Kondo Mao, Sekine Tomoe, Miyakoshi Taku, Kitajima Keiichi, Egawa Shiro, Seki Ryohei, Abe Gembu, Tamura Koji
1Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, 980-8578 Japan.
2Mammalian Genetics Laboratory, Genetic Strains Research Center, National Institute of Genetics, 1111 Yata, Mishima, Shizuoka, 411-8540 Japan.
Zoological Lett. 2018 Jan 16;4:2. doi: 10.1186/s40851-017-0085-4. eCollection 2018.
Flight feathers, a type of feather that is unique to extant/extinct birds and some non-avian dinosaurs, are the most evolutionally advanced type of feather. In general, feather types are formed in the second or later generation of feathers at the first and following molting, and the first molting begins at around two weeks post hatching in chicken. However, it has been stated in some previous reports that the first molting from the natal down feathers to the flight feathers is much earlier than that for other feather types, suggesting that flight feather formation starts as an embryonic event. The aim of this study was to determine the inception of flight feather morphogenesis and to identify embryological processes specific to flight feathers in contrast to those of down feathers.
We found that the second generation of feather that shows a flight feather-type arrangement has already started developing by chick embryonic day 18, deep in the skin of the flight feather-forming region. This was confirmed by gene expression that shows barb pattern, and the expression pattern revealed that the second generation of feather development in the flight feather-forming region seems to start by embryonic day 14. The first stage at which we detected a specific morphology of the feather bud in the flight feather-forming region was embryonic day 11, when internal invagination of the feather bud starts, while the external morphology of the feather bud is radial down-type.
The morphogenesis for the flight feather, the most advanced type of feather, has been drastically modified from the beginning of feather morphogenesis, suggesting that early modification of the embryonic morphogenetic process may have played a crucial role in the morphological evolution of this key innovation. Co-optation of molecular cues for axial morphogenesis in limb skeletal development may be able to modify morphogenesis of the feather bud, giving rise to flight feather-specific morphogenesis of traits.
飞羽是现存/已灭绝鸟类以及一些非鸟类恐龙所特有的一种羽毛，是进化程度最高的羽毛类型。一般来说，羽毛类型在首次及后续换羽时的第二代或更晚代羽毛中形成，鸡的首次换羽在孵化后约两周开始。然而，之前的一些报道称，从雏羽到飞羽的首次换羽比其他羽毛类型要早得多，这表明飞羽的形成始于胚胎期。本研究的目的是确定飞羽形态发生的起始点，并识别与雏羽相比，飞羽特有的胚胎学过程。
我们发现，在雏鸡胚胎第18天，飞羽形成区域的皮肤深处就已经开始发育显示出飞羽类型排列的第二代羽毛。这通过显示羽枝模式的基因表达得到了证实，并且表达模式表明，飞羽形成区域的第二代羽毛发育似乎在胚胎第14天就开始了。我们在飞羽形成区域检测到羽毛芽特定形态的第一阶段是胚胎第11天，此时羽毛芽开始向内凹陷，而羽毛芽的外部形态是放射状雏羽型。
最先进的羽毛类型——飞羽的形态发生从羽毛形态发生开始就发生了巨大改变，这表明胚胎形态发生过程的早期改变可能在这一关键创新的形态进化中发挥了关键作用。肢体骨骼发育中轴向形态发生的分子线索的借用可能能够改变羽毛芽的形态发生，从而产生飞羽特有的性状形态发生。
