Breakage of the nuclear envelope by an extending mitotic nucleus occurs during anaphase in Schizosaccharomyces japonicus.

During open mitosis in higher eukaryotic cells, the nuclear envelope completely breaks down and then mitotic chromosomes are exposed in the cytoplasm. By contrast, mitosis in lower eukaryotes, including fungi, proceeds with the nucleus enclosed in an intact nuclear envelope. The mechanism of mitosis has been studied extensively in yeast, a closed mitosis organism. Here, we describe a form of mitosis in which the nuclear envelope is torn by elongation of the nucleus in the fission yeast Schizosaccharomyces japonicus. The mitotic nucleus of Sz. japonicus adopted a fusiform shape in anaphase, and its following extension caused separation. Finally, a tear in the nuclear envelope occurred in late anaphase. At the same time, a polarized-biased localization of nuclear pores was seen in the fusiform-shaped nuclear envelope, suggesting a compromise in the mechanical integrity of the lipid membrane. It has been known that nuclear membrane remains intact in some metazoan mitosis. We found that a similar tear of the nuclear envelope was also observed in late mitosis of the Caenorhabditis elegans embryo. These findings provide insight into the diversity of mitosis and the biological significance of breakdown of the nuclear envelope.