FIG. 6.

Adaptive mutation to Lac⁺ and reversion to Tc^r of strains with the mutant lac and tet alleles located either in cis or in trans. (A) Samples from five independent cultures of each strain were plated with FC29 scavenger cells on lactose medium. Each day, a subset of the plates was overlaid with top agar containing glycerol, X-Gal, and Tc (see Materials and Methods). The data are the mean cumulative numbers of Lac⁺ colonies or Tc^r colonies appearing from days 2 to 5 divided by the number of cells plated. The error bars, some of which are smaller than the symbols, represent the standard errors of the mean. Solid lines, Lac⁺ colonies; dashed lines, Tc^r colonies. In FC1443, the lac and tet alleles are both on the episome (squares); in FC1373, the lac and tet alleles are both on the chromosome (triangles); in FC1487, the lac allele is on the chromosome and the tet allele is on the episome (diamonds). (B) Same as panel A, except only data from strains FC1373 and FC1487 are shown with the y axis expanded.