Direct‐developing amphibians form limbs during early embryonic stages, as opposed to the later, often postembryonic limb formation of metamorphosing species. Limb patterning is dramatically altered in direct‐developing frogs, but little attention has been given to direct‐developing salamanders. We use expression patterns of two genes, sox9 and col2a1, to assess skeletal patterning during embryonic limb development in the direct‐developing salamander Plethodon cinereus. Limb patterning in P. cinereus partially resembles that described in other urodele species, with early formation of digit II and a generally anterior‐to‐posterior formation of preaxial digits. Unlike other salamanders described to date, differentiation of preaxial zeugopodial cartilages (radius/tibia) is not accelerated in relation to the postaxial cartilages, and there is no early differentiation of autopodial elements in relation to more proximal cartilages. Instead, digit II forms in continuity with the ulnar/fibular arch. This amniote‐like connectivity to the first digit that forms may be a consequence of the embryonic formation of limbs in this direct‐developing species. Additionally, and contrary to recent models of amphibian digit identity, there is no evidence of vestigial digits. This is the first account of gene expression in a plethodontid salamander and only the second published account of embryonic limb patterning in a direct‐developing salamander species.

中文翻译：

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sox9和col2a1揭示了直接发育的sal Plethodon cinereus中的早期肢体模式。

直接发展的两栖动物在胚胎的早期阶段形成肢体，这与后来变态物种的通常在胚胎后肢体形成相反。在直接发育的青蛙中，肢体模式发生了显着变化，但是对直接发育的sal的关注却很少。我们使用两个基因sox9和col2a1的表达模式来评估直接发育的P Plethodon cinereus胚胎肢体发育过程中的骨骼模式。灰质假单胞菌的肢体模式与其他urodele物种中的描述部分相似，具有早期形成的数字II和通常从前到后的前轴数字的形成。与迄今描述的其他sal不同，前轴突足软骨的分化（radi骨/胫骨）相对于后轴软骨没有加速，并且相对于更多的近端软骨，自足元件没有早期分化。相反，数字II与尺骨/腓骨弓连续形成。这种直接形成物种的四肢与胚胎形成相似的连通性可能是四肢的胚胎形成的结果。此外，与最近的两栖动物数字身份模型相反，没有证据表明有残留数字。