Type of Thesis
Ecology and Evolutionary Biology
Dr. David Stock
Dr. Barbara Demmig-Adams
Dr. Kathryn Plath
Whether the origin of novel structures requires extensive novelty at the genetic level remains an important question in Evolutionary Biology. The bony plates in the dermis (inner layer of the skin) of stickleback fishes (Order Gasterosteiformes) are novel structures that arose as a replacement for scales and whose genetic basis is becoming increasingly well-understood. Previous work has identified an enhancer (a short DNA sequence that regulates the expression of neighboring genes) that is necessary for the expression of the Ectodysplasin (Eda) gene in the bony dermal plates of the Threespine Stickleback (Gasterosteus aculeatus). This gene is necessary for the development of such plates, as well as the scales of other fishes which they replaced in evolution. In the present study, I first used phylogenetic character mapping to clarify the evolutionary origin of bony dermal plates, finding that it was equally likely that they arose directly from scales or through an intermediate condition of naked skin. To investigate the origin of the bony dermal plate enhancer of the Eda gene, I studied the corresponding gene in the Mexican Tetra (Astyanax mexicanus), a fish species possessing scales and a well-characterized genome. I first used in situ hybridization to confirm that Eda is expressed in scales of this species. I then used reporter transgenic analysis to search for a sequence corresponding to the stickleback bony dermal plate enhancer near the Mexican Tetra Eda gene. Specifically, I used genome sequence comparisons to identify a candidate enhancer, made a DNA construct in which this candidate enhancer was joined to a green fluorescent protein (Gfp) reporter gene, and injected this construct into the Zebrafish (Danio rerio), an easily manipulated model species that also possesses scales. I found that this enhancer was not capable of driving expression in scales, but did drive expression in the upper jaw and pelvic fin. The latter activity is also characteristic of the stickleback bony dermal plate enhancer. This result suggests that the stickleback enhancer arose through modification of an existing enhancer rather than de-novo. My work is therefore consistent with morphological novelty originating without the need for extensive genetic novelty.
Ghasemi, Hannah and Stock, David, "Evolution of gene regulation in association with the origin of armor in stickleback fishes" (2019). Undergraduate Honors Theses. 1831.