What are the mechanisms of spectral sensitivity in fly visual receptors? I discovered that vitamin A deprivation specifically eliminated UV sensitivity in R1-6. R1-6 have properties unique to invertebrates: they have 2 spectral maxima. My studies over the next few years were instrumental in developing the currently accepted explanation of the two-peaked spectrum in the fly, the sensitizing pigment hypothesis. A sensitizing pigment, a form of vitaminA, absorbs UV light and transfers its energy by inductive resonance to the blue absorbing rhodopsin. We also discovered a vitamin A dependent fluorescence of R1-6 visual receptors which is related to the UV visual sensitivity. This mechanism was then widely known in photosynthesis but novel in visual transduction.

Selected papers on spectral sensitivity shape, receptor fluorescence and UV sensitization:

Stark, W.S. Spectral selectivity of visual response alterations mediated by interconversions of native and intermediate photopigments in Drosophila.Journal of Comparative Physiology, 1975, 96, 343-356.

Stark, W.S., Ivanyshyn, A.M. and Hu, K.G. Spectral sensitivities and photopigments in adaptation of fly visual receptors. Die Naturwissenschaften, 1976, 63, 513-518. (Invited review).

Stark, W.S., Ivanyshyn, A.M. and Greenberg, R.M. Sensitivity and photopigments of R1-6, a two-peaked photoreceptor, in Drosophila, Calliphora and Musca. Journal of Comparative Physiology, 1977, 121, 289-305.

Stark, W.S. Stavenga, D.G. and Kruizinga, B. Fly photoreceptor fluorescence is related to UV sensitivity. Nature, 1979, 280, 581-583.

Stark, W.S. Spectral selectivity of visual response alterations mediated by interconversions of native and intermediate photopigments in Drosophila. Journal of Comparative Physiology, 1975, 96, 343-356.

Stark, W.S. and Johnson, M.A. Microspectrophotometry of Drosophila visual pigments: determinations of conversion efficiency in R1-6 receptors. Journal of Comparative Physiology, 1980, 140, 275-286.

Stark, W.S. and Tan, K.E.W.P. Ultraviolet light: Photosensitivity and other effects of the visual system. Photochemistry and Photobiology, 1982, 36,371-380. (Invited review to accompany American Society for Photobiology1981 Meeting Lecture). PubMed

A fluorescence micrograph showing vitamin A dependent fluorescence of R1-6 in the deep pseudopupil (deprived fly eye is on the right) like that from Stark et al. 1979

The evidence that vitamin A deprivation preferentially decreases UV sensitivity (from Stark et al., 1976)


To date, I had noted statements in the literature that fluorescence was not relevant to UV vision in insects. When I bought a fluorescence microscope on my second NSF grant in 1976, I had originally intended to do single cell recording and to mark cells for histology with a fluorescent dye. But I did want to "play" with UV, so I had the UV cube as part of my package. The first day I looked at the deep pseudopupil I noticed that R1-6 fluoresced. It would be a week before my vitamin A deprived flies would eclose, and my student Robert Greenberg (here is his present web page) ran them. He reported that he did not see that fluorescence of R1-6 in vitamin A deprived flies, and we included that information in our 1977 paper.

The work on fluorescence quickly became a hot topic, and I noted that Nicolas Franceschini, working in Tubingen, published several abstracts on the topic. He claimed that blue-induced fluorescence increases monitored rhodopsin to metarhodopsin conversions while yellow-induced decreases in fluorescence monitored reconversion of metarhodopsin to rhodopsin. He showed me these experiments when I visited his lab in 1978, and I knew immediately that his lights were orders of magnitude brighter than those that induced such changes. When I went to work with Doekele Stavenga in the Netherlands, I convinced him of of this discrepency. We learned that Niccolas had a Science paper in press reporting that phenomenon. Doekele invited Nichola up for a seminar so that I could demonstrate his mistake. Everything went accordinng to plan in time for Franceschini's Science paper to be revised, and my work with Doekele was published in Nature.

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