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First published online April 8, 2004
Journal of Experimental Biology 207, 1703-1713 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00949

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Visual acuity of fly photoreceptors in natural conditions - dependence on UV sensitizing pigment and light-controlling pupil

Doekele G. Stavenga

Department of Neurobiophysics, University of Groningen, Nijenborgh 4, NL 9747 AG Groningen, The Netherlands

View larger version (14K): [in a new window]   Fig. 1. Typical radiance spectrum of a UV-rich sky. The radiance in the short-wavelength range is considerably higher than that in a normal daylight spectrum.

View larger version (16K): [in a new window]   Fig. 2. Absorbance spectra of visual pigment of fly photoreceptors R1-R6. The rhodopsin, R, is sensitized by the sensitizing pigment, S. R+S and R+2S represent the cases when the rhodopsin molecule binds one and two molecules of sensitizing pigment, respectively. It is assumed that the spectra can be summed algebraically.

View larger version (18K): [in a new window]   Fig. 3. Diagrams of the optics of the fly facet lens and photoreceptor rhabdomere. (A) According to geometric optics, the size of the visual field is given by the spatial angle taken up by the rhabdomere (rh) tip. Due to diffraction at the facet lens (f1) and the waveguide optics of the rhabdomere, a Gaussian-shaped angular sensitivity results with halfwidth , the acceptance angle; is the incident light angle. (B) When the pigment granules (pg) in the photoreceptor soma (so) are near the rhabdomere, they absorb light from the boundary waves of the waveguide modes and thus function as a light-controlling pupil mechanism. The first mode (1) extends less far outside the rhabdomere than the second mode (2).

View larger version (23K): [in a new window]   Fig. 4. Acceptance angle () of fly photoreceptors as a function of wavelength for different states of adaptation of the pupil. (A) Distal rhabdomere diameter Dr=1.4 µm; (B) Dr=2.0 µm. The rhabdomere tip is assumed to be localized in the focal plane of a 25 µm facet lens with F=2.2. The rhabdomere, length 250 µm, tapers parabolic to a proximal value of 1.0 µm. The increases slightly in the UV with increasing sensitizing pigment. The numbers near the curves indicate the distance, h (see inset), of the front line of pupil granules to the rhabdomere border. With the pupil in the fully dark-adapted state (h=), approximates r, the acceptance angle predicted by geometric optics. In the fully light-adapted state, given by h=0 µm, where the higher modes are completely suppressed, approaches l, the halfwidth of the angular diffraction function. The angular sensitivity is broadened due to the non-negligible diameter of the rhabdomere.

View larger version (19K): [in a new window]   Fig. 5. Absorption spectra for different states of light adaptation, given by the pupil distance, h (Fig. 4A, inset). At all wavelengths, a uniform light source with radiance 1 W sr-1 µm-2 illuminates a 25 µm facet lens (F=2.2) with, in its focal plane, the distal end of a rhabdomere with diameter 2.0 µm. (A) Pure rhodopsin (R). Pupil closure causes an overall reduced absorption spectrum. (B) One sensitizing pigment molecule per rhodopsin (R+S). A distinct UV peak remains upon pupil closure. (C) Two sensitizing pigment molecules per rhodopsin (R+2S). The UV peak is only slightly higher than that in B due to self-screening.

View larger version (14K): [in a new window]   Fig. 6. Absorbed number of photons per second by a photoreceptor, with pure rhodopsin (R) and with one (R+S) or two (R+2S) sensitizing pigment molecules per rhodopsin, from a sky with radiance given by Fig. 1, for a fully dark-adapted photoreceptor (h=; see Fig. 4A, inset), as a function of distal rhabdomere diameter. Photon absorption increases with rhabdomere diameter. One sensitizing pigment molecule per rhodopsin increases the absorption by 14-18% with respect to pure rhodopsin, and two sensitizing pigment molecules per rhodopsin increase the absorption by 20-27%.

View larger version (26K): [in a new window]   Fig. 7. The absorption of sky light as a function of the angle of the incident light for different states of pupil closure for a photoreceptor with a distal rhabdomere diameter of 2 µm and one sensitizing pigment molecule per rhodopsin (R+S). The pupil adaptation is given by the pupil distance, h. (A) Photon absorption per second from a patch of blue sky of 1 square degree seen under an angle . (B) Normalization shows that the angular sensitivity function distinctly narrows upon light adaptation. The vertical lines indicate the angle of incident light that is focused on the rhabdomere border.

View larger version (23K): [in a new window]   Fig. 8. Acceptance angle vs pupil absorbance calculated for three visual pigment conditions, pure rhodopsin (R), one (R+S) and two (R+2S) sensitizing pigment molecules per rhodopsin, for photoreceptors with distal rhabdomere diameters 1.4-2.0 µm. For each of the seven pupil states the acceptance angle as well as the total absorption of a uniformly radiating sky were calculated. The pupil absorbance was obtained by taking minus the decadic logarithm of the absorption relative to that for the dark-adapted state (h=; the case of Fig. 6).

View larger version (16K): [in a new window]   Fig. 9. (A) Ratio between the sensitivity for 500 nm and 359 nm light determined from intracellular, electrophysiological recordings of a Musca photoreceptor adapted to orange light, the intensity of which is indicated in decadic log units (abscissa; from Vogt et al., 1982). The peak and plateau potential (dotted line, 30 s light adaptation; continuous line, 150 s adaptation) indicate the sensitivity range of the photoreceptor. (B) Pupil absorbance as a function of log adaptation intensity derived from the sensitivity ratio of A together with the relationship between the average sensitivity ratio and the pupil absorbance for photoreceptors with one sensitizing pigment molecule per rhodopsin (Fig. 10, av).

View larger version (14K): [in a new window]   Fig. 10. Ratio between the sensitivity for 500 nm and 359 nm light of photoreceptors with distal rhabdomere diameters of 1.4-2.0 µm and one sensitizing pigment molecule per rhodopsin (R+S), obtained by calculating the relative absorption of 500 nm and 359 nm light at the various pupil states (given in Fig. 4), plotted vs the pupil absorbance for sky light at the same pupil state, together with a spline fit (av). The sensitivity ratio decreases due to selective suppression of the absorption in the blue-green wavelength range by the pupil.