CRISPR/Cas9-Mediated Disruption of Duplicated Sizzled Genes Induces Twin-Tail-like Caudal Bifurcation in Goldfish (Carassius auratus)

The twin-tail phenotype of goldfish represents a striking domestication-associated remodeling of the vertebrate caudal axial system and is classically linked to disruption of Chordin/BMP-mediated dorsal–ventral patterning. Although previous knockdown studies implicated sizzled (szl) in this process, genetic evidence from targeted disruption of endogenous szl loci remains limited. Here, we used CRISPR/Cas9 to mutate conserved coding regions shared by the duplicated goldfish paralogues szlA and szlB in single-tail embryos. Sanger sequencing and ICE analysis showed that szl-sgRNA2 and szl-sgRNA3 efficiently induced indels at both loci, whereas szl-sgRNA1 was ineffective. Across three independent biological replicates, twin-tail-like caudal bifurcation was observed in 44.63–48.19% of szl-sgRNA2-injected larvae, 69.47–79.61% of szl-sgRNA3-injected larvae, and 64.29–76.19% of larvae injected with the sgRNA mixture; szl-sgRNA1-injected larvae remained single-tailed. Calcein staining further revealed separation of distal caudal fin rays and partial splitting of the caudal skeletal complex in szl-edited larvae. qRT-PCR showed selective remodeling of dorsal–ventral patterning genes, including reduced chdA and eve1 expression and increased bmp2 and nog1 expression. These findings provide direct functional evidence that szl regulates median caudal patterning in goldfish and suggest that szl-dependent modulation of the Chordin/BMP network can generate twin-tail-like caudal morphology.