DISSERTATION RESEARCH: Mechanisms of Mammalian Middle Ear Ossicle Transition from the Reptilian Jaw Joint

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The transition from reptile to mammal was one of the most important events in leading to the eventual evolution of humans. This transition is characterized by many changes, one of the most important is the evolution of the mammalian middle ear. The goal of this project is to investigate the developmental basis of the evolution of the mammalian middle ear, using an opossum species as a model organism. Because opossum are primitive mammals, they can provide significant insights into mammalian evolution, and the improved understanding of ear development that this project will generate has the potential to positively impact human health. This project will also serve as a foundation for the STEM training of several undergraduates and for community outreach events geared toward K-12 students. Results of this project are also being incorporated into a three-part PBS documentary based upon Neil Shubin's best-selling book, Your Inner Fish. The reptilian jaw consists of multiple bones, while there is only a single bone in the reptile middle ear. Conversely, the mammalian jaw consists of a single bone, while there are multiple bones in the mammal middle ear. It has been shown, via paleontology and embryology, that the extra bones in the mammalian middle ear originated from the extra bones in the reptilian jaw. Separation from the jaw would allow more flexibility in the evolution of the middle ear elements, resulting in increased hearing sensitivity and amplified frequency range. The middle ear changes observed in the transition from reptiles to mammals in the fossil record also occur during development in opossum. Opossum are born with a middle ear similar to reptiles, but in adults the middle ear is mammalian, providing a unique opportunity to study the evolutionary development of mammalian middle ear bones in an currently living organism rather than with fossils. The project is based on a detailed characterization of the transition using micro-computed tomography scans to create three-dimensional images at five day intervals, beginning at birth, and studying the cellular processes underlying the transition by cryosectioning at the noted stages, followed by immunohistochemistry for cell proliferation and apoptosis. The project will uncover the genes responsible for the transition using in situ hybridization and RNA sequencing. Alterations in the timing of expression and concentration of particular genes (e.g., TGTbr2 and Eya1) are expected to be correlated with the separation of Meckel's cartilage and functional changes in the ossicles. The research will determine if the active genes in opossum are also expressed in the middle ear region of placental mammals. The project will compare the transition that occurs during opossum development to that observed in fossil record leading to early mammal lineages.