The enamelin genes in lizard, crocodile and frog, and the pseudogene in the chicken provide new insights on enamelin evolution in tetrapods.
Résumé
Enamelin (ENAM) has been shown to be a crucial protein for enamel formation and mineralization. Previous molecular analyses have indicated a probable origin early in vertebrate evolution, which is supported by the presence of enamel/enameloid tissues in early vertebrates. In contrast to these hypotheses, ENAM was only characterized in mammals. Our aims were to 1) look for ENAM in representatives of nonmammalian tetrapods, 2) search for a pseudogene in the chicken genome, and 3) see whether the new sequences could bring new information on ENAM evolution. Using in silico approach and polymerase chain reaction, we obtained and characterized the messenger RNA sequences of ENAM in a frog, a lizard, and a crocodile; the genomic DNA sequences of ENAM in a frog and a lizard; and the putative sequence of chicken ENAM pseudogene. The comparison with mammalian ENAM sequences has revealed 1) the presence of an additional coding exon, named exon 8b, in sauropsids and marsupials, 2) a simpler 5'-untranslated region in nonmammalian ENAMs, 3) many sequence variations in the large exons while there are a few conserved regions in small exons, and 4) 25 amino acids that have been conserved during 350 million years of tetrapod evolution and hence of crucial biological importance. The chicken pseudogene was identified in a region that was not expected when considering the gene synteny in mammals. Together with the location of lizard ENAM in a homologous region, this result indicates that enamel genes were probably translocated in an ancestor of the sauropsid lineage. This study supports the origin of ENAM earlier in vertebrate evolution, confirms that tooth loss in modern birds led to the invalidation of enamel genes, and adds information on the important role played by, for example, the phosphorylated serines and the glycosylated asparagines for correct ENAM functions.