Morphodynamical Changes of the Chloroplast of Chlamydomonas reinhardtii During the 1st Round of Division

Abstract

Summary: Morphological changes of the chloroplast of Chlamydomonas reinhardtii were examined by means of the morphometry of ultrathin serial sections through cells at definite stages of development during the 1st round of division of the vegetative (asexual) life cycle. This data enabled us to determine factors expressing the extent of complexity of the plastide shape (= degree of differentiation). A maximum degree was observed during mitosis. Additional 3Dreconstruction of chloroplasts from cells at the most characteristic stages of this developmental cycle clearly illustrated that the increase in the degree of differentiation is mainly attributable to an Increase in the total number of perforations piercing the lateral part of the “cup-shaped” (during interphase) to “bowl-shaped” (during mitosis) chloroplast. These models confirmed that chloroplast division (= plastokinesis) not only starts prior to karyokinesis (Ettl 1976), but also precedes cytokinesis. The percentage 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” transforms into a bilateral symmetrical configuration. This metamorphosis is accompanied by a relocation of the daughter plastide mass toward the future base of the daughter cell. Owing to this and the cytokinesis-specific relocation of the daughter basal apparatuses (Gaffal et al. 1993), the new polarities (apicobasal axes) of the daughter cells, which are both perpendicular to the original polarity of the parent cell and to one another, begin to be established.