Latent developmental potential to form limb-like skeletal structures in zebrafish

Summary Changes in appendage structure underlie key transitions vertebrate evolution. Addition of skeletal elements along the proximal-distal axis facilitated critical transformations, including fin-to-limb transition that permitted generation diverse modes locomotion. Here, we identify zebrafish mutants form supernumerary long bones their pectoral fins. These new integrate into musculature, joints, and articulate with neighboring elements. This phenotype is caused by activating mutations previously unrecognized regulators patterning, vav2 waslb, function a common pathway. pathway required for development across vertebrates, loss Wasl mice causes defects similar to those seen murine Hox mutants. Concordantly, formation requires Hox11 function, vav2/wasl drive enhanced expression hoxa11b, indicating developmental homology forearm. Our findings reveal latent, limb-like pattern ability fins activated simple genetic perturbation.