Abstract
The progressive modification of the photoreceptor apparatus, induced by addition of streptomycin (SM) to cultures of darkβbleached Euglena, is correlated with the change of the photomotor response. One of the effects of drugtreatment is the reduction of carotenoid synthesis; the total carotenoid content decreases from 0.20 to 0.01 pg/cell after seven weeks of treatment. The parallel progressive reduction of the stigma vesicles and their carotenoid content is observed by electron microscopy. Micrographs and microscopic fluorescence observations do not indicate alterations of the paraflagellar body (PFB). During the SMβtreatment the initial positive phototactic response decreases until it disappears after five weeks.
The loss of the absorbing properties of the stigma prevents the cell from orienting itself towards the light. The probability that the cell orients seems to depend on the light modulation amplitude and the light/dark ratio on the PFB due to the shading device. After seven weeks of SMβtreatment we observe a cell movement away from the illuminated zone. The mechanism of this escape is discussed.