Architecture and evolution of dinoflagellate chromosomes: an enigmatic origin

Abstract

Dinoflagellates are a highly diversified group of unicellular protists that present fascinating nuclear features which have intrigued researchers for many years. As examples, a dense nuclear matrix accommodates permanently condensed chromosomes that are composed of fibers organized without histones and nucleosomes in stacked rows of parallel nested arches. The macromolecular chromosome structure corresponds to cholesteric liquid crystals with a constant left-handed twist. RNA acts to maintain the chromosome structure. Whole mounted chromosomes have a left-handed screw-like configuration with coils which progressively increase their pitch. This helical arrangement seems to be the result of a couple of narrow strands coiling together. Chromosomes do not show Q, G and C banding patterns. However, a roughly spherical differentiated upper end (primitive kinetochore?) and two differentiated coiling regions, the upper one composed of two to three coils where a couple of sister strands run together and parallel to each other, and the lower one where sister strands run out of phase by 180° angular difference along the immediate next turns, can be distinguished. The chromosome segregation into two daughter chromatids begins at the telomere that attaches to the nuclear envelope, follows along the chromosome axis constituting first a Y-shaped and afterwards a V-shaped chromosome, which packs the newly synthesized DNA inside the “old” chromosome. Dividing chromosomes remain highly condensed, and the diameters of the new chromatids and the undivided chromosome are similar, but the number of arches is twice as large in G1 as in G2. The nuclear envelope remains through the cell cycle and shows spindle fibers, which penetrate intranuclear cytoplasmic channels during mitosis constituting an extra nuclear spindle. These and other cytogenetic features suggest that dinoflagellates are a group of enigmatic protists, unique and different from the usual eukaryotes. In contrast, DNA sequence studies propose that dinoflagellates are true eukaryotes, closely related to Apicomplexa, and ciliates (Alveolata), suggesting that the unusual features of chromosome and nuclear organization are not primitive but derived characters. Nevertheless, dinoflagellates have reached enigmatic specific nuclear and chromosome solutions, extremely far from those of other living beings.