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Introduction
Vertebrate eyes are adapted for vision over a wide range of light intensities allowing animals to exploit nearly the full extent of the light/dark cycle (Land & Fernald, 1992). Adaptations for light detection in different species include the shape and size of the rod and cone photoreceptors, the relative abundance of photoreceptor types and their spatial distribution (Wikler & Rakic, 1990) as well as the regulation of key cellular and molecular processes, as reviewed by Puzzolo (1989). Among vertebrates, fish visual systems have been useful model systems for analysis because evolutionary adaptations for visual function are particularly evident. For example, the peak wavelengths of cone photoreceptor absorption in fish match key features of their optical environment (McFarland & Munz, 1975). Since absorption peak wavelength depends critically on key amino acids in opsin, fish visual pigment genes appear to be particularly sensitive to selective pressures (Fernald & Liebman, 1980). For the experiments reported here, we used the African cichlid fish, Haplochromis (Astatotilapia ) burtoni (H. burtoni ), a species that displays elaborate social behavior mediated primarily by visual cues, making H. burtoni particularly appropriate for analysis of visual mechanisms (Fernald, 1990).
During evolution, regular light/dark changes have had a profound impact on visual systems including the evolution of regular rhythms in biological processes ranging from behavioral to biochemical. Diurnal rhythms are synchronized to the ambient light cycle but do not persist under constant lighting conditions, while circadian rhythms persist with a cycle of ∼24 h even under constant lighting conditions (Jacklet, 1989). Examples of both diurnal and circadian rhythms are found in features of vertebrate eyes where they regulate many processes including cell addition (Chiu et al., 1995), photoreceptor position and function (Burnside, 1976), opsin synthesis (Korenbrot & Fernald, 1989; Pierce et al., 1993; von Schantz et al., 1999), and disc shedding (Besharse et al., 1977; Young, 1978).
Among the most essential and robust rhythmic processes in the retina is the renewal of the photosensitive opsin proteins. Although all cells renew their molecular constituents through removal and new synthesis, photoreceptor cells have enhanced renewal processes that are typical of sensory systems responsive to physical signals from the environment (Fernald, 1991). Discs containing photoreceptor pigments in the rod outer segment undergo daily renewal, being...