The homology of odontodes in gnathostomes: insights from Dlx gene expression in the dogfish, Scyliorhinus canicula

We present a detailed comparison of the first developing teeth and dermal denticles (caudal primary scales) of the dogfish (Scyliorhinus canicula) and show that both odontodes develop through identical stages that correspond to the common stages of oral and pharyngeal odontogenesis. We identified six Dlx paralogs in the dogfish and found that three showed strong transcription in teeth and dermal denticles (Dlx3, Dlx4 and Dlx5) whereas a weak expression was detected for Dlx1 in dermal denticles and teeth, and for Dlx2 in dermal denticles. Very few differences in Dlx expression patterns could be detected between tooth and dermal denticle development, except for the absence of Dlx2 expression in teeth.Taken together, our histological and expression data strongly suggest that teeth and dermal denticles develop from the same developmental module and under the control of the same set of Dlx genes. Teeth and dermal denticles should therefore be considered as serial homologs developing through the initiation of a common gene regulatory network (GRN) at several body locations. This mechanism of heterotopy supports the 'inside and out' model that has been recently proposed for odontode evolution.Teeth and tooth-like structures, together designated as odontodes, are thought to be serial homologs: they are repeated mineralized units composed of dentine and enamel, or enameloid, surrounding a pulp cavity [[1,2], see for review: [3]]. Odontodes can be found in various locations on the body of extant gnathostomes, such as teeth in jaws and different bones in the oral and pharyngeal cavity, but also as dermal denticles (also called placoid scales) on the body surface in chondrichthyans [4]. Teeth (oral or pharyngeal) contrast with denticles (pharyngeal or dermal) in their ability to regenerate through a typical renewing process [5]. There has been a long and recently revitalized debate concerning the origin and evolution of odontodes. Due to their mineralized composition, they are