A UNIQUE MITOTIC VARIATION IN THE MARINE DINOFLAGELLATE OXYRRHIS MARINA (PYRROPHYTA)1

Abstract

ABSTRACTMitosis is described in the flagellate Oxyrrhis marina Dujardin and is compared in related genera. Dense plaques develop in the nuclear envelope at prophase and give rise to an intranuclear spindle. Some of the microtubules associate with the chromosomes while others extend across the nucleus. The basal bodies migrate toward the poles early in division and retain a position lateral to the nuclear poles throughout mitosis. Microtubules are not present between the nucleus and the basal bodies. The nucleolus is persistent and elongates throughout anaphase and telophase. Chromosomal separation is accomplished by sliding of non‐chromosomal microtubules and by elongation of the nuclear envelope rather than by shortening of the spindle microtubules. The nuclear envelope begins to constrict in the center early in anaphase. Continued constriction of the envelope and elongation of the nucleus leads to the formation of a dumbbell‐shaped nucleus by late telophase. Mitosis culminates by the constriction of the nucleus into two daughter nuclei. The taxonomic position of Oxyrrhis marina is discussed in light of these findings.