Wnt signalling, cell fate determination and anteroposterior polarity of the skate gill arch skeleton

The gill arch skeleton of cartilaginous fishes (sharks, skates, rays and holocephalans) exhibits anterior–posterior polarity, with a series of appendages (branchial rays) projecting from the posterior margin of the gill arch cartilages. We previously demonstrated in the skate (Leucoraja erinacea) that branchial rays derive from a posterior domain of pharyngeal arch mesenchyme, which is responsive to Shh signalling from a distal gill arch epithelial ridge (GAER) signalling centre. However, how branchial ray progenitors are specified exclusively within posterior gill arch mesenchyme is not known. Here we show that the GAER of the skate is of endodermal origin, arises at the endoderm–ectoderm boundary, and is a source of Fgf and Shh signals that are transduced broadly and posteriorly, respectively, within underlying arch mesenchyme. Using RNAseq, we discover that genes encoding several Wnt ligands are expressed in the ectoderm immediately adjacent to the GAER, and that these Wnt signals are transduced largely in the anterior arch mesenchyme. These tissue origin and gene expression features are largely conserved in the hyoid arch of the chick and are therefore likely an ancestral feature of jawed vertebrates. Finally, using pharmacological manipulations in skate, we show that loss of Wnt signalling results in an anterior expansion of Shh signal transduction within pharyngeal arch epithelium, and the formation of ectopic anterior branchial rays. Our findings demonstrate that Wnt signalling restricts chondrogenesis to the posterior gill arch and highlights the importance of signalling interactions at embryonic tissue boundaries for cell fate determination in pharyngeal arches.