Concanavalin A as a pan-eukaryotic nuclear envelope marker for expansion microscopy
Across eukaryotes, the nuclear envelope exhibits distinct remodelling strategies during mitosis: complete breakdown (open mitosis), partial disruption (intermediate), or full retention (closed). However, both light and electron microscopy of the nuclear envelope, at sufficient resolution to determine integrity and mode of remodelling, have proven technically challenging. Here, Concanavalin A (ConA), a glycan-binding plant lectin, provides discernible nuclear envelope/endoplasmic reticulum (NE/ER) labelling in Ultrastructure Expansion Microscopy (U-ExM). Applied post-expansion, ConA circumvents antibody optimisation and labels the NE across mammalian cells and diverse microbial eukaryotes. By imaging four opisthokont and amoebozoan species with established mitotic strategies, C. perkinsii, D. discoideum, S. pombe, and S. arctica, we distinguished the full spectrum of NE remodelling. Applied to species with poorly characterised mitotic strategies, ConA revealed intermediate mitosis with polar fenestrae in the multinucleate stramenopile A. limacinum and a life cycle-coupled switch between open and closed mitosis in the amoebozoan P. polycephalum. These observations support the hypothesis that multinucleated life cycles favour closed or intermediate mitosis, protecting chromosomes from capture by neighbouring spindles during synchronous divisions. Together, these results establish ConA in combination with expansion microscopy as a broadly applicable tool for uncovering the diversity and evolution of NE remodelling across eukaryotes.