Replicability and recurrence in the experimental evolution of a group I ribozyme

In order to explore the variety of possible responses available to a ribozy me population evolving a novel phenotype, five Tetrahymena thermophila grou p I intron ribozyme pools were evolved in parallel for cleavage of a DNA ol igonucleotide. These ribozyme populations were propagated under identical c onditions and characterized when they reached apparent phenotypic plateaus; the populations that reached the highest plateau showed a near 100-fold im provement in DNA cleavage activity. A detailed characterization of the evol ved response in these populations reveals at least two distinct phenotypic trajectories emerging as a result of the imposed selection. Not only do the se distinct solutions exhibit differential DNA cleavage activity, but they also exhibit a very different correlation with a related, but unselected, p henotype: RNA cleavage activity. In turn, each of these trajectories is und erwritten by differing genotypic profiles. This study underscores the compl ex network of possible trajectories through sequence space available to an evolving population and uncovers the diversity of solutions that result whe n the process of experimental evolution is repeated multiple times in a sim ple, engineered system.