Ref ID : 783
Hans Buchner; Studies on the control of heterogeneous reproduction in rotifers III. The loss of mictic potential in Brachionus urceolaris. Arch.Hydrobiol. 109(3):333-354, 1987
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The mictic potential of heterogonic rotifers, i.e. the strength with which they react to mixis-inducing environmental conditions, shows a wide range of variation between individuals of the same clone. The aim of this study was to determine the causes of this variation. Two clones of Brachionus urceolaris were isolated from each of two ponds a few km apart: A1, A4 and W1, W2, respectively. To test whether genetically identical individuals react identically to the same conditions, clone A1 was split into three lines, clone W1 into two. At 10 degrees C, A1 and A4 were almost exclusively amictic whereas W1 and W2 always produced both amictic and mictic females, the latter often at a high rate. Shifting the temperature to 20 degrees C elicited mixis in both A clones: after low rates of mixis at the beginning, they gradually reached a maximum which was followed by a decrease again. The W clones showed the same reaction but usually at lower rates. When the cultures were continued at 20 degrees C, mixis rates fluctuated irregularly in all clones. But in W1 and W2, The rates gradually decreased until all animals reproduced by parthenogenesis. In A1, this gradual decreases also occurred but much slower than in the W clones, and there were always some rare mictic females. In two successive series of experiments the time needed to reach the final amictic situation proved to be different: In the first series it took several months, in the second only a few weeks (A clones) or a few days (W clones). The difference was related to the duration of culture at 10 degrees C prior to the temperature shift: For the first series, the cold period had lasted for several years (8.5 for A1, 6 for W1 and W2), for the second series only months (3 for A1, & 0.5 for W1 and W2). Since later experiments gave the same results, it is concluded that the mictic potential of these clones is not a constant but depends on temperature: at continued warm temperature, the mictic potential is gradually lost, in the cold it is reactivated again. The changes are reversible and graded. There were distinct differences between A1 and both W clones: in the latter, the change in mictic potential was always stronger and occurred more rapidly. The often high proportion of mictic females in the 10 degrees C-cultures of W1 and W2 is probably due to this strong increase of the mictic potential which thus may bring about a determination to mixis at this low temperature. Clone A4 behaved differently: Individuals were (almost) completely amictic for years when kept at 10 degrees C but reacted strongly to a shift to 20 degrees C. Continued culture at 20 degrees C never led to a decrease of the rate of mixis. In other words, there was no weakening or loss of the mictic potential as in the other clones. Each clone maintained its characteristic type of reaction throughout the years of observation. Therefore it is probable that the mictic potential has a definite generic basis. Nevertheless, in many instances there were significant differences in strength and speed of reaction between the various lines of A1 and W1. Since the lines of each clone were genetically identical and also the environmental conditions were the same, it is assumed that the mictic potential was under the influence of additional modifying variables of unknown origin.