MORPHOGENETIC PLASTID MIGRATION AND MICROTUBULE ARRAYS IN MITOSIS ANDCYTOKINESIS IN THE GREEN-ALGA COLEOCHAETE ORBICULARIS

This study provides data on cell division in Coleochaete orbicularis, an important taxon in evolutionary theories deriving land plants from green algae. Vegetative growth in discoid species of Coleochaete resul ts from marginal cell division in two planes-radial and circumferentia l. Like many algae and certain of the simple land plants, Coleochaete is monoplastidic. Prior to mitosis, the single plastid migrates to a p osition where it will divide and be distributed into the daughter cell s. Unlike monoplastidic cell division in hornworts, mosses, and lycops ids; microtubule nucleation is not intimately associated with the plas tids. Instead, microtubule organization is associated with centriolar centrosomes throughout the cell cycle, as is common in algae. The cyto kinetic apparatus lacks preprophase bands of microtubules, but include s typical phragmoplasts consisting of brushlike arrays of microtubules on either side of a dark zone. However, the origin and role of phragm oplasts is unusual. Phragmoplasts appear to develop among microtubules that emanate from the polar centrosomes rather than from nuclear enve lopes and/or plastids. The function of phragmoplasts in Coleochaete is unclear, as the process of cytokinesis is not strictly centrifugal. S ome infurrowing occurs in radial division, and cytokinesis appears to be entirely centripetal by infurrowing in circumferential division. Th e cortical arrays of microtubules differ from those typical of land pl ants in that they develop as a network in association with centrosomes after mitosis.