Conservation of amelogenin gene expression during tetrapod evolution.

Well studied in mammals, amelogenesis is less known at the molecular level in reptiles and amphibians. In the course of extensive studies of enamel matrix protein (EMP) evolution in tetrapods, we look for correlation between changes in protein sequences and temporospatial protein gene expression during amelogenesis, using an evo-devo approach. Our target is the major EMP, amelogenin (AMEL) that plays a crucial role in enamel structure. We focused here our attention to an amphibian, the salamander Pleurodeles waltl. RNAs were extracted from the lower jaws of a juvenile P. waltl and the complete AMEL sequence was obtained using PCR and RACE PCR. The alignment of P. waltl AMEL with other tetrapodan (frogs, reptiles and mammals) sequences revealed residue conservation in the N- and C-terminal regions, and a highly variable central region. Using sense and anti-sense probes synthetized from the P. waltl AMEL sequence, we performed in situ hybridization on sections during amelogenesis in larvae, juveniles, and adults. We demonstrated that (i) AMEL expression was always found to be restricted to ameloblasts, (ii) the expression pattern was conserved through ontogeny, even in larvae where enameloid is present in addition to enamel, and (iii) the processes are similar to those described in lizards and mammals. These findings indicate that high variations in the central region of AMEL have not modified its temporospatial expression during amelogenesis for 360 million years of tetrapod evolution.