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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 3143

Hans Buchner; [Untersuchungen uber die Bedingungen der heterogonen Fortpflanzungsarten bei den Radertieren IV. Uber die Reaktivierung der miktischen Potenz bei Brachionus urceolaris] (Studies on the Control of Heterogonous Reproduction in Rotifers IV. The Reactivation of Mictic Potential in Brachionus urceolaris). Zool.Jb.Physiol. 96:97-165, 1992

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The mictic potential denotes the capacity of amictic eggs to react to mixis-inducing environmental conditions. This potential of three clones (A1, W1, and W2) of Brachionus urceolaris was lost step by step when the animals kept in mixis-inducing conditions (20 degrees C) for long time. Finally, they were completely amictic. But in a mixis-suppressing environment (10 degrees C), they slowly regained their mictic potential. It is the aim of this paper to describe the quantitative relations between the time of exposure to low temperature (10 degrees C) and the reactivation of the mictic potential. The animals of clone A1 are almost completely amictic at 10 degrees C. A transfer to 20 degrees C causes a mictic reaction: After a short time, the percentage of mictic females increases to a maximum and then decreases again to lower values. These percentages measure the mictic potential, which is determined by the time, they were kept at 10 degrees C. When the cultures were continued at 20 degrees C, the mictic percentage reaches zero. The time needed until this happens, is again determined by the duration of preceeding culture at 10 degrees C. But this relation is not very strict. Therefore the time the animals were kept at 10 degrees C cannot be attributed to definite percentages or definite rates of their decrease: The process of reactivation is very variable. So the different experiments agree only qualitatively, but not in the precise percentages and times. When the animals of the two W-clones were continuously cultivated at 20 degrees C, they became purely amictic, too. But here, contrary to A1, mictic females appeared immediately after the transfer of these amictic females to 10 degrees C. The percentage of mictic females stayed rather high for some years. The physiological mechanism causing this mixis could not be clarified as yet. In any case, it is not identical with the mictic potential, which admits mixis in warm environment. For, if amictic females were transferred back to warm environment during the first 4 weeks of cold cultivation, then they and all their descendants were unable mixis, although their sisters continuously produced mictic daughters at 10 degrees C. Only after 4 weeks of cold cultivation, the rise of temperature induced a weak reaction that increased with the duration of the cultivation at 10 degrees C. Also the time needed for the clones to become amictic after the first reaction, depends on the duration of the preceeding cold cultivation. As for A1, the mictic potential was very variable. So parallel experiments run under the same conditions gave quantitatively different results. A fourth clone A4 continuously produced mictic females although it was cultivated at 20 degrees C for years. So contrary to other clones, it did not lose its mictic potential. At 10 degrees C, the animals became amictic and could produce mictic daughters immediately after they were put back into warm environment. The material basis of the mictic potential is not known. Its variability is controlled by external conditions. This shows that it is not located in the chromosomes, but in the plasma. Each of its states may be transferred to the daughter cells. Also at every mictic reaction, the mictic potential is changed, as two antagonistic processes occur and generate the sinuoidal shape of the reaction. The mictic determination is a process, which sets up the mictic inductor in several steps by means of the energy supplied by an external impulse. The intermediate steps admit only ameiotic oogenesis. Only the final step stimulates meiosis. As the intermediate steps can be inherited, the determination may drag on over some generations even without a new impulse. It seems that occasionally it can start also spontaneously, i.e. without an external impulse.