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Unleashing Natural Competence in Lactococcus lactis by Induction of the Competence Regulator ComX. Appl Environ Microbiol 2017; 83:AEM.01320-17. [PMID: 28778888 DOI: 10.1128/aem.01320-17] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2017] [Accepted: 08/01/2017] [Indexed: 11/20/2022] Open
Abstract
In biotechnological workhorses like Streptococcus thermophilus and Bacillus subtilis, natural competence can be induced, which facilitates genetic manipulation of these microbes. However, in strains of the important dairy starter Lactococcus lactis, natural competence has not been established to date. However, in silico analysis of the complete genome sequences of 43 L. lactis strains revealed complete late competence gene sets in 2 L. lactis subsp. cremoris strains (KW2 and KW10) and at least 10 L. lactis subsp. lactis strains, including the model strain IL1403 and the plant-derived strain KF147. The remainder of the strains, including all dairy isolates, displayed genomic decay in one or more of the late competence genes. Nisin-controlled expression of the competence regulator comX in L. lactis subsp. lactis KF147 resulted in the induction of expression of the canonical competence regulon and elicited a state of natural competence in this strain. In contrast, comX expression in L. lactis NZ9000, which was predicted to encode an incomplete competence gene set, failed to induce natural competence. Moreover, mutagenesis of the comEA-EC operon in strain KF147 abolished the comX-driven natural competence, underlining the involvement of the competence machinery. Finally, introduction of nisin-inducible comX expression into nisRK-harboring derivatives of strains IL1403 and KW2 allowed the induction of natural competence in these strains also, expanding this phenotype to other L. lactis strains of both subspecies.IMPORTANCE Specific bacterial species are able to enter a state of natural competence in which DNA is taken up from the environment, allowing the introduction of novel traits. Strains of the species Lactococcus lactis are very important starter cultures for the fermentation of milk in the cheese production process, where these bacteria contribute to the flavor and texture of the end product. The activation of natural competence in this industrially relevant organism can accelerate research aiming to understand industrially relevant traits of these bacteria and can facilitate engineering strategies to harness the natural biodiversity of the species in optimized starter strains.
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52
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Mitigating Mitochondrial Genome Erosion Without Recombination. Genetics 2017; 207:1079-1088. [PMID: 28893855 DOI: 10.1534/genetics.117.300273] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2017] [Accepted: 09/01/2017] [Indexed: 01/08/2023] Open
Abstract
Mitochondria are ATP-producing organelles of bacterial ancestry that played a key role in the origin and early evolution of complex eukaryotic cells. Most modern eukaryotes transmit mitochondrial genes uniparentally, often without recombination among genetically divergent organelles. While this asymmetric inheritance maintains the efficacy of purifying selection at the level of the cell, the absence of recombination could also make the genome susceptible to Muller's ratchet. How mitochondria escape this irreversible defect accumulation is a fundamental unsolved question. Occasional paternal leakage could in principle promote recombination, but it would also compromise the purifying selection benefits of uniparental inheritance. We assess this tradeoff using a stochastic population-genetic model. In the absence of recombination, uniparental inheritance of freely-segregating genomes mitigates mutational erosion, while paternal leakage exacerbates the ratchet effect. Mitochondrial fusion-fission cycles ensure independent genome segregation, improving purifying selection. Paternal leakage provides opportunity for recombination to slow down the mutation accumulation, but always at a cost of increased steady-state mutation load. Our findings indicate that random segregation of mitochondrial genomes under uniparental inheritance can effectively combat the mutational meltdown, and that homologous recombination under paternal leakage might not be needed.
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53
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Ueda M, Takeuchi N, Kaneko K. Stronger selection can slow down evolution driven by recombination on a smooth fitness landscape. PLoS One 2017; 12:e0183120. [PMID: 28809951 PMCID: PMC5557360 DOI: 10.1371/journal.pone.0183120] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2017] [Accepted: 07/31/2017] [Indexed: 11/18/2022] Open
Abstract
Stronger selection implies faster evolution—that is, the greater the force, the faster the change. This apparently self-evident proposition, however, is derived under the assumption that genetic variation within a population is primarily supplied by mutation (i.e. mutation-driven evolution). Here, we show that this proposition does not actually hold for recombination-driven evolution, i.e. evolution in which genetic variation is primarily created by recombination rather than mutation. By numerically investigating population genetics models of recombination, migration and selection, we demonstrate that stronger selection can slow down evolution on a perfectly smooth fitness landscape. Through simple analytical calculation, this apparently counter-intuitive result is shown to stem from two opposing effects of natural selection on the rate of evolution. On the one hand, natural selection tends to increase the rate of evolution by increasing the fixation probability of fitter genotypes. On the other hand, natural selection tends to decrease the rate of evolution by decreasing the chance of recombination between immigrants and resident individuals. As a consequence of these opposing effects, there is a finite selection pressure maximizing the rate of evolution. Hence, stronger selection can imply slower evolution if genetic variation is primarily supplied by recombination.
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Affiliation(s)
- Masahiko Ueda
- Department of Basic Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan
- * E-mail:
| | - Nobuto Takeuchi
- Research Center for Complex Systems Biology, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan
| | - Kunihiko Kaneko
- Department of Basic Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan
- Research Center for Complex Systems Biology, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan
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54
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Recombination-Driven Genome Evolution and Stability of Bacterial Species. Genetics 2017; 207:281-295. [PMID: 28751420 DOI: 10.1534/genetics.117.300061] [Citation(s) in RCA: 30] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2016] [Accepted: 07/18/2017] [Indexed: 01/21/2023] Open
Abstract
While bacteria divide clonally, horizontal gene transfer followed by homologous recombination is now recognized as an important contributor to their evolution. However, the details of how the competition between clonality and recombination shapes genome diversity remains poorly understood. Using a computational model, we find two principal regimes in bacterial evolution and identify two composite parameters that dictate the evolutionary fate of bacterial species. In the divergent regime, characterized by either a low recombination frequency or strict barriers to recombination, cohesion due to recombination is not sufficient to overcome the mutational drift. As a consequence, the divergence between pairs of genomes in the population steadily increases in the course of their evolution. The species lacks genetic coherence with sexually isolated clonal subpopulations continuously formed and dissolved. In contrast, in the metastable regime, characterized by a high recombination frequency combined with low barriers to recombination, genomes continuously recombine with the rest of the population. The population remains genetically cohesive and temporally stable. Notably, the transition between these two regimes can be affected by relatively small changes in evolutionary parameters. Using the Multi Locus Sequence Typing (MLST) data, we classify a number of bacterial species to be either the divergent or the metastable type. Generalizations of our framework to include selection, ecologically structured populations, and horizontal gene transfer of nonhomologous regions are discussed as well.
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55
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Chu HY, Sprouffske K, Wagner A. The role of recombination in evolutionary adaptation of Escherichia coli to a novel nutrient. J Evol Biol 2017; 30:1692-1711. [PMID: 28612351 DOI: 10.1111/jeb.13132] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2017] [Revised: 05/11/2017] [Accepted: 06/05/2017] [Indexed: 12/11/2022]
Abstract
The benefits and detriments of recombination for adaptive evolution have been studied both theoretically and experimentally, with conflicting predictions and observations. Most pertinent experiments examine recombination's effects in an unchanging environment and do not study its genomewide effects. Here, we evolved six replicate populations of either highly recombining R+ or lowly recombining R- E. coli strains in a changing environment, by introducing the novel nutrients L-arabinose or indole into the environment. The experiment's ancestral strains are not viable on these nutrients, but 130 generations of adaptive evolution were sufficient to render them viable. Recombination conferred a more pronounced advantage to populations adapting to indole. To study the genomic changes associated with this advantage, we sequenced the genomes of 384 clones isolated from selected replicates at the end of the experiment. These genomes harbour complex changes that range from point mutations to large-scale DNA amplifications. Among several candidate adaptive mutations, those in the tryptophanase regulator tnaC stand out, because the tna operon in which it resides has a known role in indole metabolism. One of the highly recombining populations also shows a significant excess of large-scale segmental DNA amplifications that include the tna operon. This lineage also shows a unique and potentially adaptive combination of point mutations and DNA amplifications that may have originated independently from one another, to be joined later by recombination. Our data illustrate that the advantages of recombination for adaptive evolution strongly depend on the environment and that they can be associated with complex genomic changes.
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Affiliation(s)
- H-Y Chu
- Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland
| | - K Sprouffske
- Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland
| | - A Wagner
- Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland.,The Swiss Institute of Bioinformatics, Quartier Sorge, Batiment Genopode, Lausanne, Switzerland.,The Santa Fe Institute, Santa Fe, NM, USA
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56
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Québatte M, Christen M, Harms A, Körner J, Christen B, Dehio C. Gene Transfer Agent Promotes Evolvability within the Fittest Subpopulation of a Bacterial Pathogen. Cell Syst 2017. [PMID: 28624614 PMCID: PMC5496983 DOI: 10.1016/j.cels.2017.05.011] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
The Bartonella gene transfer agent (BaGTA) is an archetypical example for domestication of a phage-derived element to permit high-frequency genetic exchange in bacterial populations. Here we used multiplexed transposon sequencing (TnSeq) and single-cell reporters to globally define the core components and transfer dynamics of BaGTA. Our systems-level analysis has identified inner- and outer-circle components of the BaGTA system, including 55 regulatory components, as well as an additional 74 and 107 components mediating donor transfer and recipient uptake functions. We show that the stringent response signal guanosine-tetraphosphate (ppGpp) restricts BaGTA induction to a subset of fast-growing cells, whereas BaGTA particle uptake depends on a functional Tol-Pal trans-envelope complex that mediates outer-membrane invagination upon cell division. Our findings suggest that Bartonella evolved an efficient strategy to promote genetic exchange within the fittest subpopulation while disfavoring exchange of deleterious genetic information, thereby facilitating genome integrity and rapid host adaptation.
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Affiliation(s)
- Maxime Québatte
- Focal Area Infection Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland
| | - Matthias Christen
- Institute of Molecular Systems Biology, ETH Zürich, Auguste-Piccard-Hof 1, HPT E71, 8093 Zürich, Switzerland
| | - Alexander Harms
- Focal Area Infection Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland
| | - Jonas Körner
- Focal Area Infection Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland
| | - Beat Christen
- Institute of Molecular Systems Biology, ETH Zürich, Auguste-Piccard-Hof 1, HPT E71, 8093 Zürich, Switzerland.
| | - Christoph Dehio
- Focal Area Infection Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland.
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57
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Frozen Accident Pushing 50: Stereochemistry, Expansion, and Chance in the Evolution of the Genetic Code. Life (Basel) 2017; 7:life7020022. [PMID: 28545255 PMCID: PMC5492144 DOI: 10.3390/life7020022] [Citation(s) in RCA: 41] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2017] [Revised: 05/19/2017] [Accepted: 05/20/2017] [Indexed: 12/31/2022] Open
Abstract
Nearly 50 years ago, Francis Crick propounded the frozen accident scenario for the evolution of the genetic code along with the hypothesis that the early translation system consisted primarily of RNA. Under the frozen accident perspective, the code is universal among modern life forms because any change in codon assignment would be highly deleterious. The frozen accident can be considered the default theory of code evolution because it does not imply any specific interactions between amino acids and the cognate codons or anticodons, or any particular properties of the code. The subsequent 49 years of code studies have elucidated notable features of the standard code, such as high robustness to errors, but failed to develop a compelling explanation for codon assignments. In particular, stereochemical affinity between amino acids and the cognate codons or anticodons does not seem to account for the origin and evolution of the code. Here, I expand Crick’s hypothesis on RNA-only translation system by presenting evidence that this early translation already attained high fidelity that allowed protein evolution. I outline an experimentally testable scenario for the evolution of the code that combines a distinct version of the stereochemical hypothesis, in which amino acids are recognized via unique sites in the tertiary structure of proto-tRNAs, rather than by anticodons, expansion of the code via proto-tRNA duplication, and the frozen accident.
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58
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Variation in Recombination Rate: Adaptive or Not? Trends Genet 2017; 33:364-374. [DOI: 10.1016/j.tig.2017.03.003] [Citation(s) in RCA: 100] [Impact Index Per Article: 14.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2016] [Revised: 03/06/2017] [Accepted: 03/07/2017] [Indexed: 01/30/2023]
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59
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Khowal S, Siddiqui MZ, Ali S, Khan MT, Khan MA, Naqvi SH, Wajid S. A report on extensive lateral genetic reciprocation between arsenic resistant Bacillus subtilis and Bacillus pumilus strains analyzed using RAPD-PCR. Mol Phylogenet Evol 2016; 107:443-454. [PMID: 27956257 DOI: 10.1016/j.ympev.2016.12.010] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2016] [Revised: 10/26/2016] [Accepted: 12/08/2016] [Indexed: 12/16/2022]
Abstract
The study involves isolation of arsenic resistant bacteria from soil samples. The characterization of bacteria isolates was based on 16S rRNA gene sequences. The phylogenetic consanguinity among isolates was studied employing rpoB and gltX gene sequence. RAPD-PCR technique was used to analyze genetic similarity between arsenic resistant isolates. In accordance with the results Bacillus subtilis and Bacillus pumilus strains may exhibit extensive horizontal gene transfer. Arsenic resistant potency in Bacillus sonorensis and high arsenite tolerance in Bacillus pumilus strains was identified. The RAPD-PCR primer OPO-02 amplified a 0.5kb DNA band specific to B. pumilus 3ZZZ strain and 0.75kb DNA band specific to B. subtilis 3PP. These unique DNA bands may have potential use as SCAR (Sequenced Characterized Amplified Region) molecular markers for identification of arsenic resistant B. pumilus and B. subtilis strains.
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Affiliation(s)
- Sapna Khowal
- Department of Biotechnology, Faculty of Science, Hamdard University (Jamia Hamdard), New Delhi 110 062, India
| | - Md Zulquarnain Siddiqui
- Department of Biotechnology, Faculty of Science, Hamdard University (Jamia Hamdard), New Delhi 110 062, India
| | - Shadab Ali
- Department of Biotechnology, Faculty of Science, Hamdard University (Jamia Hamdard), New Delhi 110 062, India
| | - Mohd Taha Khan
- Department of Biotechnology, Faculty of Science, Hamdard University (Jamia Hamdard), New Delhi 110 062, India
| | - Mather Ali Khan
- 247, Bond Life Sciences Centre, 1201 Rollins Street, University of Missouri-Columbia, Columbia, MO 65211, USA
| | | | - Saima Wajid
- Department of Biotechnology, Faculty of Science, Hamdard University (Jamia Hamdard), New Delhi 110 062, India.
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60
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Benefits of a Recombination-Proficient Escherichia coli System for Adaptive Laboratory Evolution. Appl Environ Microbiol 2016; 82:6736-6747. [PMID: 27613685 DOI: 10.1128/aem.01850-16] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2016] [Accepted: 09/02/2016] [Indexed: 11/20/2022] Open
Abstract
Adaptive laboratory evolution typically involves the propagation of organisms asexually to select for mutants with the desired phenotypes. However, asexual evolution is prone to competition among beneficial mutations (clonal interference) and the accumulation of hitchhiking and neutral mutations. The benefits of horizontal gene transfer toward overcoming these known disadvantages of asexual evolution were characterized in a strain of Escherichia coli engineered for superior sexual recombination (genderless). Specifically, we experimentally validated the capacity of the genderless strain to reduce the mutational load and recombine beneficial mutations. We also confirmed that inclusion of multiple origins of transfer influences both the frequency of genetic exchange throughout the chromosome and the linkage of donor DNA. We built a simple kinetic model to estimate recombination frequency as a function of transfer size and relative genotype enrichment in batch transfers; the model output correlated well with the experimental data. Our results provide strong support for the advantages of utilizing the genderless strain over its asexual counterpart during adaptive laboratory evolution for generating beneficial mutants with reduced mutational load. IMPORTANCE Over 80 years ago Fisher and Muller began a debate on the origins of sexual recombination. Although many aspects of sexual recombination have been examined at length, experimental evidence behind the behaviors of recombination in many systems and the means to harness it remain elusive. In this study, we sought to experimentally validate some advantages of recombination in typically asexual Escherichia coli and determine if a sexual strain of E. coli can become an effective tool for strain development.
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61
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Abstract
Almost all cellular life forms are hosts to diverse genetic parasites with various levels of autonomy including plasmids, transposons and viruses. Theoretical modeling of the evolution of primordial replicators indicates that parasites (cheaters) necessarily evolve in such systems and can be kept at bay primarily via compartmentalization. Given the (near) ubiquity, abundance and diversity of genetic parasites, the question becomes pertinent: are such parasites intrinsic to life? At least in prokaryotes, the persistence of parasites is linked to the rate of horizontal gene transfer (HGT). We mathematically derive the threshold value of the minimal transfer rate required for selfish element persistence, depending on the element duplication and loss rates as well as the cost to the host. Estimation of the characteristic gene duplication, loss and transfer rates for transposons, plasmids and virus-related elements in multiple groups of diverse bacteria and archaea indicates that most of these rates are compatible with the long term persistence of parasites. Notably, a small but non-zero rate of HGT is also required for the persistence of non-parasitic genes. We hypothesize that cells cannot tune their horizontal transfer rates to be below the threshold required for parasite persistence without experiencing highly detrimental side-effects. As a lower boundary to the minimum DNA transfer rate that a cell can withstand, we consider the process of genome degradation and mutational meltdown of populations through Muller's ratchet. A numerical assessment of this hypothesis suggests that microbial populations cannot purge parasites while escaping Muller's ratchet. Thus, genetic parasites appear to be virtually inevitable in cellular organisms.
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Affiliation(s)
- Jaime Iranzo
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda
| | - Pere Puigbò
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda Present address: Department of Biology, University of Turku, Finland
| | - Alexander E Lobkovsky
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda
| | - Yuri I Wolf
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda
| | - Eugene V Koonin
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda
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62
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Koonin EV. Horizontal gene transfer: essentiality and evolvability in prokaryotes, and roles in evolutionary transitions. F1000Res 2016; 5. [PMID: 27508073 PMCID: PMC4962295 DOI: 10.12688/f1000research.8737.1] [Citation(s) in RCA: 93] [Impact Index Per Article: 11.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Accepted: 07/18/2016] [Indexed: 01/01/2023] Open
Abstract
The wide spread of gene exchange and loss in the prokaryotic world has prompted the concept of ‘lateral genomics’ to the point of an outright denial of the relevance of phylogenetic trees for evolution. However, the pronounced coherence congruence of the topologies of numerous gene trees, particularly those for (nearly) universal genes, translates into the notion of a statistical tree of life (STOL), which reflects a central trend of vertical evolution. The STOL can be employed as a framework for reconstruction of the evolutionary processes in the prokaryotic world. Quantitatively, however, horizontal gene transfer (HGT) dominates microbial evolution, with the rate of gene gain and loss being comparable to the rate of point mutations and much greater than the duplication rate. Theoretical models of evolution suggest that HGT is essential for the survival of microbial populations that otherwise deteriorate due to the Muller’s ratchet effect. Apparently, at least some bacteria and archaea evolved dedicated vehicles for gene transfer that evolved from selfish elements such as plasmids and viruses. Recent phylogenomic analyses suggest that episodes of massive HGT were pivotal for the emergence of major groups of organisms such as multiple archaeal phyla as well as eukaryotes. Similar analyses appear to indicate that, in addition to donating hundreds of genes to the emerging eukaryotic lineage, mitochondrial endosymbiosis severely curtailed HGT. These results shed new light on the routes of evolutionary transitions, but caution is due given the inherent uncertainty of deep phylogenies.
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Affiliation(s)
- Eugene V Koonin
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD, USA
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63
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Hynes AP, Shakya M, Mercer RG, Grüll MP, Bown L, Davidson F, Steffen E, Matchem H, Peach ME, Berger T, Grebe K, Zhaxybayeva O, Lang AS. Functional and Evolutionary Characterization of a Gene Transfer Agent's Multilocus "Genome". Mol Biol Evol 2016; 33:2530-43. [PMID: 27343288 PMCID: PMC5026251 DOI: 10.1093/molbev/msw125] [Citation(s) in RCA: 43] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023] Open
Abstract
Gene transfer agents (GTAs) are phage-like particles that can package and transfer a random piece of the producing cell’s genome, but are unable to transfer all the genes required for their own production. As such, GTAs represent an evolutionary conundrum: are they selfish genetic elements propagating through an unknown mechanism, defective viruses, or viral structures “repurposed” by cells for gene exchange, as their name implies? In Rhodobacter capsulatus, production of the R. capsulatus GTA (RcGTA) particles is associated with a cluster of genes resembling a small prophage. Utilizing transcriptomic, genetic and biochemical approaches, we report that the RcGTA “genome” consists of at least 24 genes distributed across five distinct loci. We demonstrate that, of these additional loci, two are involved in cell recognition and binding and one in the production and maturation of RcGTA particles. The five RcGTA “genome” loci are widespread within Rhodobacterales, but not all loci have the same evolutionary histories. Specifically, two of the loci have been subject to frequent, probably virus-mediated, gene transfer events. We argue that it is unlikely that RcGTA is a selfish genetic element. Instead, our findings are compatible with the scenario that RcGTA is a virus-derived element maintained by the producing organism due to a selective advantage of within-population gene exchange. The modularity of the RcGTA “genome” is presumably a result of selection on the host organism to retain GTA functionality.
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Affiliation(s)
- Alexander P Hynes
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Migun Shakya
- Department of Biological Sciences, Dartmouth College
| | - Ryan G Mercer
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Marc P Grüll
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Luke Bown
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Fraser Davidson
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Ekaterina Steffen
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Heidi Matchem
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Mandy E Peach
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Tim Berger
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Katherine Grebe
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
| | - Olga Zhaxybayeva
- Department of Biological Sciences, Dartmouth College Department of Computer Science, Dartmouth College
| | - Andrew S Lang
- Department of Biology, Memorial University of Newfoundland, St John's, NL, Canada
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64
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Defying Muller's Ratchet: Ancient Heritable Endobacteria Escape Extinction through Retention of Recombination and Genome Plasticity. mBio 2016; 7:mBio.02057-15. [PMID: 27329757 PMCID: PMC4916391 DOI: 10.1128/mbio.02057-15] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
Abstract
Heritable endobacteria, which are transmitted from one host generation to the next, are subjected to evolutionary forces that are different from those experienced by free-living bacteria. In particular, they suffer consequences of Muller’s ratchet, a mechanism that leads to extinction of small asexual populations due to fixation of slightly deleterious mutations combined with the random loss of the most-fit genotypes, which cannot be recreated without recombination. Mycoplasma-related endobacteria (MRE) are heritable symbionts of fungi from two ancient lineages, Glomeromycota (arbuscular mycorrhizal fungi) and Mucoromycotina. Previous studies revealed that MRE maintain unusually diverse populations inside their hosts and may have been associated with fungi already in the early Paleozoic. Here we show that MRE are vulnerable to genomic degeneration and propose that they defy Muller’s ratchet thanks to retention of recombination and genome plasticity. We suggest that other endobacteria may be capable of raising similar defenses against Muller’s ratchet.
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65
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Boto L. Evolutionary change and phylogenetic relationships in light of horizontal gene transfer. J Biosci 2016; 40:465-72. [PMID: 25963270 DOI: 10.1007/s12038-015-9514-8] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
Horizontal gene transfer has, over the past 25 years, become a part of evolutionary thinking. In the present paper I discuss horizontal gene transfer (HGT) in relation to contingency, natural selection, evolutionary change speed and the Tree-of-Life endeavour, with the aim of contributing to the understanding of the role of HGT in evolutionary processes. In addition, the challenges that HGT imposes on the current view of evolution are emphasized.
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Affiliation(s)
- Luis Boto
- Departamento de Biodiversidad y Biologia Evolutiva, Museo Nacional Ciencias Naturales, CSIC, C/ Jose Gutierrez Abascal 2, 28006, Madrid, Spain,
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66
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Abstract
The diversification of prokaryotes is accelerated by their ability to acquire DNA from other genomes. However, the underlying processes also facilitate genome infection by costly mobile genetic elements. The discovery that cells can uptake DNA by natural transformation was instrumental to the birth of molecular biology nearly a century ago. Surprisingly, a new study shows that this mechanism could efficiently cure the genome of mobile elements acquired through previous sexual exchanges. Natural transformation was thought to provide new genetic information to bacteria. Instead, a new study suggests it cures the genome of deleterious mobile elements.
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Affiliation(s)
- Eduardo P. C. Rocha
- Microbial Evolutionary Genomics, Institut Pasteur, Paris, France
- Centre National de la Recherche Scientifique (CNRS), UMR3525, Paris, France
- * E-mail:
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67
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Abstract
The genus Neisseria contains two pathogenic species of prominant public health concern: Neisseria gonorrhoeae and Neisseria meningitidis. These pathogens display a notable ability to undergo frequent programmed recombination events. The recombination-mediated pathways of transformation and pilin antigenic variation in the Neisseria are well-studied systems that are critical for pathogenesis. Here we will detail the conserved and unique aspects of transformation and antigenic variation in the Neisseria. Transformation will be followed from initial DNA binding through recombination into the genome with consideration to the factors necessary at each step. Additional focus is paid to the unique type IV secretion system that mediates donation of transforming DNA in the pathogenic Neisseria. The pilin antigenic variation system uses programmed recombinations to alter a major surface determinant, which allows immune avoidance and promotes infection. We discuss the trans- and cis- acting factors which facilitate pilin antigenic variation and present the current understanding of the mechanisms involved in the process.
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68
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Lou Q, Ma Y, Qu D. Two-component signal transduction system SaeRS is involved in competence and penicillin susceptibility in Staphylococcus epidermidis. J Basic Microbiol 2016; 56:358-68. [PMID: 26898187 DOI: 10.1002/jobm.201500488] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2015] [Accepted: 12/23/2015] [Indexed: 12/28/2022]
Abstract
Staphylococcus epidermidis, which is a causative pathogen of nosocomial infection, expresses its virulent traits such as biofilm and autolysis regulated by two-component signal transduction system SaeRS. In this study, the S. epidermidis SaeRS was identified to negatively regulate the expression of genes involved in competence (comF, murF), cytolysis (lrgA), and autolysis (lytS) by DNA microarray or real-time RT-PCR analysis. In addition, saeRS mutant showed increased competence and higher susceptibility to antibiotics such as penicillin and oxacillin than the wild-type strain. The study will be helpful for understanding the characterization of the SaeRS in S. epidermidis.
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Affiliation(s)
- Qiang Lou
- Henan Engineering Lab of Antibody Medicine, Key Laboratory of Cellular and Molecular Immunology, Medical College of Henan University, Kaifeng, China
| | - Yuanfang Ma
- Henan Engineering Lab of Antibody Medicine, Key Laboratory of Cellular and Molecular Immunology, Medical College of Henan University, Kaifeng, China
| | - Di Qu
- Key Laboratory of Medical Molecular Virology of the Ministry of Education and Ministry of Public Health, Institute of Medical Microbiology and Institutes of Biomedical Sciences, Shanghai Medical College of Fudan University, Shanghai, China
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69
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Goldberg GW, Marraffini LA. Resistance and tolerance to foreign elements by prokaryotic immune systems - curating the genome. Nat Rev Immunol 2016; 15:717-24. [PMID: 26494050 DOI: 10.1038/nri3910] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
To engage in adaptive symbioses or genetic exchange, organisms must interact with foreign, non-self elements despite the risks of predation and parasitism. By surveying the interface between self and non-self, immune systems can help ensure the benevolence of these interactions without isolating their hosts altogether. In this Essay, we examine prokaryotic restriction-modification and CRISPR-Cas (clustered, regularly interspaced palindromic repeat-CRISPR-associated proteins) activities and discuss their analogy to mammalian immune pathways. We further explain how their capacities for resistance and tolerance are optimized to reduce parasitism and immunopathology during encounters with non-self.
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Affiliation(s)
- Gregory W Goldberg
- Laboratory of Bacteriology, The Rockefeller University, 1230 York Avenue, New York City, New York 10065, USA
| | - Luciano A Marraffini
- Laboratory of Bacteriology, The Rockefeller University, 1230 York Avenue, New York City, New York 10065, USA
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70
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Carrasco B, Serrano E, Sánchez H, Wyman C, Alonso JC. Chromosomal transformation in Bacillus subtilis is a non-polar recombination reaction. Nucleic Acids Res 2016; 44:2754-68. [PMID: 26786319 PMCID: PMC4824099 DOI: 10.1093/nar/gkv1546] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2015] [Accepted: 12/29/2015] [Indexed: 11/13/2022] Open
Abstract
Natural chromosomal transformation is one of the primary driving forces of bacterial evolution. This reaction involves the recombination of the internalized linear single-stranded (ss) DNA with the homologous resident duplex via RecA-mediated integration in concert with SsbA and DprA or RecO. We show that sequence divergence prevents Bacillus subtilis chromosomal transformation in a log-linear fashion, but it exerts a minor effect when the divergence is localized at a discrete end. In the nucleotide bound form, RecA shows no apparent preference to initiate recombination at the 3′- or 5′-complementary end of the linear duplex with circular ssDNA, but nucleotide hydrolysis is required when heterology is present at both ends. RecA·dATP initiates pairing of the linear 5′ and 3′ complementary ends, but only initiation at the 5′-end remains stably paired in the absence of SsbA. Our results suggest that during gene transfer RecA·ATP, in concert with SsbA and DprA or RecO, shows a moderate preference for the 3′-end of the duplex. We show that RecA-mediated recombination initiated at the 3′- or 5′-complementary end might have significant implication on the ecological diversification of bacterial species with natural transformation.
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Affiliation(s)
- Begoña Carrasco
- Department of Microbial Biotechnology, Centro Nacional de Biotecnología, CNB-CSIC, 28049 Madrid, Spain
| | - Ester Serrano
- Department of Microbial Biotechnology, Centro Nacional de Biotecnología, CNB-CSIC, 28049 Madrid, Spain
| | - Humberto Sánchez
- Department of Genetics, Cancer Genomics Netherlands, Erasmus University Medical Center, Rotterdam, The Netherlands
| | - Claire Wyman
- Department of Genetics, Cancer Genomics Netherlands, Erasmus University Medical Center, Rotterdam, The Netherlands Department of Radiation Oncology, Erasmus University Medical Center, Rotterdam, The Netherlands
| | - Juan C Alonso
- Department of Microbial Biotechnology, Centro Nacional de Biotecnología, CNB-CSIC, 28049 Madrid, Spain
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71
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Chen XJ, Han K, Feng J, Zhuo L, Li YJ, Li YZ. The complete genome sequence and analysis of a plasmid-bearing myxobacterial strain Myxococcus fulvus 124B02 (M 206081). Stand Genomic Sci 2016; 11:1. [PMID: 26734118 PMCID: PMC4700575 DOI: 10.1186/s40793-015-0121-y] [Citation(s) in RCA: 49] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2015] [Accepted: 09/11/2015] [Indexed: 11/12/2023] Open
Abstract
Myxobacteria, phylogenetically located in the delta division of the Proteobacteria, are well known for characterized social behaviors and large genomes of more than 9 Mb in size. Myxococcus fulvus is a typical species of the genus Myxococcus in the family Myxococcaceae. M. fulvus 124B02, originally isolated from a soil sample collected in Northeast China, is the one and only presently known myxobacterial strain that harbors an endogenous autonomously replicating plasmid, named pMF1. The endogenous plasmid is of importance for understanding the genome evolution of myxobacteria, as well as for the development of genetic engineering tools in myxobacteria. Here we describe the complete genome sequence of this organism. M. fulvus 124B02 consists of a circular chromosome with a total length of 11,048,835 bp and a circular plasmid of 18,634 bp. Comparative genomic analyses suggest that pMF1 has a longstanding sustention within myxobacteria, and probably contributes to the genome expansion of myxobacteria.
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Affiliation(s)
- Xiao-jing Chen
- State Key Laboratory of Microbial Technology, School of Life Science, Shandong University, Jinan, 250100 China
| | - Kui Han
- State Key Laboratory of Microbial Technology, School of Life Science, Shandong University, Jinan, 250100 China
| | - Jing Feng
- State Key Laboratory of Microbial Technology, School of Life Science, Shandong University, Jinan, 250100 China
| | - Li Zhuo
- State Key Laboratory of Microbial Technology, School of Life Science, Shandong University, Jinan, 250100 China
| | - Ya-jie Li
- State Key Laboratory of Microbial Technology, School of Life Science, Shandong University, Jinan, 250100 China
| | - Yue-zhong Li
- State Key Laboratory of Microbial Technology, School of Life Science, Shandong University, Jinan, 250100 China
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72
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van Dijk B, Hogeweg P. In Silico Gene-Level Evolution Explains Microbial Population Diversity through Differential Gene Mobility. Genome Biol Evol 2015; 8:176-88. [PMID: 26710854 PMCID: PMC4758251 DOI: 10.1093/gbe/evv255] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Microbial communities can show astonishing ecological and phylogenetic diversity. What is the role of pervasive horizontal gene transfer (HGT) in shaping this diversity in the presence of clonally expanding “killer strains”? Does HGT of antibiotic production and resistance genes erase phylogenetic structure? To answer these questions, we study a spatial eco-evolutionary model of prokaryotes, inspired by recent findings on antagonistic interactions in Vibrionaceae populations. We find toxin genes evolve to be highly mobile, whereas resistance genes minimize mobility. This differential gene mobility is a requirement to maintain a diverse and dynamic ecosystem. The resistance gene repertoire acts as a core genome that corresponds to the phylogeny of cells, whereas toxin genes do not follow this phylogeny and have a patchy distribution. We also show that interstrain HGT makes the emergent phylogenetic structure robust to selective sweeps. Finally, in this evolved ecosystem we observe antagonistic interactions between, rather than within, spatially structure subpopulations, as has been previously observed for prokaryotes in soils and oceans. In contrast to ascribing the diversification and evolution of microbial communities to clonal dynamics, we show that multilevel evolution can elegantly explain the observed phylogenetic structure and ecosystem diversity.
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Affiliation(s)
- Bram van Dijk
- Department of Theoretical Biology and Bioinformatics, Utrecht University, The Netherlands
| | - Paulien Hogeweg
- Department of Theoretical Biology and Bioinformatics, Utrecht University, The Netherlands
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73
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Abstract
Evolution and maintenance of genetic recombination and its relation to the mutational process is a long-standing, fundamental problem in evolutionary biology that is linked to the general problem of evolution of evolvability. We explored a stochastic model of the evolution of recombination using additive fitness and infinite allele assumptions but no assumptions on the sign or magnitude of the epistasis and the distribution of mutation effects. In this model, fluctuating negative epistasis and predominantly deleterious mutations arise naturally as a consequence of the additive fitness and a reservoir from which new alleles arrive with a fixed distribution of fitness effects. Analysis of the model revealed a nonmonotonic effect of recombination intensity on fitness, with an optimal recombination rate value which maximized fitness in steady state. The optimal recombination rate depended on the mutation rate and was evolvable, that is, subject to selection. The predictions of the model were compatible with the observations on the dependence between genome rearrangement rate and gene flux in microbial genomes.
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Affiliation(s)
- Alexander E Lobkovsky
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, Maryland
| | - Yuri I Wolf
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, Maryland
| | - Eugene V Koonin
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, Maryland
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74
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Arnoldt H, Strogatz SH, Timme M. Toward the Darwinian transition: Switching between distributed and speciated states in a simple model of early life. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 92:052909. [PMID: 26651764 DOI: 10.1103/physreve.92.052909] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/13/2014] [Indexed: 06/05/2023]
Abstract
It has been hypothesized that in the era just before the last universal common ancestor emerged, life on earth was fundamentally collective. Ancient life forms shared their genetic material freely through massive horizontal gene transfer (HGT). At a certain point, however, life made a transition to the modern era of individuality and vertical descent. Here we present a minimal model for stochastic processes potentially contributing to this hypothesized "Darwinian transition." The model suggests that HGT-dominated dynamics may have been intermittently interrupted by selection-driven processes during which genotypes became fitter and decreased their inclination toward HGT. Stochastic switching in the population dynamics with three-point (hypernetwork) interactions may have destabilized the HGT-dominated collective state and essentially contributed to the emergence of vertical descent and the first well-defined species in early evolution. A systematic nonlinear analysis of the stochastic model dynamics covering key features of evolutionary processes (such as selection, mutation, drift and HGT) supports this view. Our findings thus suggest a viable direction out of early collective evolution, potentially enabling the start of individuality and vertical Darwinian evolution.
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Affiliation(s)
- Hinrich Arnoldt
- Network Dynamics, Max Planck Institute for Dynamics and Self-Organization, 37077 Göttingen, Germany
| | - Steven H Strogatz
- Department of Mathematics, Cornell University, Ithaca, New York 14853, USA
| | - Marc Timme
- Network Dynamics, Max Planck Institute for Dynamics and Self-Organization, 37077 Göttingen, Germany
- Institute for Nonlinear Dynamics, Faculty of Physics, Georg August University Göttingen, 37077 Göttingen, Germany
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75
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Abstract
The impressive body of work on the major evolutionary transitions in the last 20 y calls for a reconstruction of the theory although a 2D account (evolution of informational systems and transitions in individuality) remains. Significant advances include the concept of fraternal and egalitarian transitions (lower-level units like and unlike, respectively). Multilevel selection, first without, then with, the collectives in focus is an important explanatory mechanism. Transitions are decomposed into phases of origin, maintenance, and transformation (i.e., further evolution) of the higher level units, which helps reduce the number of transitions in the revised list by two so that it is less top-heavy. After the transition, units show strong cooperation and very limited realized conflict. The origins of cells, the emergence of the genetic code and translation, the evolution of the eukaryotic cell, multicellularity, and the origin of human groups with language are reconsidered in some detail in the light of new data and considerations. Arguments are given why sex is not in the revised list as a separate transition. Some of the transitions can be recursive (e.g., plastids, multicellularity) or limited (transitions that share the usual features of major transitions without a massive phylogenetic impact, such as the micro- and macronuclei in ciliates). During transitions, new units of reproduction emerge, and establishment of such units requires high fidelity of reproduction (as opposed to mere replication).
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Affiliation(s)
- Eörs Szathmáry
- Center for the Conceptual Foundations of Science, Parmenides Foundation, D-82049 Munich, Germany; Department of Plant Systematics, Ecology and Theoretical Biology, Biological Institute, Eötvös University, H-1117 Budapest, Hungary; and MTA-ELTE Theoretical Biology and Evolutionary Ecology Research Group, H-1117 Budapest, Hungary
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76
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Koonin EV. The Turbulent Network Dynamics of Microbial Evolution and the Statistical Tree of Life. J Mol Evol 2015; 80:244-50. [PMID: 25894542 PMCID: PMC4472940 DOI: 10.1007/s00239-015-9679-7] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2015] [Accepted: 04/08/2015] [Indexed: 08/30/2023]
Abstract
The wide spread and high rate of gene exchange and loss in the prokaryotic world translate into “network genomics”. The rates of gene gain and loss are comparable with the rate of point mutations but are substantially greater than the duplication rate. Thus, evolution of prokaryotes is primarily shaped by gene gain and loss. These processes are essential to prevent mutational meltdown of microbial populations by stopping Muller’s ratchet and appear to trigger emergence of major novel clades by opening up new ecological niches. At least some bacteria and archaea seem to have evolved dedicated devices for gene transfer. Despite the dominance of gene gain and loss, evolution of genes is intrinsically tree-like. The significant coherence between the topologies of numerous gene trees, particularly those for (nearly) universal genes, is compatible with the concept of a statistical tree of life, which forms the framework for reconstruction of the evolutionary processes in the prokaryotic world.
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Affiliation(s)
- Eugene V Koonin
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD, 20894, USA,
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77
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Endosymbiotic gene transfer from prokaryotic pangenomes: Inherited chimerism in eukaryotes. Proc Natl Acad Sci U S A 2015; 112:10139-46. [PMID: 25733873 DOI: 10.1073/pnas.1421385112] [Citation(s) in RCA: 80] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Endosymbiotic theory in eukaryotic-cell evolution rests upon a foundation of three cornerstone partners--the plastid (a cyanobacterium), the mitochondrion (a proteobacterium), and its host (an archaeon)--and carries a corollary that, over time, the majority of genes once present in the organelle genomes were relinquished to the chromosomes of the host (endosymbiotic gene transfer). However, notwithstanding eukaryote-specific gene inventions, single-gene phylogenies have never traced eukaryotic genes to three single prokaryotic sources, an issue that hinges crucially upon factors influencing phylogenetic inference. In the age of genomes, single-gene trees, once used to test the predictions of endosymbiotic theory, now spawn new theories that stand to eventually replace endosymbiotic theory with descriptive, gene tree-based variants featuring supernumerary symbionts: prokaryotic partners distinct from the cornerstone trio and whose existence is inferred solely from single-gene trees. We reason that the endosymbiotic ancestors of mitochondria and chloroplasts brought into the eukaryotic--and plant and algal--lineage a genome-sized sample of genes from the proteobacterial and cyanobacterial pangenomes of their respective day and that, even if molecular phylogeny were artifact-free, sampling prokaryotic pangenomes through endosymbiotic gene transfer would lead to inherited chimerism. Recombination in prokaryotes (transduction, conjugation, transformation) differs from recombination in eukaryotes (sex). Prokaryotic recombination leads to pangenomes, and eukaryotic recombination leads to vertical inheritance. Viewed from the perspective of endosymbiotic theory, the critical transition at the eukaryote origin that allowed escape from Muller's ratchet--the origin of eukaryotic recombination, or sex--might have required surprisingly little evolutionary innovation.
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78
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Koonin EV, Wolf YI. Evolution of the CRISPR-Cas adaptive immunity systems in prokaryotes: models and observations on virus-host coevolution. MOLECULAR BIOSYSTEMS 2015; 11:20-7. [PMID: 25238531 PMCID: PMC5875448 DOI: 10.1039/c4mb00438h] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
CRISPR-Cas is an adaptive immunity system in prokaryotes that functions via a unique mechanism which involves incorporation of foreign DNA fragments into CRISPR arrays and subsequent utilization of transcripts of these inserts (known as spacers) as guide RNAs to cleave the cognate selfish element genome. Multiple attempts have been undertaken to explore the coevolution of viruses and microbial hosts carrying CRISPR-Cas using mathematical models that employ either systems of differential equations or an agent-based approach, or combinations thereof. Analysis of these models reveals highly complex co-evolutionary dynamics that ensues from the combination of the heritability of the CRISPR-mediated adaptive immunity with the existence of different degrees of immunity depending on the number of cognate spacers and the cost of carrying a CRISPR-Cas locus. Depending on the details of the models, a variety of testable, sometimes conflicting predictions have been made on the dependence of the degree of immunity and the benefit of maintaining CRISPR-Cas on the abundance and diversity of hosts and viruses. Some of these predictions have already been directly validated experimentally. In particular, both the reality of the virus-host arms race, with viruses escaping resistance and hosts reacquiring it through the capture of new spacers, and the fitness cost of CRISPR-Cas due to the curtailment of beneficial HGT have been reproduced in the laboratory. However, to test the predictions of the models more specifically, detailed studies of coevolving populations of microbes and viruses both in nature and in the laboratory are essential. Such analyses are expected to yield disagreements with the predictions of the current, oversimplified models and to trigger a new round of theoretical developments.
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Affiliation(s)
- Eugene V Koonin
- National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USA.
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79
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Abstract
Horizontal gene transfer (HGT) is a main driving force of bacterial evolution and innovation. This phenomenon was long thought to be marginal in mycoplasmas, a large group of self-replicating bacteria characterized by minute genomes as a result of successive gene losses during evolution. Recent comparative genomic analyses challenged this paradigm, but the occurrence of chromosomal exchanges had never been formally addressed in mycoplasmas. Here, we demonstrated the conjugal transfer of large chromosomal regions within and among ruminant mycoplasma species, with the incorporation of the incoming DNA occurring by homologous recombination into the recipient chromosome. By combining classical mating experiments with high-throughput next-generation sequencing, we documented the transfer of almost every position of the mycoplasma chromosome. Mycoplasma conjugation relies on the occurrence of an integrative conjugative element (ICE) in at least one parent cell. While ICE propagates horizontally from ICE-positive to ICE-negative cells, chromosomal transfers (CTs) occurred in the opposite direction, from ICE-negative to ICE-positive cells, independently of ICE movement. These findings challenged the classical mechanisms proposed for other bacteria in which conjugative CTs are driven by conjugative elements, bringing into the spotlight a new means for rapid mycoplasma innovation. Overall, they radically change our current views concerning the evolution of mycoplasmas, with particularly far-reaching implications given that over 50 species are human or animal pathogens. Horizontal gene transfers (HGT) shape bacterial genomes and are key contributors to microbial diversity and innovation. One main mechanism involves conjugation, a process that allows the simultaneous transfer of significant amounts of DNA upon cell-to-cell contact. Recognizing and deciphering conjugal mechanisms are thus essential in understanding the impact of gene flux on bacterial evolution. We addressed this issue in mycoplasmas, the smallest and simplest self-replicating bacteria. In these organisms, HGT was long thought to be marginal. We showed here that nearly every position of the Mycoplasma agalactiae chromosome could be transferred via conjugation, using an unconventional mechanism. The transfer involved DNA blocks containing up to 80 genes that were incorporated into the host chromosome by homologous recombination. These findings radically change our views concerning mycoplasma evolution and adaptation with particularly far-reaching implications given that over 50 species are human or animal pathogens.
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80
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Mortimer TD, Pepperell CS. Genomic signatures of distributive conjugal transfer among mycobacteria. Genome Biol Evol 2014; 6:2489-500. [PMID: 25173757 PMCID: PMC4202316 DOI: 10.1093/gbe/evu175] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
Distributive conjugal transfer (DCT) is a newly described mechanism of lateral gene transfer (LGT) that results in a mosaic transconjugant structure, similar to the products of meiosis. We have tested popular LGT detection methods on whole-genome sequence data from experimental DCT transconjugants and used the best performing methods to compare genomic signatures of DCT with those of LGT through natural transformation, conjugative plasmids, and mobile genetic elements (MGE). We found that DCT results in transfer of larger chromosomal segments, that these segments are distributed more broadly around the chromosome, and that a greater proportion of the chromosome is affected by DCT than by other mechanisms of LGT. We used the best performing methods to characterize LGT in Mycobacterium canettii, the mycobacterial species most closely related to Mycobacterium tuberculosis. Patterns of LGT among M. canettii were highly distinctive. Gene flow appeared unidirectional, from lineages with minimal evidence of LGT to isolates with a substantial proportion (6–13%) of sites identified as recombinant. Among M. canettii isolates with evidence of LGT, recombinant fragments were larger and more evenly distributed relative to bacteria that undergo LGT through natural transformation, conjugative plasmids, and MGE. Spatial bias in M. canettii was also unusual in that patterns of recombinant fragment sharing mirrored overall phylogenetic structure. Based on the proportion of recombinant sites, the size of recombinant fragments, their spatial distribution and lack of association with MGE, as well as unidirectionality of DNA transfer, we conclude that DCT is the predominant mechanism of LGT among M. canettii.
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Affiliation(s)
- Tatum D Mortimer
- Department of Medical Microbiology and Immunology, University of Wisconsin-Madison Microbiology Doctoral Training Program, University of Wisconsin-Madison
| | - Caitlin S Pepperell
- Department of Medical Microbiology and Immunology, University of Wisconsin-Madison Department of Medicine, Division of Infectious Diseases, University of Wisconsin-Madison
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81
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Overballe-Petersen S, Willerslev E. Horizontal transfer of short and degraded DNA has evolutionary implications for microbes and eukaryotic sexual reproduction. Bioessays 2014; 36:1005-10. [PMID: 25143190 PMCID: PMC4255686 DOI: 10.1002/bies.201400035] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/02/2022]
Abstract
Horizontal gene transfer in the form of long DNA fragments has changed our view of bacterial evolution. Recently, we discovered that such processes may also occur with the massive amounts of short and damaged DNA in the environment, and even with truly ancient DNA. Although it presently remains unclear how often it takes place in nature, horizontal gene transfer of short and damaged DNA opens up the possibility for genetic exchange across distinct species in both time and space. In this essay, we speculate on the potential evolutionary consequences of this phenomenon. We argue that it may challenge basic assumptions in evolutionary theory; that it may have distant origins in life's history; and that horizontal gene transfer should be viewed as an evolutionary strategy not only preceding but causally underpinning the evolution of sexual reproduction.
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82
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Yadav T, Carrasco B, Serrano E, Alonso JC. Roles of Bacillus subtilis DprA and SsbA in RecA-mediated genetic recombination. J Biol Chem 2014; 289:27640-52. [PMID: 25138221 DOI: 10.1074/jbc.m114.577924] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Bacillus subtilis competence-induced RecA, SsbA, SsbB, and DprA are required to internalize and to recombine single-stranded (ss) DNA with homologous resident duplex. RecA, in the ATP · Mg(2+)-bound form (RecA · ATP), can nucleate and form filament onto ssDNA but is inactive to catalyze DNA recombination. We report that SsbA or SsbB bound to ssDNA blocks the RecA filament formation and fails to activate recombination. DprA facilitates RecA filamentation; however, the filaments cannot engage in DNA recombination. When ssDNA was preincubated with SsbA, but not SsbB, DprA was able to activate DNA strand exchange dependent on RecA · ATP. This work demonstrates that RecA · ATP, in concert with SsbA and DprA, catalyzes DNA strand exchange, and SsbB is an accessory factor in the reaction. In contrast, RecA · dATP efficiently catalyzes strand exchange even in the absence of single-stranded binding proteins or DprA, and addition of the accessory factors marginally improved it. We proposed that the RecA-bound nucleotide (ATP and to a lesser extent dATP) might dictate the requirement for accessory factors.
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Affiliation(s)
- Tribhuwan Yadav
- From the Department of Microbial Biotechnology, Centro Nacional de Biotecnología, CSIC, 28049 Madrid, Spain
| | - Begoña Carrasco
- From the Department of Microbial Biotechnology, Centro Nacional de Biotecnología, CSIC, 28049 Madrid, Spain
| | - Ester Serrano
- From the Department of Microbial Biotechnology, Centro Nacional de Biotecnología, CSIC, 28049 Madrid, Spain
| | - Juan C Alonso
- From the Department of Microbial Biotechnology, Centro Nacional de Biotecnología, CSIC, 28049 Madrid, Spain
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83
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Zaccaria E, van Baarlen P, de Greeff A, Morrison DA, Smith H, Wells JM. Control of competence for DNA transformation in streptococcus suis by genetically transferable pherotypes. PLoS One 2014; 9:e99394. [PMID: 24968201 PMCID: PMC4072589 DOI: 10.1371/journal.pone.0099394] [Citation(s) in RCA: 50] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2014] [Accepted: 05/14/2014] [Indexed: 11/18/2022] Open
Abstract
Here we show that S. suis, a major bacterial pathogen of pigs and emerging pathogen in humans responds to a peptide pheromone by developing competence for DNA transformation. This species does not fall within any of the phylogenetic clusters of streptococci previously shown to regulate competence via peptide pheromones suggesting that more species of streptococci may be naturally competent. Induction of competence was dependent on ComX, a sigma factor that controls the streptococcal late competence regulon, extracellular addition of a comX-inducing peptide (XIP), and ComR, a regulator of comX. XIP was identified as an N-terminally truncated variant of ComS. Different comS alleles are present among strains of S. suis. These comS alleles are not functionally equivalent and appear to operate in conjuction with a cognate ComR to regulate comX through a conserved comR-box promoter. We demonstrate that these ‘pherotypes’ can be genetically transferred between strains, suggesting that similar approaches might be used to control competence induction in other lactic acid bacteria that lack ComR/ComS homologues but possess comX and the late competence regulon. The approaches described in this paper to identify and optimize peptide-induced competence may also assist other researchers wishing to identify natural competence in other bacteria. Harnessing natural competence is expected to accelerate genetic research on this and other important streptococcal pathogens and to allow high-throughput mutation approaches to be implemented, opening up new avenues for research.
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Affiliation(s)
- Edoardo Zaccaria
- Host-Microbe Interactomics, Animal Sciences, Wageningen University, Wageningen, The Netherlands
| | - Peter van Baarlen
- Host-Microbe Interactomics, Animal Sciences, Wageningen University, Wageningen, The Netherlands
| | - Astrid de Greeff
- Central Veterinary Institute, Animal Sciences, Wageningen University, Lelystad, The Netherlands
| | - Donald A. Morrison
- Biological Sciences, University of Illinois at Chicago, Chicago, Illinois, United States of America
| | - Hilde Smith
- Central Veterinary Institute, Animal Sciences, Wageningen University, Lelystad, The Netherlands
| | - Jerry M. Wells
- Host-Microbe Interactomics, Animal Sciences, Wageningen University, Wageningen, The Netherlands
- * E-mail:
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