DO PHOTOTACTIC CILIATES MAKE USE OF DIRECTIONAL ANTENNAS TO TRACK THEDIRECTION OF LIGHT

Phototaxis describes the directed movement of motile microorganisms wi th respect to the direction of light. In ciliates, phototactic orienta tion has been reported for more than a dozen species. Some, like Stent er and Blepharisma, are brightly coloured due to their numerous pigmen t granules. In Chlamydodon, a stigma appears in conjunction with a beh avioural change from negative to positive phototaxis. The histophagous ciliates Ophryoglena and Porpostoma, which either possess a watchglas s organelle or a composed crystalline organelle, display different pho totactic behaviour at different stages of their life cydes. It has bee n suggested that the characteristic pigments and cell organelles of ph ototactic ciliates play important roles in the detection of the light direction. Bleached cells with a reduced number of pigment granules, t ransparent cells without a stigma, and cells that lacked a watchglass organelle due to micromanipulation were found to be little sensitive t o light, or showed inaccurate or no phototactic orientation. While the pigment granules of the heterotrichs presumedly contain the photorece ptor molecules mediating photo-orientation, the stigma of Chlamydodon, the watch-glass organelle of Ophryoglena, and the composed crystallin e organelle of Porpostoma are suspected shading or light reflecting st ructures. Interacting with the sites of sensory transduction, which sh ould be located in the cell membrane nearby, but are unidentified, con spicuous organelles could be used by the cells as direction-sensitive light detectors.