Ectodysplasin overexpression reveals spatiotemporally dynamic tooth formation competency in stickleback and zebrafish.

Organ initiation is often driven by extracellular signaling molecules that activate precursor cells competent to receive and respond to a given signal, yet little is known about the dynamics of competency in space and time during development. Teeth are excellent organs to study cellular competency because they can be activated with the addition of a single signaling ligand, Ectodysplasin (Eda). To investigate the role of Eda in tooth specification, we generated transgenic sticklebacks and zebrafish with heat shock-inducible Eda overexpression. We find that stickleback Eda can drive ectopic tooth formation in at least eight distinct morphological domains. Both zebrafish and stickleback exhibit maximal responsiveness to Eda overexpression during pioneer tooth initiation. Analysis of candidate receptor expression in sticklebacks reveals that ectopic tooth formation in the pharynx correlates with Edar and Troy expression, while only Troy expression was detected in regions of the face where teeth can form, suggesting competency may involve spatially restricted receptor expression. These findings underscore the latent developmental potential, i.e. competency, of the vertebrate dentition and provide insights into organ competency during embryonic and post-embryonic development.