Rebuttal: adaptive mutation in Escherichia coli (Foster).

Abstract

If mutations arose in nongrowing cells, as proposed in Foster’s paper (1), then stress-induced mutagenesis would seem inescapable. The plated population certainly grows very little. However, no evidence is provided that reversion occurs in this population and no effort is made to counter evidence to the contrary (see below)—that mutations actually arise in cells growing under selection within developing clones (rendering mutagenesis dispensable). (i) Reversion requires that the lac allele be leaky and lactose be present in the selection medium, suggesting a growth requirement. (ii) General mutagenesis is not induced by simple starvation (3) but is seen in the Cairns system when lactose is provided, suggesting dependence on growth. (iii) Clonally related unstable and stable lac cells are found within all revertant colonies, consistent with both types arising sequentially within a growing colony. (iv) Revertant number is strongly reduced if one inhibits growth of cells carrying a lac amplification. (v) Revertant number increases if a lac duplication is provided in the parent cells.