Evidence against tetrapod-wide digit identities and for a limited frame shift in bird wings

In crown group tetrapods, individual digits are homologized in relation to a pentadactyl ground plan. However, testing hypotheses of digit homology is challenging because it is unclear whether digits develop with distinct and conserved gene regulatory states. Here we show dramatic evolutionary dynamism in the gene expression profiles of digits, challenging the notion that five digit identities are conserved across amniotes. Transcriptomics of developing limbs shows diversity in the patterns of genetic differentiation of digits, although the anterior-most digit has a unique, conserved expression profile. Further, we identify a core set of transcription factors that are differentially expressed among the digits of amniote limbs; their spatial expression domains, however, vary between species. In light of these results, we reevaluate the frame shift hypothesis of avian wing evolution and conclude that only the anterior-most digit has shifted position, suggesting a 1,3,4 digit identity in the bird wing. Examination of mRNA profiles of developing digits and their associated posterior interdigital messenchyme from forelimbs of mouse, green anole (Anolis), and American alligator.在冠群四足类（crown group tetrapods）中，单个指/趾的同源性判定均以五趾型基础构型（pentadactyl ground plan）为参照。然而，验证指/趾同源性假说颇具挑战，因为目前尚不清楚指/趾是否以独特且保守的基因调控状态（gene regulatory states）完成发育。本研究揭示了指/趾基因表达谱中显著的演化动态性，对"羊膜动物中五种指/趾特征身份（digit identity）均保守"的观点提出了质疑。对发育肢体的转录组学（Transcriptomics）分析显示，各指/趾的遗传分化模式存在多样性，但最靠前的指/趾拥有独特且保守的表达谱。此外，本研究鉴定出一套核心转录因子（transcription factors），它们在羊膜动物肢体的各指/趾间差异表达，但其空间表达域在不同物种间存在差异。基于上述结果，我们重新评估了鸟类翼演化的框架移位假说（frame shift hypothesis），并得出结论：仅最靠前的指/趾发生了位置移位，这表明鸟类翼的指/趾特征身份为1、3、4号。本研究对小鼠、绿安乐蜥（Anolis）以及美国短吻鳄（American alligator）前肢的发育中指/趾及其相关后部趾间间质（interdigital mesenchyme）的mRNA表达谱进行了检测。