Graviresponses of certain ciliates and flagellates

Protozoa are eukaryotic cells and represent suitable model systems to study the mechanisms of gravity perception and signal transduction due to their clear gravity-induced responses (gravitaxis and gravikinesis). Among protis ts, parallel evolution for graviperception mechanisms have been identified: either sensing by distinct state-organelles (e.g., the Muller vesicles of the ciliate Loxodes) or by sensing the density difference between the whole cytoplasm and the extracellular medium (as proposed for Paramecium and Eug lena), These two models are supported by experiments in density-adjusted me dia, as the gravitaxis of Loxodes was not affected, whereas the orientation of Paramecium and Euglena was completely disturbed. Both models include th e involvement of ion channels in the cell membrane. Diverse experiments gav e new information on the mechanism of graviperception in unicellular system s, such as threshold values in the range of 10% of gravity, relaxation of t he responses after removal of the stimulus, and no visible adaptation pheno mena during exposure to hypergravity or microgravity conditions for up to 1 2 days.