The medaka zic1/zic4 mutant provides molecular insights into teleost caudal fin evolution

Teleosts have an asymmetrical caudal fin skeleton formed by the upward bending of the caudal-most portion of the body axis, the ural region [1-3]. 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 [4-6]. 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.