1 Color and polarized light vision: The Drosophila retinal mosaic

Although the types of eyes found across the animal kingdom vary extensively in general organization and complexity, all visual systems use similar mechanisms to respond to environmental cues. In particular, all animals use related opsin photopigment proteins in their photoreceptor cells to capture photons (for review, see Arendt & Wittbrodt 2001). The visual system serves several major purposes. At the origin, it likely served as a light detector for photophobic or photophilic behaviors. It then rapidly acquired an additional function as a light detector for entraining the circadian clock. These uses did not require complex optics or sophisticated neural processing, and in fact some of these functions were performed by using non opsin photodetectors. The most important use for the sophisticated eyes of many higher vertebrates (from fish to mammals) and many invertebrates (from the camera-like eye of squids to the compound eye of arthropods), is a dual role in image formation/motion detection but also in the detection of the quality of light, i.e. the wavelengths (color) or vector of skylight polarization for navigation. This requires the ability to form images with complex optics (image formation), the specialization of photoreceptors to detect particular types of light (color or polarization detection), as well as the building of a sophisticated processing machinery in the central nervous system. The different subclasses of photoreceptors exhibit important morphological and molecular differences as well as characteristic distribution patterns through the retina in order to maximize the amount of information extracted from the environment. Development of the retina and correlative connections to the optic lobes must be exquisitely regulated to achieve the impressive mosaic of photoreceptors observed in many retinas. Patterning of the retina involves a series of highly coordinated and organized processes. Many of the factors involved in the formation of th