Gene expression controls the development of skin appendages

The University of Geneva (UNIGE) researchers have made a groundbreaking discovery regarding vertebrate skin appendages. These appendages, such as scales, spines, feathers, and hair, exhibit remarkable diversity in their forms. However, despite their distinct appearances, these appendages share common developmental processes during the embryonic stage. By manipulating the expression of specific genes, the scientists have successfully transformed the scales on chicken feet into feathers, providing new insights into the mechanisms behind significant evolutionary transitions in various species.

The skin of land-dwelling vertebrates showcases an array of keratinized appendages, including feathers, hair, and scales. Despite the variations observed within and across species, the initial development of these skin appendages follows a similar pattern during embryogenesis. It begins with the activation of certain cells that create localized thickening on the skin surface and express specific genes.

Among these genes, one called Sonic hedgehog (Shh) plays a vital role in a signaling pathway—a communication system facilitating message transmission within and between cells. Shh signaling is crucial in the development of various structures, such as the neural tube, limb buds, and skin appendages.

A common ancestor

Michel Milinkovitch, a professor in the Department of Genetics and Evolution at the Faculty of Science, UNIGE, leads a laboratory dedicated to investigating the biological and physical processes that contribute to the diverse range of skin appendages found in vertebrates. One of their significant findings is the identification of hair, feathers, and scales as homologous structures inherited from a common reptilian ancestor.

To study the development of skin appendages, scientists often use feathers from chicken embryos as a model system. Certain chicken breeds, such as the “Brahma” and “Sablepoot” varieties, possess feathered legs and dorsal foot surfaces, but the precise genetic mechanisms underlying this trait remain incompletely understood.

A transient modification for a permanent change

To uncover the signaling pathways involved in the transformation of skin appendages, Michel Milinkovitch’s team conducted research on the potential role of the Shh pathway. Employing a well-established method called “egg candling,” the scientists illuminated the blood vessels within the eggshell using a powerful torch. This technique enabled them to precisely administer a molecule that activates the Shh pathway directly into the bloodstream of chicken embryos. Rory Cooper, a co-author of the study and a post-doctoral researcher in Michel Milinkovitch’s laboratory, explains the methodology and its significance.

Credit: University of Geneva

The researchers made an intriguing observation during their study: a single targeted treatment at a specific stage of development was adequate to stimulate the growth of abundant juvenile down-type feathers in regions that would typically be covered with scales. Surprisingly, these experimentally induced feathers were similar to those found on the rest of the body. They exhibited regenerative properties and were subsequently autonomously replaced by adult feathers.

Comparative analysis using RNA sequencing was conducted between embryos injected with the active molecule and those injected with a “control” solution lacking the active component. The results revealed that the Shh pathway was immediately and continuously activated following the injection of the active molecule. This finding provides solid evidence that the activation of the Shh pathway is responsible for the conversion of scales into feathers.

Michel Milinkovitch emphasizes the significance of these findings, stating, “Our results demonstrate that the transition from scales to feathers, a significant evolutionary leap, does not necessitate extensive changes in the genome’s composition or expression. Instead, a transient alteration in the expression of a single gene, Shh, can trigger a series of developmental events leading to feather formation instead of scales.” This research, initially focused on understanding the development of scales and feathers, holds substantial implications for unraveling the evolutionary mechanisms that give rise to the remarkable diversity of animal forms observed in the natural world.

Source: University of Geneva

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