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.

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Teleost fishes display a remarkable diversity of adult dentitions; this diversity is all the more remarkable in light of the uniformity of first-generation dentitions. Few studies have quantitatively documented the transition between generalized first-generation dentitions and specialized adult dentitions in teleosts. We investigated this transition in the Mexican tetra, Astyanax mexicanus (Characidae), by measuring aspects of the dentition in an ontogenetic series of individuals from embryos to 160 days old, in addition to adults of unknown age. The first-generation dentition and its immediate successors consist of small, unicuspid teeth that develop extraosseously. Multicuspid teeth first appear during the second tooth replacement event, and are derived from single tooth germs, rather than from the fusion of multiple conical tooth germs. We document that the transition from unicuspid to multicuspid teeth corresponds to a change in the location of developing tooth germs (from extraosseous to intraosseous) and in patterns of tooth replacement (from haphazard to simultaneous within a jaw quadrant). In addition, while the size of the largest teeth scales with positive allometry to fish size, the transition to multicuspid teeth is accompanied by an exceptionally large increase in tooth size. Yamamoto Y, Espinasa L, Stock DW, Jeffery WR. 2003. Development and evolution of craniofacial patterning is mediated by eye-dependent and -independent processes in the cavefish Astyanax. Evolution and Development 5: 435-446. Yamamoto Y, Stock DW, Jeffery WR. 2004. Hedgehog signaling controls eye degeneration in blind cavefish. Nature 431: 844-847.

The endemic cichlids of the Cuatro Cienegas Basin, Coahuila, Mexico are currently grouped in a single polymorphic species, Cichlasoma minckleyi.Two morphs of C. minckleyi were distinguished largely by features of the trophic apparatus, especially the pharyngeal dentition. Variation in body shape, based upon analysis of a set of linear measures, was continuous and did not allow recognition of discrete morphs. Individuals raised in the laboratory on several different diets indicated that trophic morphology had an important genetic component. Individuals raised in the laboratory, however, did not differentiate to the degree seen in comparably sized individuals collected at Cuatro Cienegas. This may be because snails used as food in the experiments were not as hard as endemic snails and indicated that some aspects of trophic morphology were also dependent upon environmental cues.