Responses of Dictyostelium discoideum to Multiple Environmental Stimuli

The movement responses of the cellular slime mold Dictyostelium discoideum to multiple stimuli were investigated. The responses were found to differ depending on the developmental stage of the organism. A novel response, positive gravitaxis, was found in Dictyostelium slugs but not in amoebae. In the presence of a simultaneous light stimulus, gravitaxis is effective only at low fluence rates. Slugs showed positive thermotaxis in a thermal gradient (0.2 °C cm^?1) and ignored the simultaneous light stimulus at low fluence rates (< 10^?3 W m^?2), while at higher fluence rates they moved toward the light source. With a combination of a thermal gradient and gravity Dictyostelium slugs clearly oriented thermotactically ignoring the gravistimulus.