Numerical, morphological and topographical heterogeneity of the chondriome during the vegetative life cycle of Polytoma papillatum

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Abstract
Numerical, morphological, and topographical changes in the mitochondrial inventory (= chondriome) during the vegetative (= asexual) life cycle of Polytoma papillatum were examined by means of the serial-sectioning technique. At the onset of interphase the chondriome consists of primarily of one highly reticulated basket-shaped complex, which lines the periphery of the cell. Up to 3 additional fragments could be analysed, which, however, were much smaller and either spherical to ovoid, or elongated and poorly branched. During interphase growth both the size of the mitochondrial basket and the number of additional fragments (ca. 40) increases. During mitosis I (I. division cycle) the multiplication of the number of mitochondria gradually continues; the maximum (ca. 250) was counted at late telephase I and early cytokinesis I, but it cannot be excluded that the sparser short-lived meta- and anaphase cells, which were unfortunately missed, contain an even higher number of mitochondrial units. Concomitant with mitosis I, the mitochondrial basket is subdivided into several fragments; at the same time the mitochondria tend to form clusters and to occupy no longer merely peripheral but also more central regions. An irregular distribution of the chondriome results, with either local accumulations or local absence of mitochondria within definite regions of the cell. After completion of cytokinesis I the total number of mitochondria (ca. 80) is drastically reduced but it seems to increase again during the 2. division cycle. Irrespective of the number of preceding division cycles, the chondriome of young daughter cells enclosed in the sporangium usually resembles that of young interphase cells. Our results are discussed in relation to some of the current problems on mitochondriogenesis.