The changes in ultrastructure during fertilization of the colourless flagellate Polytoma papillatum with special reference to the configural changes of their mitochondria.

Changes in the morphology of the mitochondrial inventory (= chondriome), the nucleus and the flagellar apparatus during the generative (sexual) life cycle of Polytoma papillatum were examined by means of the serial sectioning technique. At the onset of copulation gametes do not differ obviously from interphase cells of the vegetative (asexual) life cycle, in that, both primarily contain one basket-shaped mitochondrion. The quadriflagellate and binucleate zygote exhibits a chondriome which consists of one large highly reticulated basket at the periphery of the zygote and 33 smaller mitochondrial units. Therefore, the basket clearly results from fusion of the two gamete chondriomes. The smaller mitochondrial fragments are either spherical to ovoid or elongated and poorly branched; they tend to occupy more central regions of the zygote. After karyogamy the mitochondrial basket disintegrates into several fragments of various shapes and sizes. Most of the mitochondrial fragments are located at the periphery. At the onset of karyogamy the nuclei and the flagellar apparatuses do not differ significantly from those of the gametes and vegetative interphase cells. The diploid nucleus, however, is characterized by: 1. many spherical bodies (diameter: ca. 200 to 600 nm) which are found both in the nucleoplasm and in the nucleolus. The major part of these bodies consists of material whose ultrastructure resembles that of the "pars fibrosa" in the nucleolus; 2. three deep invaginations of the nuclear membrane, two of which extend to the nucleolus; 3. an increase of nucleoplasm-filled cavities in the nucleolar "pars granulosa". The four flagella are considerably shortened; the basal bodies bound to the flagella have lost their striated connection and the roots have nearly completely disappeared. The results are compared with those obtained from investigations in Chlamydomonas; their significance in extranuclear genetics and in the systematics of Volvocales is discussed.