Morphogenesis of the plastidome and the flagellar apparatus during the vegetative life cycle of the colourless phytoflagellate Polytoma papillatum.

Serial-section reconstructions were used to study changes in morphology of the leucoplast and of flagellar apparatus during the vegetative life cycle of Polytoma papillatum. During interphase growth the volume of highly perforated leucoplast nearly doubled. The leucoplast of late interphase cells still had a deep cup shape (cyathiform), but several structural modifications were observed compared to the leucoplast of young interphase cells: either a thin and less perforated sheet-like area, taking up ca 25% of the external surface, and several double-layered areas occur sporadically at the tube-shaped part of the cup, or a bladder-shaped structure consisting of several very thin concentrically ringed layers occurred at the base of the tube. During preprophase the cyathiform leucoplast was transformed into a massive bowl-shaped (= poculiform) leucoplast, which was also characterized by thin-layered bladder-like or concentrically-ringed structures and by duplications in the form of pockets or chambers. The bipartition of the poculiform leucoplast occurred by furrowing, which started at preprophase and was completed only at the end of cytokinesis. The dynamics of the processes involved in these configural changes are discussed. During preprophase of the first division cycle the flagellar apparatus was subjected to several changes. The apical papilla which gave rise to the flagella was tied off into small 'extracytoplasmic' vesicles. This event caused a change in the orientation of the flagella to one another; the angle of ca 140 degrees during interphase was reduced to ca 90 degrees. The basal bodies detached from their axonemes and both the striated connection and the roots disintegrated. During prophase of the second division cycle the separation of the four basal bodies into two pairs and their migration towards the poles of the nucleus was completed; the outgrowth of the new daughter flagella also started. During late cytokinesis of the second division cycle the basal bodies had already duplicated and resynthesis of the striated connection was completed, whereas resynthesis of the roots was still in progress. The results are compared with those obtained from investigation of other algae.