Gravikinesis in Stylonychia mytilus is based on membrane potential changes

Abstract

The graviperception of the hypotrichous ciliate Stylonchia mytilus was investigated using electrophysiological methods and behavioural analysis. It is shown that Stylonchia can sense gravity and thereby compensates sedimentation rate by a negative gravikinesis. The graviresponse consists of a velocity-regulating physiological component (negative gravikinesis) and an additional orientational component. The latter is largely based on a physical mechanism but might, in addition, be affected by the frequency of ciliary reversals, which is under physiological control. We show that the external stimulus of gravity is transformed to a physilogical signal, activating mechanosensitive calcium and potassium channels. Earlier electrophysical experiments revealed that these ion channels are distributed in the manner of two opposing gradients over the surface membrane. Here, we show, for the first time, records of gravireceptor potentials in Stylonchia that are presumably based on this two-gradient system of ion channels. The gravireceptor potentials had maximum amplitudes of approximately 4mV and slow activation characteristics (0.03 mV/s-1). The presumptive number of involved graviperceptive ion channels was calculated and correlates with the analysis of the locomotive behaviour.