STOCK, D.W.*; TRAPANI, J.; JACKMAN, W.R.; Univ. of Colorado, Boulder: A developmental genetic investigation of dentition reduction in cypriniform fishes

Evolutionary reduction of vertebrate dentitions is a frequent phenomenon, while regain of lost teeth is rare. We have used loss of oral teeth in cypriniform fishes as a model to investigate whether this trend is the result of selection, ease of tooth loss relative to gain, or a combination of the two. Despite a diversity of species and feeding modes, as well as the retention of teeth in the pharyngeal region, no cypriniform lineage has re-acquired oral teeth in the clade�s 55 million year history. To identify whether the mechanism of cypriniform tooth loss has rendered it irreversible, we compared gene expression and function in a representative species, the zebrafish (Danio rerio) and one of its closest relatives retaining oral teeth, the Mexican tetra (Astyanax mexicanus: Characidae). The most significant difference we found was absence of expression of the transcription factors dlx2a and dlx2b in the oral region of D. rerio, despite their expression from early stages in the oral tooth-forming region of A. mexicanus. Because these genes arose by genome duplication prior to cypriniform tooth loss, the most parsimonious explanation for loss of expression of both is change in a common upstream regulator. We used the Fibroblast growth factor (Fgf) signaling pathway inhibitor Su5402 to identify Fgf signaling as a candidate upstream regulatory process; treatment of A. mexicanus with Su5402 resulted in loss of oral teeth and associated dlx2a and dlx2b expression. Gain-of-function experiments are being carried out to test the hypothesis that loss of Fgf signaling was responsible for cypriniform tooth loss and may yield insight into whether this loss is reversible.