Monitoring the Rate and Dynamics of Concerted Evolution in the Ribosomal DNA Repeats of Saccharomyces cerevisiae Using Experimental Evolution

Concerted evolution describes the unusual evolutionary pattern exhibited by certain repetitive sequences, whereby all the repeats are maintained in the genome with very similar sequences but differ between related species. The pattern of concerted evolution is thought to result from continual turnover of repeats by recombination, a process known as homogenization. Approaches to studying concerted evolution have largely been observational because of difficulties investigating repeat evolution in an experimental setting with large arrays of identical repeats. Here, we establish an experimental evolution approach to look at the rate and dynamics of concerted evolution in the ribosomal DNA (rDNA) repeats. A small targeted mutation was made in the spacer of a single rDNA unit in Saccharomyces cerevisiae so we could monitor the fate of this unit without the need for a selectable marker. The rate of loss of this single unit was determined, and the frequency of duplication was also estimated. The results show that duplication and deletion events occur at similar rates and are very common: An rDNA unit may be gained or lost as frequently as once every cell division. Investigation of the spatial dynamics of rDNA turnover showed that when the tagged repeat unit was duplicated, the copy predominantly, but not exclusively, ended up near to the tagged repeat. This suggests that variants in the rDNA spread in a semiclustered fashion. Surprisingly, large deletions that remove a significant fraction of total rDNA repeats were frequently found. We propose these large deletions are a driving force of concerted evolution, acting to increase homogenization efficiency over-and-above that afforded by turnover of individual rDNA units. Thus, the results presented here enhance our understanding of concerted evolution by offering insights into both the spatial and temporal dynamics of the homogenization process and suggest an important new aspect in our understanding of concerted evolution.