Khannoon Eraqi R, Russell Anthony P, Tucker Abigail S
Zoology Department, Faculty of Science, Fayoum University, Fayoum, 63514 Egypt ; King's College London, Floor 27 Guy's Tower, Guy's Hospital, Great Maze Pond, London Bridge, London, SE1 9RT UK.
Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4 Canada.
Evodevo. 2015 Apr 10;6:8. doi: 10.1186/s13227-015-0003-9. eCollection 2015.
The limb and autopodium are frequently employed to study pattern formation during embryonic development, providing insights into how cells give rise to complex anatomical structures. With regard to the differentiation of structures at the distal tips of digits, geckos constitute an attractive clade, because within their ranks they exhibit multiple independent occurrences of claw loss and reduction, these being linked to the development of adhesive pads. The developmental patterns that lead to claw loss, however, remain undescribed. Among geckos, Tarentola is a genus characterized by large claws on digits III and IV of the manus and pes, with digits I, II, and V bearing only vestigial claws, or lacking them entirely. The variable expression of claws on different digits provides the opportunity to investigate the processes leading to claw reduction and loss within a single species.
Here, we document the embryonic developmental dynamics that lead to this intraspecifically variable pattern, focusing on the cellular processes of proliferation and cell death. We find that claws initially develop on all digits of all autopodia, but, later in development, those of digits I, II, and V regress, leading to the adult condition in which robust claws are evident only on digits III and IV. Early apoptotic activity at the digit tips, followed by apoptosis of the claw primordium, premature ossification of the terminal phalanges, and later differential proliferative activity are collectively responsible for claw regression in particular digits.
Claw reduction and loss in Tarentola result from differential intensities of apoptosis and cellular proliferation in different digits, and these processes have already had some effect before visible signs of claw development are evident. The differential processes persist through later developmental stages. Variable expression of iteratively homologous structures between digits within autopodia makes claw reduction and loss in Tarentola an excellent vehicle for exploring the developmental mechanisms that lead to evolutionary reduction and loss of structures.
肢体和手足常用于研究胚胎发育过程中的模式形成，有助于深入了解细胞如何形成复杂的解剖结构。关于指（趾）远端结构的分化，壁虎是一个有吸引力的类群，因为在它们之中，爪子的缺失和退化多次独立出现，这些与粘性垫的发育有关。然而，导致爪子缺失的发育模式仍未得到描述。在壁虎中，地中海壁虎属的特征是手部和足部的第三和第四指上有大爪子，而第一、第二和第五指只有退化的爪子，或者完全没有爪子。不同指上爪子的可变表达为研究导致单个物种内爪子退化和缺失的过程提供了机会。
在这里，我们记录了导致这种种内可变模式的胚胎发育动态，重点关注增殖和细胞死亡的细胞过程。我们发现，所有手足的所有指最初都发育出爪子，但在发育后期，第一、第二和第五指的爪子退化，导致成年状态下只有第三和第四指有明显的强壮爪子。指端早期的凋亡活动，随后是爪原基的凋亡、末端指骨的过早骨化，以及后期不同的增殖活动共同导致了特定指上爪子的退化。
地中海壁虎属中爪子的退化和缺失是由于不同指上凋亡和细胞增殖强度的差异，这些过程在爪子发育的可见迹象明显之前就已经产生了一些影响。这些差异过程在后期发育阶段持续存在。手足内指之间迭代同源结构的可变表达使得地中海壁虎属中爪子的退化和缺失成为探索导致结构进化退化和缺失的发育机制的一个极好载体。
