title: Developmental genetic mechanisms of evolutionary tooth loss in cypriniform fishes.
abstract: The fossil record indicates that cypriniform fishes, a group including the zebrafish, lost oral teeth over 50 million years ago. Despite subsequent diversification of feeding modes, no cypriniform has regained oral teeth, suggesting the zebrafish as a model for studying the developmental genetic basis of evolutionary constraint. To investigate the mechanism of cypriniform tooth loss, we compared the oral expression of seven genes whose mammalian orthologs are involved in tooth initiation in the zebrafish and the Mexican tetra, Astyanax mexicanus, a related species retaining oral teeth. The most significant difference we found was an absence in zebrafish oral epithelium of expression of dlx2a and dlx2b, transcription factors that are expressed in early Astyanax odontogenic epithelium. Analysis of orthologous genes in the Japanese medaka (Oryzias latipes) and a catfish (Synodontis multipunctatus) suggests that expression was lost in cypriniforms, rather than gained in Astyanax. Treatment of Astyanax with an inhibitor of Fibroblast growth factor (Fgf) signaling produced a partial phenocopy of the zebrafish oral region, in that oral teeth, and expression of dlx2a and dlx2b, were lost, whereas shh and pitx2, genes whose expression is present in zebrafish oral epithelium, were unaffected. We hypothesize that a loss of Fgf signaling to oral epithelium was associated with cypriniform tooth loss.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309-0334, USA. David.Stock@Colorado.edu
authors.forename: David W
authors.initials: DW
authors.lastname: Stock
authors.forename: William R
authors.initials: WR
authors.lastname: Jackman
authors.forename: Josh
authors.initials: J
authors.lastname: Trapani
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D005075:Biological Evolution; D003531:Cypriniformes / Q000235:genetics* / Q000254:growth & development*; D004848:Epithelium / Q000378:metabolism; D005346:Fibroblast Growth Factors / Q000037:antagonists & inhibitors / Q000378:metabolism; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D009061:Mouth Mucosa / Q000378:metabolism; D051764:PAX9 Transcription Factor / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D016388:Tooth Loss / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D015027:Zebrafish / Q000235:genetics / Q000254:growth & development*; D000097577:Homeobox Protein PITX2
chemical_list: D005346:Fibroblast Growth Factors; D018398:Homeodomain Proteins; D051764:PAX9 Transcription Factor; D014157:Transcription Factors; D000097577:Homeobox Protein PITX2; C088334:Distal-less homeobox proteins