A Genetic Dissection of the Photophobic Response of Paramecium tetraurelia

Paramecium tetraurelia displayed two behavioral responses upon the initiation of a light stimulus at 7 x 104 lux. The cells exhibited a photophobic response in the form of behavioral avoiding reactions, followed by an increase in forward swimming velocity that was significantly higher than prior to the light stimulus activation. It was determined that an intensity of approximately 6.5 x 103 lux was required to initiate a moderate avoidance behavioral response. Following the avoiding response, a gradual increase in speed occurred as the intensity increased, indicating that increased swimming speeds are dependent on the light intensity. Two mutants, pawnA and Dancer, were utilized since they affect known Ca2+-currents of the cell. The use of pawnA cells, which lack voltage-dependent Ca2+ channel activity, showed that the two responses to light could be genetically separated, in that the cells showed no avoiding reactions, but did increase their swimming speed. The Dancer cells, which display exaggerated Ca2+ channel activity, exhibited similar initial avoiding responses as the wild type cells, however did not increase their swimming speed as the intensity of the light was increased. This phenotype as replicated in wildtype cells that had been placed in 25 μM 8-Br-cGMP. These data demonstrate that the photophobic light response of Paramecium tetraurelia can be genetically dissected as a means of elucidating the molecular mechanisms of the light response.