Evolutionary consequences of experimental transfer into yeast populations of an animal transposon

Wake Forest University, Winston Salem NC

Investigators

Abstract

This project will investigate a major process of evolutionary change, the rearrangement of genomes by transposable genetic elements. Plant, animal and fungal genome sizes vary widely. For instance, the lungfish genome is 10,000 times as large as a yeast genome. In some plants, genomes are even larger. This tremendous variation in the amount of DNA is not just related to the complexity or the size of the organism. It results mostly from variation in the abundance of short DNA sequences that spread copies of themselves among the chromosomes of their host. Known as transposable elements, these "jumping genes" can multiply within a genome until they make up most of an organism's DNA. Knowing how this occurs will allow us to understand why there is such great variation in genome size. Jumping genes are also important because they cause mutations when they jump. Because these DNA sequences can spread and diversify by exploiting rather than benefitting the organism that carries them, jumping genes require us to think beyond the function of a given gene in trying to understand the genome. This project will allow observation of a jumping gene's spread directly, as it occurs, in the genome of baker's yeast. Broader impacts of the proposed work include the training of graduate and undergraduate students in evolutionary genomics. The transposable element that will be used as an experimental invader is Hermes. It is a transposable element isolated from the housefly. It has an entirely different structure and mechanism of transposition from any of the transposable elements found in the yeast genome. This Hermes transposable element has been modified for high transposition rates in yeast under the control of a galactose genetic promoter. Once Hermes is introduced to the yeast, hundreds of yeast lineages will be evolved for hundreds of generations. Selective gene sequencing of areas surrounding the transposable elements will be used to track proliferation or movement of Hermes over time. Hypotheses will be tested concerning three questions. First, what enables a foreign transposable element to invade a new population? Second, what are the effects of a newly established transposable element on the ability of that population to respond to new selective pressures? The final question is, how quickly do new mechanisms to suppress an alien transposon evolve in a host population under strong selection from a transposable element with very high transposition rates?