Theses and Dissertations from UMD
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Item Mechanisms contributing to opsin expression divergence in the visual system of African Cichlids(2018) Nandamuri, Sri Pratima; Carleton, Karen L; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Vision is an important sensory modality, guiding fundamental tasks such as foraging, escaping from predators, identification of conspecifics and selection of mates. As such, animals exhibit a wide variety of adaptations to spectrally tune their visual systems to closely match the environment. This extensive variation in visual system tuning is achieved both via genetic changes as well as environmental induced plasticity. Lacustrine cichlids of East Africa are famous for their expedited adaptive radiations. Cichlids in these rift lakes inhabit a diverse range of light environments, from the murky red-shifted waters of Lake Victoria to clear Lake Malawi. Consequently, African cichlids have some of the most diverse visual systems among vertebrates on the planet, with species expressing different combinations of seven cone opsin genes. This differential expression is under genetic control and leads to drastic differences in the visual sensitivities between closely related species. Moreover, cichlid species often exhibit plastic changes in opsin expression due to alterations in environmental light conditions. The diversity of genetically determined visual palettes and the variation in expression due to plastic changes offers an excellent opportunity to study the proximate mechanisms governing opsin expression divergence in this group. Utilizing a hybrid cross between two species varying in opsin expression, we show that divergent expression of cone opsins is regulated by multiple quantitative trait loci (QTLs). Most of these QTLs are located in trans to the opsins, with the exception of one QTL in cis to the SWS1 opsin. Further fine-mapping of the cis-QTL revealed a deletion in the promoter of the SWS1 gene that is associated with a decrease in its expression. Additionally, performing two reciprocal experiments, we show that adult cichlids from Lake Malawi show rapid and reversible plastic changes in opsin expression due to differences in lighting conditions. These studies show that both predetermined genetic factors and environmental inputs contribute to opsin expression divergence in cichlids. These factors enable short term and ultimately long-term adaptation to changing habitats, facilitating the survival and perhaps speciation of these fantastic fishes.Item EVOLUTION, DEVELOPMENT, AND GENETICS OF OPSIN GENE EXPRESSION IN AFRICAN CICHLID FISHES(2011) O'Quin, Kelly E; Carleton, Karen L; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The molecular genetic mechanisms that underlie phenotypic evolution include mutations within protein-coding, cis-regulatory, and trans-regulatory factors. Although many studies have examined how these mutations individually contribute to phenotypic divergence and the formation of new species, none have examined how they may do so collectively. In this study, I examine how these molecular genetic mutations collectively contribute to the evolution of color vision among African cichlid fishes. I show that phenotypic divergence in cichlid color vision is achieved by mutations affecting the coding sequence and expression of seven opsin genes. After contrasting the roles of these two mechanisms, I use bioinformatic-, association-, and experimental genetic analyses to determine what role mutations in cis- and trans-regulatory DNA play in the evolution of cichlid opsin expression. Specifically, I demonstrate that: (1) Protein-coding mutations primarily affect cichlid opsins sensitive to the ends of the visible light spectrum (SWS1 [ultraviolet-sensitive] and LWS [red-sensitive]). (2) Changes in opsin gene expression contribute to large differences in color vision among closely related species. These analyses also reveal that the expression of the SWS1 and SWS2B opsins have diverged among closely related cichlids in association with foraging preferences and ambient light intensity, suggesting that their expression has evolved due to natural selection. Ancestral state reconstructions reveal that changes in opsin expression have evolved repeatedly among cichlids in Lakes Tanganyika and Malawi; further, I find that this repeated evolution has likely been achieved by repeated changes to cichlid development. (3) Bioinformatic analyses suggest that cichlids have diverged in multiple cis-regulatory sequences surrounding the opsin genes, and association mapping identified three putative single nucleotide polymorphisms upstream of the SWS2A (blue), RH2B (blue-green), and LWS (red) opsins that may contribute to cichlid opsin expression differences in cis. (4) Genetic mapping in experimental crosses suggests that divergence in multiple trans-regulatory factors also contribute to the evolution of SWS2B (violet), RH2A (green), and LWS (red) opsin expression. The contribution of these trans-regulatory factors to the evolution of cichlid opsin expression may outweigh those in cis. These results reveal that multiple molecular genetic mechanisms can contribute to phenotypic evolution among closely related species.