Light dependence of calcium and membrane potential measured in blowfly photoreceptors in vivo

Light adaptation in insect photoreceptors is caused by an increase in the cytosolic Ca2+ concentration. To better understand this process, we measured the cytosolic Ca2+ concentration in vivo as a function of adapting light intensity in the white-eyed blowfly mutant chalky. We developed a technique to measure the cytosolic Ca2+ concentration under conditions as natural as possible. The calcium indicator dyes Oregon Green 1, 2, or 5N (Molecular Probes, Inc., Eugene, OR) were iontophoretically injected via an intracellular electrode into a photoreceptor cell in the intact eye; the same electrode was also used to measure the membrane potential. The blue-induced green fluorescence of these dyes could be monitored by making use of the optics of the facet lens and the rhabdomere waveguide. The use of the different Ca2+-sensitive dyes that possess different affinities for Ca2+ allowed the quantitative determination of the cytosolic Ca2+ concentration in the steady state. Determining the cytosolic Ca2+ concentration as a function of the adapting light intensity shows that the Ca2+ concentration is regulated in a graded fashion over the whole dynamic range where a photoreceptor cell can respond to light. When a photoreceptor is adapted to bright light, the cytosolic Ca2+ concentration reaches stable values higher than 10 mu M. The data are consistent with the hypothesis that the logarithm of the increase in cytosolic Ca2+ concentration is linear with the logarithm of the light intensity. From the estimated values of the cytosolic Ca2+ concentration, we conclude that the Ca2+-buffering capacity is limited. The percentage of the Ca2+ influx that is buffered gradually decreases with increasing Ca2+ concentrations; at cytosolic Ca2+ concentration levels above 10 mu M, buffering becomes minimal

One Rhodopsin per Photoreceptor: Iro-C Genes Break the Rule

While photoreceptors usually contain a single type of rhodopsin, two rhodopsins are sometimes expressed. This bi-allelic expression appears to be under genetic control, an example of which is discussed in this Primer

Biophotonics of diversely coloured peacock tail feathers

Peacock feathers feature a rich gamut of colours, created by a most sophisticated structural colouration mechanism. The feather barbules contain biophotonic structures consisting of two-dimensionally-ordered lattices of cylindrical melanosomes and air channels embedded in keratin. Here, we study the reflectance characteristics of the various peacock tail feather colours by applying bifurcated-probe- and micro-spectrophotometry and imaging scatterometry. We compare the experimental results with published anatomical SEM and TEM data, using a transfer-matrix based effective-medium multilayer model that includes the number and diameter of the melanosome rodlets and air channels, the lattice spacing and the keratin cortex thickness, together with the recently determined wavelength-dependence of the refractive indices of keratin and melanin. Slight variations in the parameter values cause substantial changes in the spectral position and shape of the reflectance bands. We find that the number of layers crucially determines the number of peaks in the reflectance spectra. For a small number of melanosome layers, the reflectance band shape is particularly sensitive to the properties of the uppermost layer, which provides a simple mechanism for tuning the feather colours. This journal i

Functional interplay of visual, sensitizing and screening pigments in the eyes of Drosophila and other red-eyed dipteran flies

Several fly species have distinctly red-coloured eyes, meaning that the screening pigments that provide a restricted angular sensitivity of the photoreceptors may perform poorly in the longer wavelength range. The functional reasons for the red transparency and possible negative visual effects of the spectral properties of the eye-colouring screening pigments are discussed within the context of the photochemistry, arrestin binding and turnover of the visual pigments located in the various photoreceptor types. A phylogenetic survey of the spectral properties of the main photoreceptors of the Diptera indicates that the transition of the brown eye colour of the Nematocera and lower Brachycera to a much redder eye colour of the higher Brachycera occurred around the emergence of the Tabanidae family

Path integral approach to eikonal and next-to-eikonal exponentiation

We approach the issue of exponentiation of soft gauge boson corrections to scattering amplitudes from a path integral point of view. We show that if one represents the amplitude as a first quantized path integral in a mixed coordinate-momentum space representation, a charged particle interacting with a soft gauge field is represented as a Wilson line for a semi-infinite line segment, together with calculable fluctuations. Combining such line segments, we show that exponentiation in an abelian field theory follows immediately from standard path-integral combinatorics. In the non-abelian case, we consider color singlet hard interactions with two outgoing external lines, and obtain a new viewpoint for exponentiation in terms of ``webs'', with a closed form solution for their corresponding color factors. We investigate and clarify the structure of next-to-eikonal corrections.Comment: 43 pages, 16 figure

Lorentz covariance of the canonical perfect lens

The canonical perfect lens--comprising three slabs, each made of a linear, homogeneous, bianisotropic material with orthorhombic symmetry--is Lorentz covariant