PHOTOTACTIC ORIENTATION IN DICTYOSTELIUM-DISCOIDEUM AMEBAS

Amoebae of Dictyostelium discoideum strain AX-2 have been found to show a precise phototactic orientation only during the stationary phase of the growth while they did not respond to light in the early logarithmic phase. The sensitivity was not as high as in slugs and the threshold in white light was found at about 10(-4) W m-2. The cells show a positive phototaxis at low rates and a negative one at higher fluence rates; the cross over point was found at about 10-1 W m-2. Toward the end of the stationary phase the negative phototaxis to a high irradiance (62 W m-2) reversed into a positive phototaxis, while the positive phototaxis (1.6 x 10(-2) W m-2) remained stable during the whole stationary phase. Artificial ultraviolet radiation (from a transilluminator) drastically affected photoorientation and motility in Dictyostelium discoideum amoebae within about 15 min.