Sarcopterygian fin ontogeny elucidates the origin of hands with digits.

Abstract

How the hand and digits originated from fish fins during the Devonian fin-to-limb transition remains unsolved. Controversy in this conundrum stems from the scarcity of ontogenetic data from extant lobe-finned fishes. We report the patterning of an autopod-like domain by <i>hoxa13</i> during fin development of the Australian lungfish, the most closely related extant fish relative of tetrapods. Differences from tetrapod limbs include the absence of digit-specific expansion of <i>hoxd13</i> and <i>hand2</i> and distal limitation of <i>alx4</i> and <i>pax9</i>, which potentially evolved through an enhanced response to <i>shh</i> signaling in limbs. These developmental patterns indicate that the digit program originated in postaxial fin radials and later expanded anteriorly inside of a preexisting autopod-like domain during the evolution of limbs. Our findings provide a genetic framework for the transition of fins into limbs that supports the significance of classical models proposing a bending of the tetrapod metapterygial axis.