Cell Biology International Reports, Vol. 7, No. 8, August 1983 611 RESPONSES OF DICTYOSTELIUM DISCOIDEUM AMOEBAE TO LOCAL STIMULATION BY LIGHi D.-P. Haderl, M. Claviezz, R. Merkl2 and G. Gerischz 1 Philipps-Universitat Marburg, Fachbereich Biologie-Botanik, Lahnberge, D-3550 Marburg, Germany and 2Max-Planck-Institut fur Biochemie, D-8033 Martinsried, Germany ABSTRACT Single amoebae of Dictyostelium discoideum were locally stimulated with microbeams of white and monochromatic light. Low illuminance sti- mulation favored formation of pseudopodia at the irradiated parts of the cells, hi-gh illuminance stimulation locally suppressed the extension of pseudopodia. When the high illuminance light spot was placed on any portion of the cell other than the moving front, no response could be observed. The results are compatible with the assumption that, during their phototactic response, single amoebae detect the direction of light by a shadowing effect caused by pigments like cytochromes, and/or by light scattering of particles in the cytoplasm. INTRODUCTION After aggregation, cells of Dictyostelium discoideum form a slug or pseudoplasmodium, a cylindrical body consisting of about 105 cells. The slug migrates on the surface of a substratum such as agar, and orientates towards a light source over a wide range of light intensities (Bonner et al., 1950; Francis, 1964; Poff and Loomis, 1973). The phototactic response of the slug is based on a lens effect (Francis, 1964; Hader and Burkart, 1983). Laterally impinging light is focussed on the distal side of the cylinder since the cells comprising the slug have a higher refractive index than the air surrounding the slug. It has been suggested that the cells on the brighter distal side release a chemical factor into the environment, to which they respond by negative chemotaxis, and that this effect is the basis of the turning of the slug towards the light source (Fisher and Williams, 1981; Fisher et al., 1981). Before aggregation, D.discoideum cells live as single amoebae which show positive phototaxis in low illuminance (Hader and Poff, 1979a) and negative phototaxis in higher illuminance (Hader and Poff, 1979b). The action spectra of the positive and negative responses of single amoebae are 0309-I 651/83/08061 l-06/$03.0010 @ 1983 Academic Press Inc. (London) Ltd.