The sclerotome is the source of the dorsal and anal fin skeleton and its expansion is required for median fin development.

Paired locomotion appendages are hypothesized to have redeployed the developmental program of median appendages, such as the dorsal and anal fins. Compared with paired fins, and limbs, median appendages remain surprisingly understudied. Here, we report that a dominant zebrafish mutant, smoothback(smb), fails to develop a dorsal fin. Moreover, the anal fin is reduced along the antero-posterior axis, and spine defects develop. Mechanistically, smb is caused by an insertion of a sox10:Gal4VP16transgenic construct into a non-coding region. The first step in fin, and limb, induction is aggregation of undifferentiated mesenchyme at the appendage development site. In smb, this dorsal fin mesenchyme is absent. Lineage tracing demonstrates the previously unknown developmental origin of the mesenchyme, the sclerotome, which also gives rise to the spine. Strikingly, we find that there is significantly less sclerotome in smb compared to wild type. Our results give insight into the origin and modularity of understudied median fins, which have changed position, number, size, and even disappeared across evolutionary time.