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R E Lenski

Publications and source records attributed to R E Lenski.

11 recordsLinked to original sources
PubMed1992

Hierarchical analysis of linkage disequilibrium in Rhizobium populations: evidence for sex?

Many bacterial species exhibit strong linkage disequilibrium of their chromosomal genes, which apparently indicates restricted recombination between alleles at different loci. The extent to which restricted recombination reflects limited migration between geographically isolated populations versus infrequent mixis of genotypes within populations is more difficult to determine. We examined the genetic structure of Rhizobium leguminosarum biovar phaseoli populations associated with wild and cultivated beans (Phaseolus spp.) over several spatial scales, ranging from individual host plants to throughout the Western Hemisphere. We observed significant linkage disequilibrium at scales at least as small as a cultivated plot. However, the amount of disequilibrium was much greater among isolates collected throughout the Western Hemisphere than among isolates from one area of Mexico, even when disequilibrium was quantified using an index that scales for allelic diversity. This finding suggests that limited migration between populations contributes substantially to linkage disequilibrium in Rhizobium. We also compared the genetic structure for R. leguminosarum bv. phaseoli taken from a cultivated plot with that for Escherichia coli obtained from one human host in an earlier study. Even at this fine scale, linkage disequilibrium in E. coli was very near the theoretical maximum level, whereas it was much less extreme in the local population of Rhizobium. Thus, the genetic structure for R. leguminosarum bv. phaseoli does not exclude the possibility of frequent mixis within local populations.

Enzymes↗
PubMed1992

Experimental evidence for an alternative to directed mutation in the bgl operon.

The directed mutation hypothesis suggests that some mutations occur more often when selectively advantageous than when neutral or disadvantageous, challenging the principle that the selective value of a mutation does not affect the rate of its occurrence. Mutations in the bgl operon of Escherichia coli have been reported to be a case of directed mutation. E. coli K12 strains chi342LD cannot grow on salicin but derivatives with two mutations in the bgl operon, an excision of IS150 (formally called IS103) from bglF and a point mutation or insertion in bflR, grow rapidly on this sugar. When chi342LD is grown on a medium containing salicin, bglF excision mutants accumulate to a frequency of greater than 1%, even though these mutants are reportedly unable to grown on salicin, and Sal+ double mutants subsequently attain a high frequency. Comparable accumulations of excision mutants and Sal+ double mutants are not observed in the absence of salicin. As salicin is not mutagenic, it has been suggested that excision mutations in bglF might serve only to create the potential for a secondary selectively advantageous mutation. We show here, however, that these double mutants can be accounted for by spontaneous mutation to intermediate genotypes in non-growing populations, coupled with slow growth of some of these intermediates on salicin, which enables their populations to reach a size where secondary mutations allowing rapid growth on salicin become common.

Benzyl Alcohols↗
PubMed1991

Quantifying fitness and gene stability in microorganisms.

Fitness represents the combined effects of all other phenotypic properties on the capacity for survival and reproduction by a particular genotype in a particular environment. For most environmental application of genetically modified microorganisms, efficacy will be enhanced if the engineered genotype is more fit than its wild-type counterpart in the target environment. However, inadvertent spread of the engineered genotype will be less likely if it is less fit than the wild-type. Thus, the fate of a population of genetically engineered microorganisms, and the likelihood and magnitude of any environmental effects (whether beneficial or detrimental), will be strongly influenced by the relative fitnesses of modified and unmodified genotypes. In this chapter, I have presented theoretical principles and empirical methods for determining the relative fitnesses of engineered and wild-type clones. Selection coefficients were used to provide a quantitative measure of the difference in fitness between the two clones in a particular environment. Many engineered genotypes are unstable, such that their frequencies decline with time. Instability may be caused by infidelity of replication or transmission of a particular gene (which is termed segregation), or it may be caused by a difference in the fitness of genotypes that retain or have lost that gene (selection). In this chapter, I have also presented theoretical principles and empirical methods for distinguishing the effects of selection and segregation. Finally, it should be emphasized that selection coefficients and segregation rates can be estimated not only in highly simplified laboratory systems, but also in more complex natural or semi-natural systems, such as microcosms. All that is required is the ability to monitor the relative abundance of two clones (e.g., engineered and wild-type) that share a common environment.

Bacteria↗
PubMed1990

Rapid evolution in response to high-temperature selection.

Temperature is an important environmental factor affecting all organisms, and there is ample evidence from comparative physiology that species and even conspecific populations can adapt genetically to different temperature regimes. But the effect of these adaptations on fitness and the rapidity of their evolution is unknown, as is the extent to which they depend on pre-existing genetic variation rather than new mutations. We have begun a study of the evolutionary adaptation of Escherichia coli to different temperature regimes, taking advantage of the large population sizes and short generation times in experiments on this bacterial species. We report significant improvement in temperature-specific fitness of lines maintained at 42 degrees C for 200 generations (about one month). These changes in fitness are due to selection on de novo mutations and show that some biological systems can evolve rapidly in response to changes in environmental factors such as temperature.

Biological Evolution↗
PubMed1990

New data on excisions of Mu from E. coli MCS2 cast doubt on directed mutation hypothesis.

According to the directed mutation hypothesis, certain mutations in bacteria occur more frequently in environments in which the resulting phenotype is selectively favoured than in non-selective environments. This hypothesis therefore challenges the fundamental tenet that mutations occur spontaneously, irrespective of effects on the organism's fitness. One purported case of directed mutation is the excision of a Mu sequence from Escherichia coli strain MCS2 in minimal lactose-arabinose medium. Here, we show that this case can be more simply explained by an accelerated rate of excision mutation in response to non-specific physiological stresses of starvation and by slight growth of MCS2 on minimal lactose-arabinose medium.

Arabinose↗
PubMed1989

Mutation and selection in bacterial populations: alternatives to the hypothesis of directed mutation.

Bacterial populations have served as model systems for studying evolutionary processes ever since the classic experiments of Luria and Delbrück, which demonstrated the occurrence of mutations prior to selection for the traits they conferred. However, several authors have recently presented experiments suggesting that bacteria may have mechanisms for directing which mutations occur, such that the rate of adaptive mutations is enhanced. Before the hypothesis of directed mutation is accepted, it is imperative to consider alternative hypotheses that might account for the same observations. To this end, we expand upon existing mathematical theory of the dynamics of mutation and selection in clonal populations for two cases of particular interest. The first case concerns selection against mutants before plating; this selection occurs as the result of differences in growth rate between mutants and nonmutants. We demonstrate that this selection model gives rise to distributions of mutants, obtained by plating from sister cultures, that are very similar to those expected when some mutations are induced by the selective environment. The second case concerns the sequential incorporation of two mutations as the result of selection for an intermediate genotype after plating. We demonstrate that this two-step mutation model also yields distributions that are similar to those expected when some mutations are induced by the selective environment. These two cases therefore provide alternatives to the hypothesis of directed mutation. We suggest experiments that might be used to examine our alternative hypotheses. We also contrast the hypothesis of directed mutation with the notion of inheritance of acquired characteristics.

Bacteria↗
PubMed1989

Effects of carriage and expression of the Tn10 tetracycline-resistance operon on the fitness of Escherichia coli K12.

We have been examining the consequences of alternative modes of regulation of plasmid-borne, Tn10-encoded tetracycline resistance for the fitness of Escherichia coli. In a tetracycline-free environment, we measured the effects on fitness that were caused by (1) maximally induced expression of the resistance operon, (2) low-level constitutive expression of the resistance protein, (3) residual expression of the repressed resistance operon, (4) carriage of the resistance operon, (5) the remainder of the plasmid genome, and (6) hyperexpression of the repressor protein. We observed large reductions in fitness that were associated with induction and with constitutive expression of the tetracycline-resistance protein, but there was no discernible effect of hyperexpression of the repressor protein. We also observed a small reduction in fitness associated with the remainder of the plasmid genome. However, any reductions in fitness that were caused by residual expression and by carriage of the repressed operon were not more than 0.3%. We conclude that tight gene regulation has eliminated antagonistic pleiotropic effects of the resistance gene on fitness, so that possession of an inducible Tn10-encoded tetracycline-resistance operon imposes essentially no burden in the absence of antibiotic.

DNA Transposable Elements↗
PubMed1988

Evolution of a bacteria/plasmid association.

Associations between bacteria and their accessory elements (viruses, plasmids and transposons) range from antagonistic to mutualistic. A number of previous studies have demonstrated that plasmid carriage reduces bacterial fitness in the absence of selection for specific functions such as antibiotic resistance. Many studies have demonstrated increased fitness of evolving microbial populations in laboratory environments, but we are aware of only one study in which fitness gains were partitioned between a plasmid and its host. Here, we examine the evolution of an association between a plasmid and its bacterial host. Carriage of the non-conjugative plasmid pACYC184 initially reduced the fitness of Escherichia coli B in the absence of antibiotic. We then cultured plasmid-bearing bacteria for 500 generations in the presence of antibiotic. The fitness of each combination of host and plasmid, with and without the culture history, was determined by competing it against a baseline strain. The results indicate adaptation by the host genome, but no plasmid adaptation. We also competed the evolved host, transformed with the baseline plasmid, against its isogenic plasmid-free counterpart. The plasmid now increased the fitness of its host.

Adaptation, Physiological↗
PubMed1987

Effects of segregation and selection on instability of plasmid pACYC184 in Escherichia coli B.

We use a mathematical model to analyze the dynamics of loss of nonconjugative pACYC184 from populations of Escherichia coli B in glucose-limited continuous culture. This model incorporates both plasmid segregation and selection against plasmid carriage. It is concluded that there is intense selection against plasmid carriage (s = 0.3 per culture generation), which amplifies the frequency of segregants arising de novo.

Conjugation, Genetic↗