Molecular Packaging, Liquid-Crystalline Order, and Three-Dimensional Architecture of Dinoflagellate Chromosomes

Many dinoflagellates package exceptionally large nuclear genomes within the dinokaryon, a nucleus that departs from conventional eukaryotic patterns of chromosome condensation, DNA packaging, and mitosis. Their chromosomes remain visibly condensed through most or all of the cell cycle. In many core dinoflagellates, DNA exhibits cholesteric liquid-crystalline order, while canonical nucleosomal chromatin does not serve as the dominant structural framework. This review synthesizes current evidence on dinoflagellate chromosome organization, spanning molecular packaging mechanisms and three-dimensional nuclear architecture. Dinoflagellates are histone-poor rather than histone-free: divergent core histones persist at low abundance, whereas HCc/MBNP-type proteins can be highly abundant in some species, with additional contributions from dinoflagellate viral nucleoproteins, divalent cations, RNA-associated factors, and modified DNA bases. Classical electron microscopy revealed nested arch patterns consistent with cholesteric order and inspired spiral, double-helical, and toroidal chromonema models, although molecular-resolution support remains lacking and fixation-sensitive features require caution. Recent volume electron microscopy and chromosome-conformation studies are revising chromosome counts and nuclear organization. Each of four reconstructed Oxyrrhis marina nuclei contained 398-405 physically segmented chromosome bodies, focused ion beam scanning electron microscopy (FIB-SEM) reconstruction of a Prorocentrum cordatum CCMP 1329 nucleus resolved 62 tightly packed chromosome bodies together with an organized nucleolus and clustered nuclear pores, and high-throughput chromosome conformation capture (Hi-C) studies in Symbiodiniaceae revealed chromosome-scale scaffolds and transcription-dependent domains. Integrating standardized volume electron microscopy, chromosome-scale genomics, cryo-electron tomography, and biochemical reconstitution should connect molecular packaging to whole-nucleus architecture and improve understanding of ecologically important microbial eukaryotes.