Ran-GTP assembles a specialized spindle structure for accurate chromosome segregation in medaka early embryos

Despite drastic cellular changes during cleavage divisions, a mitotic spindle is assembled in each blastomere to accurately segregate duplicated chromosomes. Recent studies indicate that early embryonic divisions are highly error-prone in bovines and humans. However, processes and mechanisms of embryonic spindle assembly remain little understood in vertebrates. Here, we established live functional assay systems in medaka fish (Oryzias latipes) embryos by combining CRISPR knock-in with an auxin-inducible degron technology. In contrast to mammals, mitoses during cleavage divisions are very rapid (<12 min), but segregation errors are rarely observed. Importantly, we found that the Ran-GTP gradient assembles a specialized, dense microtubule network at the spindle midplane during metaphase, which is essential for faithful chromosome segregation in early embryos. In contrast, Ran-GTP becomes dispensable for chromosome segregation in later stages, where spindles are morphologically remodeled into short, somatic-like spindles lacking the dense microtubule network. We propose that the specialized Ran-based spindle structure ensures high fidelity of chromosome segregation in large, vertebrate early embryos.