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Abstract
My research addresses two fundamental problems in evolutionary biology: the evolution of dominance and the genetics of speciation.
In Chapter 1, I focus on the problem of dominance. Kacser and Burns metabolic theory provides a paradigmatic explanation of why large-effect mutations are recessive. In Chapter 1, I use gene deletion data from Saccharomyces cerevisiae to reveal a negative correlation between dominance and selection coefficients among mutations. Surprisingly, this negative correlation holds for all classes of genes, including those that do not encode enzymes. Because the metabolic theory depends on the mathematics of enzyme kinetics, it should hold only for mutations at genes that encode enzymes, but not for other classes of genes. My findings show that Kacser and Burns's theory, in its present form, cannot provide a complete explanation of dominance.
I next focus on the problem of speciation, the process by which one species splits into two. The Dobzhansky-Muller model shows how hybrid incompatibilities that cause postzygotic isolation can evolve unimpeded by natural selection. However, previous researchers have identified very few genes that cause hybrid sterility or inviability. Furthermore, we know very little about the particular ecological or genomic forces that drive the observed rapid evolution of these genes. In Chapter 2, I perform a high-resolution analysis of a chromosomal region having a large effect on hybrid sterility and segregation distortion in the Drosophila pseudoobscura Bogota-USA hybridization. Using transgenic experiments, I show that a single gene causes both hybrid sterility and segregation distortion. This work provides direct evidence that genetic conflict may drive the rapid evolution of postzygotic isolation.
In Chapter 3, I perform quantitative trait locus mapping of the X and second chromosomes to identify regions that cause hybrid sterility and segregation distortion in the Bogota-USA hybridization. I find few chromosomal regions that have a large effect on both hybrid sterility and segregation distortion. I further tentatively map an X-linked locus with a large effect on hybrid sterility to a 50kb region that includes only two genes. This work lays the foundation for the eventual identification and characterization of additional hybrid sterility and segregation distortion genes.
