On the evolution of new metabolic functions in diploid organisms.

Evolution of lactose utilization via the ebg system of Escherichia coli requires both structural gene (ebgA) and regulatory gene (ebgR) mutations. Because evolution of new metabolic functions in diploids might be subject to constraints not present in haploid organisms, merodiploid strains carrying a wild-type and an evolved ebgA allele, or a wild-type and an evolved ebgR allele were constructed. I show that heterozygosity at ebgA does not significantly affect the selective advantage of the evolved ebgA allele; whereas heterozygosity at ebgR eliminates the selective advantage of the evolved ebgR allele. Is is suggested that, in diploid organisms, evolution of new functions for systems under negative control would be very difficult.