Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan.
Biol Lett. 2010 Aug 23;6(4):517-20. doi: 10.1098/rsbl.2009.1038. Epub 2010 Feb 10.
Frogs have highly conserved hand and foot morphology, possessing four fingers and five toes. As an exception, two Japanese ranid frog species, the Otton frog Babina subaspera and the dagger frog Babina holsti, possess a unique thumb-like structure (the pseudothumb) in the forelimb, giving an appearance of a total of five fingers on the hand. To obtain insights into the developmental mechanisms that generate this novel character, we investigated the hand morphogenesis of the Otton frog. The unique morphological pattern of the pseudothumb was already established in juveniles. Surprisingly, the bud-like structure, which is similar to the area of inductive activity (e.g. feather buds in birds and the carapacial ridge in turtles), was detected over the site where the future prepollex develops in larvae. By contrast, this bud-like structure was not found in larvae of other ranid species. We discuss possible scenarios that would favour the evolution of this very unusual trait in frogs.
青蛙具有高度保守的手和脚形态，拥有四个手指和五个脚趾。作为一个例外，两种日本雨蛙属物种，奥托雨蛙 Babina subaspera 和短剑蛙 Babina holsti，在前肢拥有一个独特的拇指状结构（伪拇指），使得手部看起来共有五个手指。为了深入了解产生这种新特征的发育机制，我们研究了奥托雨蛙的手部形态发生。伪拇指的独特形态模式在幼体中就已经建立。令人惊讶的是，在未来的近节指骨发育的部位检测到了类似芽状结构，类似于诱导活性的区域（例如鸟类的羽芽和龟鳖类的背甲脊）。相比之下，在其他雨蛙属物种的幼体中没有发现这种芽状结构。我们讨论了可能有利于这种非常不寻常的青蛙特征进化的情况。
