The <i>Shh</i>/<i>Gli3</i> gene regulatory network precedes the origin of paired fins and reveals the deep homology between distal fins and digits.

Abstract

One of the central problems of vertebrate evolution is understanding the relationship among the distal portions of fins and limbs. Lacking comparable morphological markers of these regions in fish and tetrapods, these relationships have remained uncertain for the past century and a half. Here we show that Gli3 functions in controlling the proliferative expansion of distal progenitors are shared among dorsal and paired fins as well as tetrapod limbs. Mutant knockout <i>gli3</i> fins in medaka (<i>Oryzias latipes</i>) form multiple radials and rays, in a pattern reminiscent of the polydactyly observed in <i>Gli3</i>-null mutant mice. In limbs, <i>Gli3</i> controls both anterior-posterior patterning and cell proliferation, two processes that can be genetically uncoupled. In situ hybridization, quantification of proliferation markers, and analysis of regulatory regions reveal that in paired and dorsal fins, <i>gli3</i> plays a main role in controlling proliferation but not in patterning. Moreover, <i>gli3</i> down-regulation in <i>shh</i> mutant fins rescues fin loss in a manner similar to how <i>Gli3</i> deficiency restores digits in the limbs of <i>Shh</i> mutant mouse embryos. We hypothesize that the <i>Gli3/Shh</i> gene pathway preceded the origin of paired appendages and was originally involved in modulating cell proliferation. Accordingly, the distal regions of dorsal fins, paired fins, and limbs retain a deep regulatory and functional homology that predates the origin of paired appendages.