MORPHODYNAMICAL CHANGES OF THE CHLOROPLAST OF CHLAMYDOMONAS-REINHARDTII DURING THE 1ST ROUND OF DIVISION

Morphological changes of the chloroplast of Chlamydomonas reinhardtii were examined by means of the morphometry of ultrathin serial sections th rough cells at definite stages of development during the Ist round of division of the vegetative (asexual) life cycle. This data enabled us to determine factors expressing the extent of complexity of the pl astide shape (= degree of differentiation). A maximum degree was obser ved during mitosis. Additional 3D-reconstruction of chloroplasts from cells at the most characteristic stages of this developmental cycle cl early illustrated that the increase in the degree of differentiation i s mainly attributable to an increase in the total number of perforatio ns piercing the lateral part of the ''cup-shaped'' (during interphase) to ''bowl-shaped'' (during mitosis) chloroplast. These models confirm ed that chloroplast division (= plastokinesis) not only starts prior t o karyokinesis (ETTL 1976), but also precedes cytokinesis. The percent age of the chloroplast in the cell volume (51 +/- 3%; n = 14) remains constant during mitosis; and this organelle mass is apportioned to the daughter cells in equal amounts. As early as at late cytokinesis the asymmetrical configuration of each half of the bisected ''bowl'' trans forms into a bilateral symmetrical configuration. This metamorphosis i s accompanied by a relocation of the daughter plastide mass toward the future base of the daughter cell. Owing to this and the cytokinesis-s pecific relocation of the daughter basal apparatuses (GAFFAL et al. 19 93), the new polarities (apicobasal axes) of the daughter cells, which are both perpendicular to the original polarity of the parent cell an d to one another, begin to be established.