1
|
Abstract
We discuss the genetic, demographic, and selective forces that are likely to be at play in restricting observed levels of DNA sequence variation in natural populations to a much smaller range of values than would be expected from the distribution of census population sizes alone-Lewontin's Paradox. While several processes that have previously been strongly emphasized must be involved, including the effects of direct selection and genetic hitchhiking, it seems unlikely that they are sufficient to explain this observation without contributions from other factors. We highlight a potentially important role for the less-appreciated contribution of population size change; specifically, the likelihood that many species and populations may be quite far from reaching the relatively high equilibrium diversity values that would be expected given their current census sizes.
Collapse
Affiliation(s)
- Brian Charlesworth
- Institute of Ecology and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom
| | - Jeffrey D Jensen
- School of Life Sciences, Arizona State University, Tempe, AZ, USA
| |
Collapse
|
2
|
Gardon H, Biderre-Petit C, Jouan-Dufournel I, Bronner G. A drift-barrier model drives the genomic landscape of a structured bacterial population. Mol Ecol 2020; 29:4143-4156. [PMID: 32920913 DOI: 10.1111/mec.15628] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2020] [Revised: 08/26/2020] [Accepted: 08/28/2020] [Indexed: 01/05/2023]
Abstract
Bacterial populations differentiate over time and space to form distinct genetic units. The mechanisms governing this diversification are presumed to result from the ecological context of living units to adapt to specific niches. Recently, a model assuming the acquisition of advantageous genes among populations rather than whole genome sweeps has emerged to explain population differentiation. However, the characteristics of these exchanged, or flexible, genes and whether their evolution is driven by adaptive or neutral processes remain controversial. By analysing the flexible genome of single-amplified genomes of co-occurring populations of the marine Prochlorococcus HLII ecotype, we highlight that genomic compartments - rather than population units - are characterized by different evolutionary trajectories. The dynamics of gene fluxes vary across genomic compartments and therefore the effectiveness of selection depends on the fluctuation of the effective population size along the genome. Taken together, these results support the drift-barrier model of bacterial evolution.
Collapse
Affiliation(s)
- Hélène Gardon
- Laboratoire Microorganismes: Génome et Environnement, Université Clermont Auvergne, CNRS, Clermont-Ferrand, France
| | - Corinne Biderre-Petit
- Laboratoire Microorganismes: Génome et Environnement, Université Clermont Auvergne, CNRS, Clermont-Ferrand, France
| | - Isabelle Jouan-Dufournel
- Laboratoire Microorganismes: Génome et Environnement, Université Clermont Auvergne, CNRS, Clermont-Ferrand, France
| | - Gisèle Bronner
- Laboratoire Microorganismes: Génome et Environnement, Université Clermont Auvergne, CNRS, Clermont-Ferrand, France
| |
Collapse
|
3
|
Abstract
The genomes of bacteria contain fewer genes and substantially less noncoding DNA than those of eukaryotes, and as a result, they have much less raw material to invent new traits. Yet, bacteria are vastly more taxonomically diverse, numerically abundant, and globally successful in colonizing new habitats compared to eukaryotes. Although bacterial genomes are generally considered to be optimized for efficient growth and rapid adaptation, nonadaptive processes have played a major role in shaping the size, contents, and compact organization of bacterial genomes and have allowed the establishment of deleterious traits that serve as the raw materials for genetic innovation.
Collapse
Affiliation(s)
- Paul C Kirchberger
- Department of Integrative Biology, University of Texas at Austin, Texas 78712, USA; ; ;
| | - Marian L Schmidt
- Department of Integrative Biology, University of Texas at Austin, Texas 78712, USA; ; ;
| | - Howard Ochman
- Department of Integrative Biology, University of Texas at Austin, Texas 78712, USA; ; ;
| |
Collapse
|
4
|
Bobay LM. CoreSimul: a forward-in-time simulator of genome evolution for prokaryotes modeling homologous recombination. BMC Bioinformatics 2020; 21:264. [PMID: 32580695 PMCID: PMC7315543 DOI: 10.1186/s12859-020-03619-x] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2020] [Accepted: 06/19/2020] [Indexed: 12/26/2022] Open
Abstract
Background Prokaryotes are asexual, but these organisms frequently engage in homologous recombination, a process that differs from meiotic recombination in sexual organisms. Most tools developed to simulate genome evolution either assume sexual reproduction or the complete absence of DNA flux in the population. As a result, very few simulators are adapted to model prokaryotic genome evolution while accounting for recombination. Moreover, many simulators are based on the coalescent, which assumes a neutral model of genomic evolution, and those are best suited for organisms evolving under weak selective pressures, such as animals and plants. In contrast, prokaryotes are thought to be evolving under much stronger selective pressures, suggesting that forward-in-time simulators are better suited for these organisms. Results Here, I present CoreSimul, a forward-in-time simulator of core genome evolution for prokaryotes modeling homologous recombination. Simulations are guided by a phylogenetic tree and incorporate different substitution models, including models of codon selection. Conclusions CoreSimul is a flexible forward-in-time simulator that constitutes a significant addition to the limited list of available simulators applicable to prokaryote genome evolution.
Collapse
Affiliation(s)
- Louis-Marie Bobay
- Department of Biology, University of North Carolina Greensboro, 321 McIver Street, PO Box 26170, Greensboro, NC, 27402, USA.
| |
Collapse
|
5
|
Martinez-Gutierrez CA, Aylward FO. Strong Purifying Selection Is Associated with Genome Streamlining in Epipelagic Marinimicrobia. Genome Biol Evol 2020; 11:2887-2894. [PMID: 31539038 PMCID: PMC6798728 DOI: 10.1093/gbe/evz201] [Citation(s) in RCA: 23] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/08/2019] [Indexed: 12/21/2022] Open
Abstract
Marine microorganisms inhabiting nutrient-depleted waters play critical roles in global biogeochemical cycles due to their abundance and broad distribution. Many of these microbes share similar genomic features including small genome size, low % G + C content, short intergenic regions, and low nitrogen content in encoded amino acid residue side chains (N-ARSC), but the evolutionary drivers of these characteristics are unclear. Here, we compared the strength of purifying selection across the Marinimicrobia, a candidate phylum which encompasses a broad range of phylogenetic groups with disparate genomic features, by estimating the ratio of nonsynonymous and synonymous substitutions (dN/dS) in conserved marker genes. Our analysis reveals that epipelagic Marinimicrobia that exhibit features consistent with genome streamlining have significantly lower dN/dS values when compared with their mesopelagic counterparts. We also found a significant positive correlation between median dN/dS values and % G + C content, N-ARSC, and intergenic region length. We did not identify a significant correlation between dN/dS ratios and estimated genome size, suggesting the strength of selection is not a primary factor shaping genome size in this group. Our findings are generally consistent with genome streamlining theory, which postulates that many genomic features of abundant epipelagic bacteria are the result of adaptation to oligotrophic nutrient conditions. Our results are also in agreement with previous findings that genome streamlining is common in epipelagic waters, suggesting that microbes inhabiting this region of the ocean have been shaped by strong selection together with prevalent nutritional constraints characteristic of this environment.
Collapse
Affiliation(s)
| | - Frank O Aylward
- Department of Biological Sciences, Virginia Tech, Blacksburg, Virginia
| |
Collapse
|
6
|
Becher H, Jackson BC, Charlesworth B. Patterns of Genetic Variability in Genomic Regions with Low Rates of Recombination. Curr Biol 2019; 30:94-100.e3. [PMID: 31866366 DOI: 10.1016/j.cub.2019.10.047] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2019] [Revised: 10/09/2019] [Accepted: 10/23/2019] [Indexed: 12/19/2022]
Abstract
The amount of DNA sequence variability in a genomic region is often positively correlated with its rate of crossing over (CO) [1-3]. This pattern is caused by selection acting on linked sites, which reduces genetic variability and biases the frequency distribution of segregating variants toward more rare variants than are expected without selection (skew). These effects may involve the spread of beneficial mutations (selective sweeps [SSWs]), the elimination of deleterious mutations (background selection [BGS]), or both, and are expected to be stronger with lower CO rates [1-3]. However, in a recent study of human populations, the skew was reduced in the lowest CO regions compared with regions with somewhat higher CO rates [4]. A low skew in very low CO regions, compared with theoretical predictions, is seen in the population genomic studies of Drosophila simulans described here and in other Drosophila species. Here, we propose an explanation for lower than expected skew in low CO regions, and validate it using computer simulations; explanations for higher skew with higher CO rates, as in D. simulans, will be explored elsewhere. Partially recessive, linked deleterious mutations can increase neutral variability when the product of the effective population size (Ne) and the selection coefficient against homozygous carriers of mutations (s) is ≤1, i.e., there is associative overdominance (AOD) rather than BGS [5]. AOD can operate in low CO regions, producing a lower skew than in its absence. This opens up a new perspective on how selection affects patterns of variability at linked sites.
Collapse
Affiliation(s)
- Hannes Becher
- Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Charlotte Auerbach Road, Edinburgh EH9 3FL, UK.
| | - Benjamin C Jackson
- Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Charlotte Auerbach Road, Edinburgh EH9 3FL, UK
| | - Brian Charlesworth
- Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Charlotte Auerbach Road, Edinburgh EH9 3FL, UK
| |
Collapse
|
7
|
Buppan P, Seethamchai S, Kuamsab N, Harnyuttanakorn P, Putaporntip C, Jongwutiwes S. Multiple Novel Mutations in Plasmodium falciparum Chloroquine Resistance Transporter Gene during Implementation of Artemisinin Combination Therapy in Thailand. Am J Trop Med Hyg 2019; 99:987-994. [PMID: 30141388 DOI: 10.4269/ajtmh.18-0401] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
Mutations in the chloroquine resistance transporter gene of Plasmodium falciparum (Pfcrt) are associated with drug susceptibility status of chloroquine and other antimalarials that interfere with heme detoxification process including artemisinin. We aim to investigate whether an increase in duration of artemisinin combination therapy (ACT) in Thailand could affect mutations in Pfcrt. The complete coding sequences of Pfcrt and dihydrofolate reductase (Pfdhfr), and size polymorphisms of the merozoite surface proteins-1 and 2 (Pfmsp-1 and Pfmsp-2) of 189 P. falciparum isolates collected during 1991 and 2016 were analyzed. In total, 12 novel amino acid substitutions and 13 novel PfCRT haplotypes were identified. The most prevalent haplotype belonged to the Dd2 sequence and no wild type was found. A significant positive correlation between the frequency of Pfcrt mutants and the year of sample collection was observed during nationwide ACT implementation (r = 0.780; P = 0.038). The number of haplotypes and nucleotide diversity of isolates collected during 3-day ACT (2009-2016) significantly outnumbered those collected before this treatment regimen. Positive Darwinian selection occurred in the transmembrane domains only among isolates collected during 3-day ACT but not among those collected before this period. No remarkable change was observed in the molecular indices for other loci analyzed when similar comparisons were performed. An increase in the duration of artesunate in combination therapy in Thailand could exert selective pressure on the Pfcrt locus, resulting in emergence of novel variants. The impact of these novel haplotypes on antimalarial susceptibilities requires further study.
Collapse
Affiliation(s)
- Pattakorn Buppan
- Molecular Biology of Malaria and Opportunistic Parasites Research Unit, Department of Parasitology, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand.,Inter-Department Program of Biomedical Sciences, Faculty of Graduate School, Chulalongkorn University, Bangkok, Thailand
| | - Sunee Seethamchai
- Department of Biology, Faculty of Science, Naresuan University, Pitsanulok Province, Thailand
| | - Napaporn Kuamsab
- Molecular Biology of Malaria and Opportunistic Parasites Research Unit, Department of Parasitology, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand
| | | | - Chaturong Putaporntip
- Molecular Biology of Malaria and Opportunistic Parasites Research Unit, Department of Parasitology, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand
| | - Somchai Jongwutiwes
- Molecular Biology of Malaria and Opportunistic Parasites Research Unit, Department of Parasitology, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand
| |
Collapse
|
8
|
Bobay LM, Ochman H. Factors driving effective population size and pan-genome evolution in bacteria. BMC Evol Biol 2018; 18:153. [PMID: 30314447 PMCID: PMC6186134 DOI: 10.1186/s12862-018-1272-4] [Citation(s) in RCA: 77] [Impact Index Per Article: 12.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2018] [Accepted: 10/04/2018] [Indexed: 02/07/2023] Open
Abstract
Background Knowledge of population-level processes is essential to understanding the efficacy of selection operating within a species. However, attempts at estimating effective population sizes (Ne) are particularly challenging in bacteria due to their extremely large census populations sizes, varying rates of recombination and arbitrary species boundaries. Results In this study, we estimated Ne for 153 species (152 bacteria and one archaeon) defined under a common framework and found that ecological lifestyle and growth rate were major predictors of Ne; and that contrary to theoretical expectations, Ne was unaffected by recombination rate. Additionally, we found that Ne shapes the evolution and diversity of total gene repertoires of prokaryotic species. Conclusion Together, these results point to a new model of genome architecture evolution in prokaryotes, in which pan-genome sizes, not individual genome sizes, are governed by drift-barrier evolution. Electronic supplementary material The online version of this article (10.1186/s12862-018-1272-4) contains supplementary material, which is available to authorized users.
Collapse
Affiliation(s)
- Louis-Marie Bobay
- Department of Integrative Biology, University of Texas at Austin, Austin, TX, 78712, USA. .,Department of Biology, University of North Carolina at Greensboro, 321 McIver Street, PO Box 26170, Greensboro, NC, 27402, USA.
| | - Howard Ochman
- Department of Integrative Biology, University of Texas at Austin, Austin, TX, 78712, USA
| |
Collapse
|
9
|
Abstract
Some bacteria can transfer to new host species, and this poses a risk to human health. Indeed, an estimated 60% of all human pathogens have originated from other animal species. Similarly, human-to-animal transitions are recognized as a major threat to sustainable livestock production, and emerging pathogens impose an increasing burden on crop yield and global food security. Recent advances in high-throughput sequencing technologies have enabled comparative genomic analyses of bacterial populations from multiple hosts. Such studies are providing new insights into the evolutionary processes that underpin the establishment of bacteria in new host niches. A better understanding of the genetic and mechanistic basis for bacterial host adaptation may reveal novel targets for controlling infection or inform the design of approaches to limit the emergence of new pathogens.
Collapse
Affiliation(s)
- Samuel K Sheppard
- Milner Centre for Evolution, Department of Biology & Biotechnology, University of Bath, Claverton Down, Bath, UK
| | - David S Guttman
- Department of Cell & Systems Biology, University of Toronto, Toronto, Ontario, Canada
- Centre for the Analysis of Genome Evolution & Function, University of Toronto, Toronto, Ontario, Canada
| | - J Ross Fitzgerald
- The Roslin Institute, University of Edinburgh, Easter Bush Campus, Edinburgh, UK.
| |
Collapse
|
10
|
Sprouffske K, Aguilar-Rodríguez J, Sniegowski P, Wagner A. High mutation rates limit evolutionary adaptation in Escherichia coli. PLoS Genet 2018; 14:e1007324. [PMID: 29702649 PMCID: PMC5942850 DOI: 10.1371/journal.pgen.1007324] [Citation(s) in RCA: 54] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2017] [Revised: 05/09/2018] [Accepted: 03/21/2018] [Indexed: 11/19/2022] Open
Abstract
Mutation is fundamental to evolution, because it generates the genetic variation on which selection can act. In nature, genetic changes often increase the mutation rate in systems that range from viruses and bacteria to human tumors. Such an increase promotes the accumulation of frequent deleterious or neutral alleles, but it can also increase the chances that a population acquires rare beneficial alleles. Here, we study how up to 100-fold increases in Escherichia coli's genomic mutation rate affect adaptive evolution. To do so, we evolved multiple replicate populations of asexual E. coli strains engineered to have four different mutation rates for 3000 generations in the laboratory. We measured the ability of evolved populations to grow in their original environment and in more than 90 novel chemical environments. In addition, we subjected the populations to whole genome population sequencing. Although populations with higher mutation rates accumulated greater genetic diversity, this diversity conveyed benefits only for modestly increased mutation rates, where populations adapted faster and also thrived better than their ancestors in some novel environments. In contrast, some populations at the highest mutation rates showed reduced adaptation during evolution, and failed to thrive in all of the 90 alternative environments. In addition, they experienced a dramatic decrease in mutation rate. Our work demonstrates that the mutation rate changes the global balance between deleterious and beneficial mutational effects on fitness. In contrast to most theoretical models, our experiments suggest that this tipping point already occurs at the modest mutation rates that are found in the wild.
Collapse
Affiliation(s)
- Kathleen Sprouffske
- Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland
- The Swiss Institute of Bioinformatics, Lausanne, Switzerland
| | - José Aguilar-Rodríguez
- Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland
- The Swiss Institute of Bioinformatics, Lausanne, Switzerland
| | - Paul Sniegowski
- Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
| | - Andreas Wagner
- Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland
- The Swiss Institute of Bioinformatics, Lausanne, Switzerland
- The Santa Fe Institute, Santa Fe, New Mexico, United States of America
| |
Collapse
|
11
|
Carrasco-Hernandez R, Jácome R, López Vidal Y, Ponce de León S. Are RNA Viruses Candidate Agents for the Next Global Pandemic? A Review. ILAR J 2017; 58:343-358. [PMID: 28985316 PMCID: PMC7108571 DOI: 10.1093/ilar/ilx026] [Citation(s) in RCA: 124] [Impact Index Per Article: 17.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2017] [Revised: 07/14/2017] [Accepted: 07/15/2017] [Indexed: 12/16/2022] Open
Abstract
Pathogenic RNA viruses are potentially the most important group involved in zoonotic disease transmission, and they represent a challenge for global disease control. Their biological diversity and rapid adaptive rates have proved to be difficult to overcome and to anticipate by modern medical technology. Also, the anthropogenic change of natural ecosystems and the continuous population growth are driving increased rates of interspecies contacts and the interchange of pathogens that can develop into global pandemics. The combination of molecular, epidemiological, and ecological knowledge of RNA viruses is therefore essential towards the proper control of these emergent pathogens. This review outlines, throughout different levels of complexity, the problems posed by RNA viral diseases, covering some of the molecular mechanisms allowing them to adapt to new host species-and to novel pharmaceutical developments-up to the known ecological processes involved in zoonotic transmission.
Collapse
Affiliation(s)
- R Carrasco-Hernandez
- R. Carrasco-Hernandez, PhD, is a postdoctoral research fellow at the Microbiome Laboratory in the Postgraduate Division of the Faculty of Medicine at the Universidad Nacional Autónoma de México, CDMX
| | - Rodrigo Jácome
- Rodrigo Jácome, MD, PhD, is a postdoctoral research fellow at the Microbiome Laboratory in the Postgraduate Division of the Faculty of Medicine at the Universidad Nacional Autónoma de México, CDMX
| | - Yolanda López Vidal
- Yolanda López-Vidal, MD, PhD, is an associate professor “C” and is responsible for the Program of Microbial Molecular Immunology in the Department of Microbiology and Parasitology of the Faculty of Medicine at the Universidad Nacional Autónoma de México, CDMX
| | - Samuel Ponce de León
- Samuel Ponce-de-León, MD, MSc, is an associate professor “C”, is responsible for the Microbiome Laboratory and Coordinator of the University Program for Health Research of the Faculty of Medicine at the Universidad Nacional Autónoma de México, CDMX
| |
Collapse
|
12
|
Bobay LM, Ochman H. The Evolution of Bacterial Genome Architecture. Front Genet 2017; 8:72. [PMID: 28611826 PMCID: PMC5447742 DOI: 10.3389/fgene.2017.00072] [Citation(s) in RCA: 83] [Impact Index Per Article: 11.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2017] [Accepted: 05/12/2017] [Indexed: 11/15/2022] Open
Abstract
The genome architecture of bacteria and eukaryotes evolves in opposite directions when subject to genetic drift, a difference that can be ascribed to the fact that bacteria exhibit a mutational bias that deletes superfluous sequences, whereas eukaryotes are biased toward large insertions. Expansion of eukaryotic genomes occurs through the addition of non-functional sequences, such as repetitive sequences and transposable elements, whereas variation in bacterial genome size is largely due to the acquisition and loss of functional accessory genes. These properties create the situation in which eukaryotes with very similar numbers of genes can have vastly different genome sizes, while in bacteria, gene number scales linearly with genome size. Some bacterial genomes, however, particularly those of species that undergo bottlenecks due to recent association with hosts, accumulate pseudogenes and mobile elements, conferring them a low gene content relative to their genome size. These non-functional sequences are gradually eroded and eliminated after long-term association with hosts, with the result that obligate symbionts have the smallest genomes of any cellular organism. The architecture of bacterial genomes is shaped by complex and diverse processes, but for most bacterial species, genome size is governed by a non-adaptive process, i.e., genetic drift coupled with a mutational bias toward deletions. Thus, bacteria with small effective population sizes typically have the smallest genomes. Some marine bacteria counter this near-universal trend: despite having immense population sizes, selection, not drift, acts to reduce genome size in response to metabolic constraints in their nutrient-limited environment.
Collapse
Affiliation(s)
- Louis-Marie Bobay
- Department of Integrative Biology, University of Texas, AustinTX, United States
| | - Howard Ochman
- Department of Integrative Biology, University of Texas, AustinTX, United States
| |
Collapse
|
13
|
A Comparison of the Costs and Benefits of Bacterial Gene Expression. PLoS One 2016; 11:e0164314. [PMID: 27711251 PMCID: PMC5053530 DOI: 10.1371/journal.pone.0164314] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2016] [Accepted: 09/22/2016] [Indexed: 11/19/2022] Open
Abstract
To study how a bacterium allocates its resources, we compared the costs and benefits of most (86%) of the proteins in Escherichia coli K-12 during growth in minimal glucose medium. The cost or investment in each protein was estimated from ribosomal profiling data, and the benefit of each protein was measured by assaying a library of transposon mutants. We found that proteins that are important for fitness are usually highly expressed, and 95% of these proteins are expressed at above 13 parts per million (ppm). Conversely, proteins that do not measurably benefit the host (with a benefit of less than 5% per generation) tend to be weakly expressed, with a median expression of 13 ppm. In aggregate, genes with no detectable benefit account for 31% of protein production, or about 22% if we correct for genetic redundancy. Although some of the apparently unnecessary expression could have subtle benefits in minimal glucose medium, the majority of the burden is due to genes that are important in other conditions. We propose that at least 13% of the cell's protein is "on standby" in case conditions change.
Collapse
|
14
|
Price MN, Arkin AP. A Theoretical Lower Bound for Selection on the Expression Levels of Proteins. Genome Biol Evol 2016; 8:1917-28. [PMID: 27289091 PMCID: PMC4943197 DOI: 10.1093/gbe/evw126] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
We use simple models of the costs and benefits of microbial gene expression to show that changing a protein's expression away from its optimum by 2-fold should reduce fitness by at least [Formula: see text], where P is the fraction the cell's protein that the gene accounts for. As microbial genes are usually expressed at above 5 parts per million, and effective population sizes are likely to be above 10(6), this implies that 2-fold changes to gene expression levels are under strong selection, as [Formula: see text], where Ne is the effective population size and s is the selection coefficient. Thus, most gene duplications should be selected against. On the other hand, we predict that for most genes, small changes in the expression will be effectively neutral.
Collapse
Affiliation(s)
- Morgan N Price
- Environmental Genomics and Systems Biology, Lawrence Berkeley National Lab
| | - Adam P Arkin
- Environmental Genomics and Systems Biology, Lawrence Berkeley National Lab
| |
Collapse
|
15
|
Indirect Fitness Benefits Enable the Spread of Host Genes Promoting Costly Transfer of Beneficial Plasmids. PLoS Biol 2016; 14:e1002478. [PMID: 27270455 PMCID: PMC4896427 DOI: 10.1371/journal.pbio.1002478] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2016] [Accepted: 05/09/2016] [Indexed: 11/19/2022] Open
Abstract
Bacterial genes that confer crucial phenotypes, such as antibiotic resistance, can spread horizontally by residing on mobile genetic elements (MGEs). Although many mobile genes provide strong benefits to their hosts, the fitness consequences of the process of transfer itself are less clear. In previous studies, transfer has been interpreted as a parasitic trait of the MGEs because of its costs to the host but also as a trait benefiting host populations through the sharing of a common gene pool. Here, we show that costly donation is an altruistic act when it spreads beneficial MGEs favoured when it increases the inclusive fitness of donor ability alleles. We show mathematically that donor ability can be selected when relatedness at the locus modulating transfer is sufficiently high between donor and recipients, ensuring high frequency of transfer between cells sharing donor alleles. We further experimentally demonstrate that either population structure or discrimination in transfer can increase relatedness to a level selecting for chromosomal transfer alleles. Both mechanisms are likely to occur in natural environments. The simple process of strong dilution can create sufficient population structure to select for donor ability. Another mechanism observed in natural isolates, discrimination in transfer, can emerge through coselection of transfer and discrimination alleles. Our work shows that horizontal gene transfer in bacteria can be promoted by bacterial hosts themselves and not only by MGEs. In the longer term, the success of cells bearing beneficial MGEs combined with biased transfer leads to an association between high donor ability, discrimination, and mobile beneficial genes. However, in conditions that do not select for altruism, host bacteria promoting transfer are outcompeted by hosts with lower transfer rate, an aspect that could be relevant in the fight against the spread of antibiotic resistance. Altruistic host bacteria can preferentially enhance the horizontal transfer of beneficial plasmids (such as those conferring antibiotic resistance or virulence) to others of their kind. In bacteria, genes can move between cells, sometimes with the donor host cell actively involved in the gene transfer mechanisms. This movement of genes is called horizontal gene transfer, and it increases the prevalence of mobile genes in bacterial populations. However, it is not clear if donor host cells benefit from gene spread, or are simply exploited by selfish genes. Here, we show with both modelling and experiments that for the donor host, investing in the transfer of beneficial genes—such as those conferring antibiotic resistance—can be understood as an altruistic behaviour. This behaviour is costly to the donor but beneficial to recipients and can be selected for if a sufficient proportion of recipient cells share the donors’ transfer allele. Preferential transfer from donors towards recipients that share this allele occurs when dispersal is limited or if discrimination mechanisms are present. Our work suggests that both processes are likely to be widespread in nature, promoting horizontal gene spread by host donor cells. As many antimicrobial resistance and virulence genes are mobile, our work further implies that the spread of harmful traits among human pathogens may be modulated by host bacteria in a direction that depends on the bacterial ability to transfer the traits specifically to their kind.
Collapse
|