The CRISPR-Cas (clustered regularly interspaced short palindromic repeats, CRISPR-associated genes) is an adaptive immunity system in bacteria and archaea that functions via a distinct self-non-self recognition mechanism that is partially analogous to the mechanism of eukaryotic RNA interference (RNAi). The CRISPR-Cas system incorporates fragments of virus or plasmid DNA into the CRISPR repeat cassettes and employs the processed transcripts of these spacers as guide RNAs to cleave the cognate foreign DNA or RNA. The Cas proteins, however, are not homologous to the proteins involved in RNAi and comprise numerous, highly diverged families. The majority of the Cas proteins contain diverse variants of the RNA recognition motif (RRM), a widespread RNA-binding domain. Despite the fast evolution that is typical of the cas genes, the presence of diverse versions of the RRM in most Cas proteins provides for a simple scenario for the evolution of the three distinct types of CRISPR-cas systems. In addition to several proteins that are directly implicated in the immune response, the cas genes encode a variety of proteins that are homologous to prokaryotic toxins that typically possess nuclease activity. The predicted toxins associated with CRISPR-Cas systems include the essential Cas2 protein, proteins of COG1517 that, in addition to a ligand-binding domain and a helix-turn-helix domain, typically contain different nuclease domains and several other predicted nucleases. The tight association of the CRISPR-Cas immunity systems with predicted toxins that, upon activation, would induce dormancy or cell death suggests that adaptive immunity and dormancy/suicide response are functionally coupled. Such coupling could manifest in the persistence state being induced and potentially providing conditions for more effective action of the immune system or in cell death being triggered when immunity fails.
CRISPR-Cas; adaptive immunity; innate immunity; programmed cell death; dormancy; RRM domain
CRISPR-Cas adaptive immunity systems of bacteria and archaea insert fragments of virus or plasmid DNA as spacer sequences into CRISPR repeat loci. Processed transcripts encompassing these spacers guide the cleavage of the cognate foreign DNA or RNA. Most CRISPR-Cas loci, in addition to recognized cas genes, also include genes that are not directly implicated in spacer acquisition, CRISPR transcript processing or interference. Here we comprehensively analyze sequences, structures and genomic neighborhoods of one of the most widespread groups of such genes that encode proteins containing a predicted nucleotide-binding domain with a Rossmann-like fold, which we denote CARF (CRISPR-associated Rossmann fold). Several CARF protein structures have been determined but functional characterization of these proteins is lacking. The CARF domain is most frequently combined with a C-terminal winged helix-turn-helix DNA-binding domain and “effector” domains most of which are predicted to possess DNase or RNase activity. Divergent CARF domains are also found in RtcR proteins, sigma-54 dependent regulators of the rtc RNA repair operon. CARF genes frequently co-occur with those coding for proteins containing the WYL domain with the Sm-like SH3 β-barrel fold, which is also predicted to bind ligands. CRISPR-Cas and possibly other defense systems are predicted to be transcriptionally regulated by multiple ligand-binding proteins containing WYL and CARF domains which sense modified nucleotides and nucleotide derivatives generated during virus infection. We hypothesize that CARF domains also transmit the signal from the bound ligand to the fused effector domains which attack either alien or self nucleic acids, resulting, respectively, in immunity complementing the CRISPR-Cas action or in dormancy/programmed cell death.
CRISPR; Rossmann fold; beta barrel; DNA-binding proteins; phage defense
The relationship between the selection affecting codon usage and selection on protein sequences of orthologous genes in diverse groups of bacteria and archaea was examined by using the Alignable Tight Genome Clusters database of prokaryote genomes. The codon usage bias is generally low, with 57.5% of the gene-specific optimal codon frequencies (Fopt) being below 0.55. This apparent weak selection on codon usage contrasts with the strong purifying selection on amino acid sequences, with 65.8% of the gene-specific dN/dS ratios being below 0.1. For most of the genomes compared, a limited but statistically significant negative correlation between Fopt and dN/dS was observed, which is indicative of a link between selection on protein sequence and selection on codon usage. The strength of the coupling between the protein level selection and codon usage bias showed a strong positive correlation with the genomic GC content. Combined with previous observations on the selection for GC-rich codons in bacteria and archaea with GC-rich genomes, these findings suggest that selection for translational fine-tuning could be an important factor in microbial evolution that drives the evolution of genome GC content away from mutational equilibrium. This type of selection is particularly pronounced in slowly evolving, “high-status” genes. A significantly stronger link between the two aspects of selection is observed in free-living bacteria than in parasitic bacteria and in genes encoding metabolic enzymes and transporters than in informational genes. These differences might reflect the special importance of translational fine-tuning for the adaptability of gene expression to environmental changes. The results of this work establish the coupling between protein level selection and selection for translational optimization as a distinct and potentially important factor in microbial evolution.
Selection affects the evolution of microbial genomes at many levels, including both the structure of proteins and the regulation of their production. Here we demonstrate the coupling between the selection on protein sequences and the optimization of codon usage in a broad range of bacteria and archaea. The strength of this coupling varies over a wide range and strongly and positively correlates with the genomic GC content. The cause(s) of the evolution of high GC content is a long-standing open question, given the universal mutational bias toward AT. We propose that optimization of codon usage could be one of the key factors that determine the evolution of GC-rich genomes. This work establishes the coupling between selection at the level of protein sequence and at the level of codon choice optimization as a distinct aspect of genome evolution.
A new study shows that the expression of two classes of repetitive elements in the mouse genome is controlled through two complementary mechanisms: DNA methylation and p53-mediated transcription suppression.¹ When both lines of defense fail, expression of the repeats yields large quantities of double-stranded RNA, triggering interferon response that leads to caspase-dependent cell death. These notable findings highlight two fundamental trends: tight coupling of defense and cell death mechanisms that appears to be universal in cellular life and the exploitation of the expression of “junk” DNA as a signal triggering “altruistic” cell suicide.
p53; transposable elements; SINE repeats; DNA methylation; interferon response
The nucleocytoplasmic large DNA viruses (NCLDVs) comprise a monophyletic group of viruses that infect animals and diverse unicellular eukaryotes. The NCLDV group includes the families Poxviridae, Asfarviridae, Iridoviridae, Ascoviridae, Phycodnaviridae, Mimiviridae and the proposed family “Marseilleviridae”. The family Mimiviridae includes the largest known viruses, with genomes in excess of one megabase, whereas the genome size in the other NCLDV families varies from 100 to 400 kilobase pairs. Most of the NCLDVs replicate in the cytoplasm of infected cells, within so-called virus factories. The NCLDVs share a common ancient origin, as demonstrated by evolutionary reconstructions that trace approximately 50 genes encoding key proteins involved in viral replication and virion formation to the last common ancestor of all these viruses. Taken together, these characteristics lead us to propose assigning an official taxonomic rank to the NCLDVs as the order “Megavirales”, in reference to the large size of the virions and genomes of these viruses.
Single-stranded (ss)DNA viruses are extremely widespread, infect diverse hosts from all three domains of life and include important pathogens. Most ssDNA viruses possess small genomes that replicate by the rolling-circle-like mechanism initiated by a distinct virus-encoded endonuclease. However, viruses of the family Bidnaviridae, instead of the endonuclease, encode a protein-primed type B DNA polymerase (PolB) and hence break this pattern. We investigated the provenance of all bidnavirus genes and uncover an unexpected turbulent evolutionary history of these unique viruses. Our analysis strongly suggests that bidnaviruses evolved from a parvovirus ancestor from which they inherit a jelly-roll capsid protein and a superfamily 3 helicase. The radiation of bidnaviruses from parvoviruses was probably triggered by integration of the ancestral parvovirus genome into a large virus-derived DNA transposon of the Polinton (polintovirus) family resulting in the acquisition of the polintovirus PolB gene along with terminal inverted repeats. Bidnavirus genes for a receptor-binding protein and a potential novel antiviral defense modulator are derived from dsRNA viruses (Reoviridae) and dsDNA viruses (Baculoviridae), respectively. The unusual evolutionary history of bidnaviruses emphasizes the key role of horizontal gene transfer, sometimes between viruses with completely different genomes but occupying the same niche, in the emergence of new viral types.
Accurate inference of orthologous genes is a pre-requisite for most comparative genomics studies, and is also important for functional annotation of new genomes. Identification of orthologous gene sets typically involves phylogenetic tree analysis, heuristic algorithms based on sequence conservation, synteny analysis, or some combination of these approaches. The most direct tree-based methods typically rely on the comparison of an individual gene tree with a species tree. Once the two trees are accurately constructed, orthologs are straightforwardly identified by the definition of orthology as those homologs that are related by speciation, rather than gene duplication, at their most recent point of origin. Although ideal for the purpose of orthology identification in principle, phylogenetic trees are computationally expensive to construct for large numbers of genes and genomes, and they often contain errors, especially at large evolutionary distances. Moreover, in many organisms, in particular prokaryotes and viruses, evolution does not appear to have followed a simple ‘tree-like’ mode, which makes conventional tree reconciliation inapplicable. Other, heuristic methods identify probable orthologs as the closest homologous pairs or groups of genes in a set of organisms. These approaches are faster and easier to automate than tree-based methods, with efficient implementations provided by graph-theoretical algorithms enabling comparisons of thousands of genomes. Comparisons of these two approaches show that, despite conceptual differences, they produce similar sets of orthologs, especially at short evolutionary distances. Synteny also can aid in identification of orthologs. Often, tree-based, sequence similarity- and synteny-based approaches can be combined into flexible hybrid methods.
homolog; ortholog; paralog; xenolog; orthologous groups; tree reconciliation; comparative genomics
Diverse transposable elements are abundant in genomes of cellular organisms from all three domains of life. Although transposons are often regarded as junk DNA, a growing body of evidence indicates that they are behind some of the major evolutionary innovations. With the growth in the number and diversity of sequenced genomes, previously unnoticed mobile elements continue to be discovered.
We describe a new superfamily of archaeal and bacterial mobile elements which we denote casposons because they encode Cas1 endonuclease, a key enzyme of the CRISPR-Cas adaptive immunity systems of archaea and bacteria. The casposons share several features with self-synthesizing eukaryotic DNA transposons of the Polinton/Maverick class, including terminal inverted repeats and genes for B family DNA polymerases. However, unlike any other known mobile elements, the casposons are predicted to rely on Cas1 for integration and excision, via a mechanism similar to the integration of new spacers into CRISPR loci. We identify three distinct families of casposons that differ in their gene repertoires and evolutionary provenance of the DNA polymerases. Deep branching of the casposon-encoded endonuclease in the Cas1 phylogeny suggests that casposons played a pivotal role in the emergence of CRISPR-Cas immunity.
The casposons are a novel superfamily of mobile elements, the first family of putative self-synthesizing transposons discovered in prokaryotes. The likely contribution of capsosons to the evolution of CRISPR-Cas parallels the involvement of the RAG1 transposase in vertebrate immunoglobulin gene rearrangement, suggesting that recruitment of endonucleases from mobile elements as ready-made tools for genome manipulation is a general route of evolution of adaptive immunity.
Mobile genetic elements; CRISPR-Cas system; Adaptive immunity; Transposons; Archaea; DNA polymerases
Gene evolution is traditionally considered within the framework of the molecular clock (MC) model whereby each gene is characterized by an approximately constant rate of evolution. Recent comparative analysis of numerous phylogenies of prokaryotic genes has shown that a different model of evolution, denoted the Universal PaceMaker (UPM), which postulates conservation of relative, rather than absolute evolutionary rates, yields a better fit to the phylogenetic data. Here, we show that the UPM model is a better fit than the MC for genome wide sets of phylogenetic trees from six species of Drosophila and nine species of yeast, with extremely high statistical significance. Unlike the prokaryotic phylogenies that include distant organisms and multiple horizontal gene transfers, these are simple data sets that cover groups of closely related organisms and consist of gene trees with the same topology as the species tree. The results indicate that both lineage-specific and gene-specific rates are important in genome evolution but the lineage-specific contribution is greater. Similar to the MC, the gene evolution rates under the UPM are strongly overdispersed, approximately 2-fold compared with the expectation from sampling error alone. However, we show that neither Drosophila nor yeast genes form distinct clusters in the tree space. Thus, the gene-specific deviations from the UPM, although substantial, are uncorrelated and most likely depend on selective factors that are largely unique to individual genes. Thus, the UPM appears to be a key feature of genome evolution across the history of cellular life.
molecular clock; genome evolution; phylogenetic trees; relative evolution rates
A stochastic, agent-based mathematical model of the coevolution of the archaeal and bacterial adaptive immunity system, CRISPR-Cas, and lytic viruses shows that CRISPR-Cas immunity can stabilize the virus-host coexistence rather than leading to the extinction of the virus. In the model, CRISPR-Cas immunity does not specifically promote viral diversity, presumably because the selection pressure on each single proto-spacer is too weak. However, the overall virus diversity in the presence of CRISPR-Cas grows due to the increase of the host and, accordingly, the virus population size. Above a threshold value of total viral diversity, which is proportional to the viral mutation rate and population size, the CRISPR-Cas system becomes ineffective and is lost due to the associated fitness cost. Our previous modeling study has suggested that the ubiquity of CRISPR-Cas in hyperthermophiles, which contrasts its comparative low prevalence in mesophiles, is due to lower rates of mutation fixation in thermal habitats. The present findings offer a complementary, simpler perspective on this contrast through the larger population sizes of mesophiles compared to hyperthermophiles, because of which CRISPR-Cas can become ineffective in mesophiles. The efficacy of CRISPR-Cas sharply increases with the number of proto-spacers per viral genome, potentially explaining the low information content of the proto-spacer-associated motif (PAM) that is required for spacer acquisition by CRISPR-Cas because a higher specificity would restrict the number of spacers available to CRISPR-Cas, thus hampering immunity. The very existence of the PAM might reflect the tradeoff between the requirement of diverse spacers for efficient immunity and avoidance of autoimmunity.
Alternative splicing (AS), alternative transcription initiation (ATI) and alternative transcription termination (ATT) create the extraordinary complexity of transcriptomes and make key contributions to the structural and functional diversity of mammalian proteomes. Analysis of mammalian genomic and transcriptomic data shows that contrary to the traditional view, the joint contribution of ATI and ATT to the transcriptome and proteome diversity is quantitatively greater than the contribution of AS. Although the mean numbers of protein-coding constitutive and alternative nucleotides in gene loci are nearly identical, their distribution along the transcripts is highly non-uniform. On average, coding exons in the variable 5′ and 3′ transcript ends that are created by ATI and ATT contain approximately four times more alternative nucleotides than core protein-coding regions that diversify exclusively via AS. Short upstream exons that encompass alternative 5′-untranslated regions and N-termini of proteins evolve under strong nucleotide-level selection whereas in 3′-terminal exons that encode protein C-termini, protein-level selection is significantly stronger. The groups of genes that are subject to ATI and ATT show major differences in biological roles, expression and selection patterns.
This article was reviewed by Lakshminarayan M. Iyer and I. King Jordan. For complete reviews, see the Reviewers’ Reports section.
Polintons (also known as Mavericks) and Tlr elements of Tetrahymena thermophila represent two families of large DNA transposons widespread in eukaryotes. Here, we show that both Polintons and Tlr elements encode two key virion proteins, the major capsid protein with the double jelly-roll fold and the minor capsid protein, known as the penton, with the single jelly-roll topology. This observation along with the previously noted conservation of the genes for viral genome packaging ATPase and adenovirus-like protease strongly suggests that Polintons and Tlr elements combine features of bona fide viruses and transposons. We propose the name ‘Polintoviruses’ to denote these putative viruses that could have played a central role in the evolution of several groups of DNA viruses of eukaryotes.
Polintons; Mavericks; Transposable elements; Double jelly-roll fold; Capsid proteins; Virus evolution
We have recently reconstructed the ‘hatcheries’ of the
first cells by combining geochemical analysis with phylogenomic scrutiny of the
inorganic ion requirements of universal components of modern cells (Mulkidjanian
et al.: Origin of first cells at terrestrial, anoxic
geothermal fields. Proc Natl Acad Sci USA 2012,
109:E821–830). These ubiquitous, and by inference primordial, proteins
and functional systems show affinity to and functional requirement for
K+, Zn2+, Mn2+, and phosphate. Thus,
protocells must have evolved in habitats with a high
K+/Na+ ratio and relatively high concentrations of Zn,
Mn and phosphorous compounds. Geochemical reconstruction shows that the ionic
composition conducive to the origin of cells could not have existed in marine
settings but is compatible with emissions of vapor-dominated zones of inland
geothermal systems. Under anoxic, CO2-dominated atmosphere, the ionic
composition of pools of cool, condensed vapor at anoxic geothermal fields would
resemble the internal milieu of modern cells. Such pools would be lined with
porous silicate minerals mixed with metal sulfides and enriched in K+
ions and phosphorous compounds.
Here we address some questions that have appeared in print after the
publication of our anoxic geothermal field scenario. We argue that anoxic
geothermal fields, which were identified as likely cradles of life by using a
top-down approach and phylogenomics analysis as a tool, could provide
geochemical conditions similar to those which were suggested as most conducive
for the emergence of life by the chemists who pursuit the complementary
The CRISPR-Cas systems of archaeal and bacterial adaptive immunity are classified into three types that differ by the repertoires of CRISPR-associated (cas) genes, the organization of cas operons and the structure of repeats in the CRISPR arrays. The simplest among the CRISPR-Cas systems is type II in which the endonuclease activities required for the interference with foreign deoxyribonucleic acid (DNA) are concentrated in a single multidomain protein, Cas9, and are guided by a co-processed dual-tracrRNA:crRNA molecule. This compact enzymatic machinery and readily programmable site-specific DNA targeting make type II systems top candidates for a new generation of powerful tools for genomic engineering. Here we report an updated census of CRISPR-Cas systems in bacterial and archaeal genomes. Type II systems are the rarest, missing in archaea, and represented in ∼5% of bacterial genomes, with an over-representation among pathogens and commensals. Phylogenomic analysis suggests that at least three cas genes, cas1, cas2 and cas4, and the CRISPR repeats of the type II-B system were acquired via recombination with a type I CRISPR-Cas locus. Distant homologs of Cas9 were identified among proteins encoded by diverse transposons, suggesting that type II CRISPR-Cas evolved via recombination of mobile nuclease genes with type I loci.
Any scenario of the transition from chemistry to biology should include an “energy module” because life can exist only when supported by energy flow(s). We addressed the problem of primordial energetics by combining physico-chemical considerations with phylogenomic analysis. We propose that the first replicators could use abiotically formed, exceptionally photostable activated nucleotides both as building blocks and as the main energy source. Nucleoside triphosphates could replace cyclic nucleotides as the principal energy-rich compounds at the stage of the first cells, presumably because the metal chelates of nucleoside triphosphates penetrated membranes much better than the respective metal complexes of nucleoside monophosphates. The ability to exploit natural energy flows for biogenic production of energy-rich molecules could evolve only gradually, after the emergence of sophisticated enzymes and ion-tight membranes. We argue that, in the course of evolution, sodium-dependent membrane energetics preceded the proton-based energetics which evolved independently in bacteria and archaea.
Viruses are the most abundant biological entities on earth and encompass a vast amount of genetic diversity. The recent rapid increase in the number of sequenced viral genomes has created unprecedented opportunities for gaining new insight into the structure and evolution of the virosphere. Here, we present an update of the phage orthologous groups (POGs), a collection of 4,542 clusters of orthologous genes from bacteriophages that now also includes viruses infecting archaea and encompasses more than 1,000 distinct virus genomes. Analysis of this expanded data set shows that the number of POGs keeps growing without saturation and that a substantial majority of the POGs remain specific to viruses, lacking homologues in prokaryotic cells, outside known proviruses. Thus, the great majority of virus genes apparently remains to be discovered. A complementary observation is that numerous viral genomes remain poorly, if at all, covered by POGs. The genome coverage by POGs is expected to increase as more genomes are sequenced. Taxon-specific, single-copy signature genes that are not observed in prokaryotic genomes outside detected proviruses were identified for two-thirds of the 57 taxa (those with genomes available from at least 3 distinct viruses), with half of these present in all members of the respective taxon. These signatures can be used to specifically identify the presence and quantify the abundance of viruses from particular taxa in metagenomic samples and thus gain new insights into the ecology and evolution of viruses in relation to their hosts.
Bacteria and archaea face continual onslaughts of rapidly diversifying viruses and plasmids. Many prokaryotes maintain adaptive immune systems known as clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated genes (Cas). CRISPR-Cas systems are genomic sensors that serially acquire viral and plasmid DNA fragments (spacers) that are utilized to target and cleave matching viral and plasmid DNA in subsequent genomic invasions, offering critical immunological memory. Only 50% of sequenced bacteria possess CRISPR-Cas immunity, in contrast to over 90% of sequenced archaea. To probe why half of bacteria lack CRISPR-Cas immunity, we combined comparative genomics and mathematical modeling. Analysis of hundreds of diverse prokaryotic genomes shows that CRISPR-Cas systems are substantially more prevalent in thermophiles than in mesophiles. With sequenced bacteria disproportionately mesophilic and sequenced archaea mostly thermophilic, the presence of CRISPR-Cas appears to depend more on environmental temperature than on bacterial-archaeal taxonomy. Mutation rates are typically severalfold higher in mesophilic prokaryotes than in thermophilic prokaryotes. To quantitatively test whether accelerated viral mutation leads microbes to lose CRISPR-Cas systems, we developed a stochastic model of virus-CRISPR coevolution. The model competes CRISPR-Cas-positive (CRISPR-Cas+) prokaryotes against CRISPR-Cas-negative (CRISPR-Cas−) prokaryotes, continually weighing the antiviral benefits conferred by CRISPR-Cas immunity against its fitness costs. Tracking this cost-benefit analysis across parameter space reveals viral mutation rate thresholds beyond which CRISPR-Cas cannot provide sufficient immunity and is purged from host populations. These results offer a simple, testable viral diversity hypothesis to explain why mesophilic bacteria disproportionately lack CRISPR-Cas immunity. More generally, fundamental limits on the adaptability of biological sensors (Lamarckian evolution) are predicted.
A remarkable recent discovery in microbiology is that bacteria and archaea possess systems conferring immunological memory and adaptive immunity. Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated genes (CRISPR-Cas) are genomic sensors that allow prokaryotes to acquire DNA fragments from invading viruses and plasmids. Providing immunological memory, these stored fragments destroy matching DNA in future viral and plasmid invasions. CRISPR-Cas systems also provide adaptive immunity, keeping up with mutating viruses and plasmids by continually acquiring new DNA fragments. Surprisingly, less than 50% of mesophilic bacteria, in contrast to almost 90% of thermophilic bacteria and Archaea, maintain CRISPR-Cas immunity. Using mathematical modeling, we probe this dichotomy, showing how increased viral mutation rates can explain the reduced prevalence of CRISPR-Cas systems in mesophiles. Rapidly mutating viruses outrun CRISPR-Cas immune systems, likely decreasing their prevalence in bacterial populations. Thus, viral adaptability may select against, rather than for, immune adaptability in prokaryotes.
The question whether adaptation follows a deterministic route largely prescribed by the environment or can proceed along a large number of alternative trajectories has engaged extensive research over the recent years. Experimental evolution studies enabled by advances in high throughput techniques for genome sequencing and manipulation, along with increasingly detailed mathematical modeling of fitness landscapes, are beginning to allow quantitative exploration of the repeatability of evolutionary trajectories. It is becoming clear that evolutionary trajectories in static correlated fitness landscapes are substantially non-random but the relative contributions of determinism and stochasticity in the evolution of specific phenotypes strongly depend on the specific conditions, particularly the magnitude of the selective pressure and the number of available beneficial mutations.
evolutionary trajectory; predictability of evolution; fitness landscape; divergence of trajectories
In 2009, we are celebrating the 200th anniversary of Charles Darwin and the 150th jubilee of his masterpiece, the Origin of Species. Darwin developed the first coherent and compelling narrative of biological evolution and thus founded evolutionary biology—and modern biology in general, remembering the famous dictum of Dobzhansky. It is, however, counter-productive, and ultimately, a disservice to Darwin’s legacy, to define modern evolutionary biology as neo-Darwinism. The current picture of evolution, informed, in particular, by results of comparative genomics and systems biology, is by far more complex than that presented in the Origin of Species, so that Darwinian principles, including natural selection, are incorporated into the evolving new synthesis as important but certainly not all-embracing tenets. This expansion of evolutionary biology does not denigrate Darwin in the least but rather emphasizes the fertility of his ideas.
Darwin’s anniversary; Darwinism; modern synthesis; genome evolution; systems biology; horizontal gene transfer; Tree of Life
Horizontal gene transfer (HGT) is a major factor in the evolution of prokaryotes. An intriguing question is whether HGT is maintained during evolution of prokaryotes owing to its adaptive value or is a byproduct of selection driven by other factors such as consumption of extracellular DNA (eDNA) as a nutrient. One hypothesis posits that HGT can restore genes inactivated by mutations and thereby prevent stochastic, irreversible deterioration of genomes in finite populations known as Muller’s ratchet. To examine this hypothesis, we developed a population genetic model of prokaryotes undergoing HGT via homologous recombination. Analysis of this model indicates that HGT can prevent the operation of Muller’s ratchet even when the source of transferred genes is eDNA that comes from dead cells and on average carries more deleterious mutations than the DNA of recipient live cells. Moreover, if HGT is sufficiently frequent and eDNA diffusion sufficiently rapid, a subdivided population is shown to be more resistant to Muller’s ratchet than an undivided population of an equal overall size. Thus, to maintain genomic information in the face of Muller’s ratchet, it is more advantageous to partition individuals into multiple subpopulations and let them “cross-reference” each other’s genetic information through HGT than to collect all individuals in one population and thereby maximize the efficacy of natural selection. Taken together, the results suggest that HGT could be an important condition for the long-term maintenance of genomic information in prokaryotes through the prevention of Muller’s ratchet.
environmental DNA; evolution of transformation; competence; structured population; soil bacteria
The widespread exchange of genes among prokaryotes, known as horizontal gene transfer (HGT), is often considered to “uproot” the Tree of Life (TOL). Indeed, it is by now fully clear that genes in general possess different evolutionary histories. However, the possibility remains that the TOL concept can be reformulated and remain valid as a statistical central trend in the phylogenetic “Forest of Life” (FOL). This article describes a computational pipeline developed to chart the FOL by comparative analysis of thousands of phylogenetic trees. This analysis reveals a distinct, consistent phylogenetic signal that is particularly strong among the Nearly Universal Trees (NUTs), which correspond to genes represented in all or most of the analyzed organisms. Despite the substantial amount of apparent HGT seen even among the NUTs, these gene transfers appear to be distributed randomly and do not obscure the central tree-like trend.
Genome-wide comparison of phylogenetic trees is becoming an increasingly common approach in evolutionary genomics, and a variety of approaches for such comparison have been developed. In this article we present several methods for comparative analysis of large numbers of phylogenetic trees. To compare phylogenetic trees taking into account the bootstrap support for each internal branch, the Boot-Split Distance (BSD) method is introduced as an extension of the previously developed Split Distance (SD) method for tree comparison. The BSD method implements the straightforward idea that comparison of phylogenetic trees can be made more robust by treating tree splits differentially depending on the bootstrap support. Approaches are also introduced for detecting tree-like and net-like evolutionary trends in the phylogenetic Forest of Life (FOL), i.e., the entirety of the phylogenetic trees for conserved genes of prokaryotes. The principal method employed for this purpose includes mapping quartets of species onto trees to calculate the support of each quartet topology and so to quantify the tree and net contributions to the distances between species. We describe the applications methods used to analyze the FOL and the results obtained with these methods. These results support the concept of the Tree of Life (TOL) as a central evolutionary trend in the FOL as opposed to the traditional view of the TOL as a ‘species tree’.
Forest of life; tree of life; phylogenomic methods; tree comparison; map of quartets
The CRISPR-Cas-derived RNA-guided Cas9 endonuclease is the key element of an emerging promising technology for genome engineering in a broad range of cells and organisms. The DNA-targeting mechanism of the type II CRISPR-Cas system involves maturation of tracrRNA:crRNA duplex (dual-RNA), which directs Cas9 to cleave invading DNA in a sequence-specific manner, dependent on the presence of a Protospacer Adjacent Motif (PAM) on the target. We show that evolution of dual-RNA and Cas9 in bacteria produced remarkable sequence diversity. We selected eight representatives of phylogenetically defined type II CRISPR-Cas groups to analyze possible coevolution of Cas9 and dual-RNA. We demonstrate that these two components are interchangeable only between closely related type II systems when the PAM sequence is adjusted to the investigated Cas9 protein. Comparison of the taxonomy of bacterial species that harbor type II CRISPR-Cas systems with the Cas9 phylogeny corroborates horizontal transfer of the CRISPR-Cas loci. The reported collection of dual-RNA:Cas9 with associated PAMs expands the possibilities for multiplex genome editing and could provide means to improve the specificity of the RNA-programmable Cas9 tool.
The recently discovered CRISPR-Cas adaptive immune system is present in almost all archaea and many bacteria. It consists of cassettes of CRISPR repeats that incorporate spacers homologous to fragments of viral or plasmid genomes that are employed as guide RNAs in the immune response, along with numerous CRISPR-associated (cas) genes that encode proteins possessing diverse, only partially characterized activities required for the action of the system. Here, we investigate the evolution of the cas genes and show that they evolve under purifying selection that is typically much weaker than the median strength of purifying selection affecting genes in the respective genomes. The exceptions are the cas1 and cas2 genes that typically evolve at levels of purifying selection close to the genomic median. Thus, although these genes are implicated in the acquisition of spacers from alien genomes, they do not appear to be directly involved in an arms race between bacterial and archaeal hosts and infectious agents. These genes might possess functions distinct from and additional to their role in the CRISPR-Cas-mediated immune response. Taken together with evidence of the frequent horizontal transfer of cas genes reported previously and with the wide-spread microscale recombination within these genes detected in this work, these findings reveal the highly dynamic evolution of cas genes. This conclusion is in line with the involvement of CRISPR-Cas in antiviral immunity that is likely to entail a coevolutionary arms race with rapidly evolving viruses. However, we failed to detect evidence of strong positive selection in any of the cas genes.
The recently discovered Pandoraviruses are by far the largest viruses known, with their 2 megabase genomes exceeding in size the genomes of numerous bacteria and archaea. Pandoraviruses show a distant relationship with other nucleocytoplasmic large DNA viruses (NCLDV) of eukaryotes, lack some of the NCLDV core genes and in particular do not appear to be specifically related to the other, better characterized family of giant viruses, the Mimiviridae. Here we report phylogenetic analysis of 6 core NCLDV genes that confidently places Pandoraviruses within the family Phycodnaviridae, with an apparent specific affinity with Coccolithoviruses. We conclude that, despite their many unusual characteristics, Pandoraviruses are highly derived phycodnaviruses. These findings imply that giant viruses have independently evolved from smaller NCLDV on at least two occasions.
This article was reviewed by Patrick Forterre and Lakshminarayan Iyer. For the full reviews, see the Reviewers’ reports section.