Optical properties of Dictyostelium discoideum pseudoplasmodia responsible for phototactic orientation

Abstract

Pseudoplasmodia of Dictyostelium discoideum detect the light direction measuring an internal, nonuniform distribution of light. The organism acts as a cylindrical lens which focuses parallel lateral light onto the distal flank of the organism. The focal band can be demonstrated optically and can be calculated mathematically based on geometrical optics. The refractive index of homogenized pseudoplasmodia is 1.369 ± 0.001 (SEM), while the calculated refractive index, based on measurements of the focal bands and using an iterative technique, is 1.371 ± 0.001 (SEM). The nonuniform internal light distribution produced by the lens effect can be reversed by incorporating an internal absorbing screen pigment, neutral red, into the pseudoplasmodia or by focusing the laterally impinging parallel light onto the proximal flank, both of which cause a negative phototactic orientation.