Factors Affecting Genetic Variation in Tetrahymena Thermophila

In order to understand how evolution proceeds, we must determine how heritable variation is generated, passed from one generation to the next, and acted on by various evolutionary forces. Here, I review three factors that influence how genetic variation progresses through populations and can be acted on by various evolutionary forces. Specifically, I will discuss 1) biogeography, 2) genome architecture, and 3) reproductive strategies. I highlight how these factors apply to the ciliated microbial eukaryote Tetrahymena. 1) I found that this species has a distribution restricted to the Eastern United States, with high diversity in the northeast and low diversity across the rest of its distribution, supporting the moderate endemity model of protist biogeography. 2) To assess the effects of the unusual genome architecture of T. thermophila on the maintenance of genetic variation, I compared variation in fitness in the micronuclear and macronuclear genomes of natural isolates and found that T. thermophila genome architecture allows for genetic variation in fitness to be hidden in the micronuclear genome. 3) Among asexual lineages maintained in Tetrahymena, I expected that genes involved in sexual reproduction would no longer be functional, and would thus be under relaxed selection. In contrast to this prediction, I found evidence that Tetrahymena amicronucleate lineages are experiencing purifying selection at the tm regions of the mat locus, the only known function of which is in mating type specification for sexual conjugation. These results suggest that the tm regions of the mat locus may serve some function in Tetrahymena asexual amicronucleates that remains to be discovered.Biology and Biochemistry, Department o