Fröbisch Nadia B, Carroll Robert L, Schoch Rainer R
Redpath Museum, McGill University, 859 Sherbrooke Street West, Montreal H3A 2K6, Canada.
Evol Dev. 2007 Jan-Feb;9(1):69-75. doi: 10.1111/j.1525-142X.2006.00138.x.
Despite the wide range of shapes and sizes that accompany a vast variety of functions, the development of tetrapod limbs follows a conservative pattern of de novo condensation, branching, and segmentation. Development of the zeugopodium and digital arch typically occurs in a posterior to anterior sequence, referred to as postaxial dominance, with a digital sequence of 4-3-5-2-1. The only exception to this pattern in all of living Tetrapoda can be found in salamanders, which display a preaxial dominance in limb development, a de novo condensation of a basale commune (distal carpal/tarsal 1+2) and a precoccial development of digits I and II. These divergent patterns have puzzled researchers for over a century leading to various explanatory hypotheses. Despite many advances in research on tetrapod limb development, the divergent evolution of these two pathways and its causes are still not understood. Based on an extensive ontogenetic series we investigated the pattern of limb development of the 300 Ma old branchiosaurid amphibian Apateon. This revealed a preaxial dominance in limb development that was previously believed to be unique and derived for modern salamanders. The Branchiosauridae are favored as close relatives of extant salamanders in most phylogenetic hypotheses of the highly controversial origins and relationships of extant amphibians. The findings provide new insights into the evolution of developmental pathways in tetrapod limb development, the relationships of modern amphibians with possible Paleozoic antecedents, and their initial timing of divergence.
尽管四足动物的肢体具有各种各样的形状和大小，以适应广泛的功能，但它们的发育遵循从头凝聚、分支和分段的保守模式。近端四肢和指弓的发育通常按从后向前的顺序进行，即轴后优势，指的顺序为4-3-5-2-1。在所有现存的四足动物中，这种模式的唯一例外见于蝾螈，它们在肢体发育中表现出轴前优势，基底总骨（远端腕骨/跗骨1+2）从头凝聚，以及第一和第二指的早熟发育。这些不同的模式困扰了研究人员一个多世纪，导致了各种解释性假说。尽管在四足动物肢体发育的研究方面取得了许多进展，但这两种途径的趋异进化及其原因仍不清楚。基于一系列广泛的个体发育研究，我们调查了生活在3亿年前的枝龙科两栖动物阿氏螈的肢体发育模式。这揭示了肢体发育中的轴前优势，这种优势以前被认为是现代蝾螈所特有的。在现存两栖动物极具争议的起源和关系的大多数系统发育假说中，枝龙科被认为是现存蝾螈的近亲。这些发现为四足动物肢体发育中发育途径的进化、现代两栖动物与可能的古生代祖先的关系以及它们最初的分化时间提供了新的见解。
