Chromosomal changes during experimental evolution in laboratory populations of Escherichia coli

Abstract

Short-term rates of chromosome evolution were analyzed in experimental populations of Escherichia coli B that had been propagated for 2,000 generations under four thermal regimens. Chromosome alterations were monitored in 24 independent populations by pulsed-field gel electrophoresis of DNA treated with five rare-cutting restriction enzymes. A total of 11 changes, 8 affecting chromosome size and 3 altering restriction sites, were observed in these populations, with none occurring in strains cultured at 37 degreesC. Considering the changes detected in these experimental populations, the rate of chromosome alteration of E. coli is estimated to be half of that observed in experimental populations of yeast.

FIG. 1

Genealogical relationships and growth conditions of E. coli strains. Strains 2037 to 2042 were grown at 32°C; strains 2043 to 2046, 2143, and 2144 were grown at 37°C; and strains 2047 to 2051 and 2142 were grown at 42°C. Strains 2052 to 2057 were grown at temperatures that alternated daily between 32 and 42°C. Chromosomal changes occurring in these strains are denoted as follows: deletions, del; duplications, dupl; inversions, inv; point mutations, pt. Strains were grown in 10 ml of glucose limited minimal medium, and each day, 0.1 ml of culture was transferred to 9.9 ml of fresh medium (4). At 37°C, the population size of the ancestral strain in this medium is 4 × 10⁸ cells in stationary phase (4). The common ancestor of all populations, strain 1206, was derived from a population, seeded with strain 606, that had been evolving for 2,000 generations at 37°C (14).