Continued structural dissection of male meiosis in C. elegans

• Applicant: Professor Dr. Thomas Müller-Reichert
• Subject Area: Cell Biology ; Structural Biology
• Term: since 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 282354882

Meiosis is a fundamental biological process in all sexually reproducing organisms. Restricted to germ cells, meiosis promotes genetic variability and halves the number of chromosomes in producing gametes. Previously, we have analyzed the meiotic segregation of chromosomes in males of the animal model Caenorhabditis elegans. We found that the lagging of the single X chromosome, a distinctive feature of anaphase I in males, is due to a lack of chromosome pairing. The unpaired chromosome at anaphase in wild-type males remains tethered to centrosomes by kinetochore microtubules, which are under tension, suggesting that a ‘tug of war’ reliably resolves lagging Taking advantage of the worm as one of the major model systems for male meiosis, we aim to continue with the structural dissection of this process by extending our fully developed combination of live-cell imaging and large-scale electron tomography to dynein mutant males. Dynein is a ‘universal’ minus-end-directed molecular motor that is essential for mitosis and meiosis, but its precise role in chromosome segregation remains incompletely understood. We propose to: 1) reconstruct the ultrastructure of late anaphase I spindles in wild-type males in three dimensions (3D), 2) perform live-cell imaging of meiosis in dynein mutant males, 3) analyze the 3D ultrastructure of anaphase I spindles in dynein-mutant males, 4) determine the role of membrane ingression in specifying the direction of X chromosome segregation in wild-type male spermatocytes, and 5) refine our proposed ‘tug-of-war' model on X chromosome partitioning. For wild-type males, we expect to find a difference in the number of kinetochore microtubules attached to both sides of the X in late anaphase I. As predicted by our model, such a difference in the number kinetochore microtubules is supposed to initiate the segregation of the X to one of the spindle poles. The side with more attached kinetochore microtubules is supposed to ‘win’ the tug-of-war. In addition, we expect that spermatocytes in dynein mutant males will show a change in chromosome segregation dynamics and an impaired attachment of microtubules to kinetochores. Furthermore, we anticipate that membrane ingression during cytokinesis ‘finalizes’ the decision on the direction of X chromosome segregation. The results will significantly extend our understanding of male meiotic spindle architecture and function in C. elegans and will have important implications for the role of dynein in meiotic chromosome segregation in other systems, including human cells.

• DFG Programme: Research Grants