Abstract
Microvillar photoreceptors are intrinsically capable of detecting the orientation of e-vector of linearly polarized light. They provide most invertebrates with an additional sensory channel to detect important features of their visual environment. However, polarization sensitivity (PS) of photoreceptors may lead to the detection of polarization-induced false colors and intensity contrasts. Most insect photoreceptors are thus adapted to have minimal PS. Flies have twisted rhabdomeres with microvilli rotated along the length of the ommatidia to reduce PS. The additional UV-absorbing sensitizing pigment on their opsin minimizes PS in the ultraviolet. We recorded voltage from Drosophila photoreceptors R1–6 to measure the spectral dependence of PS and found that PS in the UV is invariably negligible but can be substantial above 400 nm. Using modeling, we demonstrate that in R1–6 without the sensitizing pigment, PS in the UV (PSUV) would exceed PS in the visible part of the spectrum (PSVIS) by a factor PSUV/PSVIS = 1.2–1.8, as lower absorption of Rh1 rhodopsin reduces self-screening. We use polarimetric imaging of objects relevant to fly polarization vision to show that their degree of polarization outdoors is highest in the short-wavelength part of the spectrum. Thus, under natural illumination, the sensitizing pigment in R1–6 renders even those cells with high PS in the visible part unsuitable for proper polarization vision. We assume that fly ventral polarization vision can be mediated by R7 alone, with R1–6 serving as an unpolarized reference channel.
Highlights
Microvillar or rhabdomeric photoreceptors have the structural prerequisite to sense the orientation of e-vector of incident light (De Vries et al, 1953; Snyder and Laughlin, 1975)
Polarization Vision sensitivity (PS) of R1–6 is always very low in the UV, rendering R1– 6 useless as the opponent analyzer in the pair with R7, the rare putative cases of R1–6 with high PS in the UV might have been missed in our recordings
Due to the sensitizing pigment, even those R1–6 with high PS in the blue cannot function as an opponent channel to R8 while observing outdoor reflections
Summary
Microvillar or rhabdomeric photoreceptors have the structural prerequisite to sense the orientation of e-vector of incident light (De Vries et al, 1953; Snyder and Laughlin, 1975). Sensitizing Pigment in Fly Polarization Vision sensitivity (PS), which is smaller than ∆ due to self-screening, except in the special case of a crustacean-type, interdigitated rhabdom (Snyder, 1973), found in the horsefly retina (Wunderer et al, 1990). In Drosophila, PS in the ventral retina (VPS) is probably mediated by a subpopulation of the ommatidial type named pale (p), which contains broadband or blue-sensitive R1–6, UVsensitive R7p, and blue-sensitive R8p receptors (Wernet et al, 2012). This receptor combination is clearly not optimized for a spectrally balanced polarization vision. We conclude that it is very unlikely that under natural illumination, Drosophila R1–6 could analyze polarized visual signals
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