Experimental and evolutionary analysis of the retinal mechanisms mediating UV polarization vision




Ramsden, Samuel David

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Numerous techniques have been used to characterize ultraviolet polarization sensitivity (PS) in teleost fishes. To date PS in the rainbow trout (Oncorhynchus mykiss) has been characterized using compound action potential (CAP) recordings from the optic nerve. CAP recordings have also been used in conjunction with chromatic adaptation to isolate the individual cone photoreceptor mechanisms responsible for PS detection. The primary goal of this thesis was to discover the ancestry of PS within the Salmoninae clade containing Salmo, Salvelinus and Oncorhynchus, by characterizing and mapping the cone photoreceptor mechanisms mediating PS onto the phylogeny. I developed a research paradigm that used electroretinogram (ERG) recordings to characterize PS in rainbow trout. Substantial differences were found between CAP and ERG recordings. When identical stimuli were used, ERG recordings produced two additional peaks of maximal sensitivity at 45° and 135°. Chromatic adaptation was used to explore differences in bipolar cell and optic nerve generated PS curves. Under chromatic adaptation, with the use of ERG recordings, the additional peaks shifted consistently with the adaptation of either the horizontal or vertical polarization detector mechanism. These findings have contributed to the understanding of how ultraviolet polarized light is processed within the visual system I then used CAP and ERG recordings to characterize PS in Atlantic salmon (Salmo salar), because of its basal relationship within the Salmo, Salvelinus and Oncorhynchus clade. PS results obtained from CAP and ERG recordings were similar for Atlantic salmon and rainbow trout. CAP recordings generated a `W'-shaped PS curve maximally sensitive at 0°, 90°, and 180°, while ERG recordings generated a tuning curve with the additional peaks of maximal sensitivity at 0°, 45°, 90°, 135° and 180°. Therefore, it was concluded that both PS and the mechanisms of retinal processing underlying it are ancestral within the clade making up Salmo, Salvelinus and Oncorhychus. This has implications for the sensory abilities available when diadromous life histories evolved in Salmonidae.



salmonidae, evolution