Fly photoreceptor fluorescence is related to UV sensitivity

Abstract

THE predominant photoreceptor type in flies, R1-6, contains a rhodopsin which absorbs maximally at around 480 nm. Light converts rhodopsin into a thermostable metarhodopsin peaking at around 580 nm (refs. 1–8). A curious discrepancy is the finding that the electrophysiological sensitivity is highest in the UV^5,9—completely at variance with known rhodopsin absorption curves. Several possible explanations of the high UV-peak, including the presence of a UV-rhodopsin and waveguide effects, have recently been excluded^5–10. Kirschfeld and co-workers have presented an alternative explanation^6,11, that R1-6 also contains a photostable pigment that absorbs UV light and converts the photopigment by energy transfer. This hypothesised sensitising pigment would most probably be a carotenoid, since vitamin A deprivation preferentially suppresses UV sensitivity^8,12,13. Of course, vitamin A deprivation also lowers sensitivity by reducing the photopigment level^14,15. Here we present further evidence for the existence of a vitamin A-dependent non-photopigment substance in fly photoreceptors. We measured fluorescence while simultaneously monitoring photopigment conversions and argue that the fluorescence is probably related to the UV-sensitivity.