Chromosome segregation occurs by microtubule pushing in oocytes

Abstract

During cell division, spindle microtubules ensure an equal repartition of chromosomes between the two daughter cells. While the kinetochore-dependent mechanisms that drive mitotic chromosome segregation are well understood, in oocytes of most species atypical spindles assembled in absence of centrosomes entail poorly understood mechanisms of chromosome segregation. In particular, the structure(s) responsible for force generation during meiotic chromosome separation in oocytes is unclear. Using quantitative light microscopy, electron tomography, laser-mediated ablation, and genetic perturbations in the Caenorhabditis elegans oocyte, we studied the mechanism of chromosome segregation in meiosis. We find spindle poles are largely dispensable, and in fact act as brakes for chromosome segregation. Instead, our results suggest that CLS-2-dependent microtubules of the meiotic central spindle, located between the segregating chromosomes and aligned along the axis of segregation, are essential. Our results support a model in which inter-chromosomal microtubules of the central spindle push chromosomes apart during meiotic anaphase in oocytes.

Sumoylation regulates central spindle protein dynamics during chromosome segregation in oocytes

“…Another key protein during meiotic chromosome segregation is the CLASP orthologue CLS-2 (Dumont et al, 2010), whose presence in the central spindle is required for homologues to segregate during anaphase I (Dumont et al, 2010;Laband et al, 2017). We did not detect CLS-2 in the midbivalent ring domain during metaphase I or II (Supplementary Figure 3A).…”

“…To assess the role of SUMO during anaphase I progression, we investigated the localisation and role of two proteins shown to play key roles during meiotic chromosome segregation: BUB-1 and AIR-2 (Dumont et al, 2010;Kaitna et al, 2002;Rogers et al, 2002;Muscat et al, 2015;Laband et al, 2017). AIR-2 concentrates in the midbivalent ring domain (Kaitna et al, 2002;Rogers et al, 2002), while BUB-1 is present in the ring domain and also in kinetochores (Monen et al, 2005;Dumont et al, 2010;Laband et al, 2017). In agreement with this, we observed a strong AIR-2 and BUB-1 co-localisation in the midbivalent ring ( Figure 2A, cyan arrows).…”

“…Acute depletion of BUB-1 and CLS-2 during oocyte meiosis. The role of BUB-1 and CLS-2, as well as other proteins, has been mainly addressed using protein depletion by means of RNAi, either on its own or in depletion/rescue experiments (Davis-Roca et al, 2018;Dumont et al, 2010;Laband et al, 2017;Muscat et al, 2015;Wignall and Villeneuve, 2009). These experiments use RNAi mostly for between 12 and 48 hours, but in some cases RNAi incubation have reached up to 72 hours.…”

“…During anaphase, while chromosomes are segregating, MTs populate the interchromosomal region creating the central-spindle. While many studies of MT-dependent chromosome movement focused on pulling forces generated by kinetochore MTs (kMTs) making end-on contacts with chromosomes (Cheeseman, 2014), there is also evidence for pushing forces that are exerted on the segregating chromosomes (Khodjakov et al, 2004;Nahaboo et al, 2015;Laband et al, 2017;Vukušić et al, 2017;Yu et al, 2019).…”

“…Furthermore, the role of this ring domain itself during chromosome segregation has remained elusive. During anaphase the ring domain stretches and its composition changes rapidly, leading to the recruitment of SEP-1 Separase (Muscat et al, 2015), MDF-1 M ad1 (Moyle et al, 2014), and CLS-2 CLASP (Dumont et al, 2010;Laband et al, 2017). CLS-2 exhibits a BUB-1-dependent kinetochore localisation until metaphase I and localises within the central spindle during anaphase.…”