A MOLECULAR INVESTIGATION OF GENOTYPE BY ENVIRONMENT INTERACTIONS

The fitnesses conferred by seven lactose operons, which had been trans duced into a common genetic background from natural isolates of Escher ichia coli, were determined during competition for growth rate-limitin g quantities of galactosyl-glycerol, a naturally occurring galactoside . The fitnesses of these same operons hare been previously determined on lactose and three artificial galactosides, lactulose, methyl-galact oside and galactosyl-arabinose. Analysis suggests that although marked genotype by environment interactions occur, changes in the fitness ra nkings are rare. The relative activities of the beta-galactosidases an d the permeases were determined on galactosyl-glycerol, lactose, lactu lose and methyl-galactoside. Both enzymes display considerable kinetic variation. The beta-galactosidase alleles provide no evidence for gen otype by environment interactions at the revel of enzyme activity. The permease alleles display genotype by environment interactions with a few causing changes in activity rankings. The contributions to fitness made by the permeases and the beta-galactosidases were partitioned us ing metabolic control analysis. Most of the genotype by environment in teraction at the level of fitness is generated by changes in the distr ibution of control among steps in the pathway particularly at the perm ease where large control coefficients ensure that its kinetic variatio n has marked fitness effects. Indeed, changes in activity rankings at the permease account for the few changes in fitness rankings. In contr ast, the control coefficients of the beta-galactosidase are sufficient ly small that its kinetic variation is in, or close to, the neutral li mit. The selection coefficients are larger on the artificial galactosi des because the control coefficients of the permease and beta-galactos idase are larger. The flux summation theorem requires that control coe fficients associated with other steps in the pathway must be reduced, implying that the selection at these steps will be less intense on the artificial galactosides. This suggests that selection intensities nee d not be greater in novel environments.