Conservation and divergence of Bmp2a, Bmp2b, and Bmp4 expression patterns within and between dentitions of teleost fishes.

The diversity of tooth location in teleost fishes provides an excellent system for comparing genetic divergence between teeth in different species (phylogenetic homologs) with divergence between teeth within one species (iterative homologs). We have chosen to examine the expression of three members of the bone morphogenetic protein (Bmp) family because they are known to play multiple roles in tooth development and evolution in tetrapod vertebrates. We characterized expression of Bmp2a, Bmp2b, and Bmp4 during the development of oral and pharyngeal dentitions in three species of teleost fishes, the zebrafish (Danio rerio), Mexican tetra (Astyanax mexicanus), and Japanese medaka (Oryzias latipes). We found that expression in teleosts is generally highly conserved, with minor differences found among both iteratively homologous and phylogenetically homologous teeth. Expression of orthologous genes differs in several ways between the teeth of teleost fishes and those of the mouse, but between these vertebrate groups the summed expression pattern of Bmp genes is highly conserved. Significantly, the toothless oral region of the zebrafish lacks Bmp expression domains found in teleosts with oral teeth, implicating these genes in evolutionary tooth loss. We conclude that Bmp expression has been largely conserved in vertebrate tooth development over evolutionary time, and that loss of Bmp expression is correlated with region-specific loss of the dentition in a major group of fishes.