Date of Award
Master of Arts (MA)
Ecology & Evolutionary Biology
David W. Stock
The extent to which constraints on adaptive evolution are imposed by the genetic and developmental architecture of organisms is a fundamental question in evolutionary biology. The evolution of dentition in cypriniform fish presents a unique opportunity to study such constraints. Teeth in ray-finned fishes are commonly found on the jaws as well as in the posterior pharynx (throat). In cypriniforms, they are restricted to a single pair of bones in the pharynx as a result of tooth loss in evolution. That the mechanisms of tooth loss or subsequent genetic changes represent constraints on the reappearance of lost teeth is suggested by the conservation of reduced dentition even in species with feeding modes that would likely benefit from additional teeth. The present study investigated a potential role of modification of Wnt signaling in the reduction of cypriniform dentition, a process that might contribute to a constraint on regaining lost teeth. The expression of the transcription factors lef1 and tcf7, two downstream targets of Wnt signaling, was compared between a representative cypriniform, the zebrafish (Danio rerio), and a member of a related order with a more complete dentition, the Mexican cave tetra, Astyanax mexicanus. Both genes were found to be expressed in all tooth germs examined and to have lost their expression in regions from which teeth were lost in the zebrafish lineage. To determine whether such loss of expression was the cause of cypriniform dentition reduction, the necessity of Wnt signaling for tooth development in both species was examined. Injection of morpholino antisense oligonucleotides and application of pharmacological inhibitors revealed that Wnt signaling is necessary for the formation of tooth germs, as evidenced by the blocking of tooth germ molecular markers. However, some markers retained their expression, suggesting that the constraint on regaining teeth lost in cypriniforms is likely the alteration of genetic pathways in addition to Wnt signaling.
Szymaszek, Julie Françoise, "The Dentition of Cypriniform Fishes as a Model for the Nature of Developmental Constraints on Evolution" (2014). Ecology & Evolutionary Biology Graduate Theses & Dissertations. 2.