Department of Biological Sciences, Graduate School of Science, University of Tokyo, Tokyo, Japan.
Curr Biol. 2012 Apr 10;22(7):601-7. doi: 10.1016/j.cub.2012.01.063. Epub 2012 Mar 1.
Teleosts have an asymmetrical caudal fin skeleton formed by the upward bending of the caudal-most portion of the body axis, the ural region. This homocercal type of caudal fin ensures powerful and complex locomotion and is regarded as one of the most important innovations for teleosts during adaptive radiation in an aquatic environment. However, the mechanisms that create asymmetric caudal fin remain largely unknown. The spontaneous medaka (teleost fish) mutant, Double anal fin (Da), exhibits a unique symmetrical caudal skeleton that resembles the diphycercal type seen in Polypterus and Coelacanth. We performed a detailed analysis of the Da mutant to obtain molecular insight into caudal fin morphogenesis. We first demonstrate that a large transposon, inserted into the enhancer region of the zic1 and zic4 genes (zic1/zic4) in Da, is associated with the mesoderm-specific loss of their transcription. We then show that zic1/zic4 are strongly expressed in the dorsal part of the ural mesenchyme and thereby induce asymmetric caudal fin development in wild-type embryos, whereas their expression is lost in Da. Comparative analysis further indicates that the dorsal mesoderm expression of zic1/zic4 is conserved in teleosts, highlighting the crucial role of zic1/zic4 in caudal fin morphogenesis.
硬骨鱼的尾鳍骨骼呈不对称结构，这是由身体轴最末端的尾部区域（尾区）向上弯曲形成的。这种同源尾鳍类型确保了硬骨鱼强有力且复杂的运动，被认为是硬骨鱼在水生环境中适应辐射过程中最重要的创新之一。然而，产生不对称尾鳍的机制在很大程度上仍不清楚。自发出现的日本青鳉（硬骨鱼）突变体双臀鳍（Double anal fin，Da）表现出一种独特的对称尾骨骼，类似于多鳍鱼和腔棘鱼的双齿型。我们对 Da 突变体进行了详细分析，以获得对尾鳍形态发生的分子见解。我们首先证明，一个大的转座子插入到 Da 中 zic1 和 zic4 基因（zic1/zic4）的增强子区域，与它们在中胚层中的转录特异性缺失有关。然后我们表明，zic1/zic4 在尾区间质的背侧强烈表达，从而诱导野生型胚胎产生不对称的尾鳍发育，而在 Da 中则失去表达。比较分析进一步表明，zic1/zic4 在硬骨鱼中的背侧中胚层表达是保守的，突出了 zic1/zic4 在尾鳍形态发生中的关键作用。
