Kif11-haploinsufficient oocytes reveal spatially differential requirements for chromosome biorientation in the spindle

Bipolar spindle assembly and chromosome biorientation are prerequisites for chromosome segregation during cell division. The kinesin motor KIF11 drives spindle bipolarization by sliding antiparallel microtubules bidirectionally, elongating a spherical spindle into a bipolar-shaped structure in oocytes. This process stretches chromosomes, establishing chromosome biorientation at the spindle equator. The quantitative requirement for KIF11 in spindle bipolarization and chromosome biorientation remains unclear. Here, using a genetic strategy to modulate KIF11 expression levels, we show that Kif11 haploinsufficiency impairs spindle elongation, leading to the formation of a partially bipolarized spindle during meiosis I in mouse oocytes. While the partially bipolarized spindle allows chromosome stretching in the inner region of its equator, it fails to do so in the outer region. These findings demonstrate the necessity of biallelic functional Kif11 for bipolar spindle assembly in oocytes and reveal a spatially differential requirement for chromosome biorientation within the spindle.