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Title: Rudimentary organs and genes : evolution of the visual system in caecilians
Author: Mohun, Samantha Mila
ISNI:       0000 0004 2674 1966
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2008
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As Darwin (1859 pp143-146) noted, each major group of animals has evolved a visual system exhibiting unique adaptations to visual tasks. Caecilians are elongate, snake-like amphibians (the order Gymnophiona) that, except for the secondarily aquatic Typhlonectidae, are to some extent fossorial as adults. Caecilian eye morphology shows various degrees of rudimentation but the literature contains many contradictory reports, and phylogenetic characters based on eye morphology are controversial. New physiological, molecular and morphological data are presented here in order to better characterise the caecilian visual system and to investigate the utility of visual gene sequences in phylogenetics. I provide the first description of the morphology of the eye of a member of the Rhinatrematidae, the sister group of all other caecilians and additional observations on a range of caecilian species. Morphological discoveries include the presence of a poorly developed iris in both the Rhinatrematidae and Ichthyophiidae. I elucidate the previously unknown complete sequence of the caecilian rod opsin gene and present new data on the physiology caecilian rhodopsin. The obvious reduction in visual system morphology in caecilians is associated with a change in the genes involved in visual function. The rod opsin is the only visual pigment detected in caecilians suggesting a loss of colour and diurnal vision, which are probably unnecessary in these predominately burrowing vertebrates. A change in the maximum absorbance of the caecilian rhodopsin compared to other amphibians is reported and experimental confirmation that this is caused by amino-acid substitutions in the rod opsin gene is presented. The pattern of substitutions implies a previously unknown mechanism of spectral tuning. Phylogenetic analyses of caecilian rod opsin sequences give results that reflect current understanding of caecilian relationships and suggest both coding and non-coding regions are useful markers for inferring phylogenetic relationships within caecilians.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available