Thyroid hormone regulates proximodistal identity in the fin skeleton

Across the ∼30,000 species of ray-finned fish, fins show incredible diversity in overall shape and in the patterning of the supportive bony rays. Fin length mutant zebrafish have provided critical insights into the developmental pathways that regulate relative fin size. However, the processes that govern skeletal patterning along the proximodistal axis of the fin have remained less well understood. Here, we show that thyroid hormone regulates proximodistal identity of fin rays, distalizing gene expression profiles, morphogenetic processes during outgrowth, and ultimate morphology of the fin. This role for thyroid hormone in specifying proximodistal identity appears conserved between development and regeneration, in all the fins, and between species. We demonstrate that proximodistal identity is regulated independently from pathways that determine size, and we show that modulating proximodistal patterning relative to growth can recapitulate the spectrum of fin ray diversity found in nature.