Meiotic spindle organization and chromosome condensation in Drosophila oocytes
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Errors in chromosome segregation during the first division of female meiosis are very common in humans and result in aneuploidy leading to reproduction problems. Chromosome segregation depends on the formation and function of the meiotic spindle as well as the structure of chromosomes, which need to condense to be able to orient and segregate properly. It is important to understand the mechanisms underlying the female meiotic spindle function and chromosome condensation to gain insight into female fertility problems. The female meiotic spindle assembles without centrosomes, so the mechanisms ensuring microtubule nucleation, spindle assembly and establishment of bipolarity act differently from those of mitosis or male meiosis. I identified a set of genes that are required for microtubule nucleation, spindle maintenance and centromere orientation in Drosophila female meiosis. This was accomplished by mapping previously uncharacterized Drosophila mutants and depleting already known genes by RNAi. I discovered that several proteins have a different role in female meiosis as compared to mitosis, which provides insight into the major differences between these systems. Little is known about the molecular mechanisms of chromosome condensation. The roles of only a few factors, such as condensin complexes, have been studied previously, and the evidence suggests that there are more molecular players required for chromosome condensation. To discover molecular mechanisms critical to this process, I depleted various chromosomal proteins by RNAi and screened for abnormalities of metaphase chromosome morphology in Drosophila oocytes by immunostaining and live imaging. I found that the conserved kinase NHK-1 plays a role in chromosome condensation in female meiosis. BAF is a critical NHK-1 substrate in this process and its phosphorylation is required for detachment of the chromosomes from the nuclear envelope to allow proper condensation. Also, I discovered that the nucleosome remodelling complex NuRD is crucial for chromosome condensation, especially for the chromosome arms. As a result of my PhD project I identified multiple factors required for meiotic spindle function. I also discovered two novel pathways of chromosome condensation that require the NuRD complex and NHK-1 activity.