Figure 2.

The Cairns–Foster reversion experiment. Cells of the strain described above are pregrown on glycerol, washed, and plated on lactose medium. The few revertant colonies that appear on lactose within 2 days are initiated by fully lac⁺ revertant cells that arose during prior nonselective pregrowth. More Lac⁺ colonies accumulate on the plate over the next 4 days. On day 5, 90% of new late colonies are made up of stable lac⁺ cells that have acquired a compensating frameshift mutation and, therefore, a functional lac⁺allele. The other 10% of revertant colonies are made up of cells with an amplified array of the original mutant lac allele. Amplification-bearing cells are unstable and frequently lose their Lac⁺ phenotype. When these revertants are streaked on rich, nonselective medium containing the chromogenic LacZ substrate X-gal, they form sectored (blue/white) colonies that reveal their frequent loss of ability to use lactose (see Fig. 3).