ジュネーブ大学の研究者らは、ソニック・ヘッジホッグ（Shh）の遺伝子発現を一時的に改変することで鶏の鱗を羽毛に変え、ゲノムに大きな変化がなくても重要な進化の移行が起こり得ることを明らかにした。 この研究は、動物の形態の幅広い多様性に関与するメカニズムを明らかにします。 クレジット: © UNIGE / クーパー & ミリンコビッチ
UNIGE チームは、遺伝子発現を特異的に変更することで、鶏の鱗が羽に置き換わる仕組みを示しました。
鱗、背骨、羽毛、毛髪は脊椎動物の皮膚付属器の例であり、非常に多様な微小器官のグループを構成しています。 自然に多様な形態があるにもかかわらず、これらの付属器官は胎生期の初期の発生過程を共有しています。 ジュネーブ大学 (UNIGE) の 2 人の研究者は、特定の遺伝子の発現を特異的に変更することにより、通常ニワトリの足を覆う鱗を永久的に羽毛に変える方法を発見しました。 これらの結果は雑誌に掲載されました species.
The skin of terrestrial vertebrates is adorned with diverse keratinized appendages, such as hair, feathers, and scales. Despite the diversity of forms within and among species, the embryonic development of skin appendages typically begins in a very similar way. Indeed, all of these structures develop from cells that produce a localized thickening on the skin surface and express particular genes. One of these genes, called Sonic hedgehog (Shh), controls a signaling pathway — a communication system that allows the transmission of messages within and between cells. Shh signaling is involved in the development of diverse structures, including the neural tube, limb buds, and skin appendages.
A common ancestor
The laboratory of Michel Milinkovitch, professor in the Department of Genetics and Evolution at the Faculty of Science of the 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 reptilian common ancestor.
A transient change in expression of one gene (Shh) can produce a cascade of developmental events leading to the formation of feathers instead of scales. Credit: © UNIGE / Cooper & Milinkovitch
Feathers of the chicken embryo are used by scientists as a model system to understand skin appendage development. While it is known that certain breeds of chickens, such as the ‘Brahma’ and ‘Sablepoot’ varieties, exhibit feathered legs and dorsal foot surfaces, the genetic determinism of this trait is not fully understood.
A transient modification for a permanent change
As the signaling pathways responsible for this transformation have not been fully determined, Michel Milinkovitch’s group investigated the potential role of the Shh pathway. “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,’’ explains Rory Cooper, a post-doctoral researcher in Michel Milinkovitch’s laboratory and co-author of the study.
The two scientists observed that this single stage-specific treatment is sufficient to trigger the formation of abundant juvenile down-type feathers, in areas that would normally be covered with scales. Remarkably, these experimentally-induced feathers are comparable to those covering the rest of the body, as they are regenerative and are subsequently and autonomously 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 both immediately and persistently activated following injection of the molecule. This confirms that activation of the Shh pathway underlies the conversion of scales into 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 scales,’’ says Michel Milinkovitch. This research, initially focused on the study of the development of scales and feathers, therefore has important implications for understanding the evolutionary mechanisms generating the enormous diversity of animal forms observed in nature.
Reference: “Transient agonism of the sonic hedgehog pathway triggers a permanent transition of skin appendage fate in the chicken embryo” by Rory L. Cooper and Michel C. Milinkovitch, 17 May 2023, Science Advances.
DOI: 10.1126/sciadv.adg9619