Novel ultrastructural features of the nucleus of the ancestral dinoflagellate Oxyrrhis marina as revealed by freeze substitution fixation and volume electron microscopy Authors

Yasuhiro Fukuda ¹ Toshinobu Suzaki ² Kazuyoshi Murata ³ Chihong Song ^4*

• ¹ Graduate School of Agricultural Science, Tohoku University, Sendai, Japan
• ² Graduate School of Sciences, Faculty of Science, Kobe University, Kobe, Hyōgo, Japan
• ³ National Institute for Physiological Sciences (NIPS), Okazaki, Aichi, Japan
• ⁴ School of Medicine, Pusan National University, Busan, Republic of Korea

The ultrastructural features of the nucleus, nucleolus, and chromosomes of dinoflagellate Oxyrrhis marina are described in 3D. The nuclear morphological features of O. marina using electron microscopy were described about half a century ago.However, it was suspected that the cells fixed using conventional chemical methods in those descriptions may have included ultrastructural artifacts, such as cytoplasmic gaps and organelle swelling, as there were inconsistencies with observations by light microscopy. Therefore, freeze-substitution fixation and volume electron microscopy were used to observe the number of chromosomes, the relationship between nucleoli and chromosomes, and the morphology of nucleofilaments. Dinoflagellate nuclei are characterized by a "dinokaryon" containing permanently condensed chromosomes that lack histone proteins. O. marina is the species that early diverged from other dinoflagellates. Thus, its nucleus is regarded to represent the ancestral (or primitive) form of the dinokaryon. Our findings may contribute to elucidating the circumstances of its establishment. For these reasons, we consider that our manuscript is appropriate for publication in Frontiers in Protistology.