Microscopy of multiple visual receptor types in Drosophila.

Purpose: To take advantage of specialized microscopic methods and transgenic stocks, to understand the properties of each rhodopsin now that Drosophila's six rhodopsins (Rh1-Rh6) have been isolated.

Methods: The visual pigment containing organelles, the rhabdomeres, were imaged in live flies with the pseudopupil in standard and confocal microscopes. Five transgenic Drosophila strains in which Rh2-Rh6 replaced the native Rh1 in R1-6 receptors were compared with normal controls (Rh1 in R1-6) for two lines of work: (1) autofluorescence of rhodopsin; and (2) imaging rhodopsin. Other transgenic Drosophila in which the Rh1, Rh3, and Rh4 promoters drive the green fluorescent protein (GFP) reporter were used for other purposes, especially distinguishing the R7/8 types.

Results: We show, for the first time, that visual pigment appears pink in white light, especially for Rh1 and Rh6. While showing that rhodopsin-metarhodopsin conversions were understood by their respective wavelengths, we discovered that, for Rh6, rhodopsin and metarhodopsin could not be spectrally separated. Relative fluorescent emission, Rh1=Rh5>Rh6>Rh2>Rh4>Rh3, was of little value in explaining differences between bright and dim autofluorescence in R7. Rather, analysis of GFP driven by Rh3 and Rh4 promoters show that the rhabdomeres with bright autofluorescence are the ones that contain Rh4.

Conclusions: Careful imaging provides a useful approach to analyzing Drosophila rhodopsins. Amid a considerable body of microscopic data, we identify the sources of bright and dim R7 rhabdomeres, and we demonstrate the unique properties of Rh6.