The distal photoreceptors in the tiered retina of Papilio exhibit different spectral sensitivities. There are at least two types of short-wavelength sensitive receptors: an ultraviolet receptor with a normal spectral shape and a violet receptor with a very narrow spectral bandwidth. Furthermore, a blue receptor, a double-peaked green receptor and a single-peaked green receptor exist. The violet receptor and single-peaked green receptor are only found in ommatidia that fluoresce under ultraviolet illumination. About 28% of the ommatidia in the ventral half of the retina exhibit the UV-induced fluorescence. The fluorescence originates from an ultraviolet-absorbing pigment, located in the most distal 70 microns of the ommatidium, that acts as an absorption filter, both for a UV visual pigment, causing the narrow spectral sensitivity of the violet receptor, and for a green visual pigment, causing a single-peaked green receptor.

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Photoreceptor cell death is the hallmark of a group of human inherited retinal degeneration. Although the causative genetic mutations are often known, the mechanisms leading to photoreceptor degeneration remain poorly defined. Here, we show that Semaphorin 4A (Sema4A), a member of axonal guidance molecule semaphorin, plays a role in Rab11/FIP2-mediated endosomal sorting in retinal pigment epithelial cells to support photoreceptor function. In response to oxidative stress, Sema4A switches the endosomal sorting of the lysosomal precursor protein prosaposin from the lysosome to the exosomal release, which prevents light-induced photoreceptor apoptosis. In the absence of oxidative stress, Sema4A sorts retinoid-binding proteins with retinoids between the cell surface and endoplasmic reticulum, by which 11-cis-retinal, a chromophore for phototransduction, is regenerated and transported back to photoreceptors. Owing to defects in these processes, Sema4A-deficient mice exhibit marked photoreceptor degeneration. Our findings therefore indicate that Sema4A regulates two distinct endosomal-sorting pathways that are critical for photoreceptor survival and phototransduction during the transition between daylight and darkness.

In the class Insecta, retinal and 3-hydroxyretinal are used as chromophores of visual pigments, but the absolute structure of the 3-hydroxyretinal chromophore has yet to be clarified. This study investigates the chirality of 3-hydroxyretinal in the compound eyes of five representative orders of insects. In the orders Odonata, Hemiptera, Neuroptera and Lepidoptera, and suborders Nematocera and Brachycera of the Diptera, only (3R)-3-hydroxyretinal isomers were detected, but dipterans of the suborder Cyclorrhapha (higher flies) had the (3S)-ll-cis enantiomer and a mixture of (3R)-alltrans and (3S)-all-trans 3-hydroxyretinal enantiomers; the ratio of the (3R) enantiomer to the sum of both enantiomers of the all-trans isomer was in the range 9-32%. Despite differences in feeding habits, including one species that is a butterfly parasite, all higher flies analysed to date share the same pattern of 3-hydroxyretinal enantiomers, making them a unique group with regard to the nature of the visual pigment chromophore.In vision, the step that triggers the transduction cascade in a photoreceptor cell is the absorbtion of a photon by a visual pigment, which is composed of a protein moiety called opsin and the 11-cis isomer of a light-sensitive prosthetic group referred to as the chromophore. There are only four known kinds of visual pigment chromophore in the animal kingdom, namely retinal and its congeners, 3,4-didehydroretinal, 3-hydroxyretinal and 4-hydroxyretinal [ 1, 21. Two of these chromophores, retinal and 3-hydroxyretinal, are used in the class Insecta [3-81. The functional constraints, associated environmental pressures and/or genetic factors influencing the selection of the insect visual pigment chromophores have yet to be elucidated [8].As 3-hydroxyretinal, in contrast to retinal, has a chiral center at C3 in the P-end group, there are two possible stereoisomers, (3R)-3-hydroxyretinal and (3S)-3-hydroxyretinal. Although in vitro recombination of 3-hydroxyretinal with an insect opsin has not been reported, the combination of different 11 -cis 3-hydroxyretinal enantiomers ( Fig. 1) with an insect opsin should produce two diastereomeric visual pigments differing in characteristics such as the wavelength of maximal light absorbance [9]. The clarification of which enantiomer is used as visual pigment chromophore among insects is therefore an important problem to consider when examining the selection of the visual pigment chromophore in Insecta.Pioneering work by Vogt [lo] first revealed the use of 3-hydroxyretinal as a chromophore in the fly, and the name xanthopsin was proposed for visual pigments based on 3-hydroxyretinal [4, 10, 111 as a mnemonic for the derivation Correspondence to

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