Warm Microgradients Elicit Adaptive Behavior in Isotropically Cooled, Inert Populations of Oxytricha bifaria (Ciliophora, Hypotrichida)

Not-adaptive behavior of isotropically heated, inert populations of Oxytricha bifaria (Ciliophora, Stichotrichia)

The physiological effects on isotropically heated populations of Oxytricha bifaria cultured at 24 degrees C were investigated. At 34.6 degrees C ciliates became inert, and did not adaptively react to either cold or warm microgradients; they neither moved towards the favorable cold thermal source nor escaped from the unfavorable warm one. The inert oxytrichas were only able to perform the Side-Stepping Reaction (SSR) on the same spot. However, mobile ciliates at 31.6 degrees C reacted to the cold microgradient by immediately orienting themselves towards its source, without accelerating but reducing their SSR frequency. Moreover, in a warm microgradient such ciliates immediately increased their SSR frequency, then moved away from the thermal source. At 34.6 degrees C the behavior of ciliates was not-adaptive--not acting to guide the organisms to more favorable conditions--whereas at 31.6 degrees C it was still clearly adaptive. Therefore, the locomotory inertness of the oxytrichas at 34.6 degrees C was the result of thermal stress rather than their behavioral response to the environmental isotropy, in contrast to populations of the same species made inert at 9 degrees C.

Cold microgradients elicit adaptive behavior in isotropically cooled, inert populations of Oxytricha bifaria (Ciliophora, Hypotrichida)

To complete our investigations on the oriented behavioral response of isotropically cooled, inert populations of Oxytricha bifaria to a warm thermal gradient, their physiological potentialities under cold microgradient conditions arising at 8.5 degrees C were studied. We monitored the behavior of the experimental populations, both at the level of the passing cold wave front, and afterwards when the thermal gradient stabilized, evaluating (i) their distribution in general, (ii) their relative centroids, (iii) the percentage of both backward creeping and immobile ciliates, and (iv) the numerical indices and rates of their creeping tracks. At the arrival of the cold wave front, the oxytrichas react immediately to the thermal stimulus, creep backwards at very high velocity along uninterrupted linear tracks, and thus move away from the cooling source. No specific behavioral response was ever observed in the static microgradient conditions. At 8.5 degrees C, despite their inertness, the ciliates are still able to behave adaptively, reacting immediately and orientatedly, once a directional factor (the cold-repelling thermal gradient) arises in an isotropic environment. This is similar to their behavior in the symmetric warm attracting thermal gradient.

The effects of heating on the behaviour ofOxytricha bifaria(Ciliophora, Hypotrichida)

Experimental populations of the ciliate Oxytricha bifaria grown at 24°C were exposed to progressively higher temperatures in a thermally isotropic environment. Their behaviour was monitored and an ethogram was drawn for each step of the warming process (24, 29, and 34°C), as well as for three further periods of time after the temperature was brought back to the initial 24°C. As the temperature increased, (i) the percentage of mobile cells decreased (significantly at 34°C), (ii) their periods of immobilisation were more frequent; (iii) their average creeping velocity did not change; (iv) the radius of the arcs and the length of both the linear segments and the arcs decreased; (v) the frequency of the "side-stepping reaction" increased progressively, and (vi) it was often performed as bursts of reactions. Moreover, (vii) a new behavioural pattern, the "maximum-rotation reaction," was performed. When the ciliates were brought back to 24°C, their behavioural parameters, except for general mobility, did not return to normal values during the first 90 min. Linear heating of the experimental populations of O. bifaria induced nonlinear and time-stable effects on their behaviour (12 out of 17 parameters actually changed at 34°C).