Chromatin structure in the unicellular algae Olisthodiscus luteus, Crypthecodinium cohnii and Peridiniun balticum

Isolated nuclei of the unicellular alga Olisthodiscus luteus, the uninucleate dinoflagellate Crypthecodinium cohnii and the binucleate dinoflagellate Peridinium balticum were lysed and deposited on grids by the microcentrifugation technique. The ultrastructure of the released chromatin fibers was compared to that of mouse liver nuclei. Chromatin from nuclei of Olisthodiscus luteus and the "eukaryotic" nuclei of Peridinium balticum, appeared as linear arrays of regularly repeating subunits which were identical in size and morphology to mouse nucleosomes. In contrast, the chromatin fibers from Crypthecodinium cohnii nuclei appeared as smoothe threads with a diameter of about 6.5 nm. Nuclear preparations containing mixtures of "dinokaryotic" and "eukaryotic" nuclei of Peridinium balticum also contained smooth fibers which most likely originated from the dinokaryotic nuclei. These and other results demonstrating the presence of nucleosomes in lower eukaryotes suggest that the subunit structure of chromatin arose very early in the evolution of the eukaryotic cell.