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W.S. Stark; Drosophila's six opsins:visualizing visual pigment and rhodopsin–metarhodopsin conversions in live animals . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3629.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Drosophila has long been a model for multiple spectral receptor types, ultraviolet (UV) vision and, possibly, color vision (Harris, Stark & Walker, 1976, J. Physiol.256, 415–439). Rh1, the predominant opsin, residing in R1–6 receptors in the compound eye, was cloned in 1985 (O'Tousa et al. Cell 839–850; Zuker et al. Cell 40 851–858). Eventually, Rh2 – Rh6 were isolated. Rh2 normally resides in ocelli, simple eyes. Rh3 and Rh4 are the UV rhodopsins of R7. Rh5 and Rh6 are R8 opsins. The availability of transgenics expressing these minor opsins in R1–6 permitted an examination of rhodopsins' properties using microscopy and specialized optics in living flies. Methods: Transgenic Drosophila with Rh2 – Rh6 replacing Rh1 in R1–6 were provided through the generosity of Charles Zuker and Steven Britt. The deep pseudopupil in white eyed flies permitted visualization of the rhabdomeres (visual organelles) of R1–6. Actinic stimulations to convert short wavelength rhodopsins to their stable longer wavelength metarhodopsins were applied through the incident illuminator of a fluorescence microscope. Results: Viewing with white light, R1–6 looked pink especially for Rh1, Rh5, and Rh6 but not for Rh2, Rh3, and Rh4. For Rh1 to Rh5, conversions of rhodopsin to metarhodopsin were witnessed by R1–6 darkening at a long wavelength when a short wavelength was applied: Rh1 579 nm 460 nm; Rh2 520 nm 436 nm; Rh3 480 nm 350 nm; Rh3 480 nm 376 nm; Rh5 505 nm 405 nm. Wavelength selection was based on knowledge of rhodopsin – metarhodopsin from the literature (Feiler et al., 1988, Nature 333, 737–741; Feiler et al, 1992, J. Neurosci. 12, 3862–3868, Salcedo et al., 1999, J. Neurosci. 19, 10716–10727). For Rh6, rhabdomeres were dark when transilluminated with 514 nm even without actinic light. Conclusions: This is the first report that a pink color of visual pigment can be easily imaged in Drosophila. The pink color for Rh1, Rh5, and Rh6, and its absence for Rh2, Rh3, and Rh4 is explained when the absorbance of the rhodopsin–metarhodopsin combination covers much of the "visible" spectrum except long wavelengths. For Rh1 to Rh5, the absorbance increase of the photoreceptive organelle was as expected from what was known about the rhodopsin and metarhodopsin spectra. The data for Rh6 represent an interesting discovery. Possibly Rh6 rhodopsin and metarhodopsin spectra overlap so much that they cannot be spectrally separated (as suggested by Harris et al., 1976) even though Salcedo et al. (1999) concluded that the rhodopsin peak was 508 nm and the metarhodopsin peak was 468 nm.
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