Stage-dependent functions of retinoic acid (RA) signaling during amphioxus pharyngeal development. Expression of the main mediators of RA signaling in the developing endoderm is indicated at three developmental stages: the neurula, the late embryo and the early larva. Shown are the domains of retinoic acid receptor (RAR), RA synthesizing enzymes (RALDH), RA degradation enzymes (Cyp26) and TR2/4, a competitive inhibitor of RA signaling. Posterior elongation of the embryo and larva in the course of development is indicated with a red arrow. As the embryo and larva elongate, the source of endogenous RA as well as the expression domain of RAR move posteriorly, hence creating a permissive environment for the patterning and development of additional pharyngeal structures, chiefly gill slits.