A New Form of Inherited Red-Blindness Identified in Zebrafish

Abstract

A red-blind zebrafish mutant, partial optokinetic response b (pob), has been isolated by measuring eye movements of larvae in a three-generation screen for recessive mutations affecting the visual system. pob larvae exhibit eye movements in response to rotating black and white stripes illuminated with white light, but they do not move their eyes when the stripes are illuminated with red light. Physiological, immunohistochemical, and in situ hybridization analyses of pob retinas showed a selective loss of red-sensitive cones at 5 days postfertilization (dpf ). At 3 dpf, cells expressing red opsin are present, suggesting that red-sensitive cones form initially but then disappear rapidly, whereas other photoreceptors remain. Linkage analysis indicated that the mutation identified in the pob mutant is not at the red opsin locus. Because red opsin is the only known molecule unique to red cones, these data suggest that a novel gene is required for the maintenance or function of red cones. Key words: zebrafish; retina; red opsin; vision; behavior; mutantWe have initiated a genetic analysis of zebrafish (Danio rerio) so that we can use its sophisticated visual system to identify genes required for normal vertebrate visual responses. Zebrafish are tetrachromatic; in addition to rods, their retinas contain four types of cones that absorb light maximally in the red (570 nm), green (480 nm), blue (415 nm), and UV (362 nm) regions of the spectrum . Early in development, zebrafish vision is primarily photopic, and it is not until 12 days postfertilization (dqf ) that scotopic visual behavior is measurable (Clark, 1981;Branchek, 1984). In adults, the cones are arranged in a row mosaic (Branchek and Bremiller, 1984;Robinson et al., 1993); red-and green-sensitive cones alternate in sequence and are separated by either a single blue cone (always adjacent to a red-sensitive cone) or a single UV cone (always adjacent to a green-sensitive cone) . Red opsin expression begins at ϳ52 hr postfertilization, followed by blue and then by UV opsin (Raymond et al., 1995) (E. Schmitt and J. Dowling, unpublished observation). By ϳ3 dpf, zebrafish have opsinexpressing cones distributed throughout the photoreceptor layer of the retina.We described earlier how a rapid behavioral assay based on the optokinetic response (OKR) can be used to isolate zebrafish larvae with vision defects but with no obvious external morphological abnormalities (Brockerhoff et al., 1995). Zebrafish larvae, by 5 dpf, reliably display smooth pursuit and saccade eye movements in response to illuminated rotating stripes (see http:// weber.u.washington.edu/ϳjbhurley/ Movies.html for a demonstration of the OKR). As a secondary assay, we record the electroretinogram (ERG) from fish that lack an OKR to determine whether the defect resides in the outer retina. Using these two assays we have been able to isolate visual mutants with subtle retinal defects.In this study we conducted a similar screen using the same behavioral assay; however, we specifically searched for recessive m...

Zebrafish—on the move towards ophthalmological research

“…48 In addition, OKR was also useful in isolating a colour-blind mutant by changing the colour of visual stimulus. 49 In summary, the OKR appears to result in more a robust, reliable, and quantifiable behavioural data compared with other methods described below, especially after automated commercial systems have become available. 3,49 Optomotor response (OMR)…”

Behavioral genetic approaches to visual system development and function in zebrafish

“…In such a screen, Brockerhoff et al (1997) identified the red-blind mutant partial optokinetic response b (pob) by screening for OKR behavior under red illumination. Homozygous pob larva showed a normal response to white light but failed to execute an OKR under red illumination.…”

Quantitative measurements of the optokinetic response in adult fish

“…Common behavioral tests that were already used to investigate the visual system of zebrafish larvae or to isolate recessive mutations in large-scale, forward genetic screens include the optokinetic and the optomotor response (e.g. Brockerhoff et al, 1997;Gross et al, 2005;Muto et al, 2005;Neuhauss et al, 1999). These assays are both robust and therefore well suited for high-throughput screening of larvae.…”

“…The optokinetic response (OKR) has proven to be particularly useful for testing vision of zebrafish larvae (Beck et al, 2004;Brockerhoff et al, 1997;Gross et al, 2005;Muto et al, 2005;Neuhauss et al, 1999;Rinner et al, 2005). The OKR is a stereotyped eye movement elicited by whole-field motion of the visual surround.…”