Meeting Abstract
34.3 Monday, Jan. 5 Ectopic expression of Fgf ligands results in supernumerary and fused teeth in zebrafish larvae JACKMAN, W.R.*; STOCK, D.W.; Bowdoin College; Univ. of Colorado wjackman@bowdoin.edu
Fibroblast growth factor (Fgf) signaling is required for teeth to progress beyond the earliest stages of development in both mammals and zebrafish. Whether Fgf signaling is sufficient for tooth initiation remains unclear. To investigate further the role of Fgfs in tooth initiation and delimit the regions competent to respond to them, we used the heat-inducible hsp70 promoter to drive the ectopic expression of fgf10a in zebrafish embryos. Teeth in zebrafish are restricted to the posterior pharynx. A single tooth is visible on each side of the midline at 4 dpf, and these sites are marked two days prior by restricted foci of dlx2b expression. We found that larvae injected with the hsp70:fgf10a construct and exposed to heat shock exhibited dramatic local expansion of dlx2b expression early in development and formed up to five teeth on a side by 4 dpf. Bicuspid teeth, which we interpret as the result of fusion of teeth that initiated simultaneously and in close proximity, were also observed. Despite the induction of Fgf expression throughout injected embryos, supernumerary teeth were limited to the posterior pharynx. We tested additional Fgf ligands for similar activity, and preliminary results indicate that some but not all exhibit this property. These ectopic tooth phenotypes qualitatively resemble those predicted by reaction-diffusion models for the control of the spacing of vertebrate ectodermal appendages (e.g. feathers and hairs), in which Fgf signaling serves as an activator of placode formation. Finally, the inability of ectopic Fgf signaling to induce tooth formation in the anterior pharyngeal and oral cavities raises the possibility that evolutionary reduction of dentition in the zebrafish and other cypriniform fishes was caused by the restriction of competence to respond to tooth initiation signals.
