Mutator genomes decay, despite sustained fitness gains, in a long-term experiment with bacteria

Bacterial genomes are extremely diverse in size and composition. Biologists have long sought to explain such variability based on present-day selective and mutational forces. However, mutation rates can change dramatically over time, and experiments with hypermutable bacteria show that their genomes rapidly decay when propagated under the near absence of selection. Whether selection can prevent this decay is unclear. Here, we document the rapid genome decay of hypermutable bacteria even during tens of thousands of generations of sustained adaptation to a laboratory environment. These findings suggest the need to reexamine current ideas about the evolution of bacterial genomes, and they have implications for other hypermutable systems such as viruses and cancer cells.