UC San Diego

When a gene experiences new selective pressures from its environment, adaptative mutations may arise that alter its amino acid sequence that produce a protein better suited to the new pressure, or through mutations that tune the gene’s expression to adjust to the new environment. Here we report evolution experiments to understand the second mode of adaptation by studying the evolution of λ, a bacterial virus or bacteriophage that infects E. coli, under controlled laboratory settings. We pressured  to alter its lysis timing, which is controlled by a well-studied gene regulatory system, primarily driven by the S gene. For 10 days, we pressured λ towards shortened or lengthened lysis times and found that  evolved shorter lysis times repeatedly in both treatments. Lysis time evolved in most replicates without evolving cis-regulatory S mutations, suggesting more genes are involved in lysis timing circuit than previously reported. We found a single S regulatory mutation among 12 experimental populations. This mutation account for some of the fitness gains  evolved, and the mutation also increased the stochasticity of lysis timing. We hypothesize that the stochasticity helped increase phenotypic variance and allowed  to adapt to changing pressures. Altogether we found that  rapidly adapts to selective pressure on lysis timing, but in unintuitive ways. Clearly a more sophisticated understanding how gene regulation networks respond to evolutionary pressure is required to predict adaptation.