Thesis advisor: Sarah McMenamin / Pectoral fins are evolutionarily homologous to tetrapod limbs and can serve as useful models for studying the genetic and hormonal factors regulating appendage development. To test the roles of thyroid hormone (TH) in the development of these appendages, we examined the morphogenesis of zebrafish pectoral fins under euthyroid (EuTH) or hypothyroid (hypoTH) conditions. TH plays an important role in modulating the development of the pectoral fin endoskeleton, as well as the proximo-distal patterning of the fin rays. Additionally, in HypoTH fish, shh was expressed in the same domains, but at ~50% of WT expression levels. Nuclear TH acts by binding to dual-action receptors, including Thrab, which represses or activates expression depending on interaction with the TH ligand. When Thrab was absent in HypoTH fish, we found that many elements of the HypoTH phenotype were rescued, suggesting TH relieves Thrab- mediated repression. We also found that TH modulates the development of the musculature surrounding the pectoral fin. Lastly, we generated CRISPR-Mediated knockouts of dio1 and dio3b, to learn how the deiodination of TH may be affecting the development of the pectoral fin. / Thesis (BS) — Boston College, 2023. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Departmental Honors. / Discipline: Biology.

Polypterus senegalus, an extant member of the ray-finned fishes, can both swim in water and walk overland. Both environments impose different locomotor requirements on Polypterus fins. In an aquatic environment, forward propulsion is largely generated through oscillations of the pectoral fins working in sync with each other. On land, the pectoral fins are engaged in a contralateral gait, and are involved in lifting the body off the ground while simultaneously balancing the body. Polypterus have been shown to undergo behavioural, anatomical, and physiological changes during both short- and long-term exposure to land. Differences in force environments and locomotor behaviour between aquatic and terrestrial environments are hypothesized to be the cause of these plastic changes observed in the musculoskeletal tissues of Polypterus. Despite these observable changes, it is unclear exactly how the pectoral fins are experiencing ground reaction forces (GRF) during terrestrial locomotion. By measuring and quantifying force production during walking in Polypterus, this thesis provides a first look at the relationship between GRFs produced and experienced during walking and the pectoral fins of the amphibious fish, Polypterus. The kinematics of the pectoral fins and fore body were analyzed during terrestrial locomotion, and strategic points across both pectoral fins and body were digitized. Kinematics were compared with GRFs in the thrust (X), stabilizing (Y) and lifting (Z) planes to understand how impact forces travel through the fin tissues. Further analysis, using inverse dynamics, is required to determine how these impact forces travel through the musculature of the pectoral fins, perhaps providing potential hypotheses as to the effects of GRFs and their role in not only how terrestrial locomotion affects the behavioural, anatomical, and physiological plasticity observed in Polypterus, but also the limbs of tetrapods during the evolutionary transition from aquatic to terrestrial environments.