Newswise — Scales, spurs, plumage, and fur are instances of vertebrate dermal extensions, forming an incredibly varied array of tiny structures. Despite their inherent diversity, these extensions exhibit shared initial developmental pathways during embryogenesis. Two scientists from the University of Geneva (UNIGE) have unveiled a technique to permanently convert the scales typically found on chicken feet into feathers, accomplished by targeted alterations in gene expression. Published in the journal Science Advances, these findings present fresh opportunities for investigating the mechanisms facilitating profound evolutionary transformations in morphology across different species.
The integument of land-dwelling vertebrates exhibits an assortment of distinct keratinized outgrowths, namely hair, feathers, and scales. Despite the multitude of variations found both within and between species, the initial embryonic development of these dermal appendages follows a remarkably similar pattern. In fact, all of these structures originate from specialized cells that induce a localized thickening on the surface of the skin and activate specific genetic expression. One key gene, known as Sonic hedgehog (Shh), governs a signaling pathway—a means of cellular communication facilitating message transmission within and between cells. Shh signaling plays a crucial role in the formation of diverse structures, encompassing the neural tube, limb buds, and skin appendages.
A common ancestor
Michel Milinkovitch, a professor at the Faculty of Science at UNIGE, heads a laboratory focused on investigating the physical and biological mechanisms responsible for the remarkable variety of skin appendages observed in vertebrates. Specifically, his team has previously provided evidence showcasing that hair, feathers, and scales are homologous structures, inherited from a shared reptilian ancestor.
Scientists utilize chicken embryo feathers as a model system to gain insights into the development of skin appendages. Although it is recognized that specific chicken breeds, such as the 'Brahma' and 'Sablepoot' varieties, possess feathers on their legs and dorsal foot surfaces, the precise genetic factors governing this trait remain incompletely understood.
A transient modification for a permanent change
Due to the incomplete understanding of the signaling pathways involved in this transformation, Michel Milinkovitch's research group conducted an investigation into the possible involvement of the Shh pathway. To explore this, they employed a conventional technique called "egg candling," where a strong light source illuminates the blood vessels within the eggshell. This enabled precise treatment of chicken embryos by directly injecting a molecule that specifically activates the Shh pathway into the bloodstream. Rory Cooper, a post-doctoral researcher in Michel Milinkovitch's laboratory and a co-author of the study, elaborates on the methodology.
During their investigation, the two scientists made an intriguing observation. They found that a single, stage-specific treatment with the molecule that activates the Shh pathway was adequate to induce the development of numerous down-type feathers in regions that would typically be covered by scales. Notably, these experimentally-induced feathers exhibited remarkable similarity to the feathers found elsewhere on the body. They displayed regenerative properties and were subsequently replaced by adult feathers in a self-sustained manner.
Through a comparative analysis with embryos injected with a "control" solution lacking the active molecule, the researchers conducted RNA sequencing analysis. The results revealed that the Shh pathway was not only immediately but also continuously activated following the injection of the molecule. This significant finding serves as confirmation that the activation of the Shh pathway is indeed responsible for the transformation of scales into feathers.
Michel Milinkovitch emphasizes the findings by stating, "Our results suggest that the transition from scales to feathers in evolution does not necessitate substantial alterations in the genome's composition or expression. Rather, a temporary modification in the expression of a single gene, Shh, can initiate a series of developmental processes that culminate in the formation of feathers instead of scales." This research, originally aimed at investigating scale and feather development, carries significant implications for comprehending the evolutionary mechanisms responsible for the vast array of animal forms observed in the natural world.
RSS