Ultrastructural investigations of three stages of the sexual life history of Peridinium balticum (Pyrrhophyta)

Abstract

Peridinium balticum (Levander) Lemmermann is a binucleate dinoflagellate that harbors a photosynthetic chrysophyte. Two stages of the sexual life history, i.e. the gamete and zygote, were investigated using light and electron microscopy. Cytological features of these stages were compared with morphological and ultrastructural characteristics of the vegetative cell using information in the literature and from this study. A re-examination of the vegetative cell resulted in new information about five organelles. These included the endoplasmic reticulum (ER), trichocysts, peripheral vacuoles, Golgi apparatus, and cytoskeletal system. Gamete production was induced by transferring vegetative cells to a nitrogen deficient medium. Gametes were 10-15 μm in size and gymnodinioid in shape. Newly emerged gametes had a clear cytoplasm and few chloroplasts. Ultrastructurally, gametes were similar to vegetative cells, the major difference being chloroplast structure and thylakoid arrangement. Thylakoids were composed mostly of single lamellae. Often osmiophilic globules and pyrenoids were absent. Two morphologically distinct stages of the zygote were recognized and designated stage A and stage B. Stage A zygotes represented an early stage of fusion between gametes. They were a figure-8 in shape and had a pinwheel spin. Gamete position during fusion was specific, i.e. sulcal region ot apical region. Thecal plates invaginated along the length of the zygote and were dissolved, as were plates at the point of gamete contact. The dinokaryotic nuclei fused first and dissected chromosomes were observed in these nuclei. Stage B zygotes were elongated and were characterized by the two X-shaped fused cingula. Division of the fused dinokaryotic nucleus occurred at this stage; fusion of the eukaryotic nuclei followed. Changes observed concerning the fused eukaryotic nuclei included: (1) condensation of chromatin, (2) crystalline rod formation, and (3) a concentration of a less dense symbiont cytoplasm around the eukaryotic nucleus. This study provides evidence that significant morphological changes occur in the eukaryotic nucleus during zygote development. Therefore, it may be inferred that this nucleus is not completely passive in sexual reproduction, as opposed to its reported passive state during mitotic division.