, Volume 126, Issue 1, pp 15-24

Electrophysiological characterization ofDrosophila ocelli

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Drosophila have 3 simple eyes, ocelli, located on the vertex of the head, in addition to 2 compound eyes. We determined ocellar function using the electroretinogram (ERG) and vision mutants.

The ERG waveform in response to a 1 s stimulus was a slow negative (receptor depolarizing) potential followed by a return to baseline at light-off (Fig. 1). During long stimuli there was an exponential decay to baseline after the initial negative deflection. At light-off there was a positive overshoot and return to baseline (Fig. 2). During a long stimulus the ERG reflected stepwise changes in intensity, with a negative response to an increase in intensity and positive polarization to a decrease (Fig. 3). The ERG thus reflects extraction of intensity change information.

The ocellar ERG waveforms (Fig. 4) and intensity-response functions (Fig. 5) were similar across wavelengths ranging from 370 to 520 nm. The ocellar spectral sensitivity peaks around 350–370 nm (ultraviolet) and 445 nm (blue) (Fig. 6). Chromatic adaptation with intense 370 or 445 nm did not selectively reduce 370 or 445 nm peak sensitivities (Fig. 7). These findings offer no support for more than one ocellar receptor type.

In additional experiments, bright 570 nm adaptation was found to increase responsivity relative to bright short wavelength adaptation. These wavelength-specific effects were elicited even during anoxia suggesting wavelength-dependent photopigment interconversions (Fig. 8). Spectral adaptation data were obtained suggesting that ocelli have a stable (non-bleaching) metarhodopsin. Spectral adaptation and sensitivity data allowed an approximation of the metarhodopsin spectrum which has long wavelength and ultraviolet maxima (Fig. 7).

Comparisons between the ERG waveforms and spectral sensitivities of normal and visual mutantDrosophila suggest that the absence of compound eye receptor types has no effect on the ocellar ERG. Also, strains with or without screening pigments do not show significant differences. However, 2 mutants with abnormal compound eye receptor potentials had ocellar abnormalities.

Supported by Die Studienstiftung des deutschen Volkes and SFB 46 to H.R. and NSF Grant BNS 76-11921 to W.S.
We thank R. Chappell and R. DeVoe for their critical reviews of the manuscript. We also thank H. Broda, H. Brownell, M. Johnson, T. Labhart, A. Shearn, H.Ch. Spatz, H. Weaver and C.-F. Wu for assistance.