E. coli long-term evolution experiment

 http://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment

 

E. coli long-term evolution experiment

From Wikipedia, the free encyclopedia

The 12 evolving E. coli populations on June 25, 2008

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.^[1] The populations reached the milestone of 50,000 generations in February 2010.
Since the experiment's inception, Lenski and his colleagues have reported a wide array of genetic changes; some evolutionary adaptations have occurred in all 12 populations, while others have only appeared in one or a few populations. One particularly striking adaption was the evolution of a strain of E. coli that was able to use citric acid as a carbon source in an aerobic environment.^[2]

Contents

• 1 Experimental approach
  • 1.1 Methods
• 2 Results
  • 2.1 Evolution of aerobic citrate usage in one population
  • 2.2 Evolution of increased cell size in all twelve populations
• 3 References
• 4 See also
• 5 Further reading
• 6 External links

Experimental approach

The long-term evolution experiment was intended to provide experimental evidence for several of the central questions of evolutionary biology: how rates of evolution vary over time; the extent to which evolutionary changes are repeatable in separate populations with identical environments; and the relationship between evolution at the phenotypic and genomic levels.^[3]
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, and has made experimental procedures (refined over decades of E. coli use in molecular biology) fairly simple. The bacteria can also be frozen and preserved, creating what Lenski has described as a "frozen fossil record" that can be revived at any time (and can be used to restart recent populations in cases of contamination or other disruption of the experiment). Lenski chose an E. coli strain that reproduces only asexually, without bacterial conjugation; this limits the study to evolution based on new mutations and also allows genetic markers to persist without spreading except by common descent.^[3]

Methods

Each of the 12 populations is kept in an incubator in Lenski's laboratory at Michigan State University in a minimal growth medium. Each day, 1% of each population is transferred to a flask of fresh growth medium. Under these conditions, each population experiences 6.64 generations, or doublings, each day. Large, representative samples of each population are frozen with glycerol as a cryoprotectant at 500-generation (75 day) intervals. The bacteria in these samples remain viable, and can be revived at any time. This collection of samples is referred to as the "frozen fossil record", and provides a history of the evolution of each population through the entire experiment. The populations are also regularly screened for changes in mean fitness, and supplemental experiments are regularly performed to study interesting developments in the populations.^[4] As of October 2012, the E. coli populations have been under study for over 56,000 generations, and are thought to have undergone enough spontaneous mutations that every possible single point mutation in the E. coli genome has occurred multiple times.^[2]
The initial strain of E. coli for Lenski's long-term evolution experiment came from "strain Bc251", as described in a 1966 paper by Seymour Lederberg, via Bruce Levin (who used it in a bacterial ecology experiment in 1972). The defining genetics traits of this strain were: T6^r, Str^r, r⁻m⁻, Ara⁻ (unable to grow on arabinose).^[1] Before the beginning of the experiment, Lenski prepared an Ara⁺ variant (a point mutation in the ara operon that enables growth on arabinose) of the strain; the initial populations consisted of 6 Ara⁻ colonies and 6 Ara⁺ colonies, which allowed the two sets of strains to be differentiated and tested for fitness against each other. Unique genetic markers have since evolved to allow identification of each strain.

Results

Growth in cell size of bacteria in the Lenski experiment

In the early years of the experiment, several common evolutionary developments were shared by the populations. The mean fitness of each population, as measured against the ancestor strain, increased, rapidly at first, but leveled off after close to 20,000 generations (at which point they grew about 70% faster than the ancestor strain). All populations evolved larger cell volumes and lower maximum population densities, and all became specialized for living on glucose (with declines in fitness relative to the ancestor strain when grown in dissimilar nutrients). Of the 12 populations, four developed defects in their ability to repair DNA, greatly increasing the rate of additional mutations in those strains. Although the bacteria in each population are thought to have generated hundreds of millions of mutations over the first 20,000 generations, Lenski has estimated that within this time frame, only 10 to 20 beneficial mutations achieved fixation in each population, with fewer than 100 total point mutations (including neutral mutations) reaching fixation in each population.^[3]

The population designated Ara-3 (center) is more turbid because that population evolved to use the citrate present in the growth medium.

Evolution of aerobic citrate usage in one population

In 2008, Lenski and his collaborators reported on a particularly important adaptation that occurred in the population called Ara-3: the bacteria evolved the ability to grow on citrate under the oxygen-rich conditions of the experiment. Wild-type E. coli cannot grow on citrate when oxygen is present due to the inability during aerobic metabolism to produce an appropriate transporter protein that can bring citrate into the cell, where it could be metabolized via the citric acid cycle. The consequent lack of growth on citrate under oxic conditions, referred to as a Cit^- phenotype, is considered a defining characteristic of the species that has been a valuable means of differentiating E. coli from pathogenic Salmonella. Around generation 33,127, the experimenters noticed a dramatically expanded population-size in one of the samples; they found clones in this population could grow on the citrate included in the growth medium to permit iron acquisition. Examination of samples of the population frozen at earlier time points led to the discovery that a citrate-using variant (Cit⁺) had evolved in the population at some point between generations 31,000 and 31,500. They used a number of genetic markers unique to this population to exclude the possibility that the citrate-using E. coli were contaminants. They also found the ability to use citrate could spontaneously re-evolve in a subset of genetically pure clones isolated from earlier time points in the population's history. Such re-evolution of citrate use was never observed in clones isolated from before generation 20,000. Even in those clones that were able to re-evolve citrate use, the function showed a rate of occurrence on the order of one occurrence per trillion cell divisions. The authors interpret these results as indicating that the evolution of citrate use in this one population depended on one or more earlier, possibly nonadaptive "potentiating" mutations that had the effect of increasing the rate of mutation to an accessible level. (The data they present further suggests that citrate use required at least two mutations subsequent to this "potentiating" mutation) More generally, the authors suggest these results indicate (following the argument of Stephen Jay Gould) "that historical contingency can have a profound and lasting impact" on the course of evolution.^[2]
In 2012, a team of researchers working under Lenski reported the results of a genomic analysis of the Cit⁺ trait that shed light on the genetic basis and evolutionary history of the trait.^[5] The researchers had sequenced the entire genomes of twenty-nine clones isolated from various time points in the Ara-3 population's history. They used these sequences to reconstruct the phylogenetic history of the population, which showed that the population had diversified into three clades by 20,000 generations. The Cit⁺ variants had evolved in one of these, which they called Clade 3. Clones that had been found to be potentiated in earlier research were distributed among all three clades, but were over-represented in Clade 3. This led the researchers to conclude that there had been at least two potentiating mutations involved in Cit⁺ evolution. The researchers also found that all Cit⁺ clones sequenced had in their genomes a duplication mutation of 2933 base pairs that involved the gene for the citrate transporter protein used in anaerobic growth on citrate, citT. The duplication is tandem, resulting in two copies that are head-to-tail with respect to each other. This duplication immediately conferred the Cit⁺ trait by creating a new regulatory module in which the normally silent citT gene is placed under the control of a promoter for an adjacent gene called rnk. The new promoter activates expression of the citrate transporter when oxygen is present, and thereby enabling aerobic growth on citrate. Movement of this new regulatory module (called the rnk-citT module) into the genome of a potentiated Cit^- clone was shown to be sufficient to produce a Cit⁺ phenotype. However, the initial Cit⁺ phenotype conferred by the duplication was very weak, and only granted a ~1% fitness benefit. The researchers found that the number of copies of the rnk-citT module had to be increased to strengthen the Cit⁺ trait sufficiently to permit the bacteria to grow well on the citrate, and that further mutations after the Cit⁺ bacteria became dominant in the population continued to accumulate that refined and improved growth on citrate. The researchers conclude that the evolution of the Cit⁺ trait suggests that new traits evolve through three stages: potentiation, in which mutations accumulate over a lineage's history that make a trait accessible; actualization, in which one or more mutations render a new trait manifest; and refinement, in which the trait is improved by further mutations.

Evolution of increased cell size in all twelve populations

All twelve of the experimental populations show an increase in cell size, and in many of the populations, a more rounded cell shape.^[6] This change was partly the result of a mutation that changed the expression of a gene for a penicillin binding protein, which allowed the mutant bacteria to outcompete ancestral bacteria under the conditions in the long-term evolution experiment. However, although this mutation increased fitness under these conditions, it also increased the bacteria's sensitivity to osmotic stress and decreased their ability to survive long periods in stationary phase cultures.^[6]