E. coli long-term evolution experiment
The E. coli long-term evolution experiment is an ongoing study in experimental evolution led by Richard Lenski that has been tracking genetic changes in 12 initially identical populations of asexual Escherichia coli bacteria since 24 February 1988. The populations reached the milestone of 50,000 generations in February 2010 and 60,000 in April 2014.Since the experiment's inception in 1988, Lenski and his colleagues have reported a wide array of genetic changes. Some changes have occurred in all 12 populations and others have only appeared in one or a few populations. For example, all 12 populations experienced improvement in fitness that decelerated over time and some of populations evolved detrimental effects such as defects in DNA repair, causing mutator phenotypes. One of the significant adaptions occurred in one strain of E. coli. In general, this bacteria is known to not being able to use citrate in an aerobic environment as an energy source, even though it could use citrate under anaerobic conditions because it already has the machinery to process citrate. This strain, though ancestrally unable to do so initially, was able to transport citrate for use as an energy source after a duplication mutation that was involved in the gene for the citrate transporter protein used in anaerobic growth. Even though all the ancestors already had a complete citric acid cycle, and thus could metabolize citrate internally for energy during aerobic growth, none of the 12 populations had a transporter protein for citrate since the beginning, which was the only barrier to being able to use citrate for energy in oxygen-rich conditions. Earlier independent studies had already reported E.Coli strains from agricultural or clinical settings that already had the ability to use citrate under aerobic conditions.A genomic study was done to investigate the history of the adaption using clones to isolate the number of mutations needed to develop the trait. It concluded that multiple mutations (at least two or more) such as duplication mutations were needed to allow the transport of citrate for use in energy. For the trait to develop and stick in a population, it needed multiple mutations at three main phases: potentiation (makes a trait possible), actualization (makes the trait manifest), and refinement (makes it effective).