A DEVIATING PATTERN OF CELL-DIVISION IN THE GREEN-ALGA MICROTHAMNION - ULTRASTRUCTURE OF VEGETATIVE CELL-DIVISION AND ZOOSPOROGENESIS

The ultrastructure of cell division of the green alga Microthamnion is described. It involves a unique mechanism for both vegetative cell di vision and zoosporogenesis. It starts with migration of the counterclo ckwise oriented centriole pair along the nuclear surface from the pola r interphase position towards a lateral prophase position. The centrio le pair duplicates and the root templates of the parental basal bodies grow out distally. Two microtubules of each of the two microtubular r oots per basal body form two four-stranded microtubular bundles runnin g along the cell wall and oriented perpendicular to the cell axis. At metaphase the separated centriole pairs have migrated to positions at approximately 180 degrees from each other apparently via a sliding mec hanism along the newly formed microtubular bundles. The microtubular b undles lie in the equatorial plane and curve over the nucleus, thus in dicating the future division plane. In this respect the microtubular b undles 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 or ientation with respect to the long axis of the cell. In this stage the unilateral septum starts to develop from the previous prophase positi on of the centriolar complex and progresses between the two four-stran ded microtubular bundles. At telophase the daughter nuclei are reforme d and the septum development proceeds. After cytokinesis the centriole s either return to the interphase position awaiting a new division cyc le (vegetative cell division) or the centrioles take the prophase posi tion in the newly formed daughter cells and duplicate in order to repe at the division cycle (zoosporogenesis). The ultrastructural features of Microthamnion are compared with the other pleurastrophycean algae a nd some chlamydomonadalean algae. However, the systematic position of Microthamnion still remains a matter of discussion.