PROJECT SUMMARY Chromosome segregation is driven by a spindle machinery that distributes copies of the genome between daughter cells. In eggs and their progenitor oocytes, chromosomes are segregated in a specialized meiotic division program. Oocytes and eggs are remarkably vulnerable to chromosome segregation errors that give rise to aneuploidy in embryos, a leading cause of spontaneous miscarriages and developmental disorders. Embryo aneuploidy can nonetheless arise even after error-free completion of meiotic chromosome segregation. However, our understanding of the underlying causes of embryo-specific aneuploidies has been restricted by a tendency to focus only on meiosis-derived aneuploidies. Until recently, it was believed that microtubules are the only cytoskeletal components required for chromosome segregation. This view was successfully challenged by our discovery of spindle F-actin in oocytes and eggs that boost chromosome-spindle attachments and prevent aneuploidy. How is spindle F-actin assembled and how does it exert its function at the chromosome-microtubule interface? These are among outstanding questions raised by this paradigm shift in our understanding of cell division. A major goal of my lab is to understand the mechanisms that safeguard accurate chromosome segregation in mammalian oocytes and embryos. Driven by our discovery that spindle F-actin constitutes one such protection mechanism, we are combining advanced microscopy assays with rapid protein degradation tools to identify proteins required for spindle F-actin assembly and function in oocytes. This approach has revealed key actin- and microtubule-binding proteins that govern oocyte chromosome segregation, some of which were independently implicated in sporadic miscarriages and developmental disorders in recent genetic studies of infertility patients. We propose to build on this progress and study the origins of embryo-specific aneuploidy by 1) expanding our candidate-based rapid protein degradation screens to a larger subset of actin-microtubule crosstalk proteins, and 2) developing a new biochemical and proteomics-coupled experimental pipeline for unbiased identification of novel spindle F-actin assembly proteins in oocytes and embryos. Furthermore, we will take direct experimental approaches of adding or removing centrosomes using microinjection and laser microsurgery tools to address why spindle-shaped F-actin structures are unique to acentrosomal spindles. Early mouse embryo mitotic divisions, which are executed without canonical centrosomes, will provide us with an attractive experimental model in which to answer this long-standing question in cell biology. When this research is completed, we will have discovered and functionally characterized spindle F-actin assembly proteins in oocytes and embryos. Overall, this study will reveal how distinct cytoskeletal systems cooperate to drive accurate chromosome segregation in early development.