1. Gene Expression and Cell Biology/Biophysics Programmes, European Molecular Biology Laboratory (EMBL), Meyerhofstrasse 1, D-69117 Heidelberg, Germany
2. Theoretical Bioinformatics B080, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 580, D-69120 Heidelberg, Germany
3. Departments of Orthodontics, Oral and Maxillofacial Surgery, Pediatric Dentistry and Advanced General Dentistry, and
4. Department of Physiology, University of Connecticut Health Center, Farmington, Connecticut 06030, USA
5. Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA
6. †Present address: Research Institute of Molecular Pathology (IMP), Dr Bohr-Gasse 7, A-1030 Vienna, Austria

Correspondence to: Jan Ellenberg¹ Correspondence and requests for materials should be addressed to J.E. (Email: jan.ellenberg@embl.de).

Abstract

Chromosome capture by microtubules is widely accepted as the universal mechanism of spindle assembly in dividing cells. However, the observed length of spindle microtubules and computer simulations of spindle assembly predict that chromosome capture is efficient in small cells, but may fail in cells with large nuclear volumes such as animal oocytes. Here we investigate chromosome congression during the first meiotic division in starfish oocytes. We show that microtubules are not sufficient for capturing chromosomes. Instead, chromosome congression requires actin polymerization. After nuclear envelope breakdown, we observe the formation of a filamentous actin mesh in the nuclear region, and find that contraction of this network delivers chromosomes to the microtubule spindle. We show that this mechanism is essential for preventing chromosome loss and aneuploidy of the egg—a leading cause of pregnancy loss and birth defects in humans.

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