Microscopic observations on karyokinesis and cytokinesis in the gametophytes of Saccharina japonica

Gametophyte clones of Saccharina japonica can be established through the induction of mitosis in gametophytes under artificial culture conditions. However, there have been few reports on the cytological characteristics of mitosis in these clones. This study presents the first observation of the cellular structure during the mitosis phases of kelp gametophytes using paraffin sections, immunofluorescence technique coupled with DAPI staining. Changes in the size and morphology of the gametophyte nucleus during the phases of mitosis were clearly illustrated by DAPI staining. The spindle fibers, originating from the centrosome, and cage-shaped spindle were successfully visualized. Ultrathin sections revealed cleavage furrows at the positions of cell plates under a transmission electron microscope. These furrows, with closed curved front ends formed by invagination of the plasmalemma, grew centripetally until they contacted and fused, leading to the construction of a new cell wall and completion of cytokinesis. The chromosomes of the gametophyte cell were prepared by pressing method, stained with hematoxylin and counterstained with DAPI. It was confirmed that there were 31 chromosomes in the gametophyte of S. japonica. The characterizations of mitosis of S. japonica gametophytes including karyokinesis and cytokinesis were firstly observed in the study, and chromosomes in the intact cell of gametophyte were also illustrated. It provided accurate data for the cytogenetic study of S. japonica, and provided new evidence for the study of cytoplasmic division of brown algae.