Genetic and optogenetic analysis of cell cleavage plane positioning

Cells form the basis of life. Every animal starts its life as a single cell, which generates all the other cells of the body through repeated rounds of division. To develop properly, a fertilized egg needs to generate both adequate cell numbers and types. Thus, development requires a balance between the generation of more cells through symmetric divisions, and the generation of diverse cell types through asymmetric divisions. Dysregulation of such divisions underlie divergent pathologies, which include developmental abnormalities and cancer. The mitotic spindle controls the outcome of cell division. This fascinating structure consists of many cable-like microtubules, which drive DNA segregation and determine where the cell will ultimately cleave. Precise positioning of the mitotic spindle is an important determinant in orchestrating asymmetric divisions. Positioning of the mitotic spindle involves the generation of pulling forces on spindle microtubules at the cell periphery, which is driven by an evolutionary conserved set of proteins. In this thesis we use early embryos of the nematode C. elegans to study how the mitotic spindle is positioned during the early development of a living animal. To this end, we apply and further improve novel approaches used in cell and developmental biology research. These include genome editing through CRISPR/Cas9, advanced live microscopy, optogenetics (controlling protein localization with light) and thorough genetic analyses. Using these tools, we have dissected the role of diverse conserved proteins in regulating mitotic spindle positioning and cell cleavage plane positioning, which are key steps in the developmental control of cell division.