Actin and Microtubules cooperate in chromosome capture
during oocyte meiosis
Yamini Vadapalli
Accurate chromosome capture and alignment are essential for error-free cell division, preventing chromosome mis-segregation and aneuploidy. In somatic cells, dynamic astral microtubules capture chromosomes via a stochastic search-and-capture mechanism. However, in large oocytes—with nuclear diameters exceeding 100 µm—this mechanism is insufficient. Instead, actin-based mechanisms appear critical for ensuring successful chromosome congression.
In starfish oocytes, a contractile Arp2/3-nucleated F-actin network forms after nuclear envelope breakdown and transports chromosomes toward the capture range (~30 µm) of astral microtubules, facilitating robust spindle assembly. In jellyfish (Clytia hemisphaerica), a basal cnidarian species, we observed a distinct yet functionally analogous mechanism. Here, microtubule asters form around the nuclear envelope rather than the cortex, and an extensive F-actin network contracts to bring chromosomes into proximity with coalescing asters. After chromosome capture, a single large aster migrates toward the cortex, enabling bipolar spindle formation and polar body extrusion. Our results suggest that actin-driven chromosome transport is a conserved and fundamental adaptation in oocytes with large nuclear volumes.
Further, to understand viscoelastic properties of oocytes, we analysed how nuclear, and cell shapes interact mechanically. Using active microrheology and deformation assays in starfish and jellyfish, we found that oocyte nuclei are much softer and more deformable than somatic nuclei. Starfish nuclei closely follow cell shape changes under stress due to a viscoelastic cytoplasm transmitting forces to a soft nucleoplasm, while jellyfish nuclei deform less because of lower cytoplasmic elasticity. These species-specific differences highlight distinct mechanical adaptations but consistently show the fluid-like nature of the oocyte nucleus.
Together, these findings help in understanding both conserved molecular mechanisms and species-specific biophysical adaptations that ensure robust chromosome capture in large oocytes.
