Behaviour of proplastids and their nucleoids in dark-organotrophically grown cells ofEuglena gracilistransferred to an inorganic medium

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Abstract
Euglena gracilis Z cells in the stationary phase of dark-organotrophic growth in a culture without agitation were rich in lipid, and contained six to eight proplastids dispersed in the cytoplasm (A-type proplastids). When these cells were transferred to an inorganic medium and aerated in darkness, they showed, with the disappearance of lipid, diphasic increase in number after an induction phase, concurrently with the development of prolamellar bodies and some primary thylakoids in their proplastids. The proplastids and their nucleoids, examined under a fluorescence microscope with and without staining with the DNA fluorochrome, 4′,6-diamidino-2-phenylindole, showed dynamic and sequential morphological changes. Before the first cell doubling, the A-type proplastids gathered to form three to four aggregates (B-type), that were themselves connected into a single elongated form with intricate windings and branchings (C-type). The nucleoid in the C-type proplastid was present as a branched string winding throughout the proplastid body. The C-type proplastid temporarily took a compact form surrounding the nucleus (D-type), and its nucleoid also showed a compact form, some parts coming close to the nucleus. Shortly before the cell division, the C-type divided into six to eight B-type proplastids, which subsequently were partitioned in dividing cells and soon coalesced into the C-type again. In that part of the cell population that performed the second cell division, the sequence of C-(D-C)-B-C seemed to be repeated. Electron microscopy revealed that some parts of the D-type proplastid were in contact with the nuclear envelope, where a chromosome was attached to the inner membrane. In late-appearing D-type proplastids, the primary thylakoids developed were localized in a region close to the site of contact with the nucleus. Possible relations of these observations with the Euglena cell division cycle in darkness and with the proplastid development were discussed.