Physical and physiological components of the graviresponses of wild-type and mutant Paramecium tetraurelia

Wild-type and the morphological mutant kin 241 of Paramecium tetraurelia sh owed improved orientation away from the centre of gravity (negative gravita xis) when accelerations were increased from 1 to 7 g, Gravitaxis was more p ronounced in the mutant. A correlation between the efficiency of orientatio n and the applied g value suggests a physical basis for gravitaxis. Transie ntly enhanced rates of reversal of the swimming direction coincided with tr ansiently enhanced gravitaxis because reversals occurred more often in down ward swimmers than in upward swimmers. The results provide evidence of a ph ysiological modulation of gravitaxis by means of the randomizing effect of depolarization-dependent swimming reversals. Gravity bimodally altered prop ulsion rates of wild-type P. tetraurelia so that sedimentation was partly a ntagonized in upward and downward swimmers (negative gravikinesis), In the mutant, only increases in propulsion were observed, although the orientatio n-dependent sensitivity of the gravikinetic response was the same as in the wild-type population. Observed swimming speed and sedimentation rates in t he wild-type and mutant cells were linearly related to acceleration, allowi ng the determination of gravikinesis as a linear (and so far non-saturating ) function of gravity.