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No description available.

Thesis (M.S.)--University of Western Australia, 2004. / Title from PDF title page (viewed on Mar. 3, 2006). Includes bibliographical references (p. 81-96).

x, 154 leaves : ill. ; 29 cm Typescript. (Another copy on microfilm is located in Archives) Thesis (Ph.D.)--University of Oregon Vita Bibliography: leaves 144-154

The role of the retinal pigment epithelial cells on the viability and renewal of photoreceptors has been well demonstrated in the Royal College of Surgeons (RCS) strain of rat. These rats are characterized by an inherited time-dependent degeneration of their photoreceptors. This degeneration is apparently due to the inability of the retinal pigment epithelial cells to adequately ingest fragments of photoreceptor membrane that are shed during the course of photoreceptor membrane renewal. The buildup of photoreceptor material in the interphotoreceptor space ultimately leads to the degeneration of photoreceptors in these animals. With regard to the pigment epithelial cells, neither the mechanism mediating the ingestion process in normal rats nor the nature of the defect of this process in RCS rats is understood.It is the goal of this proposed research to assay for the presence of phospholipase C in retinal pigment epithelial (RPE) cells and to determine possible modulators of the enzyme in an attempt to associate this with the process of phagocytosis. / Department of Biology

Understanding why organisms possess particular combinations of visual pigments (VPs) is central to visual ecology. Species that adjust spectral sensitivity by changing retinal VPs provide a powerful tool to investigate this question. Changing chromophores (A1 or A2) and altering opsin expression are mechanisms used to adjust photoreceptor maximum absorbance (λmax). My research explored the function of such dynamic systems in two teleosts, coho salmon and zebrafish. I investigated temporal VP changes in relation to life history and environmental change. In coho, I found a correlation between chromophore shifts and seasonal variations in environmental variables for freshwater and marine life history stages (seasonal hypothesis). These findings provide an alternative explanation to the migration/metamorphosis hypothesis. The latter, suggests that shifts from A-2 to AI at metamorphosis, preceding seaward migration, are preemptive to changes in photic environment. Using exogenous thyroid hormone (TH), which plays a role in coho metamorphosis, I demonstrated that under various rearing conditions and times of year, TH consistently shifted VPs towards A2 dominance. This increased A2 is opposite to that occurring at metamorphosis, further supporting the seasonal hypothesis. While TH induced changes in rod λmax were consistent with a change in A1/A2 ratio, λmax variations in middle wavelength- and long wavelength-sensitive (MWS and LWS) cones were greater than predicted by a shift in A1/A2 ratio alone. I proposed a change in opsin expression to explain MWS and LWS cone λmax variations. In support of this hypothesis, a novel RH2 opsin subtype (expressed in MWS cones) was isolated and sequenced. This second RH2 possessed an E to Q substitution at the position analogous to 122 in bovine RH1 which imposes a hypsochromic shift in X. Further investigation found that changes in coho MWS cone λmax were correlated with ontogeny and the frequency of MWS cones with below 500 nm was reduced in marine compared to freshwater stages. The combination of changes in A1/A2 ratio and opsin expression provides coho with a dynamic spectral tuning mechanism. In zebrafish, I demonstrated the presence of an A1/A2 VP pair, which shifted to A2 dominance with exogenous TH treatment, but not with temperature.

published_or_final_version / Anatomy / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Syngnathidae (seahorse, pipefish, pipehorses & seadragons) are highly visual feeders with different species feeding on specific types of prey, a behaviour that has been related to snout length. Worldwide, many species have become threatened by habitat destruction, collection for the aquarium trade and exploitation for traditional medicine, as well as recreational and commercial bycatch. Attempts to establish aquaculture programs have been of limited success. Little is known about their visual capabilities in detail. The visual systems of fishes are known to have evolved specific adaptations that can be related to the colour of water in which they live and specific visual tasks such as predator detection and acquisition of food. This study examined the ocular and retinal morphology, photoreceptor structure and spectral sensitivity of adult individuals of a local pipefish (S. argus), local seahorse (Hippocampus subelongatus) which both inhabit green water seagrass beds, and a tropical species of seahorse (Hippocampus barbouri) from blue water coral reefs. Some juveniles were also investigated. Accordingly, we developed an understanding of the features that are common to all syngnathids and those that have evolved for specific environments. Cryosections of the eyes were taken to determine morphological distinctions of this group. Lens characteristics measured using a spectrophotometer determined 50% cut-off wavelengths below 408nm for all 3 species, hence no transmission of UV light to the retina. Histological examination determined a cone dominated fovea in the ventro-temporal retina and very large rods concentrated in the peripheral retina and adjacent to the optic nerve. Microspectrophotometry measured the absorption characteristics of the visual pigments within the photoreceptors showing the presence and maximum sensitivity (λmax) of rods, SWS single cones, and a broad, complex array of LWS double/twin cones. The results are discussed in relation to the light environment inhabited by each species and their feeding requirements. The implications for the design of suitable light environments for aquarium and aquaculture programs for the Syngnathidae are also discussed. Rearing success of this family of fish, for both the aquarium trade and re-stocking programs, would be advised to take lighting regimes and specifics of the animals’ vision into account

Understanding how individual photoreceptor cells factor in the spectral sensitivity of a visual system is essential to explain how they contribute to the visual ecology of the animal in question. Existing methods that model the absorption of visual pigments use templates which correspond closely to data from thin cross-sections of photoreceptor cells. However, few modeling approaches use a single framework to incorporate physical parameters of real photoreceptors, which can be fused, and can form vertical tiers. Akaike’s information criterion (AIC c ) was used here to select absorptance models of multiple classes of photoreceptor cells that maximize information, given visual system spectral sensitivity data obtained using extracellular electroretinograms and structural parameters obtained by histological methods. This framework was first used to select among alternative hypotheses of photoreceptor number. It identified spectral classes from a range of dark-adapted visual systems which have between one and four spectral photoreceptor classes. These were the velvet worm, Principapillatus hitoyensis , the branchiopod water flea, Daphnia magna , normal humans, and humans with enhanced S-cone syndrome, a condition in which S-cone frequency is increased due to mutations in a transcription factor that controls photoreceptor expression. Data from the Asian swallowtail, Papilio xuthus , which has at least five main spectral photoreceptor classes in its compound eyes, were included to illustrate potential effects of model over-simplification on multi-model inference. The multi-model framework was then used with parameters of spectral photoreceptor classes and the structural photoreceptor array kept constant. The goal was to map relative opsin expression to visual pigment concentration. It identified relative opsin expression differences for two populations of the bluefin killifish, Lucania goodei . The modeling approach presented here will be useful in selecting the most likely alternative hypotheses of opsin-based spectral photoreceptor classes, using relative opsin expression and extracellular electroretinography.