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

Primer E ver since Morgan observed a mutant male with white eyes among the normally brilliant red-eyed specimens in his pedigree culture of fruit flies, Drosophila has been an inexhaustible source of scientific inspiration and discovery [1]. The regular, almost crystalline mosaic of the fruit fly eye has proved a powerful tool for unraveling genetic defects resulting in anatomical as well as functional modifications of normal physiological and/or behavioral patterns [2]. Developmental and cell biological studies in particular have greatly benefited from the clear and accessible organization of the fruit fly compound eye. A Drosophila eye consists of roughly 800 anatomically identical units, called the ommatidia (Figure 1A). Each ommatidium contains eight photoreceptor cells (R1–R8) and is capped by a facet lens. The lens projects incident light into each photoreceptor's rhabdomere—the specialized, light-sensitive organelle that functions as a light guide and contains the visual pigment molecules (called rhodopsins). Rhabdomeres also harbor the photoreceptor's phototransduction machinery [3,4], which is made up of signaling components that capture and transform photons from light into an electrical signal, which conveys visual information to the brain. (All the rhabdomeres in an ommatidium are collectively known as the rhabdom.) The R1–R6 rhabdomeres stretch the full length of their photoreceptors, forming a trapezoid with the tandem of the R7 and R8 rhabdomeres in the center (Figure 1B and 1C). Accordingly, the photoreceptors and their rhabdomeres can be classed as outer and inner (or peripheral and central), respectively [2,5], and these two classes serve different visual functions. R1–R6 mediate highly light-sensitive, broad spectral-band motion vision, and R7 and R8 underlie color vision, a capacity that is enabled by special optical and neural organizations [5]. The two classes of photoreceptors can be not only differentiated by the type of vision they mediate, but also by the route via which the visual information is conveyed. The different classes of visual information are collected in two consecutive optical ganglia—namely, the lamina, where information from R1–R6 is collated, and the medulla, where the R7 and R8 signals are transmitted. The specifics regarding further neural processing are unfortunately largely unknown. Extensive studies on the larger housefly (Musca) and blowfly (Calliphora) have provided considerable insight into the workings of the fruit fly's retinal organization. For example, transmission light microscopy has revealed two types of ommatidia, with the R7/R8 rhabdomeres of randomly distributed ommatidia appearing yellow, due to a blue-absorbing, photostable pigment (presumably zeaxanthin and/or …