| The ability of lost structures to reappear in the course of evolution has long been a subject of controversy. The vertebrate dentition has featured prominently in the debate, as it provides examples of losses of teeth that have never been reversed, as well as atavistic reappearance of teeth. We have sought to understand why some modifications of the dentition are apparently irreversible and others are not using reduction of the dentition in cypriniform fishes as a model system. Cypriniforms, which include carps, suckers, loaches, and minnows such as the zebrafish, have experienced complete loss of oral and upper pharyngeal dentition, retaining teeth only on the fifth ceratobranchials of the lower, posterior pharynx. Despite extensive diversification of species and feeding modes, cypriniforms have never regained their lost dentition. We investigated the TNF family ligand ectodysplasin (Eda) as a potential cause for this drastic reduction in dentition. Both this signaling molecule and its receptor are necessary for normal tooth development in the mouse, and activation of ectodysplasin signaling is sufficient to restore a vestigial tooth in this species. We found through a comparison of Eda expression between the zebrafish and a related species retaining oral and upper pharyngeal dentition, the Mexican tetra (Astyanax mexicanus), that loss of expression of this ligand is associated with loss of dentition in cypriniforms. To investigate whether restoring Eda expression is sufficient to restore oral and upper pharyngeal teeth, we constructed a transgenic line that continuously and ubiquitously overexpresses this gene. Although Eda overexpression did not restore oral dentition in this line, it resulted in the reappearance of upper pharyngeal teeth. Expression analysis of early tooth markers suggests that these upper pharyngeal teeth form de novo in response to ectopic Eda, supporting a role for this ligand as an early activator of tooth formation. Additionally, despite the loss of oral Eda expression, other genes in this pathway are expressed in the mouth, suggesting that this region is competent to respond to ectopic Eda. Therefore, the failure of Eda overexpression to restore oral teeth suggests that their loss likely involved several necessary genes and may be essentially irreversible. |
