The evolutionary value of recombination is constrained by genome modularity

Cross-species transmission, evolution and zoonotic potential of coronaviruses

Coronaviruses (CoVs) continuously evolve, crossing species barriers and spreading across host ranges. Over the last two decades, several CoVs (HCoV-229E, HCoV-NL63, HCoV-HKU1, HCoV-OC43, SARS-CoV, MERS-CoV, and SARS-CoV-2) have emerged in animals and mammals, causing significant economic and human life losses. Due to CoV cross-species transmission and the evolution of novel viruses, it is critical to identify their natural reservoiurs and the circumstances under which their transmission occurs. In this review, we use genetic and ecological data to disentangle the evolution of various CoVs in wildlife, humans, and domestic mammals. We thoroughly investigate several host species and outline the epidemiology of CoVs toward specific hosts. We also discuss the cross-species transmission of CoVs at the interface of wildlife, animals, and humans. Clarifying the epidemiology and diversity of species reservoirs will significantly impact our ability to respond to the future emergence of CoVs in humans and domestic animals.

The Role of Extensive Recombination in the Evolution of Geminiviruses

Mutation, recombination and pseudo-recombination are the major forces driving the evolution of viruses by the generation of variants upon which natural selection, genetic drift and gene flow can act to shape the genetic structure of viral populations. Recombination between related virus genomes co-infecting the same cell usually occurs via template swapping during the replication process and produces a chimeric genome. The family Geminiviridae shows the highest evolutionary success among plant virus families, and the common presence of recombination signatures in their genomes reveals a key role in their evolution. This review describes the general characteristics of members of the family Geminiviridae and associated DNA satellites, as well as the extensive occurrence of recombination at all taxonomic levels, from strain to family. The review also presents an overview of the recombination patterns observed in nature that provide some clues regarding the mechanisms involved in the generation and emergence of recombinant genomes. Moreover, the results of experimental evolution studies that support some of the conclusions obtained in descriptive or in silico works are summarized. Finally, the review uses a number of case studies to illustrate those recombination events with evolutionary and pathological implications as well as recombination events in which DNA satellites are involved.

Plant Virus Adaptation to New Hosts: A Multi-scale Approach

Viruses are studied at each level of biological complexity: from within-cells to ecosystems. The same basic evolutionary forces and principles operate at each level: mutation and recombination, selection, genetic drift, migration, and adaptive trade-offs. Great efforts have been put into understanding each level in great detail, hoping to predict the dynamics of viral population, prevent virus emergence, and manage their spread and virulence. Unfortunately, we are still far from this. To achieve these ambitious goals, we advocate for an integrative perspective of virus evolution. Focusing in plant viruses, we illustrate the pervasiveness of the above-mentioned principles. Beginning at the within-cell level, we describe replication modes, infection bottlenecks, and cellular contagion rates. Next, we move up to the colonization of distal tissues, discussing the fundamental role of random events. Then, we jump beyond the individual host and discuss the link between transmission mode and virulence. Finally, at the community level, we discuss properties of virus-plant infection networks. To close this review we propose the multilayer network theory, in which elements at different layers are connected and submit to their own dynamics that feed across layers, resulting in new emerging properties, as a way to integrate information from the different levels.

A Trait-Based Approach to Predicting Viral Host-Range Evolvability

Predicting the evolution of virus host range has proven to be extremely difficult, in part because of the sheer diversity of viruses, each with unique biology and ecological interactions. We have not solved this problem, but to make the problem more tractable, we narrowed our focus to three traits intrinsic to all viruses that may play a role in host-range evolvability: mutation rate, recombination rate, and phenotypic heterogeneity. Although each trait should increase evolvability, they cannot do so unbounded because fitness trade-offs limit the ability of all three traits to maximize evolvability. By examining these constraints, we can begin to identify groups of viruses with suites of traits that make them especially concerning, as well as ecological and environmental conditions that might push evolution toward accelerating host-range expansion.

Conserved recombination patterns across coronavirus subgenera

Recombination contributes to the genetic diversity found in coronaviruses and is known to be a prominent mechanism whereby they evolve. It is apparent, both from controlled experiments and in genome sequences sampled from nature, that patterns of recombination in coronaviruses are non-random and that this is likely attributable to a combination of sequence features that favour the occurrence of recombination break points at specific genomic sites, and selection disfavouring the survival of recombinants within which favourable intra-genome interactions have been disrupted. Here we leverage available whole-genome sequence data for six coronavirus subgenera to identify specific patterns of recombination that are conserved between multiple subgenera and then identify the likely factors that underlie these conserved patterns. Specifically, we confirm the non-randomness of recombination break points across all six tested coronavirus subgenera, locate conserved recombination hot- and cold-spots, and determine that the locations of transcriptional regulatory sequences are likely major determinants of conserved recombination break-point hotspot locations. We find that while the locations of recombination break points are not uniformly associated with degrees of nucleotide sequence conservation, they display significant tendencies in multiple coronavirus subgenera to occur in low guanine-cytosine content genome regions, in non-coding regions, at the edges of genes, and at sites within the Spike gene that are predicted to be minimally disruptive of Spike protein folding. While it is apparent that sequence features such as transcriptional regulatory sequences are likely major determinants of where the template-switching events that yield recombination break points most commonly occur, it is evident that selection against misfolded recombinant proteins also strongly impacts observable recombination break-point distributions in coronavirus genomes sampled from nature.

Myths and Realities about Genetically Modified Food: A Risk-Benefit Analysis

The development and consumption of genetically modified (GM) crops are surrounded by controversy. According to proponents, only molecular biology approaches and genetic engineering tools are realistic food shortage solutions for the world’s ever-growing population. The main purpose of this study is to review the impact of GM products on human, animal, and environmental health. People still reject GM crops not only because of safety concerns, but also for moral reasons. Toxicity, allergies, and possible horizontal gene transfer (HGT) to the environment or to other species have been associated with the marketing of GM products. Moreover, the scarce data available about the long-term implications of using GM crops is another opponent concern. Nevertheless, science has evidenced no harm from GM crops use to date but has, instead, reported several benefits that result from their commercialization, such as economic, environmental, and health benefits for the general public. Legislation and policies about GM product labeling standards are being discussed. To overcome emerging food security challenges, considering quality scientific information is essential rather than leaving the issue and merely moving toward moral discussion. Hence, a risk–benefit analysis is necessary.

WGS- versus ORF5-Based Typing of PRRSV: A Belgian Case Study

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of one of the most widespread and economically devastating diseases in the swine industry. Typing circulating PRRSV strains by means of sequencing is crucial for developing adequate control strategies. Most genetic studies only target the highly variable open reading frame (ORF) 5, for which an extensive database is available. In this study, we performed whole-genome sequencing (WGS) on a collection of 124 PRRSV-1 positive serum samples that were collected over a 5-year period (2015–2019) in Belgium. Our results show that (nearly) complete PRRSV genomes can be obtained directly from serum samples with a high success rate. Analysis of the coding regions confirmed the exceptionally high genetic diversity, even among Belgian PRRSV-1 strains. To gain more insight into the added value of WGS, we performed phylogenetic cluster analyses on separate ORF datasets as well as on a single, concatenated dataset (CDS) containing all ORFs. A comparison between the CDS and ORF clustering schemes revealed numerous discrepancies. To explain these differences, we performed a large-scale recombination analysis, which allowed us to identify a large number of potential recombination events that were scattered across the genome. As PRRSV does not contain typical recombination hot-spots, typing PRRSV strains based on a single ORF is not recommended. Although the typing accuracy can be improved by including multiple regions, our results show that the full genetic diversity among PRRSV strains can only be captured by analysing (nearly) complete genomes. Finally, we also identified several vaccine-derived recombinant strains, which once more raises the question of the safety of these vaccines.

Metagenomic Studies of Viruses in Weeds and Wild Plants: A Powerful Approach to Characterise Variable Virus Communities

High throughput sequencing (HTS) has revolutionised virus detection and discovery, allowing for the untargeted characterisation of whole viromes. Viral metagenomics studies have demonstrated the ubiquity of virus infection - often in the absence of disease symptoms - and tend to discover many novel viruses, highlighting the small fraction of virus biodiversity described to date. The majority of the studies using high-throughput sequencing to characterise plant viromes have focused on economically important crops, and only a small number of studies have considered weeds and wild plants. Characterising the viromes of wild plants is highly relevant, as these plants can affect disease dynamics in crops, often by acting as viral reservoirs. Moreover, the viruses in unmanaged systems may also have important effects on wild plant populations and communities. Here, we review metagenomic studies on weeds and wild plants to show the benefits and limitations of this approach and identify knowledge gaps. We consider key genomics developments that are likely to benefit the field in the near future. Although only a small number of HTS studies have been performed on weeds and wild plants, these studies have already discovered many novel viruses, demonstrated unexpected trends in virus distributions, and highlighted the potential of metagenomics as an approach.

Cranial shape diversification in horses: variation and covariation patterns under the impact of artificial selection

The potential of artificial selection to dramatically impact phenotypic diversity is well known. Large-scale morphological changes in domestic species, emerging over short timescales, offer an accelerated perspective on evolutionary processes. The domestic horse (Equus caballus) provides a striking example of rapid evolution, with major changes in morphology and size likely stemming from artificial selection. However, the microevolutionary mechanisms allowing to generate this variation in a short time interval remain little known. Here, we use 3D geometric morphometrics to quantify skull morphological diversity in the horse, and investigate modularity and integration patterns to understand how morphological associations contribute to cranial evolvability in this taxon. We find that changes in the magnitude of cranial integration contribute to the diversification of the skull morphology in horse breeds. Our results demonstrate that a conserved pattern of modularity does not constrain large-scale morphological variations in horses and that artificial selection has impacted mechanisms underlying phenotypic diversity to facilitate rapid shape changes. More broadly, this study demonstrates that studying microevolutionary processes in domestic species produces important insights into extant phenotypic diversity.

Evolutionary dynamics of bipartite begomoviruses revealed by complete genome analysis

Several key evolutionary events marked the evolution of geminiviruses, culminating with the emergence of divided (bipartite) genomes represented by viruses classified in the genus Begomovirus. This genus represents the most abundant group of multipartite viruses, contributing significantly to the observed abundance of multipartite species in the virosphere. Although aspects related to virus-host interactions and evolutionary dynamics have been extensively studied, the bipartite nature of these viruses has been little explored in evolutionary studies. Here, we performed a parallel evolutionary analysis of the DNA-A and DNA-B segments of New World begomoviruses. A total of 239 full-length DNA-B sequences obtained in this study, combined with 292 DNA-A and 76 DNA-B sequences retrieved from GenBank, were analysed. The results indicate that the DNA-A and DNA-B respond differentially to evolutionary processes, with the DNA-B being more permissive to variation and more prone to recombination than the DNA-A. Although a clear geographic segregation was observed for both segments, differences in the genetic structure between DNA-A and DNA-B were also observed, with cognate segments belonging to distinct genetic clusters. DNA-B coding regions evolve under the same selection pressures than DNA-A coding regions. Together, our results indicate an interplay between reassortment and recombination acting at different levels across distinct subpopulations and segments.

Recombination and Positive Selection Differentially Shaped the Diversity of Betacoronavirus Subgenera

The Betacoronavirus genus of mammal-infecting viruses includes three subgenera (Sarbecovirus, Embecovirus, and Merbecovirus), in which most known human coronaviruses, including SARS-CoV-2, cluster. Coronaviruses are prone to host shifts, with recombination and positive selection possibly contributing to their high zoonotic potential. We analyzed the role of these two forces in the evolution of viruses belonging to the Betacoronavirus genus. The results showed that recombination has been pervasive during sarbecovirus evolution, and it is more widespread in this subgenus compared to the other two. In both sarbecoviruses and merbecoviruses, recombination hotspots are clearly observed. Conversely, positive selection was a less prominent force in sarbecoviruses compared to embecoviruses and merbecoviruses and targeted distinct genomic regions in the three subgenera, with S being the major target in sarbecoviruses alone. Overall, the results herein indicate that Betacoronavirus subgenera evolved along different trajectories, which might recapitulate their host preferences or reflect the origins of the presently available coronavirus sequences.

Engineering resistance against geminiviruses: A review of suppressed natural defenses and the use of RNAi and the CRISPR/Cas system

The Geminiviridae family is one of the most successful and largest families of plant viruses that infect a large variety of important dicotyledonous and monocotyledonous crops and cause significant yield losses worldwide. This broad spectrum of host range is only possible because geminiviruses have evolved sophisticated strategies to overcome the arsenal of antiviral defenses in such diverse plant species. In addition, geminiviruses evolve rapidly through recombination and pseudo-recombination to naturally create a great diversity of virus species with divergent genome sequences giving the virus an advantage over the host recognition system. Therefore, it is not surprising that efficient molecular strategies to combat geminivirus infection under open field conditions have not been fully addressed. In this review, we present the anti-geminiviral arsenal of plant defenses, the evolved virulence strategies of geminiviruses to overcome these plant defenses and the most recent strategies that have been engineered for transgenic resistance. Although, the in vitro reactivation of suppressed natural defenses as well as the use of RNAi and CRISPR/Cas systems hold the potential for achieving broad-range resistance and/or immunity, potential drawbacks have been associated with each case.

Polymorphism and natural selection in the merozoite surface protein 3F2 (PVX_97710) locus of Plasmodium vivax among field isolates

Plasmodium vivax, the chronic relapsing human malaria parasite with the most widespread distribution, possesses proteins associated with the merozoite surface that could be targets for host immune responses and potential vaccine candidates. Of these, the merozoite surface protein 3 of P. vivax (PvMSP3) is an attractive vaccine target as well as a genetic marker for epidemiological surveillance. PvMSP3 comprises a group of protein members encoded by a multigene family. Although some protein members, i.e. PvMSP3α and PvMSP3β, have been targets for molecular and immunological investigations, the most abundantly expressed protein member during late asexual erythrocytic stages, PvMSP3F2 (PVX_97710), remains unexplored. To address domain organization and evolution of this locus, the complete coding sequences of 31 P. vivax isolates from diverse malaria endemic areas of Thailand were analyzed and compared with 10 previously reported sequences. Results revealed that all PvMSP3F2 sequences differed but could be divided into 5 repeat-containing domains flanked by 6 non-repeat domains. Repeat domains II and IV at the 5' portion and domain X at the 3' portion exhibited extensive sequence and length variation whereas repeat domains VI and VIII located at the central region were relatively conserved. Despite a repertoire of PvMSP3F2 variants, predicted coiled-coil tertiary structure and predicted B-cell epitopes seem to be maintained. Evidence of intragenic recombination has been detected among field isolates in Thailand that could enhance sequence diversity at this locus. Non-repeat domains I and IX located at the 5' end and at the 3' portion, respectively, seem to have evolved under purifying selection. Evidence of positive selection was found in non-repeat domains III, V and VII where a number of predicted HLA class I epitopes were identified. Amino acid substitutions in these predicted epitopes could alter predicted peptide binding affinity or abolish peptide epitope property, suggesting that polymorphism in these epitopes conferred host immune evasion. Further studies on PvMSP3F2 are warranted, particularly on interaction with host immune system and the potential role of this PvMSP3 protein member as a vaccine target.

Life on the Edge: Geminiviruses at the Interface Between Crops and Wild Plant Hosts

Viruses constitute the largest group of emerging pathogens, and geminiviruses (plant viruses with circular, single-stranded DNA genomes) are the major group of emerging plant viruses. With their high potential for genetic variation due to mutation and recombination, their efficient spread by vectors, and their wide host range as a group, including both wild and cultivated hosts, geminiviruses are attractive models for the study of the evolutionary and ecological factors driving virus emergence. Studies on the epidemiological features of geminivirus diseases have traditionally focused primarily on crop plants. Nevertheless, knowledge of geminivirus infection in wild plants, and especially at the interface between wild and cultivated plants, is necessary to provide a complete view of their ecology, evolution, and emergence. In this review, we address the most relevant aspects of geminivirus variability and evolution in wild and crop plants and geminiviruses' potential to emerge in crops.

Evolutionary insights of Bean common mosaic necrosis virus and Cowpea aphid-borne mosaic virus

Plant viral diseases are one of the major limitations in legume production within sub-Saharan Africa (SSA), as they account for up to 100% in production losses within smallholder farms. In this study, field surveys were conducted in the western highlands of Kenya with viral symptomatic leaf samples collected. Subsequently, next-generation sequencing was carried out to gain insights into the molecular evolution and evolutionary relationships of Bean common mosaic necrosis virus (BCMNV) and Cowpea aphid-borne mosaic virus (CABMV) present within symptomatic common bean and cowpea. Eleven near-complete genomes of BCMNV and two for CABMV were obtained from western Kenya. Bayesian phylogenomic analysis and tests for differential selection pressure within sites and across tree branches of the viral genomes were carried out. Three well-supported clades in BCMNV and one supported clade for CABMNV were resolved and in agreement with individual gene trees. Selection pressure analysis within sites and across phylogenetic branches suggested both viruses were evolving independently, but under strong purifying selection, with a slow evolutionary rate. These findings provide valuable insights on the evolution of BCMNV and CABMV genomes and their relationship to other viral genomes globally. The results will contribute greatly to the knowledge gap involving the phylogenomic relationship of these viruses, particularly for CABMV, for which there are few genome sequences available, and inform the current breeding efforts towards resistance for BCMNV and CABMV.

Tomato Yellow Leaf Curl Sardinia Virus, a Begomovirus Species Evolving by Mutation and Recombination: A Challenge for Virus Control

The tomato leaf curl disease (TYLCD) is associated with infections of several species of begomoviruses (genus Begomovirus, family Geminiviridae) and causes severe damage to tomatoes throughout tropical and sub-tropical regions of the world. Among others, the Tomato yellow leaf curl Sardinia virus (TYLCSV) species causes damage in the Mediterranean Basin since early outbreaks occurred. Nevertheless, scarce information is available about the diversity of TYLCSV. Here, we study this aspect based on the sequence information accessible in databases. Isolates of two taxonomically differentiated TYLCSV strains can be found in natural epidemics. Their evolution is mostly associated with mutation combined with selection and random genetic drift and also with inter-species recombination which is frequent in begomoviruses. Moreover, a novel putative inter-strain recombinant is reported. Although no significantly new biological behaviour was observed for this latter recombinant, its occurrence supports that as shown for other related begomoviruses, recombination continues to play a central role in the evolution of TYLCD-associated viruses and the dynamism of their populations. The confrontation of resistant tomatoes with isolates of different TYLCD-associated viruses including the novel recombinant demonstrates the existence of a variable virus x plant genotype interaction. This has already been observed for other TYLCD-associated viruses and is a challenge for the control of their impact on tomato production.

Notes on recombination and reassortment in multipartite/segmented viruses

Besides evolving through nucleotide substitution, viruses frequently also evolve by genetic recombination which can occur when related viral variants co-infect the same cells. Viruses with segmented or multipartite genomes can additionally evolve via the reassortment of genomic components. Various computational techniques are now available for identifying and characterizing recombination and reassortment. While these techniques have revealed both that all well studied segmented and multipartite virus species show some capacity for reassortment, and that recombination is common in many multipartite species, they have indicated that recombination is either rare or does not occur in species with segmented genomes. Reassortment and recombination can make it very difficult to study segmented/multipartite viruses using metagenomics-based approaches. Notable challenges include, both the accurate identification and assignment of genomic components to individual genomes, and the differentiation between natural 'real' recombination events and artifactual 'fake' recombination events arising from the inaccurate de novo assembly of genome component sequences determined using short read sequencing.

Phylogenomic relationship and evolutionary insights of sweet potato viruses from the western highlands of Kenya

Sweet potato is a major food security crop within sub-Saharan Africa where 90% of Africa production occurs. One of the major limitations of sweet potato production are viral infections. In this study, we used a combination of whole genome sequences from a field isolate obtained from Kenya and those available in GenBank. Sequences of four sweet potato viruses: Sweet potato feathery mottle virus (SPFMV), Sweet potato virus C (SPVC), Sweet potato chlorotic stunt virus (SPCSV), Sweet potato chlorotic fleck virus (SPCFV) were obtained from the Kenyan sample. SPFMV sequences both from this study and from GenBank were found to be recombinant. Recombination breakpoints were found within the Nla-Pro, coat protein and P1 genes. The SPCSV, SPVC, and SPCFV viruses from this study were non-recombinant. Bayesian phylogenomic relationships across whole genome trees showed variation in the number of well-supported clades; within SPCSV (RNA1 and RNA2) and SPFMV two well-supported clades (I and II) were resolved. The SPCFV tree resolved three well-supported clades (I-III) while four well-supported clades were resolved in SPVC (I-IV). Similar clades were resolved within the coalescent species trees. However, there were disagreements between the clades resolved in the gene trees compared to those from the whole genome tree and coalescent species trees. However the coat protein gene tree of SPCSV and SPCFV resolved similar clades to the genome and coalescent species tree while this was not the case in SPFMV and SPVC. In addition, we report variation in selective pressure within sites of individual genes across all four viruses; overall all viruses were under purifying selection. We report the first complete genomes of SPFMV, SPVC, SPCFV, and a partial SPCSV from Kenya as a mixed infection in one sample. Our findings provide a snap shot on the evolutionary relationship of sweet potato viruses (SPFMV, SPVC, SPCFV, and SPCSV) from Kenya as well as assessing whether selection pressure has an effect on their evolution.

A guide to the contained use of plant virus infectious clones

Plant virus infectious clones are important tools with wide-ranging applications in different areas of biology and medicine. Their uses in plant pathology include the study of plant-virus interactions, and screening of germplasm as part of prebreeding programmes for virus resistance. They can also be modified to induce transient plant gene silencing (Virus Induced Gene Silencing - VIGS) and as expression vectors for plant or exogenous proteins, with applications in both plant pathology and more generally for the study of plant gene function. Plant viruses are also increasingly being investigated as expression vectors for in planta production of pharmaceutical products, known as molecular farming. However, plant virus infectious clones may pose a risk to the environment due to their ability to reconstitute fully functional, transmissible viruses. These risks arise from both their inherent pathogenicity and the effect of any introduced genetic modifications. Effective containment measures are therefore required. There has been no single comprehensive review of the biosafety considerations for the contained use of genetically modified plant viruses, despite their increasing importance across many biological fields. This review therefore explores the biosafety considerations for working with genetically modified plant viruses in contained environments, with focus on plant growth facilities. It includes regulatory frameworks, risk assessment, assignment of biosafety levels, facility features and working practices. The review is based on international guidance together with information provided by plant virus researchers.

Rolling circle amplification-based analysis of Sri Lankan cassava mosaic virus isolates from Tamil Nadu, India, suggests a low level of genetic variability

Cassava mosaic disease is a widespread disease of cassava in south Asia and the African continent. In India, CMD is known to be caused by two single-stranded DNA viruses (geminiviruses), Indian cassava mosaic virus (ICMV) and Sri Lankan cassava mosdaic virus (SLCMV). Previously, the diversity of ICMV and SLCMV in India has been studied using PCR, a sequence-dependent method. To have a more in-depth study of the variability of the above viruses and to detect any novel geminiviruses associated with CMD, sequence-independent amplification using rolling circle amplification (RCA)-based methods were used. CMD affected cassava plants were sampled across eighty locations in nine districts of the southern Indian state of Tamil Nadu. Twelve complete sequence of coat protein genes of the resident geminiviruses, comprising 256 amino acid residues were generated from the above samples, which indicated changes at only six positions. RCA followed by RFLP of the 80 samples indicated that most samples (47) contained only SLCMV, followed by 8, which were infected jointly with ICMV and SLCMV. In 11 samples, the pattern did not match the expected patterns from either of the two viruses and hence, were variants. Sequence analysis of an average of 700 nucleotides from 31 RCA-generated fragments of the variants indicated identities of 97-99% with the sequence of a previously reported infectious clone of SLCMV. The evidence suggests low levels of genetic variability in the begomoviruses infecting cassava, mainly in the form of scattered single nucleotide changes.

The diversification of begomovirus populations is predominantly driven by mutational dynamics

Begomoviruses (single-stranded DNA plant viruses) are responsible for serious agricultural threats. Begomovirus populations exhibit a high degree of within-host genetic variation and evolve as quickly as RNA viruses. Although the recombination-prone nature of begomoviruses has been extensively demonstrated, the relative contribution of recombination and mutation to the genetic variation of begomovirus populations has not been assessed. We estimated the genetic variability of begomovirus datasets from around the world. An uneven distribution of genetic variation across the length of the cp and rep genes due to recombination was evident from our analyses. To estimate the relative contributions of recombination and mutation to the genetic variability of begomoviruses, we mapped all substitutions over maximum likelihood trees and counted the number of substitutions on branches which were associated with recombination (η_r) and mutation (η_μ). In addition, we also estimated the per generation relative rates of both evolutionary mechanisms (r/μ) to express how frequently begomovirus genomes are affected by recombination relative to mutation. We observed that the composition of genetic variation in all begomovirus datasets was dominated by mutation. Additionally, the low correlation between the estimates indicated that the relative contributions of recombination and mutation are not necessarily a function of their relative rates. Our results show that, although a considerable fraction of the genetic variation levels could be assigned to recombination, it was always lower than that due to mutation, indicating that the diversification of begomovirus populations is predominantly driven by mutational dynamics.

Genomic differentiation among wild cyanophages despite widespread horizontal gene transfer

Background

Genetic recombination is a driving force in genome evolution. Among viruses it has a dual role. For genomes with higher fitness, it maintains genome integrity in the face of high mutation rates. Conversely, for genomes with lower fitness, it provides immediate access to sequence space that cannot be reached by mutation alone. Understanding how recombination impacts the cohesion and dissolution of individual whole genomes within viral sequence space is poorly understood across double-stranded DNA bacteriophages (a.k.a phages) due to the challenges of obtaining appropriately scaled genomic datasets.

Results

Here we explore the role of recombination in both maintaining and differentiating whole genomes of 142 wild double-stranded DNA marine cyanophages. Phylogenomic analysis across the 51 core genes revealed ten lineages, six of which were well represented. These phylogenomic lineages represent discrete genotypic populations based on comparisons of intra- and inter- lineage shared gene content, genome-wide average nucleotide identity, as well as detected gaps in the distribution of pairwise differences between genomes. McDonald-Kreitman selection tests identified putative niche-differentiating genes under positive selection that differed across the six well-represented genotypic populations and that may have driven initial divergence. Concurrent with patterns of recombination of discrete populations, recombination analyses of both genic and intergenic regions largely revealed decreased genetic exchange across individual genomes between relative to within populations.

Conclusions

These findings suggest that discrete double-stranded DNA marine cyanophage populations occur in nature and are maintained by patterns of recombination akin to those observed in bacteria, archaea and in sexual eukaryotes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3286-x) contains supplementary material, which is available to authorized users.

Divergent evolutionary and epidemiological dynamics of cassava mosaic geminiviruses in Madagascar

BACKGROUND

Cassava mosaic disease (CMD) in Madagascar is caused by a complex of at least six African cassava mosaic geminivirus (CMG) species. This provides a rare opportunity for a comparative study of the evolutionary and epidemiological dynamics of distinct pathogenic crop-infecting viral species that coexist within the same environment. The genetic and spatial structure of CMG populations in Madagascar was studied and Bayesian phylogeographic modelling was applied to infer the origins of Madagascan CMG populations within the epidemiological context of related populations situated on mainland Africa and other south western Indian Ocean (SWIO) islands.

RESULTS

The isolation and analysis of 279 DNA-A and 117 DNA-B sequences revealed the presence in Madagascar of four prevalent CMG species (South African cassava mosaic virus, SACMV; African cassava mosaic virus, ACMV; East African cassava mosaic Kenya virus, EACMKV; and East African cassava mosaic Cameroon virus, EACMCV), and of numerous CMG recombinants that have, to date, only ever been detected on this island. SACMV and ACMV, the two most prevalent viruses, displayed low degrees of genetic diversity and have most likely been introduced to the island only once. By contrast, EACMV-like CMG populations (consisting of East African cassava mosaic virus, EAMCKV, EACMCV and complex recombinants of these) were more diverse, more spatially structured, and displayed evidence of at least three independent introductions from mainland Africa. Although there were no statistically supported virus movement events between Madagascar and the other SWIO islands, at least one mainland African ACMV variant likely originated in Madagascar.

CONCLUSIONS

Our study highlights both the complexity of CMD in Madagascar, and the distinct evolutionary and spatial dynamics of the different viral species that collectively are associated with this disease. Given that more distinct CMG species and recombinants have been found in Madagascar than any other similarly sized region of the world, the risks of recombinant CMG variants emerging on this island are likely to be higher than elsewhere. Evidence of an epidemiological link between Madagascan and mainland African CMGs suggests that the consequences of such emergence events could reach far beyond the shores of this island.

INTEGRATING PARASITES AND PATHOGENS INTO THE STUDY OF GEOGRAPHIC RANGE LIMITS

The geographic distributions of all species are limited, and the determining factors that set these limits are of fundamental importance to the fields of ecology and evolutionary biology. Plant and animal ranges have been of primary concern, while those of parasites, which represent much of the Earth's biodiversity, have been neglected. Here, we review the determinants of the geographic ranges of parasites and pathogens, and explore how parasites provide novel systems with which to investigate the ecological and evolutionary processes governing host/parasite spatial distributions. Although there is significant overlap in the causative factors that determine range borders of parasites and free-living species, parasite distributions are additionally constrained by the geographic range and ecology of the host species' population, as well as by evolutionary factors that promote host-parasite coevolution. Recently, parasites have been used to infer population demographic and ecological information about their host organisms and we conclude that this strategy can be further exploited to understand geographic range limitations of both host and parasite populations.

RNA Silencing May Play a Role in but Is Not the Only Determinant of the Multiplicity of Infection

The multiplicity of infection (MOI), i.e., the number of viral genomes that infect a cell, is an important parameter in virus evolution, which for each virus and environment may have an optimum value that maximizes virus fitness. Thus, the MOI might be controlled by virus functions, an underexplored hypothesis in eukaryote-infecting viruses. To analyze if the MOI is controlled by virus functions, we estimated the MOI in plants coinfected by two genetic variants of Tomato bushy stunt virus (TBSV); by TBSV and a TBSV-derived defective interfering RNA (DI-RNA); or by TBSV and a second tombusvirus, Cymbidium ringspot virus (CymRSV). The MOI was significantly larger in TBSV-CymRSV coinfections (~4.0) than in TBSV-TBSV or TBSV-DI-RNA coinfections (~1.7 to 2.2). Coinfections by CymRSV or TBSV with chimeras in which an open reading frame (ORF) of one virus species was replaced by that of the other identified a role of viral proteins in determining the MOI, which ranged from 1.6 to 3.9 depending on the coinfecting genotypes. However, no virus-encoded protein or genomic region was the sole MOI determinant. Coinfections by CymRSV and TBSV mutants in which the expression of the gene-silencing suppressor protein p19 was abolished also showed a possible role of gene silencing in MOI determination. Taken together, these results demonstrate that the MOI is a quantitative trait showing continuous variation and that as such it has a complex determination involving different virus-encoded functions.

IMPORTANCE

The number of viral genomes infecting a cell, or the multiplicity of infection (MOI), is an important parameter in virus evolution affecting recombination rates, selection intensity on viral genes, evolution of multipartite genomes, or hyperparasitism by satellites or defective interfering particles. For each virus and environment, the MOI may have an optimum value that maximizes virus fitness, but little is known about MOI control in eukaryote-infecting viruses. We show here that in plants coinfected by two genotypes of Tomato bushy stunt virus (TBSV), the MOI was lower than in plants coinfected by TBSV and Cymbidium ringspot virus (CymRSV). Coinfections by CymRSV or TBSV with TBSV-CymRSV chimeras showed a role of viral proteins in MOI determination. Coinfections by CymRSV and TBSV mutants not expressing the gene-silencing suppressor protein also showed a role of gene silencing in MOI determination. The results demonstrate that the MOI is a quantitative trait with a complex determination involving different viral functions.

Genetic Consequences of Antiviral Therapy on HIV-1

A variety of enzyme inhibitors have been developed in combating HIV-1, however the fast evolutionary rate of this virus commonly leads to the emergence of resistance mutations that finally allows the mutant virus to survive. This review explores the main genetic consequences of HIV-1 molecular evolution during antiviral therapies, including the viral genetic diversity and molecular adaptation. The role of recombination in the generation of drug resistance is also analyzed. Besides the investigation and discussion of published works, an evolutionary analysis of protease-coding genes collected from patients before and after treatment with different protease inhibitors was included to validate previous studies. Finally, the review discusses the importance of considering genetic consequences of antiviral therapies in models of HIV-1 evolution that could improve current genotypic resistance testing and treatments design.

Contrasting genetic structure between two begomoviruses infecting the same leguminous hosts

Begomoviruses are whitefly-transmitted, ssDNA plant viruses and are among the most damaging pathogens causing epidemics in economically important crops worldwide. Wild/non-cultivated plants play a crucial epidemiological role, acting as begomovirus reservoirs and as ‘mixing vessels' where recombination can occur. Previous work suggests a higher degree of genetic variability in begomovirus populations from non-cultivated hosts compared with cultivated hosts. To assess this supposed host effect on the genetic variability of begomovirus populations, cultivated (common bean, Phaseolus vulgaris, and lima bean, Phaseolus lunatus) and non-cultivated (Macroptilium lathyroides) legume hosts were sampled from two regions of Brazil. A total of 212 full-length DNA-A genome segments were sequenced from samples collected between 2005 and 2012, and populations of the begomoviruses Bean golden mosaic virus (BGMV) and Macroptilium yellow spot virus (MaYSV) were obtained. We found, for each begomovirus species, similar genetic variation between populations infecting cultivated and non-cultivated hosts, indicating that the presumed genetic variability of the host did not a priori affect viral variability. We observed a higher degree of genetic variation in isolates from MaYSV populations than BGMV populations, which was explained by numerous recombination events in MaYSV. MaYSV and BGMV showed distinct distributions of genetic variation, with the BGMV population (but not MaYSV) being structured by both host and geography.

Split personality of a Potyvirus: to specialize or not to specialize?

Bean yellow mosaic virus (BYMV), genus Potyvirus, has an extensive natural host range encompassing both dicots and monocots. Its phylogenetic groups were considered to consist of an ancestral generalist group and six specialist groups derived from this generalist group during plant domestication. Recombination was suggested to be playing a role in BYMV's evolution towards host specialization. However, in subsequent phylogenetic analysis of whole genomes, group names based on the original hosts of isolates within each of them were no longer supported. Also, nine groups were found and designated I-IX. Recombination analysis was conducted on the complete coding regions of 33 BYMV genomes and two genomes of the related Clover yellow vein virus (CYVV). This analysis found evidence for 12 firm recombination events within BYMV phylogenetic groups I-VI, but none within groups VII-IX or CYVV. The greatest numbers of recombination events within a sequence (two or three each) occurred in four groups, three which formerly constituted the single ancestral generalist group (I, II and IV), and group VI. The individual sequences in groups III and V had one event each. These findings with whole genomes are consistent with recombination being associated with expanding host ranges, and call into question the proposed role of recombination in the evolution of BYMV, where it was previously suggested to play a role in host specialization. Instead, they (i) indicate that recombination explains the very broad natural host ranges of the three BYMV groups which infect both monocots and dicots (I, II, IV), and (ii) suggest that the three groups with narrow natural host ranges (III, V, VI) which also showed recombination now have the potential to reduce host specificity and broaden their natural host ranges.

Extensive recombination-induced disruption of genetic interactions is highly deleterious but can be partially reversed by small numbers of secondary recombination events

Although homologous recombination can potentially provide viruses with vastly more evolutionary options than are available through mutation alone, there are considerable limits on the adaptive potential of this important evolutionary process. Primary among these is the disruption of favorable coevolved genetic interactions that can occur following the transfer of foreign genetic material into a genome. Although the fitness costs of such disruptions can be severe, in some cases they can be rapidly recouped by either compensatory mutations or secondary recombination events. Here, we used a maize streak virus (MSV) experimental model to explore both the extremes of recombination-induced genetic disruption and the capacity of secondary recombination to adaptively reverse almost lethal recombination events. Starting with two naturally occurring parental viruses, we synthesized two of the most extreme conceivable MSV chimeras, each effectively carrying 182 recombination breakpoints and containing thorough reciprocal mixtures of parental polymorphisms. Although both chimeras were severely defective and apparently noninfectious, neither had individual movement-, encapsidation-, or replication-associated genome regions that were on their own "lethally recombinant." Surprisingly, mixed inoculations of the chimeras yielded symptomatic infections with viruses with secondary recombination events. These recombinants had only 2 to 6 breakpoints, had predominantly inherited the least defective of the chimeric parental genome fragments, and were obviously far more fit than their synthetic parents. It is clearly evident, therefore, that even when recombinationally disrupted virus genomes have extremely low fitness and there are no easily accessible routes to full recovery, small numbers of secondary recombination events can still yield tremendous fitness gains. Importance: Recombination between viruses can generate strains with enhanced pathological properties but also runs the risk of producing hybrid genomes with decreased fitness due to the disruption of favorable genetic interactions. Using two synthetic maize streak virus genome chimeras containing alternating genome segments derived from two natural viral strains, we examined both the fitness costs of extreme degrees of recombination (both chimeras had 182 recombination breakpoints) and the capacity of secondary recombination events to recoup these costs. After the severely defective chimeras were introduced together into a suitable host, viruses with between 1 and 3 secondary recombination events arose, which had greatly increased replication and infective capacities. This indicates that even in extreme cases where recombination-induced genetic disruptions are almost lethal, and 91 consecutive secondary recombination events would be required to reconstitute either one of the parental viruses, moderate degrees of fitness recovery can be achieved through relatively small numbers of secondary recombination events.

Patterns of recombination in HIV-1M are influenced by selection disfavouring the survival of recombinants with disrupted genomic RNA and protein structures

Genetic recombination is a major contributor to the ongoing diversification of HIV. It is clearly apparent that across the HIV-genome there are defined recombination hot and cold spots which tend to co-localise both with genomic secondary structures and with either inter-gene boundaries or intra-gene domain boundaries. There is also good evidence that most recombination breakpoints that are detectable within the genes of natural HIV recombinants are likely to be minimally disruptive of intra-protein amino acid contacts and that these breakpoints should therefore have little impact on protein folding. Here we further investigate the impact on patterns of genetic recombination in HIV of selection favouring the maintenance of functional RNA and protein structures. We confirm that chimaeric Gag p24, reverse transcriptase, integrase, gp120 and Nef proteins that are expressed by natural HIV-1 recombinants have significantly lower degrees of predicted folding disruption than randomly generated recombinants. Similarly, we use a novel single-stranded RNA folding disruption test to show that there is significant, albeit weak, evidence that natural HIV recombinants tend to have genomic secondary structures that more closely resemble parental structures than do randomly generated recombinants. These results are consistent with the hypothesis that natural selection has acted both in the short term to purge recombinants with disrupted RNA and protein folds, and in the longer term to modify the genome architecture of HIV to ensure that recombination prone sites correspond with those where recombination will be minimally deleterious.

Identifying geographic hot spots of reassortment in a multipartite plant virus

Reassortment between different species or strains plays a key role in the evolution of multipartite plant viruses and can have important epidemiological implications. Identifying geographic locations where reassortant lineages are most likely to emerge could be a valuable strategy for informing disease management and surveillance efforts. We developed a predictive framework to identify potential geographic hot spots of reassortment based upon spatially explicit analyses of genome constellation diversity. To demonstrate the utility of this approach, we examined spatial variation in the potential for reassortment among Cardamom bushy dwarf virus (CBDV; Nanoviridae, Babuvirus) isolates in Northeast India. Using sequence data corresponding to six discrete genome components for 163 CBDV isolates, a quantitative measure of genome constellation diversity was obtained for locations across the sampling region. Two key areas were identified where viruses with highly distinct genome constellations cocirculate, and these locations were designated as possible geographic hot spots of reassortment, where novel reassortant lineages could emerge. Our study demonstrates that the potential for reassortment can be spatially dependent in multipartite plant viruses and highlights the use of evolutionary analyses to identify locations which could be actively managed to facilitate the prevention of outbreaks involving novel reassortant strains.

Estimation of the in vivo recombination rate for a plant RNA virus

Phylogenomic evidence suggested that recombination is an important evolutionary force for potyviruses, one of the larger families of plant RNA viruses. However, mixed-genotype potyvirus infections are marked by low levels of cellular coinfection, precluding template switching and recombination events between virus genotypes during genomic RNA replication. To reconcile these conflicting observations, we evaluated the in vivo recombination rate (rg) of Tobacco etch virus (TEV; genus Potyvirus, family Potyviridae) by coinfecting plants with pairs of genotypes marked with engineered restriction sites as neutral markers. The recombination rate was then estimated using two different approaches: (i) a classical approach that assumed recombination between marked genotypes can occur in the whole virus population, rendering an estimate of rg = 7.762 × 10(-8) recombination events per nucleotide site per generation, and (ii) an alternative method that assumed recombination between marked genotypes can occur only in coinfected cells, rendering a much higher estimate of rg = 3.427 × 10(-5) recombination events per nucleotide site per generation. This last estimate is similar to the TEV mutation rate, suggesting that recombination should be at least as important as point mutation in creating variability. Finally, we compared our mutation and recombination rate estimates to those reported for animal RNA viruses. Our analysis suggested that high recombination rates may be an unavoidable consequence of selection for fast replication at the cost of low fidelity.

Selection pressure and founder effects constrain genetic variation in differentiated populations of soilborne bymovirus Wheat yellow mosaic virus (Potyviridae) in China

To study the population genetic structure and forces driving the evolution of Wheat yellow mosaic virus (WYMV), the nucleotide sequences encoding the coat protein (CP) (297 sequences) or the genome-linked virion protein (VPg) (87 sequences) were determined from wheat plants growing at 11 different locations distributed in five provinces in China. There were close phylogenetic relationships between all sequences but clustering on the phylogenetic trees was congruent with their provenance, suggesting an origin-dependent population genetic structure. There were low levels of genetic diversity, ranging from 0.00035 ± 0.00019 to 0.01536 ± 0.00043 (CP), and 0.00086 ± 0.00039 to 0.00573 ± 0.00111 (VPg), indicating genetic stability or recent emergence of WYMV in China. The results may suggest that founder effects play a role in shaping the genetic structure of WYMV. Between-population diversity was consistently higher than within-population diversity, suggesting limited gene flow between subpopulations (average FST 0.6241 for the CP and 0.7981 for the VPg). Consistent amino acid substitutions correlated with the provenance of the sequences were observed at nine positions in the CP (but none in the VPg), indicating an advanced stage in population structuring. Strong negative (purifying) selection was implicated on both the CP and VPg but positive selection on a few codons in the CP, indicating an ongoing molecular adaptation.

Synonymous site variation due to recombination explains higher genetic variability in begomovirus populations infecting non-cultivated hosts

Begomoviruses are ssDNA plant viruses that cause serious epidemics in economically important crops worldwide. Non-cultivated plants also harbour many begomoviruses, and it is believed that these hosts may act as reservoirs and as mixing vessels where recombination may occur. Begomoviruses are notoriously recombination-prone, and also display nucleotide substitution rates equivalent to those of RNA viruses. In Brazil, several indigenous begomoviruses have been described infecting tomatoes following the introduction of a novel biotype of the whitefly vector in the mid-1990s. More recently, a number of viruses from non-cultivated hosts have also been described. Previous work has suggested that viruses infecting non-cultivated hosts have a higher degree of genetic variability compared with crop-infecting viruses. We intensively sampled cultivated and non-cultivated plants in similarly sized geographical areas known to harbour either the weed-infecting Macroptilium yellow spot virus (MaYSV) or the crop-infecting Tomato severe rugose virus (ToSRV), and compared the molecular evolution and population genetics of these two distantly related begomoviruses. The results reinforce the assertion that infection of non-cultivated plant species leads to higher levels of standing genetic variability, and indicate that recombination, not adaptive selection, explains the higher begomovirus variability in non-cultivated hosts.

Experimental measures of pathogen competition and relative fitness

Competition among pathogen strains for limited host resources can have a profound effect on pathogen evolution. A better understanding of the principles and consequences of competition can be useful in designing more sustainable disease management strategies. The competitive ability and relative fitness of a pathogen strain are determined by its intrinsic biological properties, the resistance and heterogeneity of the corresponding host population, the population density and genetic relatedness of the competing strains, and the physical environment. Competitive ability can be inferred indirectly from fitness components, such as basic reproduction rate or transmission rate. However, pathogen strains that exhibit higher fitness components when they infect a host alone may not exhibit a competitive advantage when they co-infect the same host. The most comprehensive measures of competitive ability and relative fitness come from calculating selection coefficients in a mixed infection in a field setting. Mark-release-recapture experiments can be used to estimate fitness costs associated with unnecessary virulence and fungicide resistance.

Reproductive clonality of pathogens: a perspective on pathogenic viruses, bacteria, fungi, and parasitic protozoa

We propose that clonal evolution in micropathogens be defined as restrained recombination on an evolutionary scale, with genetic exchange scarce enough to not break the prevalent pattern of clonal population structure, a definition already widely used for all kinds of pathogens, although not clearly formulated by many scientists and rejected by others. The two main manifestations of clonal evolution are strong linkage disequilibrium (LD) and widespread genetic clustering ("near-clading"). We hypothesize that this pattern is not mainly due to natural selection, but originates chiefly from in-built genetic properties of pathogens, which could be ancestral and could function as alternative allelic systems to recombination genes ("clonality/sexuality machinery") to escape recombinational load. The clonal framework of species of pathogens should be ascertained before any analysis of biomedical phenotypes (phylogenetic character mapping). In our opinion, this model provides a conceptual framework for the population genetics of any micropathogen.

Genome dynamics in three different geographical isolates of white spot syndrome virus (WSSV)

White spot syndrome virus (WSSV), the sole member of the monotypic family Nimaviridae, is considered an extremely lethal shrimp pathogen. Despite its impact, some essential biological characteristics related to WSSV genome dynamics, such as the synonymous codon usage pattern and selection pressure in genes, remain to be elucidated. The results show that compositional limitations and mutational pressure determine the codon usage bias and base composition in WSSV. Furthermore, different forces of selective pressure are acting across various regions of the WSSV genome. Finally, this study points out the possible occurrence of two major recombination events.

Inversion and crossover recombination contributions to the spacing between two functionally linked genes

The roles of inversion and crossover recombination in determining the spacing between two functionally linked genes on an individual strand of DNA and the resulting genetic organization throughout the population is not well understood. We employ a computer simulation to look at the spacing between functionally linked genes after many generations of a population of haploid individuals, each with a single chromosome. Simulations show that inversion and crossover recombination combine to create four attractors in gene spacing. The two major attractors include one in which the linked genes are forced to be near each other and one in which the linked genes are forced to be separated by one third of the chromosome length. Multiplicative functional linkage between two linked genes also causes a decreased average spacing compared to additive and random functional linkage.

First dating of a recombination event in mammalian tick-borne flaviviruses

The mammalian tick-borne flavivirus group (MTBFG) contains viruses associated with important human and animal diseases such as encephalitis and hemorrhagic fever. In contrast to mosquito-borne flaviviruses where recombination events are frequent, the evolutionary dynamic within the MTBFG was believed to be essentially clonal. This assumption was challenged with the recent report of several homologous recombinations within the Tick-borne encephalitis virus (TBEV). We performed a thorough analysis of publicly available genomes in this group and found no compelling evidence for the previously identified recombinations. However, our results show for the first time that demonstrable recombination (i.e., with large statistical support and strong phylogenetic evidences) has occurred in the MTBFG, more specifically within the Louping ill virus lineage. Putative parents, recombinant strains and breakpoints were further tested for statistical significance using phylogenetic methods. We investigated the time of divergence between the recombinant and parental strains in a Bayesian framework. The recombination was estimated to have occurred during a window of 282 to 76 years before the present. By unravelling the temporal setting of the event, we adduce hypotheses about the ecological conditions that could account for the observed recombination.

Recombination hotspots and host susceptibility modulate the adaptive value of recombination during maize streak virus evolution

BACKGROUND

Maize streak virus -strain A (MSV-A; Genus Mastrevirus, Family Geminiviridae), the maize-adapted strain of MSV that causes maize streak disease throughout sub-Saharan Africa, probably arose between 100 and 200 years ago via homologous recombination between two MSV strains adapted to wild grasses. MSV recombination experiments and analyses of natural MSV recombination patterns have revealed that this recombination event entailed the exchange of the movement protein - coat protein gene cassette, bounded by the two genomic regions most prone to recombination in mastrevirus genomes; the first surrounding the virion-strand origin of replication, and the second around the interface between the coat protein gene and the short intergenic region. Therefore, aside from the likely adaptive advantages presented by a modular exchange of this cassette, these specific breakpoints may have been largely predetermined by the underlying mechanisms of mastrevirus recombination. To investigate this hypothesis, we constructed artificial, low-fitness, reciprocal chimaeric MSV genomes using alternating genomic segments from two MSV strains; a grass-adapted MSV-B, and a maize-adapted MSV-A. Between them, each pair of reciprocal chimaeric genomes represented all of the genetic material required to reconstruct - via recombination - the highly maize-adapted MSV-A genotype, MSV-MatA. We then co-infected a selection of differentially MSV-resistant maize genotypes with pairs of reciprocal chimaeras to determine the efficiency with which recombination would give rise to high-fitness progeny genomes resembling MSV-MatA.

RESULTS

Recombinants resembling MSV-MatA invariably arose in all of our experiments. However, the accuracy and efficiency with which the MSV-MatA genotype was recovered across all replicates of each experiment depended on the MSV susceptibility of the maize genotypes used and the precise positions - in relation to known recombination hotspots - of the breakpoints required to re-create MSV-MatA. Although the MSV-sensitive maize genotype gave rise to the greatest variety of recombinants, the measured fitness of each of these recombinants correlated with their similarity to MSV-MatA.

CONCLUSIONS

The mechanistic predispositions of different MSV genomic regions to recombination can strongly influence the accessibility of high-fitness MSV recombinants. The frequency with which the fittest recombinant MSV genomes arise also correlates directly with the escalating selection pressures imposed by increasingly MSV-resistant maize hosts.

Complex recombination patterns arising during geminivirus coinfections preserve and demarcate biologically important intra-genome interaction networks

Genetic recombination is an important process during the evolution of many virus species and occurs particularly frequently amongst begomoviruses in the single stranded DNA virus family, Geminiviridae. As in many other recombining viruses it is apparent that non-random recombination breakpoint distributions observable within begomovirus genomes sampled from nature are the product of variations both in basal recombination rates across genomes and in the over-all viability of different recombinant genomes. Whereas factors influencing basal recombination rates might include local degrees of sequence similarity between recombining genomes, nucleic acid secondary structures and genomic sensitivity to nuclease attack or breakage, the viability of recombinant genomes could be influenced by the degree to which their co-evolved protein-protein and protein-nucleotide and nucleotide-nucleotide interactions are disreputable by recombination. Here we investigate patterns of recombination that occur over 120 day long experimental infections of tomato plants with the begomoviruses Tomato yellow leaf curl virus and Tomato leaf curl Comoros virus. We show that patterns of sequence exchange between these viruses can be extraordinarily complex and present clear evidence that factors such as local degrees of sequence similarity but not genomic secondary structure strongly influence where recombination breakpoints occur. It is also apparent from our experiment that over-all patterns of recombination are strongly influenced by selection against individual recombinants displaying disrupted intra-genomic interactions such as those required for proper protein and nucleic acid folding. Crucially, we find that selection favoring the preservation of co-evolved longer-range protein-protein and protein DNA interactions is so strong that its imprint can even be used to identify the exact sequence tracts involved in these interactions.

Genetic recombination between viruses is a form of parasexual reproduction during which two parental viruses each contribute genetic information to an offspring, or recombinant, virus. Unlike with sexual reproduction, however, recombination in viruses can even involve the transfer of sequences between the members of distantly related species. When parental genomes are very distantly related, it is anticipated that recombination between them runs the risk of producing defective offspring. The reason for this is that the interactions between different parts of genomes and the proteins they encode (such as between different viral proteins or between viral proteins and the virus genomic DNA or RNA) often depend on particular co-evolved binding sites that recognize one another. When in a recombinant genome the partners in a binding site pair are each inherited from different parents there is a possibility that they will not interact with one another properly. Here we examine recombinant genomes arising during experimental mixed infections of two distantly related viruses to detect evidence that intra-genome interaction networks are broadly preserved in these genomes. We show this preservation is so strict that patterns of recombination in these viruses can even be used to identify the interacting regions within their genomes.

Recombination in eukaryotic single stranded DNA viruses

Although single stranded (ss) DNA viruses that infect humans and their domesticated animals do not generally cause major diseases, the arthropod borne ssDNA viruses of plants do, and as a result seriously constrain food production in most temperate regions of the world. Besides the well known plant and animal-infecting ssDNA viruses, it has recently become apparent through metagenomic surveys of ssDNA molecules that there also exist large numbers of other diverse ssDNA viruses within almost all terrestrial and aquatic environments. The host ranges of these viruses probably span the tree of life and they are likely to be important components of global ecosystems. Various lines of evidence suggest that a pivotal evolutionary process during the generation of this global ssDNA virus diversity has probably been genetic recombination. High rates of homologous recombination, non-homologous recombination and genome component reassortment are known to occur within and between various different ssDNA virus species and we look here at the various roles that these different types of recombination may play, both in the day-to-day biology, and in the longer term evolution, of these viruses. We specifically focus on the ecological, biochemical and selective factors underlying patterns of genetic exchange detectable amongst the ssDNA viruses and discuss how these should all be considered when assessing the adaptive value of recombination during ssDNA virus evolution.

Distribution of the phenotypic effects of random homologous recombination between two virus species

Recombination has an evident impact on virus evolution and emergence of new pathotypes, and has generated an immense literature. However, the distribution of phenotypic effects caused by genome-wide random homologous recombination has never been formally investigated. Previous data on the subject have promoted the implicit view that most viral recombinant genomes are likely to be deleterious or lethal if the nucleotide identity of parental sequences is below 90%. We decided to challenge this view by creating a bank of near-random recombinants between two viral species of the genus Begomovirus (Family Geminiviridae) exhibiting 82% nucleotide identity, and by testing infectivity and in planta accumulation of recombinant clones randomly extracted from this bank. The bank was created by DNA-shuffling-a technology initially applied to the random shuffling of individual genes, and here implemented for the first time to shuffle full-length viral genomes. Together with our previously described system allowing the direct cloning of full-length infectious geminivirus genomes, it provided a unique opportunity to generate hundreds of "mosaic" virus genomes, directly testable for infectivity. A subset of 47 randomly chosen recombinants was sequenced, individually inoculated into tomato plants, and compared with the parental viruses. Surprisingly, our results showed that all recombinants were infectious and accumulated at levels comparable or intermediate to that of the parental clones. This indicates that, in our experimental system, despite the fact that the parental genomes differ by nearly 20%, lethal and/or large deleterious effects of recombination are very rare, in striking contrast to the common view that has emerged from previous studies published on other viruses.

Genetic diversity of symbiotic Bradyrhizobium elkanii populations recovered from inoculated and non-inoculated Acacia mangium field trials in Brazil

Acacia mangium is a legume tree native to Australasia. Since the eighties, it has been introduced into many tropical countries, especially in a context of industrial plantations. Many field trials have been set up to test the effects of controlled inoculation with selected symbiotic bacteria versus natural colonization with indigenous strains. In the introduction areas, A. mangium trees spontaneously nodulate with local and often ineffective bacteria. When inoculated, the persistence of inoculants and possible genetic recombination with local strains remain to be explored. The aim of this study was to describe the genetic diversity of bacteria spontaneously nodulating A. mangium in Brazil and to evaluate the persistence of selected strains used as inoculants. Three different sites, several hundred kilometers apart, were studied, with inoculated and non-inoculated plots in two of them. Seventy-nine strains were isolated from nodules and sequenced on three housekeeping genes (glnII, dnaK and recA) and one symbiotic gene (nodA). All but one of the strains belonged to the Bradyrhizobium elkanii species. A single case of housekeeping gene transfer was detected among the 79 strains, suggesting an extremely low rate of recombination within B. elkanii, whereas the nodulation gene nodA was found to be frequently transferred. The fate of the inoculant strains varied depending on the site, with a complete disappearance in one case, and persistence in another. We compared our results with the sister species Bradyrhizobium japonicum, both in terms of population genetics and inoculant strain destiny.

Evolution of cassava brown streak disease-associated viruses

Cassava brown streak disease (CBSD) has occurred in the Indian Ocean coastal lowlands and some areas of Malawi in East Africa for decades, and makes the storage roots of cassava unsuitable for consumption. CBSD is associated with Cassava brown streak virus (CBSV) and the recently described Ugandan cassava brown streak virus (UCBSV) [picorna-like (+)ssRNA viruses; genus Ipomovirus; family Potyviridae]. This study reports the first comprehensive analysis on how evolution is shaping the populations of CBSV and UCBSV. The complete genomes of CBSV and UCBSV (four and eight isolates, respectively) were 69.0-70.3 and 73.6-74.4% identical at the nucleotide and polyprotein amino acid sequence levels, respectively. They contained predictable sites of homologous recombination, mostly in the 3'-proximal part (NIb-HAM1h-CP-3'-UTR) of the genome, but no evidence of recombination between the two viruses was found. The CP-encoding sequences of 22 and 45 isolates of CBSV and UCBSV analysed, respectively, were mainly under purifying selection; however, several sites in the central part of CBSV CP were subjected to positive selection. HAM1h (putative nucleoside triphosphate pyrophosphatase) was the least similar protein between CBSV and UCBSV (aa identity approx. 55%). Both termini of HAM1h contained sites under positive selection in UCBSV. The data imply an on-going but somewhat different evolution of CBSV and UCBSV, which is congruent with the recent widespread outbreak of UCBSV in cassava crops in the highland areas (>1000 m above sea level) of East Africa where CBSD has not caused significant problems in the past.

Genome structure and the benefit of sex

We examine the behavior of sexual and asexual populations in modular multipeaked fitness landscapes and show that sexuals can systematically reach different, higher fitness adaptive peaks than asexuals. Whereas asexuals must move against selection to escape local optima, sexuals reach higher fitness peaks reliably because they create specific genetic variants that "skip over" fitness valleys, moving from peak to peak in the fitness landscape. This occurs because recombination can supply combinations of mutations in functional composites or "modules," that may include individually deleterious mutations. Thus when a beneficial module is substituted for another less-fit module by sexual recombination it provides a genetic variant that would require either several specific simultaneous mutations in an asexual population or a sequence of individual mutations some of which would be selected against. This effect requires modular genomes, such that subsets of strongly epistatic mutations are tightly physically linked. We argue that such a structure is provided simply by virtue of the fact that genomes contain many genes each containing many strongly epistatic nucleotides. We briefly discuss the connections with "building blocks" in the evolutionary computation literature. We conclude that there are conditions in which sexuals can systematically evolve high-fitness genotypes that are essentially unevolvable for asexuals.