Evolutionary genetics of the tidepool copepod Tigriopus californicus

This work examines the molecular population genetics of hybrid breakdown in the harpacticoid copepod Tigriopus californicus. The aim of this work was to use classical population genetic analysis of natural populations together with molecular genetics to characterize the genetic basis of reduced F2 hybrid fitness. Each of the chapters of this dissertation makes a unique contribution to our understanding of the genetic consequences of hybridization. In Chapter II, I combine time series data from natural populations with DNA sequence polymorphism data to characterize the extent of population exchange among local T. californicus populations. A primary result from this work is that gene flow among local populations is likely higher than previous data suggested. This has implications for our understanding of local population differentiation and the consequences of hybridization between T. californicus populations. In Chapters III and IV, I investigated two transcription systems, RNA polymerase I and mitochondrial RNA polymerase. Our interest in these systems primarily comes from their unusual mode of evolution. We hypothesized that rapid evolution of these transcription systems may be detrimental to the fitness of hybrid organisms. The data presented in Chapter III provide evidence of large-scale alterations in regulation of ribosomal RNA by the RNA polymerase I system in hybrids. Although the consequences of this for hybrid fitness are not clear, our results have significant implications for the molecular mechanisms responsible for the hybrid phenomenon of nucleolar dominance. In Chapter IV, results are presented on population differentiation of mitochondrial RNA polymerase, mtRPOL, and a mitochondrial transcription factor, mtTFBI. We hypothesized that elevated rates of genome evolution in T. californicus mitochondrial DNA would select for accelerated changes in the region of the polymerase responsible for promoter recognition. T. californicus populations have diverged substantially at this locus, but we found no evidence of rapid divergence in the region responsible for promoter recognition. In a separate study, I found that the mtRPOL and mtTFBI are tightly linked. A segregation study of the linkage group marked by these genes found a large viability effect of the mtRPOL/mtTFB1 locus on hybrid fitness. This effect is attributable to viability effects that are dependent on the cytoplasmic background. Finally, Appendix A presents work on variance in reproductive success in sea urchins that is unrelated to the main body of work. This was the product of research I conducted during the first two years of my dissertation and is included here as a supplement to the work conducted on T. californicus.