Strain variations in cone wavelength peaks in situ during zebrafish development.

Ralph F Nelson,Meaghan Torvund,Sara S Patterson,Tara Suresh,Annika Balraj

doi:10.1017/s0952523819000075

Abstract

There are four cone morphologies in zebrafish, corresponding to UV (U), blue (B), green (G), and red (R)-sensing types; yet genetically, eight cone opsins are expressed. How eight opsins are physiologically siloed in four cone types is not well understood, and in larvae, cone physiological spectral peaks are unstudied. We use a spectral model to infer cone wavelength peaks, semisaturation irradiances, and saturation amplitudes from electroretinogram (ERG) datasets composed of multi-wavelength, multi-irradiance, aspartate-isolated, cone-PIII signals, as compiled from many 5- to 12-day larvae and 8- to 18-month-old adult eyes isolated from wild-type (WT) or roy orbison (roy) strains. Analysis suggests (in nm) a seven-cone, U-360/B1-427/B2-440/G1-460/G3-476/R1-575/R2-556, spectral physiology in WT larvae but a six-cone, U-349/B1-414/G3-483/G4-495/R1-572/R2-556, structure in WT adults. In roy larvae, there is a five-cone structure: U-373/B2-440/G1-460/R1-575/R2-556; in roy adults, there is a four-cone structure, B1-410/G3-482/R1-571/R2-556. Existence of multiple B, G, and R types is inferred from shifts in peaks with red or blue backgrounds. Cones were either high or low semisaturation types. The more sensitive, low semisaturation types included U, B1, and G1 cones [3.0-3.6 log(quanta·μm-2·s-1)]. The less sensitive, high semisaturation types were B2, G3, G4, R1, and R2 types [4.3-4.7 log(quanta·μm-2·s-1)]. In both WT and roy, U- and B- cone saturation amplitudes were greater in larvae than in adults, while G-cone saturation levels were greater in adults. R-cone saturation amplitudes were the largest (50-60% of maximal dataset amplitudes) and constant throughout development. WT and roy larvae differed in cone signal levels, with lesser UV- and greater G-cone amplitudes occurring in roy, indicating strain variation in physiological development of cone signals. These physiological measures of cone types suggest chromatic processing in zebrafish involves at least four to seven spectral signal processing pools.

Highlights

The challenge of zebrafish spectral physiologyZebrafish is a cone athlete, and the goals in studying zebrafish color vision are to understand how the multiplicity of cone types in zebrafish serves neural circuits that process wavelength, the ecological advantages to zebrafish of such circuits, and the generation of general insights into vertebrate wavelength processing
The multiple green and red opsins are gene duplications with highly homologous sequences, so that antibodies do not readily distinguish them, yet the duplicated opsins reveal distinct spectral peaks (Chinen et al, 2003). How these 8 opsins are distributed among 4 morphological cone types and whether one or multiple opsins are physiologically active in an individual cone is not known, and further, the in situ physiological spectral peaks of the 8 opsins, within larval cones, have yet to be measured
Model fits to treatment-level spectral datasets allow access to cone spectral peaks in intact functioning retinas

Summary

Introduction

The challenge of zebrafish spectral physiologyZebrafish is a cone athlete, and the goals in studying zebrafish color vision are to understand how the multiplicity of cone types in zebrafish serves neural circuits that process wavelength, the ecological advantages to zebrafish of such circuits, and the generation of general insights into vertebrate wavelength processing. The multiple green and red opsins are gene duplications with highly homologous sequences, so that antibodies do not readily distinguish them, yet the duplicated opsins reveal distinct spectral peaks (Chinen et al, 2003). How these 8 opsins are distributed among 4 morphological cone types and whether one or multiple opsins are physiologically active in an individual cone is not known, and further, the in situ physiological spectral peaks of the 8 opsins, within larval cones, have yet to be measured. Detergent extracts of opsins generated in cell lines are often very close matches to patch-recorded photocurrent spectra from single cones, but sometimes not, as with red cones in zebrafish adults (Endeman et al, 2013)

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Conclusion