Light and Electron Microscopic Observations of Mitochondrial Fusion in Plasmodia Induced Sporulation in Physarum polycephalum

抄録

The morphological behavior of cell nuclei, mitochondria and mitochondrial nuclei during sporulation in Physarum polycephalum was investigated using light and electron microscopy. The plasmodia which reactivated from sclerotia on non-nutrient agar plates preferentially entered into sporulation. A night vision camera and time lapse VTR were available to monitor morphological changes and determine the exact time course. Sporulation processes were classified into six stages, by morphological characteristics. The initial event of sporulation was characterized by the formation of many protoplasmic knobs along plasmodium strands (protoplasmic balling stage). A papilla was projected from the apical region of each protoplasmic knob, elongated and changed to form a stalk (stalk elongation stage). The apical part of the stalk then began to expand until it reached a maximum point of stalk elongation, thereafter differentiating into the head of the sporangium (head expanding stage). After a stage without morphological changes (resting stage), the color of the sporangial head gradually changed from pale yellow to black (head blackening stage). After full blackening of the sporangial head, sporangia were completed (mature stage). 4'6-diamidino-2-phenylindole (DAPI) epifluorescence microspectrophotometry showed that the cell nuclear DNA content in sclerotia and starved plasmodia before the resting stage was the same as that in somatic diploid G2 phase nuclei. Only a single somatic nuclear division occurred at mid-resting stage. Meiotic DNA synthesis then occurred during the late resting and head blackening stages, when cytoplasmic cleavages occurred. After DNA synthesis, cell nuclei persisted in a long G2 phase of about one day and stopped at meiotic prophase I. In sclerotia and starved plasmodia, mitochondria were spherical or oval and contained one electron-dense mitochondrial nucleus (mtnucleus). Mitochondria and mt-nucleus became smaller in size in starved plasmodia. The DNA content per mitochondrial nucleus at that time was reduced by half as compared with that at the early starved plasmodium stage. Dumbbell-shaped mitochondria appeared during the resting stage; these had two mt-nuclei, each located discretely at either side of the dumbbell-shaped mitochondrion. Mitochondria containing 3-6 nuclei were also observed, at low frequency; multinucleated mitochondria persisted up to meiotic prophase I. The mitochondria in amoebae which originated from spores contained only one mt-nucleus. These results indicate that mitochondrial nuclear fusion occurs at some stage between spore germination and the amoeba stage, following mitochondrial fusion at the resting stage of sporulation in Physarum polycephalum.