de Bakker Merijn A G, Nurhidayat Luthfi, Dijkerman Alisha Kiran, Chung Wing Yu, Oudesluys Elena C, de Jager Kaylah, Willemse Joost, Richardson Michael K
Animal Science & Health, Institute of Biology Leiden (IBL), Leiden University, Leiden 2333BE, the Netherlands.
Faculty of Biology, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia.
Mol Biol Evol. 2025 Jun 4;42(6). doi: 10.1093/molbev/msaf113.
Amniotes show a great diversity of limb phenotypes, including limbs specialized for running, flying, swimming, and digging. Here, we have examined how this diversity is generated during limb development in 13 species using transcriptomics and in situ hybridization. The selected species show evolutionary changes in the number of phalanges and/or loss of claws. We first looked at genes that show cyclical expression during digit development. Significantly, we find that Gdf5 cycles more rapidly in digits developing more phalanges. We identified two novel cyclically expressed genes: Ackr3 and Wnt9a. We also identified a transition point at which phalanx formation stops and claw development begins. We found that this transition point is marked by the downregulation of multiple developmental genes in the phalanx-forming region, and upregulation of claw-related genes. The timing of this transition is conserved, taking place at the same developmental stage in all digits of all species examined-except in the clawless digits of the Chinese soft-shelled turtle, the crocodilians, and birds. We suggest a model based on transcriptional heterochrony, in which the frequency of phalanx formation and the timing of the phalanx-claw transition are evolutionary control points open to natural selection on the phenotype. Furthermore, our model suggests that relaxation of developmental constraints on the timing of the phalanx-claw transition allows the digits to develop more phalanges (hyperphalangy). This is seen in some turtles, crocodilians, and dolphins. More broadly, our findings are consistent with the hypothesis that "hotspots" in otherwise conserved developmental pathways may be targets for evolutionary tinkering.
羊膜动物的肢体表型具有高度多样性，包括专门用于奔跑、飞行、游泳和挖掘的肢体。在这里，我们使用转录组学和原位杂交技术，研究了13个物种的肢体发育过程中这种多样性是如何产生的。所选物种在指骨数量和/或爪子缺失方面表现出进化变化。我们首先研究了在指发育过程中呈现周期性表达的基因。值得注意的是，我们发现Gdf5在发育出更多指骨的指中循环更快。我们鉴定出两个新的周期性表达基因：Ackr3和Wnt9a。我们还确定了一个过渡点，在该点指骨形成停止，爪子发育开始。我们发现这个过渡点的特征是指骨形成区域中多个发育基因的下调以及爪子相关基因的上调。这个过渡的时间是保守的，在所研究的所有物种的所有指中都在相同的发育阶段发生——除了中华鳖、鳄鱼和鸟类的无爪指。我们提出了一个基于转录异时性的模型，其中指骨形成的频率和指骨 - 爪子过渡的时间是对表型进行自然选择的进化控制点。此外，我们的模型表明，指骨 - 爪子过渡时间上发育限制的放松允许指发育出更多指骨（多指症）。这在一些龟类、鳄鱼和海豚中可以看到。更广泛地说，我们的发现与以下假设一致，即在其他保守的发育途径中的“热点”可能是进化微调的目标。
