments. These “inventions” often emerged from older structures. It is assumed that this was also the case with the skin appendages of vertebrates made of the building material keratin: Spines, hair and feathers are therefore further developments of scale structures that previously developed in reptiles.
How is development regulated?
The formation of the different skin appendages is also based on similar basic processes during embryonic development: They develop from cells that produce a local thickening on the skin surface and in which certain genes are active, which ultimately lead to the formation of scales, hair or feathers. So far, however, it is unclear which control processes lead to the activation of these programs. A research group at the University of Geneva is dealing with this question. In the current study, the scientists focused on a potentially important genetic trait: it is already known that the gene called Sonic Hedgehog (Shh) is involved in the regulation of embryonic development processes.
To investigate its possible role in the formation of skin appendages, the scientists performed studies using chick embryos as a model. For the experiments with the developing beings in the egg, the scientists used a technique called "egg candling". The shell is x-rayed so intensely that the inner structures become visible – including the blood vessels. "This enabled us to precisely treat the chicken embryos with an active substance by injecting it directly into the bloodstream," says lead author Rory Cooper from the University of Geneva. As the researchers explain, the substance was a so-called agonist - a molecule that can specifically increase the genetic activity of the Shh gene.
"Spring program" permanently activated
The results of the treatment show that during a certain phase of embryonic development it leads to the activation of a signaling pathway with a clear effect: Instead of the scales that normally cover the feet of chickens, feather structures formed through the manipulation of the genetic control system. The scientists report that these experimentally produced down feathers were comparable to those that cover the rest of the body. As it turned out, it was a formative change: the treatment did not have to be repeated. The one-time administration of the active ingredient during embryonic development led to a permanent activation of the "spring program" in the chicken feet.
"The results show that variations in the signal transmission of the Shh signaling pathway probably contribute to the natural diversity and body part-specific development of skin structures in birds," the scientists summarize. Senior author Michel Milinkovitch from the University of Geneva concludes that the study can contribute to the understanding of evolutionary mechanisms: "Our results suggest that an evolutionary leap - from scales to feathers - does not require large changes in the composition or expression of the genome. Instead, a transient change in the expression of just one gene, Shh, can trigger a cascade of developmental events that lead to the formation of feathers instead of scales."
Source: University of Geneva, specialist article: Science Advances, doi: 10.1126/sciadv.adg9619