Developmental Independence of Median Fins From the larval Fin Fold Revises Their Evolutionary Origin

The median fins in modern fish that show separated forms (dorsal, anal, and caudal fins) are derived from a continuous fold-like structure, both in ontogeny and phylogeny. The median fin fold (MFF) hypothesis assumes that the median fins evolved by reducing some positions in the continuous fin fold of basal chordates, based on the classical morphological observation of developmental reduction in the larval fin folds of living fish. However, the developmental processes of median fins are still unclear at the cellular and molecular levels. Here, we describe the transition from the larval fin fold into the median fins in zebrafish at the cellar and molecular developmental level. We demonstrate that reduction does not play a role in the emergence of the dorsal fin primordium: the reduction occurs passively after the primordium formation, rather than actively scrapping the non-fin forming region by inducing cell death. We also report that the emergence of specific mesenchymal cells and their proliferation promote the dorsal fin primordium formation. Based on these results, we propose a revised hypothesis of median fin evolution in which acquisition of de novo developmental mechanisms is a crucial evolutionary component of the separated forms of median fins.