Ultrastructural study on mitosis and cytokinesis in Scytosiphon lomentaria zygotes (Scytosiphonales, Phaeophyceae) by freeze-substitution

Abstract

The ultrastructure of mitosis and cytokinesis in Scytosiphon lomentaria (Lyngbye) Link zygotes was studied by freeze fixation and substitution. During mitosis, the nuclear envelope remained mostly intact. Spindle microtubules (MTs) from the centrosome passed through the gaps of the nuclear envelope and entered the nucleoplasm. In anaphase and telophase, two daughter chromosome masses were partially surrounded with endoplasmic reticulum. After telophase, the nuclear envelope was reconstructed and two daughter nuclei formed. Then, several large vacuoles occupied the space between the daughter nuclei. MTs from the centrosomes extended toward the mid-plane between two daughter nuclei, among the vacuoles. At that time, Golgi bodies near the centrosome actively produced many vesicles. Midway between the daughter nuclei, small globular vesicles and tubular cisternae accumulated. These vesicles derived from Golgi bodies were transported from the centrosome to the future division plane. Cytokinesis then proceeded by fusion of these vesicles, but not by a furrowing of the plasma membrane. After completion of the continuity with the plasma membrane, cell wall material was deposited between the plasma membranes. The tubular cisternae were still observed at the periphery of the newly formed septum. Microfilaments could not be observed by this procedure. We conclude that cytokinesis in the brown algae proceeds by fusion of Golgi vesicles and tubular cisternae, not by a furrowing of the plasma membrane.

Asymmetric Division in Fucoid Zygotes Is Positioned by Telophase Nuclei

“…Among photosynthetic organisms, division plane specification at cytoplast boundaries defined by interdigitating radial microtubules occurs in the most ancient taxa, most notably green and red algae (Pickett-Heaps et al, 1999), and recent findings indicate that it also may be common in brown algae (Karyophyllis et al, 2000;Nagasato and Motomura, 2002). In the plant lineage, this mechanism has been retained in multinucleate tissues, such as meiocytes, nuclear endosperm, and female gametophytes (Otegui and Staehelin, 2000;Baluska et al, 2001;Brown and Lemmon, 2001), which cellularize after repeated rounds of karyokinesis.…”

Ultrastructural study of plasmodesmata in the brown alga Dictyota dichotoma (Dictyotales, Phaeophyceae)

“…Therefore, these reports suggest existence of the primary PD in brown algae. Detailed ultrastructural studies on cytokinesis of brown algae using rapid freezing/freeze substitution have been conducted (Nagasato and Motomura 2002, 2009, Katsaros et al 2009, Nagasato et al 2010). This technique revealed that novel membranous structures called "flat cisternae" (FCs), together with Golgi vesicles (GVs), contribute to partition membrane formation during cytokinesis, in a mode different from that of the phragmoplast-cell plate system of land plants (Schopfer andHepler 1991, Samuels et al 1995), Thus, the primary PD in 6 brown algae seem to be formed by a distinct process compared to that of green plants.…”

Behavior and function of paternally inherited centrioles in brown algal zygotes

“…Like animal cells, brown algal cells have a pair of centrioles throughout the cell cycle. Mitosis in brown algae has been studied in detail by electron-and immunofluorescencemicroscopy (Markey and Wilce 1975;La Claire 1982;Katsaros et al 1983; Motomura and Sakai 1985;Motomura 1991;Katsaros and Galatis 1992;Motomura 1994;Bisgrove et al 1997;Nagasato and Motomura 2002a). It has become clear that a pair of centrioles always duplicate once every cell cycle, and daughter cells inherit a centriole pair with the pericentriolar material that constitutes a complete functional centrosome in a somatic cell.…”