The katanin complex severs microtubules to regulate spindle dynamics during cell division, yet its role across the mammalian meiosis-to-mitosis transition remains unexplored.

Using oocyte-specific Katnb1 conditional knockout mice, we reveal a stage-specific requirement for microtubule severing. KATNB1 loss had minimal impact on meiotic spindle formation and oocyte development, yet caused complete female infertility through mitotic failure, predominantly at the 2-cell to 4-cell transition. Live- cell imaging showed that KO embryos assembled but failed to maintain bipolar spindles. This compromised chromosome segregation leading to failed cytokinesis and multinucleated blastomeres. Strikingly, at 2-cell metaphase we observed a spiral-shaped spindle pole morphology, which we attribute to a failure of katanin-mediated severing at the spindle poles, while microtubule nucleation persists at the kinetochores.

To investigate why meiotic spindles tolerate KATNB1 loss, we examined microtubule minus-end regulators at the spindle poles of meiotic and mitotic spindles. While KATNB1 localised to spindle poles at both stages, KIF2A, a minus-end depolymerase, was strongly expressed at meiotic spindle poles but markedly reduced in spindle poles of 2-cell embryos. We are currently testing whether KIF2A expression at the meiotic spindle functionally compensates for the loss of katanin-mediated severing. Our findings demonstrate that the molecular machinery governing microtubule minus-end regulation at spindle poles is fundamentally reorganised between meiosis and early embryonic mitosis.