Ref ID : 3725
Guy Morat; [Variations Quantitatives des Teneurs en ADN Nucleaire lors de la Reproduction Vegetative chez le Cilie Ophryoglena catenula Savoie, 1966] (Quantitative Variations of the Nuclear DNA Content during Vegetative Reproduction in the Ciliate Ophryoglena catenula Savoie, 1966). Arch.Protistenk 125:137-161, 1982
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The vegetative reproduction in Ophryoglena catenula includes a primary cycle triggered by feeding of the theront and during which the cell becomes encysted and divides, and a secondary cycle induced by starvation. In large populations the two cycles, which involve important modifications of the nuclear apparatus, frequently overlap due to asynchronous evolution of the cells in one vegetative line. 1. During the primary cycle, about 24 hr long at 21 degrees C, the successive stage: theront, trophont, tropho-tomont, tomont, tomites, generally lead to four free swimming primary theronts (exceptionally 6 or 8) which, fed again, undergo another primary cycle. Cytophotometric and autoradiographic studies on isolated and Feulgen stained nuclei and observation of micronuclear divisions show that: a) The very long macronuclear DNA synthesis, initiated in the tropho-tomont, lasts until the second bipartition of the tomont. Thus the macronuclear DNA synthesis progresses along with the digestive process. Two replications of the DNA occur during the primary cycle, but more than a two-fold increase is reached in the tomont still before the first bipartition. b) On the other hand, two complete micronuclear cycles, well separated and of unequal duration, take place. c) The large variability of the pre- and postsynthetic macronuclear DNA contents seems mainly related to the unequal distribution of chromatin between the sister cells during the two successive bipartitions of the tomont. There is no indication for a regulation of these contents, either by a discriminating synthesis in the theronts (extra-S phase when a low value is reached or partial replication from a high value), or by a periodic occurrence of a variable number of theronts during successive generations. The apparent lack of such regulatory mechanisms can probably be related to the absence of any macronuclear chromatin extrusion in the species. However, appropriately fed theronts, issued from the first secondary encystment (or from many encystements in the nature) and with low macronuclear DNA contents, seem to undergo an extra replication phase during the next cycle. 2. When the primary theront is starved, it undergoes secondary successive encystments from which secondary theronts (or abnormal cells) issue with more and more reduced size and growth ability. A heterogeneous population issued from the first encystment shows independent behaviours of micronuclei and macronucleus: a) A clear micronuclear vegetative activity takes place, including DNA replication which can be followed by asynchromous division of the micronuclei in some cells. b) On the contrary, the macronuclear DNA content decreases, a large proportion of primary theronts undergoing a last bipartition within the secondary cyst, without any previous DNA synthesis; this leads to a macronuclear "depolyploidization". In abnormal cells, the macronuclear changes and the lack of DNA synthesis can be related with the cortex and oral structure deficiencies. 3. The likely absence of sexuality and the periodic decay of cell lines let suppose the existence of a senescence phenomenon. 4. Finally this study underline a noteworthy likeness of the Ophryoglenidae and Colpodidae biological cycles.