Graduation Year

2005

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Biology

Major Professor

Brian T. Livingston.

Keywords

Molecular evolution, Sea urchin, DNA, Spicule matrix genes, Neutral evolution

Abstract

Genes present in multiple copies and genes that contain regions of repetitive sequences can undergo concerted evolution, which results in homogenization of the nucleotide sequence of the genes or repetitive regions. In regions of tandem repeats, this occurs through misalignment of repeat units followed by unequal crossover, which generates two products with differing numbers of repeat units. Gene conversion is thought to lead to one of these products becoming fixed in a species. The homogenous sequence of previously studied genes that have been thought to undergo this process has made it difficult to determine the exact models involved. Here I examine concerted evolution in SM50, a sea urchin gene that encodes a protein involved in biomineralization. The repetitive region in the SM50 gene varies in length between species, and there is variability in each repeat unit as well.

I examine the codon usage in SM50 in a variety of species, and discuss how purifying selection, substitutions, concerted evolution, and selection at the level of DNA sequence have played a role in the evolution of this gene. I also examine the structure and sequence of the repeat units, and purpose models that have led to the evolution of the repeat pattern seen in the different species examined. Finally, I have found variation in the number of repeat units within several species. This has allowed us to deduce the specific models of unequal crossover that led to this variation. The unique variation in the repetitive region of SM50 has enabled us to describe a model of how substitutions affect the model of misalignment and unequal crossover.

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