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Mon November 19, 2012
Dr. Zachary Blount, Michigan State University – Evolving Bacteria
In today’s Academic Minute, Dr. Zachary Blount of Michigan State University explains how scientists have observed bacteria evolve new capabilities over thousands of generations.
Zachary Blount is a postdoctoral researcher in the BEACON Center for the Study of Evolution in Action at Michigan State University. Blount is a research associate in the Lenski lab, where evolutionary biologists have observed and recorded changes in E. coli bacteria over more than 50,000 generations.
Dr. Zachary Blount – Evolving Bacteria
At our lab, we were able to examine the step-by-step process by which E. coli bacteria evolved to supplement a traditional diet of sugary glucose with an extra course of tangy citrate. These bacteria evolved during Richard Lenski’s long-term evolution experiment with E. coli. This experiment began in 1988 and has allowed scientists to study more than 56,000 generations of bacterial evolution in action.
Samples of the bacterial populations are frozen every 500 generations, and the bacteria in these samples remain alive. Think of it as a frozen fossil record that contains whole, living organisms! So when something interesting evolves in the experiment, scientists can go back to earlier generations, and figure out the steps along the way.
This allowed us to figure out that the ability to eat citrate evolved through three stages. The first was potentiation, during which E. coli accumulated at least two mutations that set the stage for later events. In the second, actualization, at least one more mutation allowed the bacteria to start eating citrate, but just barely. In the final stage, refinement, mutations were accumulated that greatly improved the ability to eat citrate, allowing the bacteria to wolf down their new food source, and to dominate the population.
We were particularly excited about the actualization stage. The mutation it involved is not a simple change of a letter in the bacteria’s DNA. Instead, duplication of a part of the bacterial DNA rewired some genes, and created a new regulatory module that had not existed before. The new module allows the bacteria to make a transporter that can bring citrate into the cell when oxygen is present. This is a new trick for E. coli. And the three stages by which it evolved are likely the same ones involved in the evolution of any new trait, meaning that studying this new trick in a humble bacterium has shed light on how all life has evolved.