Double Anal Fin (Da): A Medaka Mutant Exhibiting a Mirror-Image Pattern Duplication of the Dorsal–Ventral Axis

A medaka Double anal fin (Da) mutant, spontaneously isolated from the wild-type, exhibits a unique ventralizing phenotype consisting of a mirror-image duplication across the lateral midline in the dorsal trunk-tail region. No similar mutation has been reported in other model organisms. Hence, the Da mutant provides a unique model system for investigation of the mechanisms controlling the overall dorsal–ventral patterning of the vertebrate trunk-tail region. In this chapter, we introduce (1) a brief history of the Da mutant; (2) details of the morphological phenotype of Da; (3) positional cloning of the Da mutation; (4) the Da mutant in relation to fish evolution; and (5) Da as a potential model for human disease.