PhD Oral Exam - Karina Mastronardi, Biology

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Cytokinesis and morphogenesis are both regulated by cytoskeletal dynamics. In Chapter 2, we show how cytokinesis is regulated in different cell types. Cytokinesis occurs at the end of mitosis due the ingression of an actomyosin ring that cleaves the cell into two daughters. However, the pathways controlling ring assembly are not fully understood and likely vary with cell type. Our lab found that the Ran pathway spatially regulates ring position in response to chromatin in mammalian cells (Beaudet et al, 2017; Beaudet et al., 2020). We tested the hypothesis that the requirement for the Ran pathway changes with cell fate in the early C. elegans embryo. The zygote (P0) divides asymmetrically to give rise to AB and P1 cells with different fates, and we found that each cell has distinct ring assembly kinetics. The differences in AB and P1 cell kinetics is due to differences in the levels of inherited actomyosin, and the Ran pathway. Importantly, we found that the Ran pathway could regulate different downstream targets in AB vs. P1 cells that control ring assembly.
In Chapter 3, we explore C. elegans called anterior morphogenesis, where the epidermis covers the head of the embryo and helps to form an anterior lumen with the pharynx. Mechanisms must be coordinated at both the cellular and tissue level to give rise to complex structures, which have been challenging to study in vivo. We determined how cells from three different tissues are coordinated for anterior morphogenesis. By examining the localization of the polarity protein PAR-6, we observed how polarized projections from pharyngeal, neuroblasts (neuronal precursor cells) and epidermal cells form specific patterns. We proposed that the cells forming these patterns provide and/or respond to cues that regulate epidermal cell migration. In support of this, we found that disruption of the pharyngeal cells or neuroblasts affected the rate of epidermal cell migration and epithelialization of the pharynx, while altering epidermal cell fate caused lumen positioning defects. Our studies are the first to characterize anterior morphogenesis and lay the framework for identifying how cell patterns are controlled at the molecular level.