MECHANISM OF GAMETE FUSION IN DINOPHYSIS FORTII (DINOPHYCEAE, DINOPHYTA): LIGHT MICROSCOPIC AND ULTRASTRUCTURAL OBSERVATIONS1

Abstract

A variety of studies have examined the sexual life cycle of species belonging to the genus Dinophysis Ehrenberg. Here, we used TEM to investigate the mechanism of cellular fusion during the sexual life cycle in Dinophysis fortii Pavillard. We observed that fusion always occurred between a normal‐sized cell and a small cell following attachment of their ventral margins. After cell attachment, the small cell moved toward the epitheca of the normal‐sized cell, and the cingular and sulcal lists of the small cell shrunk or were almost lost. The epitheca of the normal‐sized cell then opened between the cingulum plates and the upper cingular list, after which the small cell was gradually engulfed. This is the first ultrastructural observation in a dinoflagellate of a larger cell opening its epitheca to engulf the smaller gamete. In another case, the normal‐sized cell did not open the epitheca, the cell wall of the attached small cell underwent extensive extracellular digestion, and the cytoplasm appeared to flow into the normal‐sized cell via the periflagellar area. Inflow of the nucleus was not observed in this case, suggesting that it represented a failure of sexual fusion. In both cases, membranous separations between the cytoplasm of the two cells were not observed. At the beginning of the fusion process, the nucleus of the small cell was substantially deformed. The planozygote, formed upon completion of sexual fusion, sometimes had two longitudinal flagella, but was identical to a normal vegetative cell in its cellular shape, as already mentioned by previous authors.