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Plant and Cell Physiology, 1986, Vol. 27, No. 3 533-539
© 1986

Article

An Optical Model for Phototactic Orientation in Dictyostelium discoideum Slugs

Kenneth L. Poff1, Donna R. Fontana1, Donat-P. Häder2 and Michael J. Schneider3

1Department of Energy, Plant Research Laboratory, Michigan State University East Lansing, Michigan 48824, U.S.A.
2Fachbereich Biologie-Botanik der Universität Marburg Lahnberge, D-3550 Marburg, Federal Republic of West Germany
3Department of Natural Science, The University of Michigan-Dearborn Dearborn, Michigan 48128, U.S.A.

The optical properties of an enlarged optical model of the tip of a slug (a glass tube filled with sucrose solution) of Dictyostelium discoideum were studied to validate the hypothesis that the organism orients itself with respect to lateral light using a lens effect: Light is focussed by refraction onto the distal side of the slug where the higher "signal" compared to the front side causes a turn toward the light. Light was effectively focused even when the model deviated only as little as 10° from the light direction.

Slugs of D. discoideum strains HO596 and HO813 show bidirectional phototaxis: They orient themselves at some angle on either side of the incident light beam. This angle is {small tilde}80° in HO596 and {small tilde}70° in HO813. We suggest that this phenomenon can be explained by the optical properties of the slug such as absorption and scattering, shape of the tip or location and extension of the light-sensitive zone. The angle of orientation with respect to light is regulated by a balance between a turn toward and a turn away from the light direction.

(Received October 15, 1985; Accepted February 10, 1986)
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