The Biochemistry and Evolution of the Dinoflagellate Nucleus

Dinoflagellatesare known to possess a highly aberrant nucleus—the so-called dinokaryon—that exhibits a multitude of exceptional biological features. These include: (1) Permanently condensed chromosomes; (2) DNA in a cholesteric liquid crystalline state, (3) extremely large DNA content (up to 200 pg); and, perhaps most strikingly, (4) a deficit of histones—the canonical building blocks of all eukaryotic chromatin. Dinoflagellatesbelong to the Alveolata clade ( dinoflagellates, apicomplexans, and ciliates) and, therefore, the biological oddities observed in dinoflagellatenuclei are derived character states. Understanding the sequence of changes that led to the dinokaryonhas been difficult in the past with poor resolution of dinoflagellatephylogeny. Moreover, lack of knowledge of their molecular composition has constrained our understanding of the molecular propertiesof these derived nuclei. However, recent advances in the resolution of the phylogeny of dinoflagellates, particularly of the early branching taxa; the realization that divergent histone genes are present; and the discovery of dinoflagellate-specific nuclear proteinsthat were acquired early in dinoflagellateevolution have all thrownnew light nature and evolution of the dinokaryon.