Loss of fish actinotrichia proteins and the fin-to-limb transition

The fin-to-limb transition A significant step in the evolution of tetrapod limbs was the loss of the distinctive fringe of fin-rays and fin-folds found in the fins of fishes. Marie-Andrée Akimenko and colleagues have identified two novel zebrafish proteins, actinodin 1 and 2, as essential structural components of these features. Actinodin genes are found in other fish species — including several teleost fishes and in the elephant shark — but are not present in tetrapods. The experimental loss of actinodin function leads to the absence of actinotrichia fibrils in zebrafish and to the formation of pectoral fin buds with characteristics of tetrapod limb buds. The data suggest that the loss of actinotrichia fibrils may, in part, explain the evolutionary transformation of fins into limbs.