2022
Mathé-Hubert H; Amie R; Martin M; Gaffé J; Schneider D
Evolution of Bacterial Persistence to Antibiotics during a 50,000-Generation Experiment in an Antibiotic-Free Environment Journal Article
2022-04-30, 11 (4), pp. 451, 2022, ISSN: 2079-6382.
Abstract | Links | BibTeX | Altmetric | Tags: Correlated Responses, Genotypes and Phenotypes
@article{mathe-hubert2022,
title = {Evolution of Bacterial Persistence to Antibiotics during a 50,000-Generation Experiment in an Antibiotic-Free Environment},
author = {Hugo Mathé-Hubert and Rafika Amie and Mikaël Martin and Joël Gaffé and Dominique Schneider},
url = {https://www.mdpi.com/2079-6382/11/4/451},
doi = {10.3390/antibiotics11040451},
issn = {2079-6382},
year = {2022},
date = {2022-03-27},
urldate = {2022-03-27},
journal = {2022-04-30},
volume = {11},
number = {4},
pages = {451},
abstract = {Failure of antibiotic therapies causes >700,000 deaths yearly and involves both bacterial resistance and persistence. Persistence results in the relapse of infections by producing a tiny fraction of pathogen survivors that stay dormant during antibiotic exposure. From an evolutionary perspective, persistence is either a ‘bet-hedging strategy’ that helps to cope with stochastically changing environments or an unavoidable minimal rate of ‘cellular errors’ that lock the cells in a low activity state. Here, we analyzed the evolution of persistence over 50,000 bacterial generations in a stable environment by improving a published method that estimates the number of persister cells based on the growth of the reviving population. Our results challenged our understanding of the factors underlying persistence evolution. In one case, we observed a substantial decrease in persistence proportion, suggesting that the naturally observed persistence level is not an unavoidable minimal rate of ‘cellular errors’. However, although there was no obvious environmental stochasticity, in 11 of the 12 investigated populations, the persistence level was maintained during 50,000 bacterial generations.},
keywords = {Correlated Responses, Genotypes and Phenotypes},
pubstate = {published},
tppubtype = {article}
}
Failure of antibiotic therapies causes >700,000 deaths yearly and involves both bacterial resistance and persistence. Persistence results in the relapse of infections by producing a tiny fraction of pathogen survivors that stay dormant during antibiotic exposure. From an evolutionary perspective, persistence is either a ‘bet-hedging strategy’ that helps to cope with stochastically changing environments or an unavoidable minimal rate of ‘cellular errors’ that lock the cells in a low activity state. Here, we analyzed the evolution of persistence over 50,000 bacterial generations in a stable environment by improving a published method that estimates the number of persister cells based on the growth of the reviving population. Our results challenged our understanding of the factors underlying persistence evolution. In one case, we observed a substantial decrease in persistence proportion, suggesting that the naturally observed persistence level is not an unavoidable minimal rate of ‘cellular errors’. However, although there was no obvious environmental stochasticity, in 11 of the 12 investigated populations, the persistence level was maintained during 50,000 bacterial generations.
Jordan J A; Lenski R E; Card K J
Idiosyncratic fitness costs of ampicillin-resistant mutants derived from a long-term experiment with Escherichia coli Journal Article
Antibiotics, 11 (3), pp. 347, 2022, ISSN: 2079-6382.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Jordan2022.02.06.479266,
title = {Idiosyncratic fitness costs of ampicillin-resistant mutants derived from a long-term experiment with \textit{Escherichia coli}},
author = {Jalin A. Jordan and Richard E. Lenski and Kyle J. Card},
url = {https://www.biorxiv.org/content/10.1101/2022.02.06.479266v1},
doi = {10.3390/antibiotics11030347},
issn = {2079-6382},
year = {2022},
date = {2022-03-13},
urldate = {2022-03-13},
journal = {Antibiotics},
volume = {11},
number = {3},
pages = {347},
publisher = {Cold Spring Harbor Laboratory},
abstract = {Antibiotic resistance is a growing concern that has prompted a renewed focus on drug discovery, stewardship, and evolutionary studies of the patterns and processes that underlie this phenomenon. A resistant strain’s competitive fitness relative to its sensitive counterparts in the absence of drug can impact its spread and persistence in both clinical and community settings. In a prior study, we examined the fitness of tetracycline-resistant clones that evolved from five different Escherichia coli genotypes, which had diverged during a long-term evolution experiment. In this study, we build on that work to examine whether ampicillin-resistant mutants are also less fit in the absence of the drug than their sensitive parents, and whether the cost of resistance is constant or variable among independently derived lines. Like the tetracycline-resistant lines, the ampicillin-resistant mutants were often less fit than their sensitive parents, with significant variation in the fitness costs among the mutants. This variation was not associated with the level of resistance conferred by the mutations, nor did it vary across the different parental backgrounds. In our earlier study, some of the variation in fitness costs associated with tetracycline resistance was explained by the effects of different mutations affecting the same cellular pathway and even the same gene. In contrast, the variance among the ampicillin-resistant mutants was associated with different sets of target genes. About half of the resistant clones suffered large fitness deficits, and their mutations impacted major outer-membrane proteins or subunits of RNA polymerases. The other mutants experienced little or no fitness costs and with, one exception, they had mutations affecting other genes and functions. Our findings underscore the importance of comparative studies on the evolution of antibiotic resistance, and they highlight the nuanced processes that shape these phenotypes.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
Antibiotic resistance is a growing concern that has prompted a renewed focus on drug discovery, stewardship, and evolutionary studies of the patterns and processes that underlie this phenomenon. A resistant strain’s competitive fitness relative to its sensitive counterparts in the absence of drug can impact its spread and persistence in both clinical and community settings. In a prior study, we examined the fitness of tetracycline-resistant clones that evolved from five different Escherichia coli genotypes, which had diverged during a long-term evolution experiment. In this study, we build on that work to examine whether ampicillin-resistant mutants are also less fit in the absence of the drug than their sensitive parents, and whether the cost of resistance is constant or variable among independently derived lines. Like the tetracycline-resistant lines, the ampicillin-resistant mutants were often less fit than their sensitive parents, with significant variation in the fitness costs among the mutants. This variation was not associated with the level of resistance conferred by the mutations, nor did it vary across the different parental backgrounds. In our earlier study, some of the variation in fitness costs associated with tetracycline resistance was explained by the effects of different mutations affecting the same cellular pathway and even the same gene. In contrast, the variance among the ampicillin-resistant mutants was associated with different sets of target genes. About half of the resistant clones suffered large fitness deficits, and their mutations impacted major outer-membrane proteins or subunits of RNA polymerases. The other mutants experienced little or no fitness costs and with, one exception, they had mutations affecting other genes and functions. Our findings underscore the importance of comparative studies on the evolution of antibiotic resistance, and they highlight the nuanced processes that shape these phenotypes.
Ascensao J A; Wetmore K M; Good B H; Arkin A P; Hallatschek O
Quantifying the Adaptive Potential of a Nascent Bacterial Community Journal Article Forthcoming
bioRxiv, Forthcoming.
Abstract | Links | BibTeX | Altmetric | Tags: Demography and Ecology, Descendant Experiments, Genome Evolution, Genotypes and Phenotypes
@article{Ascensao2022.02.03.475969,
title = {Quantifying the Adaptive Potential of a Nascent Bacterial Community},
author = {Joao A. Ascensao and Kelly M. Wetmore and Benjamin H. Good and Adam P. Arkin and Oskar Hallatschek},
url = {https://www.biorxiv.org/content/early/2022/02/05/2022.02.03.475969},
doi = {10.1101/2022.02.03.475969},
year = {2022},
date = {2022-01-01},
urldate = {2022-01-01},
journal = {bioRxiv},
publisher = {Cold Spring Harbor Laboratory},
abstract = {The fitness effects of all possible mutations available to an organism largely shapes the dynamics of evolutionary adaptation. Tremendous progress has been made in quantifying the strength and abundance of selected mutations available to single microbial species in simple environments, lacking strong ecological interactions. However, the adaptive potential of strains that are part of multi-strain communities remains largely unclear. We sought to fill this gap for a stable community of two closely related ecotypes (textquotedblleftLtextquotedblright and textquotedblleftStextquotedblright) shortly after they emerged within the E. coli Long-Term Evolution Experiment (LTEE). To this end, we engineered genome-wide barcoded transposon libraries and developed a computational inference pipeline to measure the fitness effects of all possible gene knockouts in the coexisting strains as well as their ancestor, for many different conditions. We found that the fitness effect of most gene knockouts sensitively depends on the genetic background and the ecological conditions, as set by environmental perturbations and the relative frequency of both ecotypes. Despite the idiosyncratic behavior of individual knockouts, we still see consistent statistical patterns of fitness effect variation across both genetic background and community composition. The background dependence of mutational effects appears to reflect widespread changes in which gene functions are important for determining fitness, for all but the most strongly interacting genes. Additionally, fitness effects are correlated with evolutionary outcomes for a number of conditions, possibly revealing shifting patterns of adaptation. Together, our results reveal how ecological and epistatic effects combine to drive adaptive potential in recently diverged, coexisting ecotypes.Competing Interest StatementThe authors have declared no competing interest.},
keywords = {Demography and Ecology, Descendant Experiments, Genome Evolution, Genotypes and Phenotypes},
pubstate = {forthcoming},
tppubtype = {article}
}
The fitness effects of all possible mutations available to an organism largely shapes the dynamics of evolutionary adaptation. Tremendous progress has been made in quantifying the strength and abundance of selected mutations available to single microbial species in simple environments, lacking strong ecological interactions. However, the adaptive potential of strains that are part of multi-strain communities remains largely unclear. We sought to fill this gap for a stable community of two closely related ecotypes (textquotedblleftLtextquotedblright and textquotedblleftStextquotedblright) shortly after they emerged within the E. coli Long-Term Evolution Experiment (LTEE). To this end, we engineered genome-wide barcoded transposon libraries and developed a computational inference pipeline to measure the fitness effects of all possible gene knockouts in the coexisting strains as well as their ancestor, for many different conditions. We found that the fitness effect of most gene knockouts sensitively depends on the genetic background and the ecological conditions, as set by environmental perturbations and the relative frequency of both ecotypes. Despite the idiosyncratic behavior of individual knockouts, we still see consistent statistical patterns of fitness effect variation across both genetic background and community composition. The background dependence of mutational effects appears to reflect widespread changes in which gene functions are important for determining fitness, for all but the most strongly interacting genes. Additionally, fitness effects are correlated with evolutionary outcomes for a number of conditions, possibly revealing shifting patterns of adaptation. Together, our results reveal how ecological and epistatic effects combine to drive adaptive potential in recently diverged, coexisting ecotypes.Competing Interest StatementThe authors have declared no competing interest.
2021
Izutsu M; Lake D M; Matson Z W D; Dodson J P; Lenski R E
Effects of periodic bottlenecks on the dynamics of adaptive evolution in microbial populations Journal Article Forthcoming
10.1101/2021.12.29.474457, Forthcoming.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments, Fitness Trajectories, Theory and Simulations
@article{,
title = {Effects of periodic bottlenecks on the dynamics of adaptive evolution in microbial populations},
author = {Minako Izutsu and Devin M. Lake and Zachary W. D. Matson and Jack P. Dodson and Richard E. Lenski},
doi = {10.1101/2021.12.29.474457},
year = {2021},
date = {2021-12-30},
journal = {10.1101/2021.12.29.474457},
publisher = {LTEE},
abstract = {Population bottlenecks are common in nature, and they can impact the rate of adaptation in evolving populations. On the one hand, each bottleneck reduces the genetic variation that fuels adaptation. On the other hand, fewer founders can undergo more generations and leave more descendants in a resource-limited environment, which allows surviving beneficial mutations to spread more quickly. Here we investigate the impact of repeated bottlenecks on the dynamics of adaptation in experimental populations of Escherichia coli. We propagated 48 populations under four dilution regimes (2-, 8-, 100-, and 1000-fold), all reaching the same final size each day, for 150 days. A simple model in which adaptation is limited by the supply rate of beneficial mutations predicts that fitness gains should be maximized with ∼8-fold dilutions. The model also assumes that selection acts only on the overall growth rate and is otherwise identical across dilution regimes. However, we found that selection in the 2-fold regime was qualitatively different from the other treatments. Moreover, we observed earlier and greater fitness gains in the populations subjected to 100- and 1000-fold dilutions than in those that evolved in the 8-fold regime. We also ran simulations using parameters estimated independently from a long-term experiment using the same ancestral strain and environment. The simulations produced dynamics similar to our empirical results under these regimes, and they indicate that the simple model fails owing to the assumption that the supply of beneficial mutations limits adaptation.},
keywords = {Descendant Experiments, Fitness Trajectories, Theory and Simulations},
pubstate = {forthcoming},
tppubtype = {article}
}
Population bottlenecks are common in nature, and they can impact the rate of adaptation in evolving populations. On the one hand, each bottleneck reduces the genetic variation that fuels adaptation. On the other hand, fewer founders can undergo more generations and leave more descendants in a resource-limited environment, which allows surviving beneficial mutations to spread more quickly. Here we investigate the impact of repeated bottlenecks on the dynamics of adaptation in experimental populations of Escherichia coli. We propagated 48 populations under four dilution regimes (2-, 8-, 100-, and 1000-fold), all reaching the same final size each day, for 150 days. A simple model in which adaptation is limited by the supply rate of beneficial mutations predicts that fitness gains should be maximized with ∼8-fold dilutions. The model also assumes that selection acts only on the overall growth rate and is otherwise identical across dilution regimes. However, we found that selection in the 2-fold regime was qualitatively different from the other treatments. Moreover, we observed earlier and greater fitness gains in the populations subjected to 100- and 1000-fold dilutions than in those that evolved in the 8-fold regime. We also ran simulations using parameters estimated independently from a long-term experiment using the same ancestral strain and environment. The simulations produced dynamics similar to our empirical results under these regimes, and they indicate that the simple model fails owing to the assumption that the supply of beneficial mutations limits adaptation.
Consuegra J; Gaffé J; Lenski R E; Hindré T; Barrick J E; Tenaillon O; Schneider D
Insertion-sequence-mediated mutations both promote and constrain evolvability during a long-term experiment with bacteria Journal Article
Nature Communications, 12 (1), pp. 980, 2021, ISSN: 2041-1723.
Abstract | Links | BibTeX | Altmetric | Tags: Fitness Trajectories, Genome Evolution, Mutation Rates
@article{Consuegra2021,
title = {Insertion-sequence-mediated mutations both promote and constrain evolvability during a long-term experiment with bacteria},
author = {Jessika Consuegra and Joël Gaffé and Richard E. Lenski and Thomas Hindré and Jeffrey E. Barrick and Olivier Tenaillon and Dominique Schneider},
url = {http://www.nature.com/articles/s41467-021-21210-7},
doi = {https://doi.org/10.1038/s41467-021-21210-7},
issn = {2041-1723},
year = {2021},
date = {2021-12-01},
urldate = {2021-12-01},
journal = {Nature Communications},
volume = {12},
number = {1},
pages = {980},
publisher = {Springer US},
abstract = {Insertion sequences (IS) are ubiquitous bacterial mobile genetic elements, and the mutations they cause can be deleterious, neutral, or beneficial. The long-term dynamics of IS elements and their effects on bacteria are poorly understood, including whether they are primarily genomic parasites or important drivers of adaptation by natural selection. Here, we investigate the dynamics of IS elements and their contribution to genomic evolution and fitness during a long-term experiment with \textit{Escherichia coli}. IS elements account for ~35% of the mutations that reached high frequency through 50,000 generations in those populations that retained the ancestral point-mutation rate. In mutator populations, IS-mediated mutations are only half as frequent in absolute numbers. In one population, an exceptionally high ~8-fold increase in IS 150 copy number is associated with the beneficial effects of early insertion mutations; however, this expansion later slowed down owing to reduced IS 150 activity. This population also achieves the lowest fitness, suggesting that some avenues for further adaptation are precluded by the IS 150 -mediated mutations. More generally, across all populations, we find that higher IS activity becomes detrimental to adaptation over evolutionary time. Therefore, IS-mediated mutations can both promote and constrain evolvability.},
keywords = {Fitness Trajectories, Genome Evolution, Mutation Rates},
pubstate = {published},
tppubtype = {article}
}
Insertion sequences (IS) are ubiquitous bacterial mobile genetic elements, and the mutations they cause can be deleterious, neutral, or beneficial. The long-term dynamics of IS elements and their effects on bacteria are poorly understood, including whether they are primarily genomic parasites or important drivers of adaptation by natural selection. Here, we investigate the dynamics of IS elements and their contribution to genomic evolution and fitness during a long-term experiment with Escherichia coli. IS elements account for ~35% of the mutations that reached high frequency through 50,000 generations in those populations that retained the ancestral point-mutation rate. In mutator populations, IS-mediated mutations are only half as frequent in absolute numbers. In one population, an exceptionally high ~8-fold increase in IS 150 copy number is associated with the beneficial effects of early insertion mutations; however, this expansion later slowed down owing to reduced IS 150 activity. This population also achieves the lowest fitness, suggesting that some avenues for further adaptation are precluded by the IS 150 -mediated mutations. More generally, across all populations, we find that higher IS activity becomes detrimental to adaptation over evolutionary time. Therefore, IS-mediated mutations can both promote and constrain evolvability.
Deatherage D E; Barrick J E
High-throughput characterization of mutations in genes that drive clonal evolution using multiplex adaptome capture sequencing Journal Article
Cell Systems, 2021, ISSN: 24054712.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments, Fitness Trajectories, Methods and Miscellaneous
@article{DEATHERAGE2021,
title = {High-throughput characterization of mutations in genes that drive clonal evolution using multiplex adaptome capture sequencing},
author = {Daniel E. Deatherage and Jeffrey E. Barrick},
url = {https://www.sciencedirect.com/science/article/pii/S2405471221003343
https://linkinghub.elsevier.com/retrieve/pii/S2405471221003343},
doi = {10.1016/j.cels.2021.08.011},
issn = {24054712},
year = {2021},
date = {2021-09-01},
urldate = {2021-09-01},
journal = {Cell Systems},
abstract = {Understanding how cells are likely to evolve can guide medical interventions and bioengineering efforts that must contend with unwanted mutations. The adaptome of a cell—the neighborhood of genetic changes that are most likely to drive adaptation in a given environment—can be mapped by tracking rare beneficial variants during the early stages of clonal evolution. We used multiplex adaptome capture sequencing (mAdCap-seq), a procedure that combines unique molecular identifiers and hybridization-based enrichment, to characterize mutations in eight \textit{Escherichia coli} genes known to be under selection in a laboratory environment. We tracked 301 mutations at frequencies as low as 0.01% and inferred the fitness effects of 240 of these mutations. There were distinct molecular signatures of selection on protein structure and function for the three genes with the most beneficial mutations. Our results demonstrate how mAdCap-seq can be used to deeply profile a targeted portion of a cell's adaptome.},
keywords = {Descendant Experiments, Fitness Trajectories, Methods and Miscellaneous},
pubstate = {published},
tppubtype = {article}
}
Understanding how cells are likely to evolve can guide medical interventions and bioengineering efforts that must contend with unwanted mutations. The adaptome of a cell—the neighborhood of genetic changes that are most likely to drive adaptation in a given environment—can be mapped by tracking rare beneficial variants during the early stages of clonal evolution. We used multiplex adaptome capture sequencing (mAdCap-seq), a procedure that combines unique molecular identifiers and hybridization-based enrichment, to characterize mutations in eight Escherichia coli genes known to be under selection in a laboratory environment. We tracked 301 mutations at frequencies as low as 0.01% and inferred the fitness effects of 240 of these mutations. There were distinct molecular signatures of selection on protein structure and function for the three genes with the most beneficial mutations. Our results demonstrate how mAdCap-seq can be used to deeply profile a targeted portion of a cell's adaptome.
van Raay K; Stolyar S; Sevigny J; Draghi J; Lenski R E; Marx C J; Kerr B; Zaman L
Evolution with private resources reverses some changes from long-term evolution with public resources Unpublished
bioRxiv, 2021.
Abstract | Links | BibTeX | Altmetric | Tags: Cell Morphology, Correlated Responses, Demography and Ecology, Descendant Experiments, Genotypes and Phenotypes, Historical Contingency, Methods and Miscellaneous
@unpublished{Raay2021,
title = {Evolution with private resources reverses some changes from long-term evolution with public resources},
author = {Katrina {van Raay} and Sergey Stolyar and Jordana Sevigny and Jeremy Draghi and Richard E. Lenski and Christopher J. Marx and Benjamin Kerr and Luis Zaman},
url = {https://www.biorxiv.org/content/10.1101/2021.07.11.451942v1},
doi = {https://doi.org/10.1101/2021.07.11.451942},
year = {2021},
date = {2021-07-12},
urldate = {2021-07-12},
journal = {bioRxiv},
pages = {2021.07.11.451942},
abstract = {A population under selection to improve one trait may evolve a sub-optimal state for another trait due to tradeoffs and other evolutionary constraints. How this evolution affects the capacity of a population to adapt when conditions change to favor the second trait is an open question. We investigated this question using isolates from a lineage spanning 60,000 generations of the Long-Term Evolution Experiment (LTEE) with \textit{Escherichia coli}, where cells have access to a shared pool of resources, and have evolved increased competitive ability and a concomitant reduction in numerical yield. Using media-in oil emulsions we shifted the focus of selection to numerical yield, where cells grew in isolated patches with private resources. We found that the time spent evolving under shared resources did not affect the ability to re-evolve toward higher numerical yield. The evolution of numerical yield commonly occurred through mutations in the phosphoenolpyruvate phosphotransferase system. These mutants exhibit slower uptake of glucose, making them poorer competitors for public resources, and produce smaller cells that release less carbon as overflow metabolites. Our results demonstrate that mutations that were not part of adaptation under one selective regime may enable access to ancestral phenotypes when selection changes to favor evolutionary reversion. },
howpublished = {bioRxiv},
keywords = {Cell Morphology, Correlated Responses, Demography and Ecology, Descendant Experiments, Genotypes and Phenotypes, Historical Contingency, Methods and Miscellaneous},
pubstate = {published},
tppubtype = {unpublished}
}
A population under selection to improve one trait may evolve a sub-optimal state for another trait due to tradeoffs and other evolutionary constraints. How this evolution affects the capacity of a population to adapt when conditions change to favor the second trait is an open question. We investigated this question using isolates from a lineage spanning 60,000 generations of the Long-Term Evolution Experiment (LTEE) with Escherichia coli, where cells have access to a shared pool of resources, and have evolved increased competitive ability and a concomitant reduction in numerical yield. Using media-in oil emulsions we shifted the focus of selection to numerical yield, where cells grew in isolated patches with private resources. We found that the time spent evolving under shared resources did not affect the ability to re-evolve toward higher numerical yield. The evolution of numerical yield commonly occurred through mutations in the phosphoenolpyruvate phosphotransferase system. These mutants exhibit slower uptake of glucose, making them poorer competitors for public resources, and produce smaller cells that release less carbon as overflow metabolites. Our results demonstrate that mutations that were not part of adaptation under one selective regime may enable access to ancestral phenotypes when selection changes to favor evolutionary reversion.
Grant N A; Maddamsetti R; Lenski R E
Maintenance of Metabolic Plasticity despite Relaxed Selection in a Long-Term Evolution Experiment with Escherichia coli Journal Article
The American Naturalist, 198 (1), pp. 93–112, 2021, ISSN: 0003-0147.
Abstract | Links | BibTeX | Altmetric | Tags: Correlated Responses, Fitness Trajectories, Genome Evolution, Genotypes and Phenotypes
@article{Grant2021b,
title = {Maintenance of Metabolic Plasticity despite Relaxed Selection in a Long-Term Evolution Experiment with \textit{Escherichia coli}},
author = {Nkrumah A. Grant and Rohan Maddamsetti and Richard E. Lenski},
url = {https://www.journals.uchicago.edu/doi/10.1086/714530},
doi = {10.1086/714530},
issn = {0003-0147},
year = {2021},
date = {2021-07-01},
urldate = {2021-07-01},
journal = {The American Naturalist},
volume = {198},
number = {1},
pages = {93--112},
abstract = {Traits that are unused in a given environment are subject to processes that tend to erode them, leading to reduced fitness in other environments. Although this general tendency is clear, we know much less about why some traits are lost while others are retained and about the roles of mutation and selection in generating different responses. We addressed these issues by examining populations of a facultative anaerobe, \textit{Escherichia coli}, that have evolved for 130 years in the presence of oxygen, with relaxed selection for anaerobic growth and the associated metabolic plasticity. We asked whether evolution led to the loss, improvement, or maintenance of anaerobic growth, and we analyzed gene expression and mutational data sets to understand the outcomes. We identified genomic signatures of both positive and purifying selection on aerobic-specific genes, while anaerobic-specific genes showed clear evidence of relaxed selection. We also found parallel evolution at two interacting loci that regulate anaerobic growth. We competed the ancestor and evolved clones from each population in an anoxic environment, and we found that anaerobic fitness had not decayed, despite relaxed selection. In summary, relaxed selection does not necessarily reduce an organism's fitness in other environments. Instead, the genetic architecture of the traits under relaxed selection and their correlations with traits under positive and purifying selection may sometimes determine evolutionary outcomes.},
keywords = {Correlated Responses, Fitness Trajectories, Genome Evolution, Genotypes and Phenotypes},
pubstate = {published},
tppubtype = {article}
}
Traits that are unused in a given environment are subject to processes that tend to erode them, leading to reduced fitness in other environments. Although this general tendency is clear, we know much less about why some traits are lost while others are retained and about the roles of mutation and selection in generating different responses. We addressed these issues by examining populations of a facultative anaerobe, Escherichia coli, that have evolved for 130 years in the presence of oxygen, with relaxed selection for anaerobic growth and the associated metabolic plasticity. We asked whether evolution led to the loss, improvement, or maintenance of anaerobic growth, and we analyzed gene expression and mutational data sets to understand the outcomes. We identified genomic signatures of both positive and purifying selection on aerobic-specific genes, while anaerobic-specific genes showed clear evidence of relaxed selection. We also found parallel evolution at two interacting loci that regulate anaerobic growth. We competed the ancestor and evolved clones from each population in an anoxic environment, and we found that anaerobic fitness had not decayed, despite relaxed selection. In summary, relaxed selection does not necessarily reduce an organism's fitness in other environments. Instead, the genetic architecture of the traits under relaxed selection and their correlations with traits under positive and purifying selection may sometimes determine evolutionary outcomes.
Karkare K; Lai H; Azevedo R B R; Cooper T F
Historical Contingency Causes Divergence in Adaptive Expression of the lac Operon Journal Article
Molecular Biology and Evolution, 38 (7), pp. 2869–2879, 2021, ISSN: 1537-1719.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{10.1093/molbev/msab077,
title = {Historical Contingency Causes Divergence in Adaptive Expression of the lac Operon},
author = {Kedar Karkare and Huei-Yi Lai and Ricardo B. R. Azevedo and Tim F. Cooper},
editor = {Patricia Wittkopp},
url = {https://academic.oup.com/mbe/article/38/7/2869/6179336},
doi = {10.1093/molbev/msab077},
issn = {1537-1719},
year = {2021},
date = {2021-06-01},
urldate = {2021-06-01},
journal = {Molecular Biology and Evolution},
volume = {38},
number = {7},
pages = {2869--2879},
abstract = {Populations of \textit{Escherichia coli} selected in constant and fluctuating environments containing lactose often adapt by substituting mutations in the lacI repressor that cause constitutive expression of the lac operon. These mutations occur at a high rate and provide a significant benefit. Despite this, eight of 24 populations evolved for 8,000 generations in environments containing lactose contained no detectable repressor mutations. We report here on the basis of this observation. We find that, given relevant mutation rates, repressor mutations are expected to have fixed in all evolved populations if they had maintained the same fitness effect they confer when introduced to the ancestor. In fact, reconstruction experiments demonstrate that repressor mutations have become neutral or deleterious in those populations in which they were not detectable. Populations not fixing repressor mutations nevertheless reached the same fitness as those that did fix them, indicating that they followed an alternative evolutionary path that made redundant the potential benefit of the repressor mutation, but involved unique mutations of equivalent benefit. We identify a mutation occurring in the promoter region of the uspB gene as a candidate for influencing the selective choice between these paths. Our results detail an example of historical contingency leading to divergent evolutionary outcomes.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
Populations of Escherichia coli selected in constant and fluctuating environments containing lactose often adapt by substituting mutations in the lacI repressor that cause constitutive expression of the lac operon. These mutations occur at a high rate and provide a significant benefit. Despite this, eight of 24 populations evolved for 8,000 generations in environments containing lactose contained no detectable repressor mutations. We report here on the basis of this observation. We find that, given relevant mutation rates, repressor mutations are expected to have fixed in all evolved populations if they had maintained the same fitness effect they confer when introduced to the ancestor. In fact, reconstruction experiments demonstrate that repressor mutations have become neutral or deleterious in those populations in which they were not detectable. Populations not fixing repressor mutations nevertheless reached the same fitness as those that did fix them, indicating that they followed an alternative evolutionary path that made redundant the potential benefit of the repressor mutation, but involved unique mutations of equivalent benefit. We identify a mutation occurring in the promoter region of the uspB gene as a candidate for influencing the selective choice between these paths. Our results detail an example of historical contingency leading to divergent evolutionary outcomes.
Card K J; Jordan J A; Lenski R E
Idiosyncratic variation in the fitness costs of tetracycline-resistance mutations in Escherichia coli Journal Article
Evolution, 75 (5), pp. 1230-1238, 2021, ISSN: 0014-3820.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{nokey,
title = {Idiosyncratic variation in the fitness costs of tetracycline-resistance mutations in \textit{Escherichia coli}},
author = {Kyle J. Card and Jalin A. Jordan and Richard E. Lenski},
url = {https://onlinelibrary.wiley.com/doi/10.1111/evo.14203},
doi = {10.1111/evo.14203},
issn = {0014-3820},
year = {2021},
date = {2021-02-25},
urldate = {2021-02-25},
journal = {Evolution},
volume = {75},
number = {5},
pages = {1230-1238},
abstract = {A bacterium's fitness relative to its competitors, both in the presence and absence of antibiotics, plays a key role in its ecological success and clinical impact. In this study, we examine whether tetracycline-resistant mutants are less fit in the absence of the drug than their sensitive parents, and whether the fitness cost of resistance is constant or variable across independently derived lines. Tetracycline-resistant lines suffered, on average, a reduction in fitness of almost 8%. There was substantial among-line variation in the fitness cost. This variation was not associated with the level of resistance conferred by the mutations, nor did it vary significantly across several genetic backgrounds. The two resistant lines with the most extreme fitness costs involved functionally unrelated mutations on different genetic backgrounds. However, there was also significant variation in the fitness costs for mutations affecting the same pathway and even different alleles of the same gene. Our findings demonstrate that the fitness costs of antibiotic resistance do not always correlate with the phenotypic level of resistance or the underlying genetic changes. Instead, these costs reflect the idiosyncratic effects of particular resistance mutations and the genetic backgrounds in which they occur.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
A bacterium's fitness relative to its competitors, both in the presence and absence of antibiotics, plays a key role in its ecological success and clinical impact. In this study, we examine whether tetracycline-resistant mutants are less fit in the absence of the drug than their sensitive parents, and whether the fitness cost of resistance is constant or variable across independently derived lines. Tetracycline-resistant lines suffered, on average, a reduction in fitness of almost 8%. There was substantial among-line variation in the fitness cost. This variation was not associated with the level of resistance conferred by the mutations, nor did it vary significantly across several genetic backgrounds. The two resistant lines with the most extreme fitness costs involved functionally unrelated mutations on different genetic backgrounds. However, there was also significant variation in the fitness costs for mutations affecting the same pathway and even different alleles of the same gene. Our findings demonstrate that the fitness costs of antibiotic resistance do not always correlate with the phenotypic level of resistance or the underlying genetic changes. Instead, these costs reflect the idiosyncratic effects of particular resistance mutations and the genetic backgrounds in which they occur.
Card K J; Thomas M D; Graves J L; Barrick J E; Lenski R E
Genomic evolution of antibiotic resistance is contingent on genetic background following a long-term experiment with Escherichia coli Journal Article
Proceedings of the National Academy of Sciences, 118 (5), pp. e2016886118, 2021, ISSN: 0027-8424.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments, Genome Evolution, Historical Contingency
@article{Card2021,
title = {Genomic evolution of antibiotic resistance is contingent on genetic background following a long-term experiment with \textit{Escherichia coli}},
author = {Kyle J. Card and Misty D. Thomas and Joseph L. Graves and Jeffrey E. Barrick and Richard E. Lenski},
url = {http://www.pnas.org/lookup/doi/10.1073/pnas.2016886118},
doi = {10.1073/pnas.2016886118},
issn = {0027-8424},
year = {2021},
date = {2021-02-01},
urldate = {2021-02-01},
journal = {Proceedings of the National Academy of Sciences},
volume = {118},
number = {5},
pages = {e2016886118},
abstract = {Antibiotic resistance is a growing health concern. Efforts to control resistance would benefit from an improved ability to forecast when and how it will evolve. Epistatic interactions between mutations can promote divergent evolutionary trajectories, which complicates our ability to predict evolution. We recently showed that differences between genetic backgrounds can lead to idiosyncratic responses in the evolvability of phenotypic resistance, even among closely related \textit{Escherichia coli} strains. In this study, we examined whether a strain's genetic background also influences the genotypic evolution of resistance. Do lineages founded by different genotypes take parallel or divergent mutational paths to achieve their evolved resistance states? We addressed this question by sequencing the complete genomes of antibiotic-resistant clones that evolved from several different genetic starting points during our earlier experiments. We first validated our statistical approach by quantifying the specificity of genomic evolution with respect to antibiotic treatment. As expected, mutations in particular genes were strongly associated with each drug. Then, we determined that replicate lines evolved from the same founding genotypes had more parallel mutations at the gene level than lines evolved from different founding genotypes, although these effects were more subtle than those showing antibiotic specificity. Taken together with our previous work, we conclude that historical contingency can alter both genotypic and phenotypic pathways to antibiotic resistance.},
keywords = {Descendant Experiments, Genome Evolution, Historical Contingency},
pubstate = {published},
tppubtype = {article}
}
Antibiotic resistance is a growing health concern. Efforts to control resistance would benefit from an improved ability to forecast when and how it will evolve. Epistatic interactions between mutations can promote divergent evolutionary trajectories, which complicates our ability to predict evolution. We recently showed that differences between genetic backgrounds can lead to idiosyncratic responses in the evolvability of phenotypic resistance, even among closely related Escherichia coli strains. In this study, we examined whether a strain's genetic background also influences the genotypic evolution of resistance. Do lineages founded by different genotypes take parallel or divergent mutational paths to achieve their evolved resistance states? We addressed this question by sequencing the complete genomes of antibiotic-resistant clones that evolved from several different genetic starting points during our earlier experiments. We first validated our statistical approach by quantifying the specificity of genomic evolution with respect to antibiotic treatment. As expected, mutations in particular genes were strongly associated with each drug. Then, we determined that replicate lines evolved from the same founding genotypes had more parallel mutations at the gene level than lines evolved from different founding genotypes, although these effects were more subtle than those showing antibiotic specificity. Taken together with our previous work, we conclude that historical contingency can alter both genotypic and phenotypic pathways to antibiotic resistance.
Marshall D J; Malerba M; Lines T; Sezmis A L; Hasan C M; Lenski R E; McDonald M J
Long-term experimental evolution decouples size and production costs in Escherichia coli Unpublished
bioRxiv, 2021.
Abstract | Links | BibTeX | Altmetric | Tags: Cell Morphology, Demography and Ecology
@unpublished{Marshall2021,
title = {Long-term experimental evolution decouples size and production costs in \emph{Escherichia coli}},
author = {Dustin J. Marshall and Martino Malerba and Thomas Lines and Aysha L. Sezmis and Chowdhury M. Hasan and Richard E. Lenski and Michael J. McDonald},
url = {https://www.biorxiv.org/content/10.1101/2021.09.28.462250v1},
doi = {10.1101/2021.09.28.462250},
year = {2021},
date = {2021-01-01},
urldate = {2021-01-01},
journal = {bioRxiv},
abstract = {Body size covaries with population dynamics across life's domains. Theory holds that metabolism imposes fundamental constraints on the coevolution of size and demography. However, studies of interspecific patterns are confounded by other factors that covary with size and demography, and experimental tests of the causal links remain elusive. Here we leverage a 60,000-generation experiment in which \textit{Escherichia coli} populations evolved larger cells to examine intraspecific metabolic scaling and correlations with demographic parameters. Metabolic theory successfully predicted the relations among size, metabolism, and maximum population density, with strong support for Damuth's law of energy equivalence in this experiment. In contrast, populations of larger cells grew faster than those of smaller cells, contradicting the fundamental assumption that costs of production should increase proportionately with size. The finding that the costs of production are substantially decoupled from size requires re-examining the evolutionary drivers and ecological consequences of biological size more generally.},
howpublished = {bioRxiv},
keywords = {Cell Morphology, Demography and Ecology},
pubstate = {published},
tppubtype = {unpublished}
}
Body size covaries with population dynamics across life's domains. Theory holds that metabolism imposes fundamental constraints on the coevolution of size and demography. However, studies of interspecific patterns are confounded by other factors that covary with size and demography, and experimental tests of the causal links remain elusive. Here we leverage a 60,000-generation experiment in which Escherichia coli populations evolved larger cells to examine intraspecific metabolic scaling and correlations with demographic parameters. Metabolic theory successfully predicted the relations among size, metabolism, and maximum population density, with strong support for Damuth's law of energy equivalence in this experiment. In contrast, populations of larger cells grew faster than those of smaller cells, contradicting the fundamental assumption that costs of production should increase proportionately with size. The finding that the costs of production are substantially decoupled from size requires re-examining the evolutionary drivers and ecological consequences of biological size more generally.
Gifford I; Dasgupta A; Barrick J E
Rates of gene conversions between Escherichia coli ribosomal operons Journal Article
G3: Genes, Genomes, Genetics, 11 (2), 2021, ISSN: 21601836.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments, Mutation Rates
@article{Gifford2021,
title = {Rates of gene conversions between \emph{Escherichia coli} ribosomal operons},
author = {Isaac Gifford and Aurko Dasgupta and Jeffrey E. Barrick},
url = {https://academic.oup.com/g3journal/article/11/2/jkaa002/5974039},
doi = {10.1093/g3journal/jkaa002},
issn = {21601836},
year = {2021},
date = {2021-01-01},
urldate = {2021-01-01},
journal = {G3: Genes, Genomes, Genetics},
volume = {11},
number = {2},
abstract = {Due to their universal presence and high sequence conservation, ribosomal RNA (rRNA) sequences are used widely in phylogenetics for inferring evolutionary relationships between microbes and in metagenomics for analyzing the composition of microbial communities. Most microbial genomes encode multiple copies of rRNA genes to supply cells with sufficient capacity for protein synthesis. These copies typically undergo concerted evolution that keeps their sequences identical, or nearly so, due to gene conversion, a type of intragenomic recombination that changes one copy of a homologous sequence to exactly match another. Widely varying rates of rRNA gene conversion have previously been estimated by comparative genomics methods and using genetic reporter assays. To more directly measure rates of rRNA intragenomic recombination, we sequenced the seven \textit{Escherichia coli} rRNA operons in 15 lineages that were evolved for ~13,750 generations with frequent single-cell bottlenecks that reduce the effects of selection. We identified 38 gene conversion events and estimated an overall rate of intragenomic recombination within the 16S and 23S genes between rRNA copies of 3.6 × 10^{−4} per genome per generation or 8.6 × 10^{6} per rRNA operon per homologous donor operon per generation. This rate varied only slightly from random expectations at different sites within the rRNA genes and between rRNA operons located at different positions in the genome. Our accurate estimate of the rate of rRNA gene conversions fills a gap in our quantitative understanding of how ribosomal sequences and other multicopy elements diversify and homogenize during microbial genome evolution.},
keywords = {Descendant Experiments, Mutation Rates},
pubstate = {published},
tppubtype = {article}
}
Due to their universal presence and high sequence conservation, ribosomal RNA (rRNA) sequences are used widely in phylogenetics for inferring evolutionary relationships between microbes and in metagenomics for analyzing the composition of microbial communities. Most microbial genomes encode multiple copies of rRNA genes to supply cells with sufficient capacity for protein synthesis. These copies typically undergo concerted evolution that keeps their sequences identical, or nearly so, due to gene conversion, a type of intragenomic recombination that changes one copy of a homologous sequence to exactly match another. Widely varying rates of rRNA gene conversion have previously been estimated by comparative genomics methods and using genetic reporter assays. To more directly measure rates of rRNA intragenomic recombination, we sequenced the seven Escherichia coli rRNA operons in 15 lineages that were evolved for ~13,750 generations with frequent single-cell bottlenecks that reduce the effects of selection. We identified 38 gene conversion events and estimated an overall rate of intragenomic recombination within the 16S and 23S genes between rRNA copies of 3.6 × 10−4 per genome per generation or 8.6 × 106 per rRNA operon per homologous donor operon per generation. This rate varied only slightly from random expectations at different sites within the rRNA genes and between rRNA operons located at different positions in the genome. Our accurate estimate of the rate of rRNA gene conversions fills a gap in our quantitative understanding of how ribosomal sequences and other multicopy elements diversify and homogenize during microbial genome evolution.
Maddamsetti R
Selection Maintains Protein Interactome Resilience in the Long-Term Evolution Experiment with Escherichia coli Journal Article
Genome Biology and Evolution, 13 (6), 2021.
Abstract | Links | BibTeX | Altmetric | Tags: Genome Evolution, Genotypes and Phenotypes
@article{Maddamsetti2021,
title = {Selection Maintains Protein Interactome Resilience in the Long-Term Evolution Experiment with \emph{Escherichia coli}},
author = {Rohan Maddamsetti},
url = {https://academic.oup.com/gbe/article/13/6/evab074/6240992},
doi = {https://doi.org/10.1093/gbe/evab074},
year = {2021},
date = {2021-01-01},
urldate = {2021-01-01},
journal = {Genome Biology and Evolution},
volume = {13},
number = {6},
abstract = {Most cellular functions are carried out by a dynamic network of interacting proteins. An open question is whether the network properties of protein interactomes represent phenotypes under natural selection. One proposal is that protein interactomes have evolved to be resilient, such that they tend to maintain connectivity when proteins are removed from the network. This hypothesis predicts that interactome resilience should be maintained by natural selection during long-term experimental evolution. I tested this prediction by modeling the evolution of protein–protein interaction (PPI) networks in Lenski’s long-term evolution experiment with \textit{Escherichia coli} (LTEE). In this test, I removed proteins affected by nonsense, insertion, deletion, and transposon mutations in evolved LTEE strains, and measured the resilience of the resulting networks. I compared the rate of change of network resilience in each LTEE population to the rate of change of network resilience for corresponding randomized networks. The evolved PPI networks are significantly more resilient than networks in which random proteins have been deleted. Moreover, the evolved networks are generally more resilient than networks in which the random deletion of proteins was restricted to those disrupted in LTEE. These results suggest that evolution in the LTEE has favored PPI networks that are, on average, more resilient than expected from the genetic variation across the evolved strains. My findings therefore support the hypothesis that selection maintains protein interactome resilience over evolutionary time.},
keywords = {Genome Evolution, Genotypes and Phenotypes},
pubstate = {published},
tppubtype = {article}
}
Most cellular functions are carried out by a dynamic network of interacting proteins. An open question is whether the network properties of protein interactomes represent phenotypes under natural selection. One proposal is that protein interactomes have evolved to be resilient, such that they tend to maintain connectivity when proteins are removed from the network. This hypothesis predicts that interactome resilience should be maintained by natural selection during long-term experimental evolution. I tested this prediction by modeling the evolution of protein–protein interaction (PPI) networks in Lenski’s long-term evolution experiment with Escherichia coli (LTEE). In this test, I removed proteins affected by nonsense, insertion, deletion, and transposon mutations in evolved LTEE strains, and measured the resilience of the resulting networks. I compared the rate of change of network resilience in each LTEE population to the rate of change of network resilience for corresponding randomized networks. The evolved PPI networks are significantly more resilient than networks in which random proteins have been deleted. Moreover, the evolved networks are generally more resilient than networks in which the random deletion of proteins was restricted to those disrupted in LTEE. These results suggest that evolution in the LTEE has favored PPI networks that are, on average, more resilient than expected from the genetic variation across the evolved strains. My findings therefore support the hypothesis that selection maintains protein interactome resilience over evolutionary time.
Maddamsetti R
Universal Constraints on Protein Evolution in the Long-Term Evolution Experiment with Escherichia coli Journal Article
Genome Biology and Evolution, 13 (6), 2021.
Abstract | Links | BibTeX | Altmetric | Tags: Genome Evolution, Genotypes and Phenotypes
@article{Maddamsetti2021b,
title = {Universal Constraints on Protein Evolution in the Long-Term Evolution Experiment with \textit{Escherichia coli}},
author = {Rohan Maddamsetti},
url = {https://academic.oup.com/gbe/article/13/6/evab070/6226398},
doi = {https://doi.org/10.1093/gbe/evab070},
year = {2021},
date = {2021-01-01},
urldate = {2021-01-01},
journal = {Genome Biology and Evolution},
volume = {13},
number = {6},
abstract = {Although it is well known that abundant proteins evolve slowly across the tree of life, there is little consensus for why this is true. Here, I report that abundant proteins evolve slowly in the hypermutator populations of Lenski’s long-term evolution experiment with \textit{Escherichia coli} (LTEE). Specifically, the density of all observed mutations per gene, as measured in metagenomic time series covering 60,000 generations of the LTEE, significantly anticorrelates with mRNA abundance, protein abundance, and degree of protein–protein interaction. The same pattern holds for nonsynonymous mutation density. However, synonymous mutation density, measured across the LTEE hypermutator populations, positively correlates with protein abundance. These results show that universal constraints on protein evolution are visible in data spanning three decades of experimental evolution. Therefore, it should be possible to design experiments to answer why abundant proteins evolve slowly.},
keywords = {Genome Evolution, Genotypes and Phenotypes},
pubstate = {published},
tppubtype = {article}
}
Although it is well known that abundant proteins evolve slowly across the tree of life, there is little consensus for why this is true. Here, I report that abundant proteins evolve slowly in the hypermutator populations of Lenski’s long-term evolution experiment with Escherichia coli (LTEE). Specifically, the density of all observed mutations per gene, as measured in metagenomic time series covering 60,000 generations of the LTEE, significantly anticorrelates with mRNA abundance, protein abundance, and degree of protein–protein interaction. The same pattern holds for nonsynonymous mutation density. However, synonymous mutation density, measured across the LTEE hypermutator populations, positively correlates with protein abundance. These results show that universal constraints on protein evolution are visible in data spanning three decades of experimental evolution. Therefore, it should be possible to design experiments to answer why abundant proteins evolve slowly.
Grant N A; abdel Magid A; Franklin J; Dufour Y; Lenski R E
Changes in cell size and shape during 50,000 generations of experimental evolution with Escherichia coli Journal Article
Journal of Bacteriology, 203 (10), pp. e00469-20, 2021.
Abstract | Links | BibTeX | Altmetric | Tags: Cell Morphology, Genotypes and Phenotypes
@article{Grant2021,
title = {Changes in cell size and shape during 50,000 generations of experimental evolution with \textit{Escherichia coli}},
author = {Nkrumah A. Grant and Ali {abdel Magid} and Joshua Franklin and Yann Dufour and Richard E. Lenski},
url = {https://journals.asm.org/doi/10.1128/JB.00469-20},
doi = {10.1128/JB.00469-20},
year = {2021},
date = {2021-01-01},
urldate = {2021-01-01},
journal = {Journal of Bacteriology},
volume = {203},
number = {10},
pages = {e00469-20},
abstract = {Bacteria adopt a wide variety of sizes and shapes, with many species exhibiting stereotypical morphologies. How morphology changes, and over what timescales, is less clear. Previous work examining cell morphology in an experiment with \textit{Escherichia coli} showed that populations evolved larger cells and, in some cases, cells that were less rod-like. That experiment has now run for over two more decades. Meanwhile, genome sequence data are available for these populations, and new computational methods enable high-throughput microscopic analyses. In this study, we measured stationary-phase cell volumes for the ancestor and 12 populations at 2,000, 10,000, and 50,000 generations, including measurements during exponential growth at the last time point. We measured the distribution of cell volumes for each sample using a Coulter counter and microscopy, the latter of which also provided data on cell shape. Our data confirm the trend toward larger cells while also revealing substantial variation in size and shape across replicate populations. Most populations first evolved wider cells but later reverted to the ancestral length-to-width ratio. All but one population evolved mutations in rod shape maintenance genes. We also observed many ghost-like cells in the only population that evolved the novel ability to grow on citrate, supporting the hypothesis that this lineage struggles with maintaining balanced growth. Lastly, we show that cell size and fitness remain correlated across 50,000 generations. Our results suggest that larger cells are beneficial in the experimental environment, while the reversion toward ancestral length-to-width ratios suggests partial compensation for the less favorable surface area-to-volume ratios of the evolved cells.},
keywords = {Cell Morphology, Genotypes and Phenotypes},
pubstate = {published},
tppubtype = {article}
}
Bacteria adopt a wide variety of sizes and shapes, with many species exhibiting stereotypical morphologies. How morphology changes, and over what timescales, is less clear. Previous work examining cell morphology in an experiment with Escherichia coli showed that populations evolved larger cells and, in some cases, cells that were less rod-like. That experiment has now run for over two more decades. Meanwhile, genome sequence data are available for these populations, and new computational methods enable high-throughput microscopic analyses. In this study, we measured stationary-phase cell volumes for the ancestor and 12 populations at 2,000, 10,000, and 50,000 generations, including measurements during exponential growth at the last time point. We measured the distribution of cell volumes for each sample using a Coulter counter and microscopy, the latter of which also provided data on cell shape. Our data confirm the trend toward larger cells while also revealing substantial variation in size and shape across replicate populations. Most populations first evolved wider cells but later reverted to the ancestral length-to-width ratio. All but one population evolved mutations in rod shape maintenance genes. We also observed many ghost-like cells in the only population that evolved the novel ability to grow on citrate, supporting the hypothesis that this lineage struggles with maintaining balanced growth. Lastly, we show that cell size and fitness remain correlated across 50,000 generations. Our results suggest that larger cells are beneficial in the experimental environment, while the reversion toward ancestral length-to-width ratios suggests partial compensation for the less favorable surface area-to-volume ratios of the evolved cells.
2020
Atolia E; Cesar S; Arjes H A; Rajendram M; Shi H; Knapp B D; Khare S; Aranda-Díaz A; Lenski R E; Huang K C
Environmental and Physiological Factors Affecting High-Throughput Measurements of Bacterial Growth Journal Article
mBio, 11 (5), pp. 1–19, 2020, ISSN: 2161-2129.
Abstract | Links | BibTeX | Altmetric | Tags: Demography and Ecology
@article{Atolia2020,
title = {Environmental and Physiological Factors Affecting High-Throughput Measurements of Bacterial Growth},
author = {Esha Atolia and Spencer Cesar and Heidi A Arjes and Manohary Rajendram and Handuo Shi and Benjamin D Knapp and Somya Khare and Andrés Aranda-Díaz and Richard E. Lenski and Kerwyn Casey Huang},
editor = {Kelly T. Hughes},
url = {https://journals.asm.org/doi/10.1128/mBio.01378-20},
doi = {10.1128/mBio.01378-20},
issn = {2161-2129},
year = {2020},
date = {2020-10-01},
urldate = {2020-10-01},
journal = {mBio},
volume = {11},
number = {5},
pages = {1--19},
abstract = {How starved bacteria adapt and multiply under replete nutrient conditions is intimately linked to their history of previous growth, their physiological state, and the surrounding environment. While automated equipment has enabled high-throughput growth measurements, data interpretation and knowledge gaps regarding the determinants of growth kinetics complicate comparisons between strains. Here, we present a framework for growth measurements that improves accuracy and attenuates the effects of growth history. We determined that background absorbance quantification and multiple passaging cycles allow for accurate growth rate measurements even in carbon-poor media, which we used to reveal growth-rate increases during long-term laboratory evolution of \textit{Escherichia coli}. Using mathematical modeling, we showed that maximum growth rate depends on initial cell density. Finally, we demonstrated that growth of Bacillus subtilis with glycerol inhibits the future growth of most of the population, due to lipoteichoic acid synthesis. These studies highlight the challenges of accurate quantification of bacterial growth behaviors.},
keywords = {Demography and Ecology},
pubstate = {published},
tppubtype = {article}
}
How starved bacteria adapt and multiply under replete nutrient conditions is intimately linked to their history of previous growth, their physiological state, and the surrounding environment. While automated equipment has enabled high-throughput growth measurements, data interpretation and knowledge gaps regarding the determinants of growth kinetics complicate comparisons between strains. Here, we present a framework for growth measurements that improves accuracy and attenuates the effects of growth history. We determined that background absorbance quantification and multiple passaging cycles allow for accurate growth rate measurements even in carbon-poor media, which we used to reveal growth-rate increases during long-term laboratory evolution of Escherichia coli. Using mathematical modeling, we showed that maximum growth rate depends on initial cell density. Finally, we demonstrated that growth of Bacillus subtilis with glycerol inhibits the future growth of most of the population, due to lipoteichoic acid synthesis. These studies highlight the challenges of accurate quantification of bacterial growth behaviors.
Maddamsetti R; Grant N A
Divergent Evolution of Mutation Rates and Biases in the Long-Term Evolution Experiment with Escherichia coli Journal Article
Genome Biology and Evolution, 12 (9), pp. 1591-1603, 2020, ISSN: 1759-6653.
Abstract | Links | BibTeX | Altmetric | Tags: Genome Evolution, Mutation Rates
@article{nokey,
title = {Divergent Evolution of Mutation Rates and Biases in the Long-Term Evolution Experiment with \textit{Escherichia coli}},
author = {Rohan Maddamsetti and Nkrumah A. Grant},
editor = {George Zhang},
url = {https://academic.oup.com/gbe/article/12/9/1591/5898197},
doi = {10.1093/gbe/evaa178},
issn = {1759-6653},
year = {2020},
date = {2020-08-21},
urldate = {2020-08-21},
journal = {Genome Biology and Evolution},
volume = {12},
number = {9},
pages = {1591-1603},
abstract = {All organisms encode enzymes that replicate, maintain, pack, recombine, and repair their genetic material. For this reason, mutation rates and biases also evolve by mutation, variation, and natural selection. By examining metagenomic time series of the Lenski long-term evolution experiment (LTEE) with \textit{Escherichia coli} (Good BH, McDonald MJ, Barrick JE, Lenski RE, Desai MM. 2017. The dynamics of molecular evolution over 60,000 generations. Nature 551(7678):45–50.), we find that local mutation rate variation has evolved during the LTEE. Each LTEE population has evolved idiosyncratic differences in their rates of point mutations, indels, and mobile element insertions, due to the fixation of various hypermutator and antimutator alleles. One LTEE population, called Ara+3, shows a strong, symmetric wave pattern in its density of point mutations, radiating from the origin of replication. This pattern is largely missing from the other LTEE populations, most of which evolved missense, indel, or structural mutations in \textit{topA}, \textit{fis}, and \textit{dusB}—loci that all affect DNA topology. The distribution of mutations in those genes over time suggests epistasis and historical contingency in the evolution of DNA topology, which may have in turn affected local mutation rates. Overall, the replicate populations of the LTEE have largely diverged in their mutation rates and biases, even though they have adapted to identical abiotic conditions.},
keywords = {Genome Evolution, Mutation Rates},
pubstate = {published},
tppubtype = {article}
}
All organisms encode enzymes that replicate, maintain, pack, recombine, and repair their genetic material. For this reason, mutation rates and biases also evolve by mutation, variation, and natural selection. By examining metagenomic time series of the Lenski long-term evolution experiment (LTEE) with Escherichia coli (Good BH, McDonald MJ, Barrick JE, Lenski RE, Desai MM. 2017. The dynamics of molecular evolution over 60,000 generations. Nature 551(7678):45–50.), we find that local mutation rate variation has evolved during the LTEE. Each LTEE population has evolved idiosyncratic differences in their rates of point mutations, indels, and mobile element insertions, due to the fixation of various hypermutator and antimutator alleles. One LTEE population, called Ara+3, shows a strong, symmetric wave pattern in its density of point mutations, radiating from the origin of replication. This pattern is largely missing from the other LTEE populations, most of which evolved missense, indel, or structural mutations in topA, fis, and dusB—loci that all affect DNA topology. The distribution of mutations in those genes over time suggests epistasis and historical contingency in the evolution of DNA topology, which may have in turn affected local mutation rates. Overall, the replicate populations of the LTEE have largely diverged in their mutation rates and biases, even though they have adapted to identical abiotic conditions.
Blount Z D; Maddamsetti R; Grant N A; Ahmed S T; Jagdish T; Baxter J A; Sommerfeld B A; Tillman A; Moore J; Slonczewski J L; Barrick J E; Lenski R E
Genomic and phenotypic evolution of Escherichia coli in a novel citrate-only resource environment Journal Article
eLife, 9 , pp. 1–64, 2020, ISSN: 2050-084X.
Abstract | Links | BibTeX | Altmetric | Tags: Citrate Evolution, Demography and Ecology, Descendant Experiments, Genotypes and Phenotypes
@article{Blount2020,
title = {Genomic and phenotypic evolution of \textit{Escherichia coli} in a novel citrate-only resource environment},
author = {Zachary D. Blount and Rohan Maddamsetti and Nkrumah A. Grant and Sumaya T. Ahmed and Tanush Jagdish and Jessica A. Baxter and Brooke A. Sommerfeld and Alice Tillman and Jeremy Moore and Joan L. Slonczewski and Jeffrey E. Barrick and Richard E. Lenski},
url = {https://elifesciences.org/articles/55414},
doi = {10.7554/eLife.55414},
issn = {2050-084X},
year = {2020},
date = {2020-05-01},
urldate = {2020-05-01},
journal = {eLife},
volume = {9},
pages = {1--64},
abstract = {Evolutionary innovations allow populations to colonize new ecological niches. We previously reported that aerobic growth on citrate (Cit^{+}) evolved in an \textit{Escherichia coli} population during adaptation to a minimal glucose medium containing citrate (DM25). Cit^{+} variants can also grow in citrate-only medium (DM0), a novel environment for \textit{E. coli}. To study adaptation to this niche, we founded two sets of Cit^{+} populations and evolved them for 2500 generations in DM0 or DM25. The evolved lineages acquired numerous parallel mutations, many mediated by transposable elements. Several also evolved amplifications of regions containing the \textit{maeA} gene. Unexpectedly, some evolved populations and clones show apparent declines in fitness. We also found evidence of substantial cell death in Cit^{+} clones. Our results thus demonstrate rapid trait refinement and adaptation to the new citrate niche, while also suggesting a recalcitrant mismatch between \textit{E. coli} physiology and growth on citrate.},
keywords = {Citrate Evolution, Demography and Ecology, Descendant Experiments, Genotypes and Phenotypes},
pubstate = {published},
tppubtype = {article}
}
Evolutionary innovations allow populations to colonize new ecological niches. We previously reported that aerobic growth on citrate (Cit+) evolved in an Escherichia coli population during adaptation to a minimal glucose medium containing citrate (DM25). Cit+ variants can also grow in citrate-only medium (DM0), a novel environment for E. coli. To study adaptation to this niche, we founded two sets of Cit+ populations and evolved them for 2500 generations in DM0 or DM25. The evolved lineages acquired numerous parallel mutations, many mediated by transposable elements. Several also evolved amplifications of regions containing the maeA gene. Unexpectedly, some evolved populations and clones show apparent declines in fitness. We also found evidence of substantial cell death in Cit+ clones. Our results thus demonstrate rapid trait refinement and adaptation to the new citrate niche, while also suggesting a recalcitrant mismatch between E. coli physiology and growth on citrate.
Barrick J E; Deatherage D E; Lenski R E
Banzhaf, Wolfgang; Cheng, Betty H C; Deb, Kalyanmoy; Holekamp, Kay E; Lenski, Richard E; Ofria, Charles; Pennock, Robert T; Punch, William F; Whittaker, Danielle J (Ed.): Evolution in Action: Past, Present and Future: A Festschrift in Honor of Erik D. Goodman, pp. 77–89, Springer International Publishing, Cham, 2020, ISBN: 978-3-030-39831-6.
Abstract | Links | BibTeX | Altmetric | Tags: Fitness Trajectories
@inbook{Barrick2020,
title = {A Test of the Repeatability of Measurements of Relative Fitness in the Long-Term Evolution Experiment with \textit{Escherichia coli}},
author = {Jeffrey E. Barrick and Daniel E. Deatherage and Richard E. Lenski},
editor = {Wolfgang Banzhaf and Betty H C Cheng and Kalyanmoy Deb and Kay E Holekamp and Richard E Lenski and Charles Ofria and Robert T Pennock and William F Punch and Danielle J Whittaker},
url = {http://link.springer.com/10.1007/978-3-030-39831-6_8},
doi = {10.1007/978-3-030-39831-6_8},
isbn = {978-3-030-39831-6},
year = {2020},
date = {2020-01-01},
urldate = {2020-01-01},
booktitle = {Evolution in Action: Past, Present and Future: A Festschrift in Honor of Erik D. Goodman},
pages = {77--89},
publisher = {Springer International Publishing},
address = {Cham},
abstract = {Experimental studies of evolution using microbes have a long tradition, and these studies have increased greatly in number and scope in recent decades. Most such experiments have been short in duration, typically running for weeks or months. A venerable exception, the long-term evolution experiment (LTEE) with \textit{Escherichia coli} has continued for 30 years and 70,000 bacterial generations. The LTEE has become one of the cornerstones of the field of experimental evolution, in general, and the BEACON Center for the Study of Evolution in Action, in particular. Science laboratories and experiments usually have finite lifespans, but we hope that the LTEE can continue far into the future. There are practical issues associated with maintaining such a long-term experiment. One issue, which we address here, is whether key measurements made at one time and place are reproducible, within reasonable limits, at other times and places. This issue comes to the forefront when one considers moving an experiment like the LTEE from one lab to another. To that end, the Barrick lab at The University of Texas at Austin, measured the fitness values of samples from the 12 LTEE populations at 2,000, 10,000, and 50,000 generations and compared the new data to data previously obtained at Michigan State University. On balance, the datasets agree very well. More generally, this finding shows the value of simplicity in experimental design, such as using a chemically defined growth medium and appropriately storing samples from microbiological experiments. Even so, one must be vigilant in checking assumptions and procedures given the potential for uncontrolled factors (e.g., water quality) to affect outcomes. This vigilance is perhaps especially important for a trait like fitness, which integrates all aspects of organismal performance and may therefore be sensitive to any number of subtle environmental influences.},
keywords = {Fitness Trajectories},
pubstate = {published},
tppubtype = {inbook}
}
Experimental studies of evolution using microbes have a long tradition, and these studies have increased greatly in number and scope in recent decades. Most such experiments have been short in duration, typically running for weeks or months. A venerable exception, the long-term evolution experiment (LTEE) with Escherichia coli has continued for 30 years and 70,000 bacterial generations. The LTEE has become one of the cornerstones of the field of experimental evolution, in general, and the BEACON Center for the Study of Evolution in Action, in particular. Science laboratories and experiments usually have finite lifespans, but we hope that the LTEE can continue far into the future. There are practical issues associated with maintaining such a long-term experiment. One issue, which we address here, is whether key measurements made at one time and place are reproducible, within reasonable limits, at other times and places. This issue comes to the forefront when one considers moving an experiment like the LTEE from one lab to another. To that end, the Barrick lab at The University of Texas at Austin, measured the fitness values of samples from the 12 LTEE populations at 2,000, 10,000, and 50,000 generations and compared the new data to data previously obtained at Michigan State University. On balance, the datasets agree very well. More generally, this finding shows the value of simplicity in experimental design, such as using a chemically defined growth medium and appropriately storing samples from microbiological experiments. Even so, one must be vigilant in checking assumptions and procedures given the potential for uncontrolled factors (e.g., water quality) to affect outcomes. This vigilance is perhaps especially important for a trait like fitness, which integrates all aspects of organismal performance and may therefore be sensitive to any number of subtle environmental influences.