A Deviating Pattern of Cell Division in the Green Alga Microthamnion: Ultrastructure of Vegetative Cell Division and Zoosporogenesis

Abstract

Summary The ultrastructure of cell division of the green alga Microthamnion is described. It involves a unique mechanism for both vegetative cell division and zoosporogenesis. It starts with migration of the counterclockwise oriented centriole pair along the nuclear surface from the polar interphase position towards a lateral prophase position. The centriole pair duplicates and the root templates of the parental basal bodies grow out distally. Two microtubules of each of the two microtubular roots per basal body form two four-stranded microtubular bundles running along the cell wall and oriented perpendicular to the cell axis. At metaphase the separated centriole pairs have migrated to positions at approximately 180° from each other apparently via a sliding mechanism along the newly formed microtubular bundles. The microtubular bundles lie in the equatorial plane and curve over the nucleus, thus indicating the future division plane. In this respect the microtubular bundles resemble the preprophase band in higher plants. At anaphase the centriole pairs seem to have slightly co-migrated with the separating chromosome halves and thus the microtubular bundles show a shifted orientation with respect to the long axis of the cell. In this stage the unilateral septum starts to develop from the previous prophase position of the centriolar complex and progresses between the two four-stranded microtubular bundles. At telophase the daughter nuclei are reformed and the septum development proceeds. After cytokinesis the centrioles either return to the interphase position awaiting a new division cycle (vegetative cell division) or the centrioles take the prophase position in the newly formed daughter cells and duplicate in order to repeat the division cycle (zoosporogenesis). The ultrastructural features of Microthamnion are compared with the other pleurastrophycean algae and some chlamydomonadalean algae. However, the systematic position of Microthamnion still remains a matter of discussion.