The E. coli long-term evolution experiment (LTEE''') is an ongoing study in experimental evolution begun by Richard Lenski at the University of California, Irvine, carried on by Lenski and colleagues at Michigan State University, and currently overseen by Jeffrey E. Barrick at the University of Texas at Austin. It has been tracking genetic changes in 12 initially identical populations of asexual Escherichia coli bacteria since 24 February 1988. Lenski performed the 10,000th transfer of the experiment on March 13, 2017. The populations reached over 73,000 generations in early 2020, shortly before being frozen because of the COVID-19 pandemic. In September 2020, the LTEE experiment was resumed using the frozen stocks.
Over the course of the experiment, Lenski and his colleagues have reported a wide array of phenotypic and genotypic changes in the evolving populations. These have included changes that have occurred in all 12 populations and others that have only appeared in one or a few populations. For example, all 12 populations showed a similar pattern of rapid improvement in fitness that decelerated over time, faster growth rates, and increased cell size. Half of the populations have evolved defects in DNA repair that have caused mutator phenotypes marked by elevated mutation rates. The most striking adaptation reported so far is the evolution of aerobic growth on citrate, which is unusual in E. coli, in one population at some point between generations 31,000 and 31,500.
On May 4, 2020, Lenski announced a 5-year renewal of the grant through the National Science Foundation's Long-Term Research in Environmental Biology (LTREB) Program that supports the LTEE. He also announced that Dr. Jeffrey E. Barrick, an associate professor of Molecular Biosciences at The University of Texas at Austin, would take over supervision of the experiment within the 5-year funding period. The experiment's time at Michigan State University ended in May 2022, when the populations reached 75,000 generations, and the experiment was transferred to Barrick's lab. The experiment was revived and restarted in Barrick's lab on June 21, 2022.
Experimental approach
The long-term evolution experiment was designed as an open-ended means of empirical examination of central features of evolution. The experiment was begun with three principal goals: To examine the dynamics of evolution, including the rate of evolutionary change. To examine the repeatability of evolution. To better understand the relationship between change on the phenotypic and genotypic levels. As the experiment has continued, its scope has grown as new questions in evolutionary biology have arisen that it can be used to address, as the populations' evolution has presented new phenomena to study, and as technology and methodological techniques have advanced.
The use of E. coli as the experimental organism has allowed many generations and large populations to be studied in a relatively short period of time. Moreover, due to the long use of E. coli as a principle model organism in molecular biology, a wide array of tools, protocols, and procedures were available for studying changes at the genetic, phenotypic, and physiological levels. The bacteria can also be frozen and preserved while remaining viable. This has permitted the creation of what Lenski describes as a "frozen fossil record" of samples of evolving populations that can be revived at any time. This frozen fossil record allows populations to be restarted in cases of contamination or other disruption in the experiment, and permits the isolation and comparison of living exemplars of ancestral and evolved clones. Lenski chose an E. coli strain that reproduces only asexually, lacks any plasmids that could permit bacterial conjugation, and has no viable prophage. As a consequence, evolution in the experiment occurs only by the core evolutionary processes of mutation, genetic drift, and natural selection. This strict asexuality also means that genetic markers persist in lineages and clades by common descent, but cannot otherwise spread in the populations.