Theory of prokaryotic genome evolution

Laboratory evolution of the bacterial genome structure through insertion sequence activation

The genome structure fundamentally shapes bacterial physiology, ecology, and evolution. Though insertion sequences (IS) are known drivers of drastic evolutionary changes in the genome structure, the process is typically slow and challenging to observe in the laboratory. Here, we developed a system to accelerate IS-mediated genome structure evolution by introducing multiple copies of a high-activity IS in Escherichia coli. We evolved the bacteria under relaxed neutral conditions, simulating those leading to IS expansion in host-restricted endosymbionts and pathogens. Strains accumulated a median of 24.5 IS insertions and underwent over 5% genome size changes within ten weeks, comparable to decades-long evolution in wild-type strains. The detected interplay of frequent small deletions and rare large duplications updates the view of genome reduction under relaxed selection from a simple consequence of the deletion bias to a nuanced picture including transient expansions. The high IS activity resulted in structural variants of IS and the emergence of composite transposons, illuminating potential evolutionary pathways for ISs and composite transposons. The extensive genome rearrangements we observed establish a baseline for assessing the fitness effects of IS insertions, genome size changes, and rearrangements, advancing our understanding of how mobile elements shape bacterial genomes.

Staphylococcus aureus promotes strain-dependent immunopathology during cutaneous leishmaniasis through induction of IL-1β

Cutaneous leishmaniasis is a parasitic infection that causes a spectrum of pathology ranging from single, self-healing lesions to disfiguring chronic wounds. In severe disease, uncontrolled inflammation exacerbates tissue damage and delays healing, though the contributing factors are unclear. We previously observed that delayed healing was associated with Staphylococcus aureus in the lesional microbiota of patients with cutaneous leishmaniasis. To investigate how S. aureus impacts immunopathology during leishmania infection, we established a murine model of S. aureus colonization with clinical isolates followed by Leishmania infection. S. aureus triggered early production of interleukin (IL)-1β during Leishmania infection, which was critical for neutrophil recruitment and cutaneous inflammation. S. aureus isolates differentially induced IL-1β and neutrophil recruitment, and isolates that induced greater neutrophil recruitment were resistant to neutrophil killing and persisted longer. We reveal a mechanism whereby S. aureus mediates immunopathology during cutaneous leishmaniasis, suggesting IL-1β as a promising immunomodulatory target for non-healing infections.

A Global Relationship Between Genome Size and Encoded Carbon Metabolic Strategies of Soil Bacteria

Microbial traits are critical for carbon sequestration and degradation in terrestrial ecosystems. Yet, our understanding of the relationship between carbon metabolic strategies and genomic traits like genome size remains limited. To address this knowledge gap, we conducted a global-scale meta-analysis of 2650 genomes, integrated whole-genome sequencing data, and performed a continental-scale metagenomic field study. We found that genome size was tightly associated with an increase in the ratio between genes encoding for polysaccharide decomposition and biomass synthesis that we defined as the carbon acquisition-to-biomass yield ratio (A/Y). We also show that horizontal gene transfer played a major evolutionary role in the expanded bacterial capacities in carbon acquisition. Our continental-scale field study further revealed a significantly negative relationship between the A/Y ratio and soil organic carbon stocks. Our work demonstrates a global relationship between genome size and the encoded carbon metabolic strategies of soil bacteria across terrestrial microbiomes.

How Big Is Big? The Effective Population Size of Marine Bacteria

Genome-reduced bacteria constitute most of the cells in surface-ocean bacterioplankton communities. Their extremely large census population sizes (N c) have been unfoundedly translated to huge effective population sizes (N e)-the size of an ideal population carrying as much neutral genetic diversity as the actual population. As N e scales inversely with the strength of genetic drift, constraining the magnitude of N e is key to evaluating whether natural selection can overcome the power of genetic drift to drive evolutionary events. Determining the N e of extant species requires measuring the genomic mutation rate, a challenging step for most genome-reduced bacterioplankton lineages. Results for genome-reduced Prochlorococcus and CHUG are surprising-their N e values are an order of magnitude lower than those of less abundant lineages carrying large genomes, such as Ruegeria and Vibrio. As bacterioplankton genome reduction commonly occurred in the distant past, appreciating their population genetic mechanisms requires constraining their ancient N e values by other methods.

Principles of Molecular Evolution: Concepts from Non-equilibrium Thermodynamics for the Multilevel Theory of Learning

We present a non-equilibrium thermodynamics approach to the multilevel theory of learning for the study of molecular evolution. This approach allows us to study the explicit time dependence of molecular evolutionary processes and their impact on entropy production. Interpreting the mathematical expressions, we can show that two main contributions affect entropy production of molecular evolution processes which can be identified as mutation and gene transfer effects. Accordingly, our results show that the optimal adaptation of organisms to external conditions in the context of evolutionary processes is driven by principles of minimum entropy production. Such results can also be interpreted as the basis of some previous postulates of the theory of learning. Although our macroscopic approach requires certain simplifications, it allows us to interpret molecular evolutionary processes using thermodynamic descriptions with reference to well-known biological processes.

Alicyclobacillus suci produces more guaiacol in media and has duplicate copies of vdcC compared to closely related Alicyclobacillus acidoterrestris

Some species of the genus Alicyclobacillus cause spoilage in juices and other beverages due to the production of guaiacol, a phenolic compound, and off-aroma. However, little is known about the genomic determinants of guaiacol production across the genus. In this study, we found that several of the genes significantly enriched in guaiacol-producing Alicyclobacillus spp. are associated with oxidative stress response, including vdcC, a phenolic acid decarboxylase putatively responsible for guaiacol synthesis. The food industry recognizes Alicyclobacillus acidoterrestris as the primary guaiacol-producing species found in beverages, though that species was recently split into two closely related yet genetically distinct species, Alicyclobacillus suci and A. acidoterrestris. We found that strains of A. suci (63.0 ± 14.2 ppm) produced significantly (P < 0.01) more guaiacol on average in media than did strains of A. acidoterrestris (25.2 ± 7.0 ppm). Additionally, A. suci and Alicyclobacillus fastidiosus genomes each had duplicate copies of vdcC, while only a single copy of vdcC was found in the genomes of A. acidoterrestris, Alicyclobacillus acidiphilus, and Alicyclobacillus herbarius. Although the food industry has not historically differentiated between A. suci and A. acidoterrestris, it may be increasingly important to target the species with greater spoilage potential. Therefore, we also demonstrated that sequencing a single locus, such as the full-length 16S region or rpoB, is sufficient to differentiate between A. acidoterrestris and A. suci.

IMPORTANCE

Microbial spoilage increases food waste. To address that challenge, it is critical to recognize and control those microbial groups with the greatest spoilage potential. Non-specific targeting of broad microbial groups (e.g., the genus of Alicyclobacillus) in which only some members cause food spoilage results in untenable, overly broad interventions. Much of the food industry does not differentiate between guaiacol-producing and non-guaiacol-producing Alicyclobacillus species. This is overly broad because Alicyclobacillus spp. which cannot produce guaiacol can be present in beverages without causing spoilage. Furthermore, no distinction is made between Alicyclobacillus suci and Alicyclobacillus acidoterrestris because A. suci is newly split from A. acidoterrestris and most of the food industry still considers them to be the same. However, these findings indicate that A. suci may have greater spoilage potential than A. acidoterrestris due to differences in their genomic determinants for guaiacol production.

Insight into Single-Molecule Imaging Techniques for the Study of Prokaryotic Genome Maintenance

Genome maintenance comprises a group of complex and interrelated processes crucial for preserving and safeguarding genetic information within all organisms. Key aspects of genome maintenance involve DNA replication, transcription, recombination, and repair. Improper regulation of these processes could cause genetic changes, potentially leading to antibiotic resistance in bacterial populations. Due to the complexity of these processes, ensemble averaging studies may not provide the level of detail required to capture the full spectrum of molecular behaviors and dynamics of each individual biomolecule. Therefore, researchers have increasingly turned to single-molecule approaches, as these techniques allow for the direct observation and manipulation of individual biomolecules, and offer a level of detail that is unattainable with traditional ensemble methods. In this review, we provide an overview of recent in vitro and in vivo single-molecule imaging approaches employed to study the complex processes involved in prokaryotic genome maintenance. We will first highlight the principles of imaging techniques such as total internal reflection fluorescence microscopy and atomic force microscopy, primarily used for in vitro studies, and highly inclined and laminated optical sheet and super-resolution microscopy, mainly employed in in vivo studies. We then demonstrate how applying these single-molecule techniques has enabled the direct visualization of biological processes such as replication, transcription, DNA repair, and recombination in real time. Finally, we will showcase the results obtained from super-resolution microscopy approaches, which have provided unprecedented insights into the spatial organization of different biomolecules within bacterial organisms.

Large-scale bacterial genomic and metagenomic analysis reveals Pseudomonas aeruginosa as potential ancestral source of tigecycline resistance gene cluster tmexCD-toprJ

BACKGROUND

The global dissemination of the multidrug resistance efflux pump gene cluster tmexCD-toprJ has greatly weakened the effects of multiple antibiotics, including tigecycline. However, the potential origin and transmission mechanisms of the gene cluster remain unclear.

METHODS

Here, we concluded a comprehensive bioinformatics analysis on integrated 73,498 bacterial genomes, including Pseudomonas spp., Klebsiella spp., Aeromonas spp., Proteus spp., and Citrobacter spp., along with 1,152 long-read metagenomic datasets to trace the origin and propagation of tmexCD-toprJ.

RESULTS

Our results demonstrated that tmexCD-toprJ was predominantly found in Pseudomonas aeruginosa sourced from human hosts in Asian countries and North American countries. Phylogenetic and genomic feature analyses showed that tmexCD-toprJ was likely evolved from mexCD-oprJ of some special clones of P. aeruginosa. Furthermore, metagenomic analysis confirmed that P. aeruginosa is the only potential ancestral bacterium for tmexCD-toprJ. A putative mobile genetic structure harboring tmexCD-toprJ, int-int-hp-hp-tnfxB-tmexCD-toprJ, was the predominant genetic context of tmexCD-toprJ across various bacterial genera, suggesting that the two integrase genes play a pivotal role in the horizontal transmission of tmexCD-toprJ.

CONCLUSIONS

Based on these findings, it is almost certain that the tmexCD-toprJ gene cluster was derived from P. aeruginosa and further spread to other bacteria.

Eco-evolutionary strategies for relieving carbon limitation under salt stress differ across microbial clades

With the continuous expansion of saline soils under climate change, understanding the eco-evolutionary tradeoff between the microbial mitigation of carbon limitation and the maintenance of functional traits in saline soils represents a significant knowledge gap in predicting future soil health and ecological function. Through shotgun metagenomic sequencing of coastal soils along a salinity gradient, we show contrasting eco-evolutionary directions of soil bacteria and archaea that manifest in changes to genome size and the functional potential of the soil microbiome. In salt environments with high carbon requirements, bacteria exhibit reduced genome sizes associated with a depletion of metabolic genes, while archaea display larger genomes and enrichment of salt-resistance, metabolic, and carbon-acquisition genes. This suggests that bacteria conserve energy through genome streamlining when facing salt stress, while archaea invest in carbon-acquisition pathways to broaden their resource usage. These findings suggest divergent directions in eco-evolutionary adaptations to soil saline stress amongst microbial clades and serve as a foundation for understanding the response of soil microbiomes to escalating climate change.

Cellular function of the GndA small open reading frame-encoded polypeptide during heat shock

Over the past 15 years, hundreds of previously undiscovered bacterial small open reading frame (sORF)-encoded polypeptides (SEPs) of fewer than fifty amino acids have been identified, and biological functions have been ascribed to an increasing number of SEPs from intergenic regions and small RNAs. However, despite numbering in the dozens in Escherichia coli, and hundreds to thousands in humans, same-strand nested sORFs that overlap protein coding genes in alternative reading frames remain understudied. In order to provide insight into this enigmatic class of unannotated genes, we characterized GndA, a 36-amino acid, heat shock-regulated SEP encoded within the +2 reading frame of the gnd gene in E. coli K-12 MG1655. We show that GndA pulls down components of respiratory complex I (RCI) and is required for proper localization of a RCI subunit during heat shock. At high temperature GndA deletion (ΔGndA) cells exhibit perturbations in cell growth, NADH+/NAD ratio, and expression of a number of genes including several associated with oxidative stress. These findings suggest that GndA may function in maintenance of homeostasis during heat shock. Characterization of GndA therefore supports the nascent but growing consensus that functional, overlapping genes occur in genomes from viruses to humans.

Microbiota succession influences nematode physiology in a beetle microcosm ecosystem

Unravelling the multifaceted and bidirectional interactions between microbiota and host physiology represents a major scientific challenge. Here, we utilise the nematode model, Pristionchus pacificus, coupled to a laboratory-simulated decay process of its insect host, to mimic natural microbiota succession and investigate associated tripartite interactions. Metagenomics reveal that during initial decay stages, the population of vitamin B-producing bacteria diminishes, potentially due to a preferential selection by nematodes. As decay progresses to nutrient-depleted stages, bacteria with smaller genomes producing less nutrients become more prevalent. Lipid utilisation and dauer formation, representing key nematode survival strategies, are influenced by microbiota changes. Additionally, horizontally acquired cellulases extend the nematodes' reproductive phase due to more efficient foraging. Lastly, the expressions of Pristionchus species-specific genes are more responsive to natural microbiota compared to conserved genes, suggesting their importance in the organisms' adaptation to its ecological niche. In summary, we show the importance of microbial successions and their reciprocal interaction with nematodes for insect decay in semi-artificial ecosystems.

A global survey of prokaryotic genomes reveals the eco-evolutionary pressures driving horizontal gene transfer

Horizontal gene transfer, the exchange of genetic material through means other than reproduction, is a fundamental force in prokaryotic genome evolution. Genomic persistence of horizontally transferred genes has been shown to be influenced by both ecological and evolutionary factors. However, there is limited availability of ecological information about species other than the habitats from which they were isolated, which has prevented a deeper exploration of ecological contributions to horizontal gene transfer. Here we focus on transfers detected through comparison of individual gene trees to the species tree, assessing the distribution of gene-exchanging prokaryotes across over a million environmental sequencing samples. By analysing detected horizontal gene transfer events, we show distinct functional profiles for recent versus old events. Although most genes transferred are part of the accessory genome, genes transferred earlier in evolution tend to be more ubiquitous within present-day species. We find that co-occurring, interacting and high-abundance species tend to exchange more genes. Finally, we show that host-associated specialist species are most likely to exchange genes with other host-associated specialist species, whereas species found across different habitats have similar gene exchange rates irrespective of their preferred habitat. Our study covers an unprecedented scale of integrated horizontal gene transfer and environmental information, highlighting broad eco-evolutionary trends.

Genome streamlining to improve performance of a fast-growing cyanobacterium Synechococcus elongatus UTEX 2973

Cyanobacteria are photosynthetic organisms that have garnered significant recognition as potential hosts for sustainable bioproduction. However, their complex regulatory networks pose significant challenges to major metabolic engineering efforts, thereby limiting their feasibility as production hosts. Genome streamlining has been demonstrated to be a successful approach for improving productivity and fitness in heterotrophs but is yet to be explored to its full potential in phototrophs. Here, we present the systematic reduction of the genome of the cyanobacterium exhibiting the fastest exponential growth, Synechococcus elongatus UTEX 2973. This work, the first of its kind in a photoautotroph, involved an iterative process using state-of-the-art genome-editing technology guided by experimental analysis and computational tools. CRISPR-Cas3 enabled large, progressive deletions of predicted dispensable regions and aided in the identification of essential genes. The large deletions were combined to obtain a strain with 55-kb genome reduction. The strains with streamlined genome showed improvement in growth (up to 23%) and productivity (by 22.7%) as compared to the wild type (WT). This streamlining strategy not only has the potential to develop cyanobacterial strains with improved growth and productivity traits but can also facilitate a better understanding of their genome-to-phenome relationships.IMPORTANCEGenome streamlining is an evolutionary strategy used by natural living systems to dispense unnecessary genes from their genome as a mechanism to adapt and evolve. While this strategy has been successfully borrowed to develop synthetic heterotrophic microbial systems with desired phenotype, it has not been extensively explored in photoautotrophs. Genome streamlining strategy incorporates both computational predictions to identify the dispensable regions and experimental validation using genome-editing tool, and in this study, we have employed a modified strategy with the goal to minimize the genome size to an extent that allows optimal cellular fitness under specified conditions. Our strategy has explored a novel genome-editing tool in photoautotrophs, which, unlike other existing tools, enables large, spontaneous optimal deletions from the genome. Our findings demonstrate the effectiveness of this modified strategy in obtaining strains with streamlined genome, exhibiting improved fitness and productivity.

Pseudogenes act as a neutral reference for detecting selection in prokaryotic pangenomes

A long-standing question is to what degree genetic drift and selection drive the divergence in rare accessory gene content between closely related bacteria. Rare genes, including singletons, make up a large proportion of pangenomes (all genes in a set of genomes), but it remains unclear how many such genes are adaptive, deleterious or neutral to their host genome. Estimates of species' effective population sizes (Ne) are positively associated with pangenome size and fluidity, which has independently been interpreted as evidence for both neutral and adaptive pangenome models. We hypothesized that pseudogenes, used as a neutral reference, could be used to distinguish these models. We find that most functional categories are depleted for rare pseudogenes when a genome encodes only a single intact copy of a gene family. In contrast, transposons are enriched in pseudogenes, suggesting they are mostly neutral or deleterious to the host genome. Thus, even if individual rare accessory genes vary in their effects on host fitness, we can confidently reject a model of entirely neutral or deleterious rare genes. We also define the ratio of singleton intact genes to singleton pseudogenes (si/sp) within a pangenome, compare this measure across 668 prokaryotic species and detect a signal consistent with the adaptive value of many rare accessory genes. Taken together, our work demonstrates that comparing with pseudogenes can improve inferences of the evolutionary forces driving pangenome variation.

Genome reduction occurred in early Prochlorococcus with an unusually low effective population size

In the oligotrophic sunlit ocean, the most abundant free-living planktonic bacterial lineages evolve convergently through genome reduction. The cyanobacterium Prochlorococcus responsible for 10% global oxygen production is a prominent example. The dominant theory known as "genome streamlining" posits that they have extremely large effective population sizes (Ne) such that selection for metabolic efficiency acts to drive genome reduction. Because genome reduction largely took place anciently, this theory builds on the assumption that their ancestors' Ne was similarly large. Constraining Ne for ancient ancestors is challenging because experimental measurements of extinct organisms are impossible and alternatively reconstructing ancestral Ne with phylogenetic models gives large uncertainties. Here, we develop a new strategy that leverages agent-based modeling to simulate the changes in the genome-wide ratio of radical to conservative nonsynonymous nucleotide substitution rate (dR/dC) in a possible range of Ne in ancestral populations. This proxy shows expected increases with decreases of Ne only when Ne falls to about 10 k - 100 k or lower, magnitudes characteristic of Ne of obligate endosymbiont species where drift drives genome reduction. Our simulations therefore strongly support a scenario where the primary force of Prochlorococcus genome reduction is drift rather than selection.

StORF-Reporter: finding genes between genes

Large regions of prokaryotic genomes are currently without any annotation, in part due to well-established limitations of annotation tools. For example, it is routine for genes using alternative start codons to be misreported or completely omitted. Therefore, we present StORF-Reporter, a tool that takes an annotated genome and returns regions that may contain missing CDS genes from unannotated regions. StORF-Reporter consists of two parts. The first begins with the extraction of unannotated regions from an annotated genome. Next, Stop-ORFs (StORFs) are identified in these unannotated regions. StORFs are open reading frames that are delimited by stop codons and thus can capture those genes most often missing in genome annotations. We show this methodology recovers genes missing from canonical genome annotations. We inspect the results of the genomes of model organisms, the pangenome of Escherichia coli, and a set of 5109 prokaryotic genomes of 247 genera from the Ensembl Bacteria database. StORF-Reporter extended the core, soft-core and accessory gene collections, identified novel gene families and extended families into additional genera. The high levels of sequence conservation observed between genera suggest that many of these StORFs are likely to be functional genes that should now be considered for inclusion in canonical annotations.

Comparison of gene clustering criteria reveals intrinsic uncertainty in pangenome analyses

BACKGROUND

A key step for comparative genomics is to group open reading frames into functionally and evolutionarily meaningful gene clusters. Gene clustering is complicated by intraspecific duplications and horizontal gene transfers that are frequent in prokaryotes. In consequence, gene clustering methods must deal with a trade-off between identifying vertically transmitted representatives of multicopy gene families, which are recognizable by synteny conservation, and retrieving complete sets of species-level orthologs. We studied the implications of adopting homology, orthology, or synteny conservation as formal criteria for gene clustering by performing comparative analyses of 125 prokaryotic pangenomes.

RESULTS

Clustering criteria affect pangenome functional characterization, core genome inference, and reconstruction of ancestral gene content to different extents. Species-wise estimates of pangenome and core genome sizes change by the same factor when using different clustering criteria, allowing robust cross-species comparisons regardless of the clustering criterion. However, cross-species comparisons of genome plasticity and functional profiles are substantially affected by inconsistencies among clustering criteria. Such inconsistencies are driven not only by mobile genetic elements, but also by genes involved in defense, secondary metabolism, and other accessory functions. In some pangenome features, the variability attributed to methodological inconsistencies can even exceed the effect sizes of ecological and phylogenetic variables.

CONCLUSIONS

Choosing an appropriate criterion for gene clustering is critical to conduct unbiased pangenome analyses. We provide practical guidelines to choose the right method depending on the research goals and the quality of genome assemblies, and a benchmarking dataset to assess the robustness and reproducibility of future comparative studies.

Genome-Wide Comparative Analysis of Lactiplantibacillus pentosus Isolates Autochthonous to Cucumber Fermentation Reveals Subclades of Divergent Ancestry

Lactiplantibacillus pentosus, commonly isolated from commercial cucumber fermentation, is a promising candidate for starter culture formulation due to its ability to achieve complete sugar utilization to an end pH of 3.3. In this study, we conducted a comparative genomic analysis encompassing 24 L. pentosus and 3 Lactiplantibacillus plantarum isolates autochthonous to commercial cucumber fermentation and 47 lactobacillales reference genomes to determine species specificity and provide insights into niche adaptation. Results showed that metrics such as average nucleotide identity score, emulated Rep-PCR-(GTG)₅, computed multi-locus sequence typing (MLST), and multiple open reading frame (ORF)-based phylogenetic trees can robustly and consistently distinguish the two closely related species. Phylogenetic trees based on the alignment of 587 common ORFs separated the L. pentosus autochthonous cucumber isolates from olive fermentation isolates into clade A and B, respectively. The L. pentosus autochthonous clade partitions into subclades A.I, A.II, and A.III, suggesting substantial intraspecies diversity in the cucumber fermentation habitat. The hypervariable sequences within CRISPR arrays revealed recent evolutionary history, which aligns with the L. pentosus subclades identified in the phylogenetic trees constructed. While L. plantarum autochthonous to cucumber fermentation only encode for Type II-A CRISPR arrays, autochthonous L. pentosus clade B codes for Type I-E and L. pentosus clade A hosts both types of arrays. L. pentosus 7.8.2, for which phylogeny could not be defined using the varied methods employed, was found to uniquely encode for four distinct Type I-E CRISPR arrays and a Type II-A array. Prophage sequences in varied isolates evidence the presence of adaptive immunity in the candidate starter cultures isolated from vegetable fermentation as observed in dairy counterparts. This study provides insight into the genomic features of industrial Lactiplantibacillus species, the level of species differentiation in a vegetable fermentation habitat, and diversity profile of relevance in the selection of functional starter cultures.

Global Transcriptome-Guided Identification of Neutral Sites for Engineering Synechococcus elongatus PCC 11801

Engineered cyanobacteria are attractive hosts for the phototrophic conversion of CO2 to chemicals. Synechococcus elongatus PCC11801, a novel, fast-growing, and stress-tolerant cyanobacterium, has the potential to be a platform cell factory, and hence, it necessitates the development of a synthetic biology toolbox. Considering the widely followed cyanobacterial engineering strategy of chromosomal integration of heterologous DNA, it is of interest to discover and validate new chromosomal neutral sites (NSs) in this strain. To that end, global transcriptome analysis was performed using RNA Seq under the conditions of high temperature (HT), carbon (HC), and salt (HS) and ambient growth conditions. We found upregulation of 445, 138, and 87 genes and downregulation of 333, 125, and 132 genes, under HC, HT, and HS, respectively. Following nonhierarchical clustering, gene enrichment, and bioinformatics analysis, 27 putative NSs were predicted. Six of them were experimentally tested, and five showed confirmed neutrality, based on unaltered cell growth. Thus, global transcriptomic analysis was effectively exploited for NS annotation and would be advantageous for multiplexed genome editing.

Human Pangenomics: Promises and Challenges of a Distributed Genomic Reference

A pangenome is a collection of the common and unique genomes that are present in a given species. It combines the genetic information of all the genomes sampled, resulting in a large and diverse range of genetic material. Pangenomic analysis offers several advantages compared to traditional genomic research. For example, a pangenome is not bound by the physical constraints of a single genome, so it can capture more genetic variability. Thanks to the introduction of the concept of pangenome, it is possible to use exceedingly detailed sequence data to study the evolutionary history of two different species, or how populations within a species differ genetically. In the wake of the Human Pangenome Project, this review aims at discussing the advantages of the pangenome around human genetic variation, which are then framed around how pangenomic data can inform population genetics, phylogenetics, and public health policy by providing insights into the genetic basis of diseases or determining personalized treatments, targeting the specific genetic profile of an individual. Moreover, technical limitations, ethical concerns, and legal considerations are discussed.

Abiotic selection of microbial genome size in the global ocean

Strong purifying selection is considered a major evolutionary force behind small microbial genomes in the resource-poor photic ocean. However, very little is currently known about how the size of prokaryotic genomes evolves in the global ocean and whether patterns reflect shifts in resource availability in the epipelagic and relatively stable deep-sea environmental conditions. Using 364 marine microbial metagenomes, we investigate how the average genome size of uncultured planktonic prokaryotes varies across the tropical and polar oceans to the hadal realm. We find that genome size is highest in the perennially cold polar ocean, reflecting elongation of coding genes and gene dosage effects due to duplications in the interior ocean microbiome. Moreover, the rate of change in genome size due to temperature is 16-fold higher than with depth up to 200 m. Our results demonstrate how environmental factors can influence marine microbial genome size selection and ecological strategies of the microbiome.

Significance of microbial genome in environmental remediation

Environmental pollutants seriously threaten the ecosystem and health of various life forms, particularly with the rapid industrialization and emerging population. Conventionally physical and chemical strategies are being opted for the removal of these pollutants. Bioremediation, through several advancements, has been a boon to combat the existing threat faced today. Microbes with enzymes degrade various pollutants and utilize them as a carbon and energy source. With the existing demand and through several research explorations, Genetically Engineered Microorganisms (GEMs) have paved to be a successful approach to abate pollution through bioremediation. The genome of the microbe determines its biodegradative nature. Thus, methods including pure culture techniques and metagenomics are used for analyzing the genome of microbes, which provides information about catabolic genes. The information obtained along with the aid of biotechnology helps to construct GEMs that are cost-effective and safer thereby exhibiting higher degradation of pollutants. The present review focuses on the role of microbes in the degradation of environmental pollutants, role of evolution in habitat and adaptation of microbes, microbial degenerative genes, their pathways, and the efficacy of recombinant DNA (rDNA) technology for creating GEMs for bioremediation. The present review also provides a gist of existing GEMs for bioremediation and their limitations, thereby providing a future scope of implementation of these GEMs for a sustainable environment.

Genomic Legacies of Ancient Adaptation Illuminate GC-Content Evolution in Bacteria

Bacterial evolution is characterized by strong purifying selection as well as rapid adaptive evolution in changing environments. In this context, the genomic GC content (genomic GC) varies greatly but presents some level of phylogenetic stability, making it challenging to explain based on current hypotheses. To illuminate the evolutionary mechanisms of the genomic GC, we analyzed the base composition and functional inventory of 11,083 representative genomes. A phylogenetically constrained bimodal distribution of the genomic GC, which mainly originated from parallel divergences in the early evolution, was demonstrated. Such variation of the genomic GC can be well explained by DNA replication and repair (DRR), in which multiple pathways correlate with the genomic GC. Furthermore, the biased conservation of various stress-related genes, especially the DRR-related ones, implies distinct adaptive processes in the ancestral lineages of high- or low-GC clades which are likely induced by major environmental changes. Our findings support that the mutational biases resulting from these legacies of ancient adaptation have changed the course of adaptive evolution and generated great variation in the genomic GC. This highlights the importance of indirect effects of natural selection, which indicates a new model for bacterial evolution. IMPORTANCE GC content has been shown to be an important factor in microbial ecology and evolution, and the genomic GC of bacteria can be characterized by great intergenomic heterogeneity, high intragenomic homogeneity, and strong phylogenetic inertia, as well as being associated with the environment. Current hypotheses concerning direct selection or mutational biases cannot well explain these features simultaneously. Our findings of the genomic GC showing that ancient adaptations have transformed the DRR system and that the resulting mutational biases further contributed to a bimodal distribution of it offer a more reasonable scenario for the mechanism. This would imply that, when thinking about the evolution of life, diverse processes of adaptation exist, and combined effects of natural selection should be considered.

Prokaryotic Pangenomes Act as Evolving Ecosystems

Understanding adaptation to the local environment is a central tenet and a major focus of evolutionary biology. But this is only part of the adaptionist story. In addition to the external environment, one of the main drivers of genome composition is genetic background. In this perspective, I argue that there is a growing body of evidence that intra-genomic selective pressures play a significant part in the composition of prokaryotic genomes and play a significant role in the origin, maintenance and structuring of prokaryotic pangenomes.

Microbial communities of stratified aquatic ecosystems of Kandalaksha Bay (White Sea) shed light on the evolutionary history of green and brown morphotypes of Chlorobiota

Anoxygenic photoautotrophic metabolism of green sulfur bacteria of the family Chlorobiaceae played a significant role in establishing the Earth's biosphere. Two known major ecological forms of these phototrophs differ in their pigment composition and, therefore, in color: the green and brown forms. The latter form often occurs in low-light environments and is specialized to harvest blue light, which can penetrate to the greatest depth in the water column. In the present work, metagenomic sequencing was used to investigate the natural population of brown Chl. phaeovibrioides ZM in a marine stratified Zeleny Mys lagoon in the Kandalaksha Bay (the White Sea) to supplement the previously obtained genomes of brown Chlorobiaceae. The genomes of brown and green Chlorobiaceae were investigated using comparative genome analysis and phylogenetic and reconciliation analysis to reconstruct the evolution of these ecological forms. Our results support the suggestion that the last common ancestor of Chlorobiaceae belonged to the brown form, i.e. it was adapted to the conditions of low illumination. However, despite the vertical inheritance of these characteristics, among modern Chlorobiaceae populations, the genes responsible for synthesizing the pigments of the brown form are subject to active horizontal transfer.

Strategies for the Biodegradation of Polyfluorinated Compounds

Many cite the strength of C-F bonds for the poor microbial biodegradability of polyfluorinated organic compounds (PFCs). However, commercial PFCs almost invariably contain more functionality than fluorine. The additional functionality provides a weak entry point for reactions that activate C-F bonds and lead to their eventual cleavage. This metabolic activation strategy is common in microbial biodegradation pathways and is observed with aromatic hydrocarbons, chlorinated compounds, phosphonates and many other compounds. Initial metabolic activation precedes critical bond breakage and assimilation of nutrients. A similar strategy with commercial PFCs proceeds via initial attack at the non-fluorinated functionalities: sulfonates, carboxylates, chlorines, phenyl rings, or phosphonates. Metabolic transformation of these non-fluorinated groups can activate the C-F bonds, allowing more facile cleavage than a direct attack on the C-F bonds. Given that virtually all compounds denoted as "PFAS" are not perfluorinated and are not alkanes, it is posited here that considering their individual chemical classes is more useful for both chemical and microbiological considerations of their fate.

Nutrition or nature: using elementary flux modes to disentangle the complex forces shaping prokaryote pan-genomes

BACKGROUND

Microbial pan-genomes are shaped by a complex combination of stochastic and deterministic forces. Even closely related genomes exhibit extensive variation in their gene content. Understanding what drives this variation requires exploring the interactions of gene products with each other and with the organism's external environment. However, to date, conceptual models of pan-genome dynamics often represent genes as independent units and provide limited information about their mechanistic interactions.

RESULTS

We simulated the stochastic process of gene-loss using the pooled genome-scale metabolic reaction networks of 46 taxonomically diverse bacterial and archaeal families as proxies for their pan-genomes. The frequency by which reactions are retained in functional networks when stochastic gene loss is simulated in diverse environments allowed us to disentangle the metabolic reactions whose presence depends on the metabolite composition of the external environment (constrained by "nutrition") from those that are independent of the environment (constrained by "nature"). By comparing the frequency of reactions from the first group with their observed frequencies in bacterial and archaeal families, we predicted the metabolic niches that shaped the genomic composition of these lineages. Moreover, we found that the lineages that were shaped by a more diverse metabolic niche also occur in more diverse biomes as assessed by global environmental sequencing datasets.

CONCLUSION

We introduce a computational framework for analyzing and interpreting pan-reactomes that provides novel insights into the ecological and evolutionary drivers of pan-genome dynamics.

Realistic Gene Transfer to Gene Duplication Ratios Identify Different Roots in the Bacterial Phylogeny Using a Tree Reconciliation Method

The rooting of phylogenetic trees permits important inferences about ancestral states and the polarity of evolutionary events. Recently, methods that reconcile discordance between gene-trees and species-trees—tree reconciliation methods—are becoming increasingly popular for rooting species trees. Rooting via reconciliation requires values for a particular parameter, the gene transfer to gene duplication ratio (T:D), which in current practice is estimated on the fly from discordances observed in the trees. To date, the accuracy of T:D estimates obtained by reconciliation analyses has not been compared to T:D estimates obtained by independent means, hence the effect of T:D upon inferences of species tree roots is altogether unexplored. Here we investigated the issue in detail by performing tree reconciliations of more than 10,000 gene trees under a variety of T:D ratios for two phylogenetic cases: a bacterial (prokaryotic) tree with 265 species and a fungal-metazoan (eukaryotic) tree with 31 species. We show that the T:D ratios automatically estimated by a current tree reconciliation method, ALE, generate virtually identical T:D ratios across bacterial genes and fungal-metazoan genes. The T:D ratios estimated by ALE differ 10- to 100-fold from robust, ALE-independent estimates from real data. More important is our finding that the root inferences using ALE in both datasets are strongly dependent upon T:D. Using more realistic T:D ratios, the number of roots inferred by ALE consistently increases and, in some cases, clearly incorrect roots are inferred. Furthermore, our analyses reveal that gene duplications have a far greater impact on ALE’s preferences for phylogenetic root placement than gene transfers or gene losses do. Overall, we show that obtaining reliable species tree roots with ALE is only possible when gene duplications are abundant in the data and the number of falsely inferred gene duplications is low. Finding a sufficient sample of true gene duplications for rooting species trees critically depends on the T:D ratios used in the analyses. T:D ratios, while being important parameters of genome evolution in their own right, affect the root inferences with tree reconciliations to an unanticipated degree.

Siderophores as an iron source for picocyanobacteria in deep chlorophyll maximum layers of the oligotrophic ocean

Prochlorococcus and Synechococcus are the most abundant photosynthesizing organisms in the oceans. Gene content variation among picocyanobacterial populations in separate ocean basins often mirrors the selective pressures imposed by the region's distinct biogeochemistry. By pairing genomic datasets with trace metal concentrations from across the global ocean, we show that the genomic capacity for siderophore-mediated iron uptake is widespread in Synechococcus and low-light adapted Prochlorococcus populations from deep chlorophyll maximum layers of iron-depleted regions of the oligotrophic Pacific and S. Atlantic oceans: Prochlorococcus siderophore consumers were absent in the N. Atlantic ocean (higher new iron flux) but constituted up to half of all Prochlorococcus genomes from metagenomes in the N. Pacific (lower new iron flux). Picocyanobacterial siderophore consumers, like many other bacteria with this trait, also lack siderophore biosynthesis genes indicating that they scavenge exogenous siderophores from seawater. Statistical modeling suggests that the capacity for siderophore uptake is endemic to remote ocean regions where atmospheric iron fluxes are the smallest, especially at deep chlorophyll maximum and primary nitrite maximum layers. We argue that abundant siderophore consumers at these two common oceanographic features could be a symptom of wider community iron stress, consistent with prior hypotheses. Our results provide a clear example of iron as a selective force driving the evolution of marine picocyanobacteria.

GeCoViz: genomic context visualisation of prokaryotic genes from a functional and evolutionary perspective

Synteny conservation analysis is a well-established methodology to investigate the potential functional role of unknown prokaryotic genes. However, bioinformatic tools to reconstruct and visualise genomic contexts usually depend on slow computations, are restricted to narrow taxonomic ranges, and/or do not allow for the functional and interactive exploration of neighbouring genes across different species. Here, we present GeCoViz, an online resource built upon 12 221 reference prokaryotic genomes that provides fast and interactive visualisation of custom genomic regions anchored by any target gene, which can be sought by either name, orthologous group (KEGGs, eggNOGs), protein domain (PFAM) or sequence. To facilitate functional and evolutionary interpretation, GeCoViz allows to customise the taxonomic scope of each analysis and provides comprehensive annotations of the neighbouring genes. Interactive visualisation options include, among others, the scaled representations of gene lengths and genomic distances, and on the fly calculation of synteny conservation of neighbouring genes, which can be highlighted based on custom thresholds. The resulting plots can be downloaded as high-quality images for publishing purposes. Overall, GeCoViz offers an easy-to-use, comprehensive, fast and interactive web-based tool for investigating the genomic context of prokaryotic genes, and is freely available at https://gecoviz.cgmlab.org.

Genome size distributions in bacteria and archaea are strongly linked to evolutionary history at broad phylogenetic scales

The evolutionary forces that determine genome size in bacteria and archaea have been the subject of intense debate over the last few decades. Although the preferential loss of genes observed in prokaryotes is explained through the deletional bias, factors promoting and preventing the fixation of such gene losses often remain unclear. Importantly, statistical analyses on this topic typically do not consider the potential bias introduced by the shared ancestry of many lineages, which is critical when using species as data points because of the potential dependence on residuals. In this study, we investigated the genome size distributions across a broad diversity of bacteria and archaea to evaluate if this trait is phylogenetically conserved at broad phylogenetic scales. After model fit, Pagel's lambda indicated a strong phylogenetic signal in genome size data, suggesting that the diversification of this trait is influenced by shared evolutionary histories. We used a phylogenetic generalized least-squares analysis (PGLS) to test whether phylogeny influences the predictability of genome size from dN/dS ratios and 16S copy number, two variables that have been previously linked to genome size. These results confirm that failure to account for evolutionary history can lead to biased interpretations of genome size predictors. Overall, our results indicate that although bacteria and archaea can rapidly gain and lose genetic material through gene transfers and deletions, respectively, phylogenetic signal for genome size distributions can still be recovered at broad phylogenetic scales that should be taken into account when inferring the drivers of genome size evolution.

Importance of RpoD- and Non-RpoD-Dependent Expression of Horizontally Acquired Genes in Cupriavidus metallidurans

The genome of the metal-resistant, hydrogen-oxidizing bacterium Cupriavidus metallidurans contains a large number of horizontally acquired plasmids and genomic islands that were integrated into its chromosome or chromid. For the C. metallidurans CH34 wild-type strain growing under nonchallenging conditions, 5,763 transcriptional starting sequences (TSSs) were determined. Using a custom-built motif discovery software based on hidden Markov models, patterns upstream of the TSSs were identified. The pattern TTGACA, −35.6 ± 1.6 bp upstream of the TSSs, in combination with a TATAAT sequence 15.8 ± 1.4 bp upstream occurred frequently, especially upstream of the TSSs for 48 housekeeping genes, and these were assigned to promoters used by RNA polymerase containing the main housekeeping sigma factor RpoD. From patterns upstream of the housekeeping genes, a score for RpoD-dependent promoters in C. metallidurans was derived and applied to all 5,763 TSSs. Among these, 2,572 TSSs could be associated with RpoD with high probability, 373 with low probability, and 2,818 with no probability. In a detailed analysis of horizontally acquired genes involved in metal resistance and not involved in this process, the TSSs responsible for the expression of these genes under nonchallenging conditions were assigned to RpoD- or non-RpoD-dependent promoters. RpoD-dependent promoters occurred frequently in horizontally acquired metal resistance and other determinants, which should allow their initial expression in a new host. However, other sigma factors and sense/antisense effects also contribute—maybe to mold in subsequent adaptation steps the assimilated gene into the regulatory network of the cell.

IMPORTANCE In their natural environment, bacteria are constantly acquiring genes by horizontal gene transfer. To be of any benefit, these genes should be expressed. We show here that the main housekeeping sigma factor RpoD plays an important role in the expression of horizontally acquired genes in the metal-resistant hydrogen-oxidizing bacterium C. metallidurans. By conservation of the RpoD recognition consensus sequence, a newly arriving gene has a high probability to be expressed in the new host cell. In addition to integrons and genes travelling together with that for their sigma factor, conservation of the RpoD consensus sequence may be an important contributor to the overall evolutionary success of horizontal gene transfer in bacteria. Using C. metallidurans as an example, this publication sheds some light on the fate and function of horizontally acquired genes in bacteria.

Horizontal gene transfer and adaptive evolution in bacteria

Horizontal gene transfer (HGT) is arguably the most conspicuous feature of bacterial evolution. Evidence for HGT is found in most bacterial genomes. Although HGT can considerably alter bacterial genomes, not all transfer events may be biologically significant and may instead represent the outcome of an incessant evolutionary process that only occasionally has a beneficial purpose. When adaptive transfers occur, HGT and positive selection may result in specific, detectable signatures in genomes, such as gene-specific sweeps or increased transfer rates for genes that are ecologically relevant. In this Review, we first discuss the various mechanisms whereby HGT occurs, how the genetic signatures shape patterns of genomic variation and the distinct bioinformatic algorithms developed to detect these patterns. We then discuss the evolutionary theory behind HGT and positive selection in bacteria, and discuss the approaches developed over the past decade to detect transferred DNA that may be involved in adaptation to new environments.

Toward a theory of evolution as multilevel learning

We apply the theory of learning to physically renormalizable systems in an attempt to outline a theory of biological evolution, including the origin of life, as multilevel learning. We formulate seven fundamental principles of evolution that appear to be necessary and sufficient to render a universe observable and show that they entail the major features of biological evolution, including replication and natural selection. It is shown that these cornerstone phenomena of biology emerge from the fundamental features of learning dynamics such as the existence of a loss function, which is minimized during learning. We then sketch the theory of evolution using the mathematical framework of neural networks, which provides for detailed analysis of evolutionary phenomena. To demonstrate the potential of the proposed theoretical framework, we derive a generalized version of the Central Dogma of molecular biology by analyzing the flow of information during learning (back propagation) and predicting (forward propagation) the environment by evolving organisms. The more complex evolutionary phenomena, such as major transitions in evolution (in particular, the origin of life), have to be analyzed in the thermodynamic limit, which is described in detail in the paper by Vanchurin et al. [V. Vanchurin, Y. I. Wolf, E. V. Koonin, M. I. Katsnelson, Proc. Natl. Acad. Sci. U.S.A. 119, 10.1073/pnas.2120042119 (2022)].

Ecological selection of bacterial taxa with larger genome sizes in response to polycyclic aromatic hydrocarbons stress

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous priority pollutants that cause great damage to the natural environment and health. Average genome size in a community is critical for shedding light on microbiome's functional response to pollution stress within an environment. Here, microcosms under different concentrations were performed to evaluate the selection of PAHs stress on the average genome size in a community. We found the distinct communities of significantly larger genome size with the increase of PAHs concentration gradients in soils, and consistent trends were discovered in soils at different latitudes. The abundance of Proteobacteria and Deinococcus-Thermus with relatively larger genomes increased along with PAHs stress and well adapted to polluted environments. In contrast, the abundance of Patescibacteria with a highly streamlined and smaller genome decreased, implying complex interactions between environmental selection and functional fitness resulted in bacteria with larger genomes becoming more abundant. Moreover, we confirmed the increased capacity for horizontal transfer of degrading genes between communities by showing an increased connection number per node positively related to the nidA gene along the concentration gradients in the co-occurrence network. Our findings suggest PAHs tend to select bacterial taxa with larger genome sizes, with significant consequences for community stability and potential biodegradation strategies.

Environmental stress leads to genome streamlining in a widely distributed species of soil bacteria

Bacteria have highly flexible pangenomes, which are thought to facilitate evolutionary responses to environmental change, but the impacts of environmental stress on pangenome evolution remain unclear. Using a landscape pangenomics approach, I demonstrate that environmental stress leads to consistent, continuous reduction in genome content along four environmental stress gradients (acidity, aridity, heat, salinity) in naturally occurring populations of Bradyrhizobium diazoefficiens (widespread soil-dwelling plant mutualists). Using gene-level network and duplication functional traits to predict accessory gene distributions across environments, genes predicted to be superfluous are more likely lost in high stress, while genes with multi-functional roles are more likely retained. Genes with higher probabilities of being lost with stress contain significantly higher proportions of codons under strong purifying and positive selection. Gene loss is widespread across the entire genome, with high gene-retention hotspots in close spatial proximity to core genes, suggesting Bradyrhizobium has evolved to cluster essential-function genes (accessory genes with multifunctional roles and core genes) in discrete genomic regions, which may stabilise viability during genomic decay. In conclusion, pangenome evolution through genome streamlining are important evolutionary responses to environmental change. This raises questions about impacts of genome streamlining on the adaptive capacity of bacterial populations facing rapid environmental change.

Prokaryote pangenomes are dynamic entities

Prokaryote pangenomes are influenced heavily by environmental factors and the opportunity for gene gain and loss events. As the field of pangenome analysis has expanded, so has the need to fully understand the complexity of how eco-evolutionary dynamics shape pangenomes. Here, we describe current models of pangenome evolution and discuss their suitability and accuracy. We suggest that pangenomes are dynamic entities under constant flux, highlighting the influence of two-way interactions between pangenome and environment. New classifications of core and accessory genes are also considered, underscoring the need for continuous evaluation of nomenclature in a fast-moving field. We conclude that future models of pangenome evolution should incorporate eco-evolutionary dynamics to fully encompass their dynamic, changeable nature.

Transposable elements promote the evolution of genome streamlining

Eukaryotes and prokaryotes have distinct genome architectures, with marked differences in genome size, the ratio of coding/non-coding DNA, and the abundance of transposable elements (TEs). As TEs replicate independently of their hosts, the proliferation of TEs is thought to have driven genome expansion in eukaryotes. However, prokaryotes also have TEs in intergenic spaces, so why do prokaryotes have small, streamlined genomes? Using an in silico model describing the genomes of single-celled asexual organisms that coevolve with TEs, we show that TEs acquired from the environment by horizontal gene transfer can promote the evolution of genome streamlining. The process depends on local interactions and is underpinned by rock-paper-scissors dynamics in which populations of cells with streamlined genomes beat TEs, which beat non-streamlined genomes, which beat streamlined genomes, in continuous and repeating cycles. Streamlining is maladaptive to individual cells, but improves lineage viability by hindering the proliferation of TEs. Streamlining does not evolve in sexually reproducing populations because recombination partially frees TEs from the deleterious effects they cause. This article is part of the theme issue 'The secret lives of microbial mobile genetic elements'.

Towards the biogeography of prokaryotic genes

Microbial genes encode the majority of the functional repertoire of life on earth. However, despite increasing efforts in metagenomic sequencing of various habitats^1-3, little is known about the distribution of genes across the global biosphere, with implications for human and planetary health. Here we constructed a non-redundant gene catalogue of 303 million species-level genes (clustered at 95% nucleotide identity) from 13,174 publicly available metagenomes across 14 major habitats and use it to show that most genes are specific to a single habitat. The small fraction of genes found in multiple habitats is enriched in antibiotic-resistance genes and markers for mobile genetic elements. By further clustering these species-level genes into 32 million protein families, we observed that a small fraction of these families contain the majority of the genes (0.6% of families account for 50% of the genes). The majority of species-level genes and protein families are rare. Furthermore, species-level genes, and in particular the rare ones, show low rates of positive (adaptive) selection, supporting a model in which most genetic variability observed within each protein family is neutral or nearly neutral.

A large-scale whole-genome sequencing analysis reveals false positives of bacterial essential genes

Essential genes are crucial for bacterial viability and represent attractive targets for novel anti-pathogen drug discovery. However, essential genes determined by the transposon insertion sequencing (Tn-seq) approach often contain many false positives. We hypothesized that some of those false positives are genes that are actually deleted from the genome, so they do not present any transposon insertion in the course of Tn-seq analysis. Based on this assumption, we performed a large-scale whole-genome sequencing analysis for the bacterium of interest. Our analysis revealed that some "essential genes" are indeed removed from the analyzed bacterial genomes. Since these genes were kicked out by bacteria, they should not be defined as essential. Our work showed that gene deletion is one of the false positive sources of essentiality determination, which is apparently underestimated in previous studies. We suggest subtracting the genome backgrounds before the evaluation of Tn-seq, and created a list of false positive gene essentiality as a reference for the downstream application. KEY POINTS: • Discovery of false positives of essential genes defined previously through the analyses of a large scale of whole-genome sequencing data • These false positives are the results of gene deletions in the studied genomes • Sequencing the target genome before Tn-seq analysis is of importance while some studies neglected it.

No one tool to rule them all: prokaryotic gene prediction tool annotations are highly dependent on the organism of study

MOTIVATION

The biases in CoDing Sequence (CDS) prediction tools, which have been based on historic genomic annotations from model organisms, impact our understanding of novel genomes and metagenomes. This hinders the discovery of new genomic information as it results in predictions being biased towards existing knowledge. To date, users have lacked a systematic and replicable approach to identify the strengths and weaknesses of any CDS prediction tool and allow them to choose the right tool for their analysis.

RESULTS

We present an evaluation framework (ORForise) based on a comprehensive set of 12 primary and 60 secondary metrics that facilitate the assessment of the performance of CDS prediction tools. This makes it possible to identify which performs better for specific use-cases. We use this to assess 15 ab initio- and model-based tools representing those most widely used (historically and currently) to generate the knowledge in genomic databases. We find that the performance of any tool is dependent on the genome being analysed, and no individual tool ranked as the most accurate across all genomes or metrics analysed. Even the top-ranked tools produced conflicting gene collections, which could not be resolved by aggregation. The ORForise evaluation framework provides users with a replicable, data-led approach to make informed tool choices for novel genome annotations and for refining historical annotations.

AVAILABILITY AND IMPLEMENTATION

Code and datasets for reproduction and customisation are available at https://github.com/NickJD/ORForise.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Pif1 helicases and the evidence for a prokaryotic origin of Helitrons

Helitrons are the only group of rolling-circle transposons that encode a transposase with a helicase domain (Hel), which belongs to the Pif1 family. Because Pif1 helicases are important components of eukaryotic genomes, it has been suggested that Hel domains probably originated after a host eukaryotic Pif1 gene was captured by a Helitron ancestor. However, the few analyses exploring the evolution of Helitron transposases (RepHel) have focused on its Rep domain, which is also present in other mobile genetic elements. Here, we used phylogenetic and nonmetric multidimensional scaling analyses to investigate the relationship between Hel domains and Pif1-like helicases from a variety of organisms. Our results reveal that Hel domains are only distantly related to genomic helicases from eukaryotes and prokaryotes, and thus are unlikely to have originated from a captured Pif1 gene. Based on this evidence, and on recent studies indicating that Rep domains are more closely related to rolling-circle plasmids and phages, we suggest that Helitrons are descendants of a RepHel-encoding prokaryotic plasmid element that invaded eukaryotic genomes before the radiation of its major groups. We discuss how a Pif1-like helicase domain might have favored the transposition of Helitrons in eukaryotes beyond simply unwinding DNA intermediates. Finally, we demonstrate that some examples in the literature describing genomic helicases from eukaryotes actually consist of Hel domains from Helitrons, a finding that underscores how transposons can hamper the analysis of eukaryotic genes. This investigation also revealed that two groups of land plants appear to have lost genomic Pif1 helicases independently.

Bacterial cooperation through horizontal gene transfer

Cooperation exists across all scales of biological organization, from genetic elements to complex human societies. Bacteria cooperate by secreting molecules that benefit all individuals in the population (i.e., public goods). Genes associated with cooperation can spread among strains through horizontal gene transfer (HGT). We discuss recent findings on how HGT mediated by mobile genetic elements promotes bacterial cooperation, how cooperation in turn can facilitate more frequent HGT, and how the act of HGT itself may be considered as a form of cooperation. We propose that HGT is an important enforcement mechanism in bacterial populations, thus creating a positive feedback loop that further maintains cooperation. To enforce cooperation, HGT serves as a homogenizing force by transferring the cooperative trait, effectively eliminating cheaters.

Genic Selection Within Prokaryotic Pangenomes

Understanding the evolutionary forces shaping prokaryotic pangenome structure is a major goal of microbial evolution research. Recent work has highlighted that a substantial proportion of accessory genes appear to confer niche-specific adaptations. This work has primarily focused on selection acting at the level of individual cells. Herein, we discuss a lower level of selection that also contributes to pangenome variation: genic selection. This refers to cases where genetic elements, rather than individual cells, are the entities under selection. The clearest examples of this form of selection are selfish mobile genetic elements, which are those that have either a neutral or a deleterious effect on host fitness. We review the major classes of these and other mobile elements and discuss the characteristic features of such elements that could be under genic selection. We also discuss how genetic elements that are beneficial to hosts can also be under genic selection, a scenario that may be more prevalent but not widely appreciated, because disentangling the effects of selection at different levels (i.e., organisms vs. genes) is challenging. Nonetheless, an appreciation for the potential action and implications of genic selection is important to better understand the evolution of prokaryotic pangenomes.

SARS-CoV-2 transgressing LncRNAs uncovers the known unknowns

SARS-CoV-2 harbors many known unknown regions in the form of hypothetical open reading frames (ORFs). Although the mechanisms underlying the disease pathogenesis are not clearly understood, molecules such as long noncoding RNAs (lncRNAs) play a key regulatory role in the viral pathogenesis from endocytosis. We asked whether or not the lncRNAs in the host are associated with the viral proteins and argue that lncRNA-mRNAs molecules related to viral infection may regulate SARS-CoV-2 pathogenesis. Toward the end of the perspective, we provide challenges and insights into investigating these transgression pathways.

Gene Duplications Are At Least 50 Times Less Frequent than Gene Transfers in Prokaryotic Genomes

The contribution of gene duplications to the evolution of eukaryotic genomes is well studied. By contrast, studies of gene duplications in prokaryotes are scarce and generally limited to a handful of genes or careful analysis of a few prokaryotic lineages. Systematic broad-scale studies of prokaryotic genomes that sample available data are lacking, leaving gaps in our understanding of the contribution of gene duplications as a source of genetic novelty in the prokaryotic world. Here, we report conservative and robust estimates for the frequency of recent gene duplications within prokaryotic genomes relative to recent lateral gene transfer (LGT), as mechanisms to generate multiple copies of related sequences in the same genome. We obtain our estimates by focusing on evolutionarily recent events among 5,655 prokaryotic genomes, thereby avoiding vagaries of deep phylogenetic inference and confounding effects of ancient events and differential loss. We find that recent, genome-specific gene duplications are at least 50 times less frequent and probably 100 times less frequent than recent, genome-specific, gene acquisitions via LGT. The frequency of gene duplications varies across lineages and functional categories. The findings improve our understanding of genome evolution in prokaryotes and have far-reaching implications for evolutionary models that entail LGT to gene duplications ratio as a parameter.

Interplay of Various Evolutionary Modes in Genome Diversification and Adaptive Evolution of the Family Sulfolobaceae

Sulfolobaceae family, comprising diverse thermoacidophilic and aerobic sulfur-metabolizing Archaea from various geographical locations, offers an ideal opportunity to infer the evolutionary dynamics across the members of this family. Comparative pan-genomics coupled with evolutionary analyses has revealed asymmetric genome evolution within the Sulfolobaceae family. The trend of genome streamlining followed by periods of differential gene gains resulted in an overall genome expansion in some species of this family, whereas there was reduction in others. Among the core genes, both Sulfolobus islandicus and Saccharolobus solfataricus showed a considerable fraction of positively selected genes and also higher frequencies of gene acquisition. In contrast, Sulfolobus acidocaldarius genomes experienced substantial amount of gene loss and strong purifying selection as manifested by relatively lower genome size and higher genome conservation. Central carbohydrate metabolism and sulfur metabolism coevolved with the genome diversification pattern of this archaeal family. The autotrophic CO2 fixation with three significant positively selected enzymes from S. islandicus and S. solfataricus was found to be more imperative than heterotrophic CO2 fixation for Sulfolobaceae. Overall, our analysis provides an insight into the interplay of various genomic adaptation strategies including gene gain-loss, mutation, and selection influencing genome diversification of Sulfolobaceae at various taxonomic levels and geographical locations.

Mobile Gene Sequence Evolution within Individual Human Gut Microbiomes Is Better Explained by Gene-Specific Than Host-Specific Selective Pressures

Pangenomes—the cumulative set of genes encoded by a population or species—arise from the interplay of horizontal gene transfer, drift, and selection. The balance of these forces in shaping pangenomes has been debated, and studies to date focused on ancient evolutionary time scales have suggested that pangenomes generally confer niche adaptation to their bacterial hosts. To shed light on pangenome evolution on shorter evolutionary time scales, we inferred the selective pressures acting on mobile genes within individual human microbiomes from 176 Fiji islanders. We mapped metagenomic sequence reads to a set of known mobile genes to identify single nucleotide variants (SNVs) and calculated population genetic metrics to infer deviations from a neutral evolutionary model. We found that mobile gene sequence evolution varied more by gene family than by human social attributes, such as household or village. Patterns of mobile gene sequence evolution could be qualitatively recapitulated with a simple evolutionary simulation without the need to invoke the adaptive value of mobile genes to either bacterial or human hosts. These results stand in contrast with the apparent adaptive value of pangenomes over longer evolutionary time scales. In general, the most highly mobile genes (i.e., those present in more distinct bacterial host genomes) tend to have higher metagenomic read coverage and an excess of low-frequency SNVs, consistent with their rapid spread across multiple bacterial species in the gut. However, a subset of mobile genes—including those involved in defense mechanisms and secondary metabolism—showed a contrasting signature of intermediate-frequency SNVs, indicating species-specific selective pressures or negative frequency-dependent selection on these genes. Together, our evolutionary models and population genetic data show that gene-specific selective pressures predominate over human or bacterial host-specific pressures during the relatively short time scales of a human lifetime.