The metamorphic transition of the frog mouth: from tadpole keratinized mouthparts to adult teeth.

Teeth have been a prominent feature of most vertebrates for 400 million years, and the core regulatory network underlying embryonic tooth formation is deeply conserved. In frogs, however, odontogenesis is delayed, occurring instead during the postembryonic metamorphosis and resulting in teeth that are restricted to the upper jaw and palate. Developmental-genetic mechanisms that underlie tooth formation in frogs are poorly understood. We assessed if the genes underlying odontogenic competence are conserved in the late-forming teeth of frogs; if unique keratinized mouthparts, which function as an alternative feeding tool in anuran larvae, impede tooth induction; and if transient tooth rudiments form in the anuran mandible. We demonstrate that the induction of tooth development is conserved in the frog upper jaw, which displays odontogenic band expression patterns comparable to those of other vertebrates. There is, however, no evidence of tooth development initiating in the mandible. Adult teeth emerge before larval mouthparts degenerate, but their location may be spatially constrained by keratin. Gene expression patterns of keratinized mouthparts and teeth overlap. We hypothesize that the novel mouthparts of tadpoles, which we characterize as ectodermal appendages, may have originated by partially co-opting the developmental program that typically mediates development of true teeth.