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 showed improved orientation away from the centre of gravity (negative gravitaxis) when accelerations were increased from 1 to 7 g. Gravitaxis was more pronounced in the mutant. A correlation between the efficiency of orientation and the applied g value suggests a physical basis for gravitaxis. Transiently enhanced rates of reversal of the swimming direction coincided with transiently enhanced gravitaxis because reversals occurred more often in downward swimmers than in upward swimmers. The results provide evidence of a physiological modulation of gravitaxis by means of the randomizing effect of depolarization-dependent swimming reversals. Gravity bimodally altered propulsion rates of wild-type P. tetraurelia so that sedimentation was partly antagonized in upward and downward swimmers (negative gravikinesis). In the mutant, only increases in propulsion were observed, although the orientation-dependent sensitivity of the gravikinetic response was the same as in the wild-type population. Observed swimming speed and sedimentation rates in the wild-type and mutant cells were linearly related to acceleration, allowing the determination of gravikinesis as a linear (and so far non-saturating) function of gravity.