Undergraduate Honors Theses

Thesis Defended

Spring 2013

Document Type

Thesis

Department

Molecular, Cellular, & Developmental Biology

First Advisor

Ken Krauter

Abstract

Deep homology is the use of similar genetic pathways in the development of structures that are not directly related by common ancestry. One such pathway is the Hedgehog (Hh) and Fibroblast growth factor (Fgf) feedback loop associated with the outgrowth of limbs, external genitalia and, as recently hypothesized, barbels in fishes. Barbels are sensory projections from the head found in a number of groups of fishes, with the most familiar example being the “whiskers” of catfishes. The present study tested whether two components of this loop, Shh and Fgf8, were sufficient to induce ectopic barbels in the zebrafish, a result that would strengthen the case for deep homology between barbels and other vertebrate appendages. Zebrafish were injected with DNA constructs for the heatYinducible expression of Fgf8 and Shh. Larvae with induced overexpression of these genes were cleared and stained for skeletal structures and examined for barbelYlike projections using light and scanning electron microscopy. Coexpression of Fgf8 and Shh was found to be sufficient for inducing ectopic cartilages in the vicinity of the jaw, which were reminiscent of the supports of barbels in some fish species. At least one of these cartilages supported a projection from the face, but the possibility that this projection represents an ectopic jaw could not be ruled out. Regardless of the identity of the ectopic cartilages/projection, the results of this study provide further evidence for the conservation of a mechanism for driving appendage outgrowth in a wide variety of species and structures.

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