Experimental evolution of pseudogenization and gene loss in a plant RNA virus

Hidden evolutionary constraints dictate the retention of coronavirus accessory genes

Coronaviruses exhibit many mechanisms of genetic innovation (1-5), including the acquisition of accessory genes that originate by capture of cellular genes or through duplication of existing viral genes (6,7). Accessory genes influence viral host range and cellular tropism, but little is known about how selection acts on these variable regions of virus genomes. We used experimental evolution of mouse hepatitis virus (MHV) encoding a cellular AKAP7 phosphodiesterase and an inactive native phosphodiesterase, NS2 (ref 8) to simulate the capture of a host gene and analyze its evolution. After courses of serial infection, the gene encoding inactive NS2, ORF2, unexpectedly remained intact, suggesting it is under cryptic constraint uncoupled from the function of NS2. In contrast, AKAP7 was retained under strong selection but rapidly lost under relaxed selection. Guided by the retention of ORF2 and similar patterns in related betacoronaviruses, we analyzed ORF8 of SARS-CoV-2, which arose via gene duplication6 and contains premature stop codons in several globally successful lineages. As with MHV ORF2, the coding-defective SARS-CoV-2 ORF8 gene remains largely intact, mirroring patterns observed during MHV experimental evolution, challenging assumptions on the dynamics of gene loss in virus genomes and extending these findings to viruses currently adapting to humans.

Expansion of viral genomes with viral protein genome linked copies

Virus protein-linked genome (VPg) proteins are required for replication. VPgs are duplicated in a subset of RNA viruses however their roles are not fully understood and the extent of viral genomes containing VPg copies has not been investigated in detail. Here, we generated a novel bioinformatics approach to identify VPg sequences in viral genomes using hidden Markov models (HMM) based on alignments of dicistrovirus VPg sequences. From metagenomic datasets of dicistrovirus genomes, we identified 717 dicistrovirus genomes containing VPgs ranging from a single copy to 8 tandem copies. The VPgs are classified into nine distinct types based on their sequence and length. The VPg types but not VPg numbers per viral genome followed specific virus clades, thus suggesting VPgs co-evolved with viral genomes. We also identified VPg duplications in aquamavirus and mosavirus genomes. This study greatly expands the number of viral genomes that contain VPg copies and indicates that duplicated viral sequences are more widespread than anticipated.

Increased Frequency of Indels in Hypervariable Regions of SARS-CoV-2 Proteins—A Possible Signature of Adaptive Selection

Most attention in the surveillance of evolving SARS-CoV-2 genome has been centered on nucleotide substitutions in the spike glycoprotein. We show that, as the pandemic extends into its second year, the numbers and ratio of genomes with in-frame insertions and deletions (indels) increases significantly, especially among the variants of concern (VOCs). Monitoring of the SARS-CoV-2 genome evolution shows that co-occurrence (i.e., highly correlated presence) of indels, especially deletions on spike N-terminal domain and non-structural protein 6 (NSP6) is a shared feature in several VOCs such as Alpha, Beta, Delta, and Omicron. Indels distribution is correlated with spike mutations associated with immune escape and growth in the number of genomes with indels coincides with the increasing population resistance due to vaccination and previous infections. Indels occur most frequently in the spike, but also in other proteins, especially those involved in interactions with the host immune system. We also showed that indels concentrate in regions of individual SARS-CoV-2 proteins known as hypervariable regions (HVRs) that are mostly located in specific loop regions. Structural analysis suggests that indels remodel viral proteins’ surfaces at common epitopes and interaction interfaces, affecting the virus’ interactions with host proteins. We hypothesize that the increased frequency of indels, the non-random distribution of them and their independent co-occurrence in several VOCs is another mechanism of response to elevated global population immunity.

Poorly Conserved P15 Proteins of Cileviruses Retain Elements of Common Ancestry and Putative Functionality: A Theoretical Assessment on the Evolution of Cilevirus Genomes

The genus Cilevirus groups enveloped single-stranded (+) RNA virus members of the family Kitaviridae, order Martellivirales. Proteins P15, scarcely conserved polypeptides encoded by cileviruses, have no apparent homologs in public databases. Accordingly, the open reading frames (ORFs) p15, located at the 5'-end of the viral RNA2 molecules, are considered orphan genes (ORFans). In this study, we have delved into ORFs p15 and the relatively poorly understood biochemical properties of the proteins P15 to posit their importance for viruses across the genus and theorize on their origin. We detected that the ORFs p15 are under purifying selection and that, in some viral strains, the use of synonymous codons is biased, which might be a sign of adaptation to their plant hosts. Despite the high amino acid sequence divergence, proteins P15 show the conserved motif [FY]-L-x(3)-[FL]-H-x-x-[LIV]-S-C-x-C-x(2)-C-x-G-x-C, which occurs exclusively in members of this protein family. Proteins P15 also show a common predicted 3D structure that resembles the helical scaffold of the protein ORF49 encoded by radinoviruses and the phosphoprotein C-terminal domain of mononegavirids. Based on the 3D structural similarities of P15, we suggest elements of common ancestry, conserved functionality, and relevant amino acid residues. We conclude by postulating a plausible evolutionary trajectory of ORFans p15 and the 5'-end of the RNA2 of cileviruses considering both protein fold superpositions and comparative genomic analyses with the closest kitaviruses, negeviruses, nege/kita-like viruses, and unrelated viruses that share the ecological niches of cileviruses.

P1 of Sweet Potato Feathery Mottle Virus Shows Strong Adaptation Capacity, Replacing P1-HCPro in a Chimeric Plum Pox Virus

Potyviridae is the largest family of plant RNA viruses. Their genomes are expressed through long polyproteins that are usually headed by the leader endopeptidase P1. This protein can be classified as type A or type B based on host proteolytic requirements and RNA silencing suppression (RSS) capacity. The main Potyviridae genus is Potyvirus, and a group of potyviruses infecting sweet potato presents an enlarged P1 protein with a polymerase slippage motif that produces an extra product termed P1N-PISPO. These two proteins display some RSS activity and are expressed followed by HCPro, which appears to be the main RNA silencing suppressor in these viruses. Here, we studied the behavior of the P1 protein of Sweet potato feathery mottle virus (SPFMV) using a viral system based on a canonical potyvirus, Plum pox virus (PPV), and discovered that this protein is able to replace both PPV P1 and HCPro. We also found that P1N-PISPO, produced after polymerase slippage, provides extra RNA silencing suppression capacity to SPFMV P1 in this viral context. In addition, the results showed that presence of two type A P1 proteins was detrimental for viral viability. The ample recombination spectrum that we found in the recovered viruses supports the strong adaptation capacity of P1 proteins and signals the N-terminal part of SPFMV P1 as essential for RSS activity. Further analyses provided data to add extra layers to the evolutionary history of sweet potato-infecting potyvirids. IMPORTANCE Plant viruses represent a major challenge for agriculture worldwide and Potyviridae, being the largest family of plant RNA viruses, is one of the primary players. P1, the leader endopeptidase, is a multifunctional protein that contributes to the successful spread of these viruses over a wide host range. Understanding how P1 proteins work, their dynamic interplay during viral infection, and their evolutionary path is critical for the development of strategic tools to fight the multiple diseases these viruses cause. We focused our efforts on the P1 protein of Sweet potato feathery mottle virus, which is coresponsible for the most devastating disease in sweet potato. The significance of our research is in understanding the capacity of this protein to perform several independent functions, using this knowledge to learn more about P1 proteins in general and the potyvirids infecting this host.

Modeling multipartite virus evolution: the genome formula facilitates rapid adaptation to heterogeneous environments†

Multipartite viruses have two or more genome segments, and package different segments into different particle types. Although multipartition is thought to have a cost for virus transmission, its benefits are not clear. Recent experimental work has shown that the equilibrium frequency of viral genome segments, the setpoint genome formula (SGF), can be unbalanced and host-species dependent. These observations have reinvigorated the hypothesis that changes in genome-segment frequencies can lead to changes in virus-gene expression that might be adaptive. Here we explore this hypothesis by developing models of bipartite virus infection, leading to a threefold contribution. First, we show that the SGF depends on the cellular multiplicity of infection (MOI), when the requirements for infection clash with optimizing the SGF for virus-particle yield per cell. Second, we find that convergence on the SGF is very rapid, often occurring within a few cellular rounds of infection. Low and intermediate MOIs lead to faster convergence on the SGF. For low MOIs, this effect occurs because of the requirements for infection, whereas for intermediate MOIs this effect is also due to the high levels of variation generated in the genome formula (GF). Third, we explored the conditions under which a bipartite virus could outcompete a monopartite one. As the heterogeneity between environments and specificity of gene-expression requirements for each environment increased, the bipartite virus was more likely to outcompete the monopartite virus. Under some conditions, changes in the GF helped to exclude the monopartite competitor, highlighting the versatility of the GF. Our results show the inextricable relationship between MOI and the SGF, and suggest that under some conditions, the cost of multipartition can be outweighed by its benefits for the rapid tuning of viral gene expression.

On the stability of sequences inserted into viral genomes

Viruses are widely used as vectors for heterologous gene expression in cultured cells or natural hosts, and therefore a large number of viruses with exogenous sequences inserted into their genomes have been engineered. Many of these engineered viruses are viable and express heterologous proteins at high levels, but the inserted sequences often prove to be unstable over time and are rapidly lost, limiting heterologous protein expression. Although virologists are aware that inserted sequences can be unstable, processes leading to insert instability are rarely considered from an evolutionary perspective. Here, we review experimental work on the stability of inserted sequences over a broad range of viruses, and we present some theoretical considerations concerning insert stability. Different virus genome organizations strongly impact insert stability, and factors such as the position of insertion can have a strong effect. In addition, we argue that insert stability not only depends on the characteristics of a particular genome, but that it will also depend on the host environment and the demography of a virus population. The interplay between all factors affecting stability is complex, which makes it challenging to develop a general model to predict the stability of genomic insertions. We highlight key questions and future directions, finding that insert stability is a surprisingly complex problem and that there is need for mechanism-based, predictive models. Combining theoretical models with experimental tests for stability under varying conditions can lead to improved engineering of viral modified genomes, which is a valuable tool for understanding genome evolution as well as for biotechnological applications, such as gene therapy.

Engineered Functional Redundancy Relaxes Selective Constraints upon Endogenous Genes in Viral RNA Genomes

Functional redundancy, understood as the functional overlap of different genes, is a double-edge sword. At the one side, it is thought to serve as a robustness mechanism that buffers the deleterious effect of mutations hitting one of the redundant copies, thus resulting in pseudogenization. At the other side, it is considered as a source of genetic and functional innovation. In any case, genetically redundant genes are expected to show an acceleration in the rate of molecular evolution. Here, we tackle the role of functional redundancy in viral RNA genomes. To this end, we have evaluated the rates of compensatory evolution for deleterious mutations affecting an essential function, the suppression of RNA silencing plant defense, of tobacco etch potyvirus (TEV). TEV genotypes containing deleterious mutations in presence/absence of engineered functional redundancy were evolved and the pattern of fitness and pathogenicity recovery evaluated. Genetically redundant genotypes suffered less from the effect of deleterious mutations and showed relatively minor changes in fitness and pathogenicity. By contrast, nongenetically redundant genotypes had very low fitness and pathogenicity at the beginning of the evolution experiment that were fully recovered by the end. At the molecular level, the outcome depended on the combination of the actual mutations being compensated and the presence/absence of functional redundancy. Reversions to wild-type alleles were the norm in the nonredundant genotypes while redundant ones either did not fix any mutation at all or showed a higher nonsynonymous mutational load.

Going, going, gone: predicting the fate of genomic insertions in plant RNA viruses

Horizontal gene transfer is common among viruses, while they also have highly compact genomes and tend to lose artificial genomic insertions rapidly. Understanding the stability of genomic insertions in viral genomes is therefore relevant for explaining and predicting their evolutionary patterns. Here, we revisit a large body of experimental research on a plant RNA virus, tobacco etch potyvirus (TEV), to identify the patterns underlying the stability of a range of homologous and heterologous insertions in the viral genome. We obtained a wide range of estimates for the recombination rate-the rate at which deletions removing the insertion occur-and these appeared to be independent of the type of insertion and its location. Of the factors we considered, recombination rate was the best predictor of insertion stability, although we could not identify the specific sequence characteristics that would help predict insertion instability. We also considered experimentally the possibility that functional insertions lead to higher mutational robustness through increased redundancy. However, our observations suggest that both functional and non-functional increases in genome size decreased the mutational robustness. Our results therefore demonstrate the importance of recombination rates for predicting the long-term stability and evolution of viral RNA genomes and suggest that there are unexpected drawbacks to increases in genome size for mutational robustness.

Emergency Services of Viral RNAs: Repair and Remodeling

Reproduction of RNA viruses is typically error-prone due to the infidelity of their replicative machinery and the usual lack of proofreading mechanisms. The error rates may be close to those that kill the virus. Consequently, populations of RNA viruses are represented by heterogeneous sets of genomes with various levels of fitness. This is especially consequential when viruses encounter various bottlenecks and new infections are initiated by a single or few deviating genomes. Nevertheless, RNA viruses are able to maintain their identity by conservation of major functional elements. This conservatism stems from genetic robustness or mutational tolerance, which is largely due to the functional degeneracy of many protein and RNA elements as well as to negative selection. Another relevant mechanism is the capacity to restore fitness after genetic damages, also based on replicative infidelity. Conversely, error-prone replication is a major tool that ensures viral evolvability. The potential for changes in debilitated genomes is much higher in small populations, because in the absence of stronger competitors low-fit genomes have a choice of various trajectories to wander along fitness landscapes. Thus, low-fit populations are inherently unstable, and it may be said that to run ahead it is useful to stumble. In this report, focusing on picornaviruses and also considering data from other RNA viruses, we review the biological relevance and mechanisms of various alterations of viral RNA genomes as well as pathways and mechanisms of rehabilitation after loss of fitness. The relationships among mutational robustness, resilience, and evolvability of viral RNA genomes are discussed.

Maize chlorotic mottle virus exhibits low divergence between differentiated regional sub-populations

Maize chlorotic mottle virus has been rapidly spreading around the globe over the past decade. The interactions of maize chlorotic mottle virus with Potyviridae viruses causes an aggressive synergistic viral condition - maize lethal necrosis, which can cause total yield loss. Maize production in sub-Saharan Africa, where it is the most important cereal, is threatened by the arrival of maize lethal necrosis. We obtained maize chlorotic mottle virus genome sequences from across East Africa and for the first time from Ecuador and Hawaii, and constructed a phylogeny which highlights the similarity of Chinese to African isolates, and Ecuadorian to Hawaiian isolates. We used a measure of clustering, the adjusted Rand index, to extract region-specific SNPs and coding variation that can be used for diagnostics. The population genetics analysis we performed shows that the majority of sequence diversity is partitioned between populations, with diversity extremely low within China and East Africa.

Evolutionary transitions during RNA virus experimental evolution

In their search to understand the evolution of biological complexity, John Maynard Smith and Eörs Szathmáry put forward the notion of major evolutionary transitions as those in which elementary units get together to generate something new, larger and more complex. The origins of chromosomes, eukaryotic cells, multicellular organisms, colonies and, more recently, language and technological societies are examples that clearly illustrate this notion. However, a transition may be considered as anecdotal or as major depending on the specific level of biological organization under study. In this contribution, I will argue that transitions may also be occurring at a much smaller scale of biological organization: the viral world. Not only that, but also that we can observe in real time how these major transitions take place during experimental evolution. I will review the outcome of recent evolution experiments with viruses that illustrate four major evolutionary transitions: (i) the origin of a new virus that infects an otherwise inaccessible host and completely changes the way it interacts with the host regulatory and metabolic networks, (ii) the incorporation and loss of genes, (iii) the origin of segmented genomes from a non-segmented one, and (iv) the evolution of cooperative behaviour and cheating between different viruses or strains during co-infection of the same host.This article is part of the themed issue 'The major synthetic evolutionary transitions'.

2b or Not 2b: Experimental Evolution of Functional Exogenous Sequences in a Plant RNA Virus

Horizontal gene transfer (HGT) is pervasive in viruses and thought to be a key mechanism in their evolution. On the other hand, strong selective constraints against increasing genome size are an impediment for HGT, rapidly purging horizontally transferred sequences and thereby potentially hindering evolutionary innovation. Here, we explore experimentally the evolutionary fate of viruses with simulated HGT events, using the plant RNA virus Tobacco etch virus (TEV), by separately introducing two functional, exogenous sequences to its genome. One of the events simulates the acquisition of a new function though HGT of a conserved AlkB domain, responsible for the repair of alkylation or methylation damage in many organisms. The other event simulates the acquisition of a sequence that duplicates an existing function, through HGT of the 2b RNA silencing suppressor from Cucumber mosaic virus. We then evolved these two viruses, tracked the maintenance of the horizontally transferred sequences over time, and for the final virus populations, sequenced their genome and measured viral fitness. We found that the AlkB domain was rapidly purged from the TEV genome, restoring fitness to wild-type levels. Conversely, the 2b gene was stably maintained and did not have a major impact on viral fitness. Moreover, we found that 2b is functional in TEV, as it provides a replicative advantage when the RNA silencing suppression domain of HC-Pro is mutated. These observations suggest a potentially interesting role for HGT of short functional sequences in ameliorating evolutionary constraints on viruses, through the duplication of functions.

High virulence does not necessarily impede viral adaptation to a new host: a case study using a plant RNA virus

BACKGROUND

Theory suggests that high virulence could hinder between-host transmission of microparasites, and that virulence therefore will evolve to lower levels. Alternatively, highly virulent microparasites could also curtail host development, thereby limiting both the host resources available to them and their own within-host effective population size. In this case, high virulence might restrain the mutation supply rate and increase the strength with which genetic drift acts on microparasite populations. Thereby, this alternative explanation limits the microparasites' potential to adapt to the host and ultimately the ability to evolve lower virulence. As a first exploration of this hypothesis, we evolved Tobacco etch virus carrying an eGFP fluorescent marker in two semi-permissive host species, Nicotiana benthamiana and Datura stramonium, for which it has a large difference in virulence. We compared the results to those previously obtained in the natural host, Nicotiana tabacum, where we have shown that carriage of eGFP has a high fitness cost and its loss serves as a real-time indicator of adaptation.

RESULTS

After over half a year of evolution, we sequenced the genomes of the evolved lineages and measured their fitness. During the evolution experiment, marker loss leading to viable virus variants was only observed in one lineage of the host for which the virus has low virulence, D. stramonium. This result was consistent with the observation that there was a fitness cost of eGFP in this host, while surprisingly no fitness cost was observed in the host for which the virus has high virulence, N. benthamiana. Furthermore, in both hosts we observed increases in viral fitness in few lineages, and host-specific convergent evolution at the genomic level was only found in N. benthamiana.

CONCLUSIONS

The results of this study do not lend support to the hypothesis that high virulence impedes microparasites' evolution. Rather, they exemplify that jumps between host species can be game changers for evolutionary dynamics. When considering the evolution of genome architecture, host species jumps might play a very important role, by allowing evolutionary intermediates to be competitive.

Different Evolutionary Paths to Complexity for Small and Large Populations of Digital Organisms

A major aim of evolutionary biology is to explain the respective roles of adaptive versus non-adaptive changes in the evolution of complexity. While selection is certainly responsible for the spread and maintenance of complex phenotypes, this does not automatically imply that strong selection enhances the chance for the emergence of novel traits, that is, the origination of complexity. Population size is one parameter that alters the relative importance of adaptive and non-adaptive processes: as population size decreases, selection weakens and genetic drift grows in importance. Because of this relationship, many theories invoke a role for population size in the evolution of complexity. Such theories are difficult to test empirically because of the time required for the evolution of complexity in biological populations. Here, we used digital experimental evolution to test whether large or small asexual populations tend to evolve greater complexity. We find that both small and large-but not intermediate-sized-populations are favored to evolve larger genomes, which provides the opportunity for subsequent increases in phenotypic complexity. However, small and large populations followed different evolutionary paths towards these novel traits. Small populations evolved larger genomes by fixing slightly deleterious insertions, while large populations fixed rare beneficial insertions that increased genome size. These results demonstrate that genetic drift can lead to the evolution of complexity in small populations and that purifying selection is not powerful enough to prevent the evolution of complexity in large populations.

Matters of Size: Genetic Bottlenecks in Virus Infection and Their Potential Impact on Evolution

For virus infections of multicellular hosts, narrow genetic bottlenecks during transmission and within-host spread appear to be widespread. These bottlenecks will affect the maintenance of genetic variation in a virus population and the prevalence of mixed-strain infections, thereby ultimately determining the strength with which different random forces act during evolution. Here we consider different approaches for estimating bottleneck sizes and weigh their merits. We then review quantitative estimates of bottleneck size during cellular infection, within-host spread, horizontal transmission, and finally vertical transmission. In most cases we find that bottlenecks do regularly occur, although in many cases they appear to be virion-concentration dependent. Finally, we consider the evolutionary implications of genetic bottlenecks during virus infection. Although on average strong bottlenecks will lead to declines in fitness, we consider a number of scenarios in which bottlenecks could also be advantageous for viruses.

Predicting the Stability of Homologous Gene Duplications in a Plant RNA Virus

One of the striking features of many eukaryotes is the apparent amount of redundancy in coding and non-coding elements of their genomes. Despite the possible evolutionary advantages, there are fewer examples of redundant sequences in viral genomes, particularly those with RNA genomes. The factors constraining the maintenance of redundant sequences in present-day RNA virus genomes are not well known. Here, we use Tobacco etch virus, a plant RNA virus, to investigate the stability of genetically redundant sequences by generating viruses with potentially beneficial gene duplications. Subsequently, we tested the viability of these viruses and performed experimental evolution. We found that all gene duplication events resulted in a loss of viability or in a significant reduction in viral fitness. Moreover, upon analyzing the genomes of the evolved viruses, we always observed the deletion of the duplicated gene copy and maintenance of the ancestral copy. Interestingly, there were clear differences in the deletion dynamics of the duplicated gene associated with the passage duration and the size and position of the duplicated copy. Based on the experimental data, we developed a mathematical model to characterize the stability of genetically redundant sequences, and showed that fitness effects are not enough to predict genomic stability. A context-dependent recombination rate is also required, with the context being the duplicated gene and its position. Our results therefore demonstrate experimentally the deleterious nature of gene duplications in RNA viruses. Beside previously described constraints on genome size, we identified additional factors that reduce the likelihood of the maintenance of duplicated genes.

Mutant swarms of a totivirus-like entities are present in the red macroalga Chondrus crispus and have been partially transferred to the nuclear genome

Chondrus crispus Stackhouse (Gigartinales) is a red seaweed found on North Atlantic rocky shores. Electrophoresis of RNA extracts showed a prominent band with a size of around 6,000 bp. Sequencing of the band revealed several sequences with similarity to totiviruses, double-stranded RNA viruses that normally infect fungi. This virus-like entity was named C. crispus virus (CcV). It should probably be regarded as an extreme viral quasispecies or a mutant swarm since low identity (<65%) was found between sequences. Totiviruses typically code for two genes: one capsid gene (gag) and one RNA-dependent RNA polymerase gene (pol) with a pseudoknot structure between the genes. Both the genes and the intergenic structures were found in the CcV sequences. A nonidentical gag gene was also found in the nuclear genome of C. crispus, with associated expressed sequence tags (EST) and upstream regulatory features. The gene was presumably horizontally transferred from the virus to the alga. Similar dsRNA bands were seen in extracts from different life cycle stages of C. crispus and from all geographic locations tested. In addition, similar bands were also observed in RNA extractions from other red algae; however, the significance of this apparently widespread phenomenon is unknown. Neither phenotype caused by the infection nor any virus particles or capsid proteins were identified; thus, the presence of viral particles has not been validated. These findings increase the known host range of totiviruses to include marine red algae.

Evolution of Genome Size in Asexual Digital Organisms

Genome sizes have evolved to vary widely, from 250 bases in viroids to 670 billion bases in some amoebas. This remarkable variation in genome size is the outcome of complex interactions between various evolutionary factors such as mutation rate and population size. While comparative genomics has uncovered how some of these evolutionary factors influence genome size, we still do not understand what drives genome size evolution. Specifically, it is not clear how the primordial mutational processes of base substitutions, insertions, and deletions influence genome size evolution in asexual organisms. Here, we use digital evolution to investigate genome size evolution by tracking genome edits and their fitness effects in real time. In agreement with empirical data, we find that mutation rate is inversely correlated with genome size in asexual populations. We show that at low point mutation rate, insertions are significantly more beneficial than deletions, driving genome expansion and the acquisition of phenotypic complexity. Conversely, the high mutational load experienced at high mutation rates inhibits genome growth, forcing the genomes to compress their genetic information. Our analyses suggest that the inverse relationship between mutation rate and genome size is a result of the tradeoff between evolving phenotypic innovation and limiting the mutational load.

Multiple Barriers to the Evolution of Alternative Gene Orders in a Positive-Strand RNA Virus

The order in which genes are organized within a genome is generally not conserved between distantly related species. However, within virus orders and families, strong conservation of gene order is observed. The factors that constrain or promote gene-order diversity are largely unknown, although the regulation of gene expression is one important constraint for viruses. Here we investigate why gene order is conserved for a positive-strand RNA virus encoding a single polyprotein in the context of its authentic multicellular host. Initially, we identified the most plausible trajectory by which alternative gene orders could evolve. Subsequently, we studied the accessibility of key steps along this evolutionary trajectory by constructing two virus intermediates: (1) duplication of a gene followed by (2) loss of the ancestral gene. We identified five barriers to the evolution of alternative gene orders. First, the number of viable positions for reordering is limited. Second, the within-host fitness of viruses with gene duplications is low compared to the wild-type virus. Third, after duplication, the ancestral gene copy is always maintained and never the duplicated one. Fourth, viruses with an alternative gene order have even lower fitness than viruses with gene duplications. Fifth, after more than half a year of evolution in isolation, viruses with an alternative gene order are still vastly inferior to the wild-type virus. Our results show that all steps along plausible evolutionary trajectories to alternative gene orders are highly unlikely. Hence, the inaccessibility of these trajectories probably contributes to the conservation of gene order in present-day viruses.

A potyvirus vector efficiently targets recombinant proteins to chloroplasts, mitochondria and nuclei in plant cells when expressed at the amino terminus of the polyprotein

Plant virus-based expression systems allow quick and efficient production of recombinant proteins in plant biofactories. Among them, a system derived from tobacco etch virus (TEV; genus potyvirus) permits coexpression of equimolar amounts of several recombinant proteins. This work analyzed how to target recombinant proteins to different subcellular localizations in the plant cell using this system. We constructed TEV clones in which green fluorescent protein (GFP), with a chloroplast transit peptide (cTP), a nuclear localization signal (NLS) or a mitochondrial targeting peptide (mTP) was expressed either as the most amino-terminal product or embedded in the viral polyprotein. Results showed that cTP and mTP mediated efficient translocation of GFP to the corresponding organelle only when present at the amino terminus of the viral polyprotein. In contrast, the NLS worked efficiently at both positions. Viruses expressing GFP in the amino terminus of the viral polyprotein produced milder symptoms. Untagged GFPs and cTP and NLS tagged amino-terminal GFPs accumulated to higher amounts in infected tissues. Finally, viral progeny from clones with internal GFPs maintained the extra gene better. These observations will help in the design of potyvirus-based vectors able to coexpress several proteins while targeting different subcellular localizations, as required in plant metabolic engineering.

Development of a Mild Viral Expression System for Gain-Of-Function Study of Phytoplasma Effector In Planta

PHYL1 and SAP54 are orthologs of pathogenic effectors of Aster yellow witches'-broom (AYWB) phytoplasma and Peanut witches'-broom (PnWB) phytoplasma, respectively. These effectors cause virescence and phyllody symptoms (hereafter leafy flower) in phytoplasma-infected plants. T0 lines of transgenic Arabidopsis expressing the PHYL1 or SAP54 genes (PHYL1 or SAP54 plants) show a leafy flower phenotype and result in seedless, suggesting that PHYL1 and SAP54 interfere with reproduction stage that restrict gain-of-function studies in the next generation of transgenic plants. Turnip mosaic virus (TuMV) mild strain (TuGK) has an Arg182Lys mutation in the helper-component proteinase (HC-ProR182K) that blocks suppression of the miRNA pathway and prevents symptom development in TuGK-infected plants. We exploited TuGK as a viral vector for gain-of-function studies of PHYL1 and SAP54 in Arabidopsis plants. TuGK-PHYL1- and TuGK-SAP54-infected Arabidopsis plants produced identical leafy flower phenotypes and similar gene expression profiles as PHYL1 and SAP54 plants. In addition, the leafy flower formation rate was enhanced in TuGK-PHYL1- or TuGK-SAP54-infected Arabidopsis plants that compared with the T0 lines of PHYL1 plants. These results provide more evidence and novel directions for further studying the mechanism of PHYL1/SAP54-mediated leafy flower development. In addition, the TuGK vector is a good alternative in transgenic plant approaches for rapid gene expression in gain-of-function studies.

Temporal dynamics of intrahost molecular evolution for a plant RNA virus

Populations of plant RNA viruses are highly polymorphic in infected plants, which may allow rapid within-host evolution. To understand tobacco etch potyvirus (TEV) evolution, longitudinal samples from experimentally evolved populations in the natural host tobacco and from the alternative host pepper were phenotypically characterized and genetically analyzed. Temporal and compartmental variabilities of TEV populations were quantified using high throughput Illumina sequencing and population genetic approaches. Of the two viral phenotypic traits measured, virulence increased in the novel host but decreased in the original one, and viral load decreased in both hosts, though to a lesser extent in the novel one. Dynamics of population genetic diversity were also markedly different among hosts. Population heterozygosity increased in the ancestral host, with a dominance of synonymous mutations fixed, whereas it did not change or even decreased in the new host, with an excess of nonsynonymous mutations. All together, these observations suggest that directional selection is the dominant evolutionary force in TEV populations evolving in a novel host whereas either diversifying selection or random genetic drift may play a fundamental role in the natural host. To better understand these evolutionary dynamics, we developed a computer simulation model that incorporates the effects of mutation, selection, and drift. Upon parameterization with empirical data from previous studies, model predictions matched the observed patterns, thus reinforcing our idea that the empirical patterns of mutation accumulation represent adaptive evolution.

Testing the independent action hypothesis of plant pathogen mode of action: a simple and powerful new approach

The independent action hypothesis is a simple model of pathogen infection that can make many useful predictions on infection kinetics and, therefore, a number of different tests of independent action have been developed. However, some of these analyses are rather sophisticated, limiting their appeal to experimentalists, and it is also unclear how well the different tests perform. Here, we developed and evaluated a simple and robust new test of independent action. Our new test is based on using a constant inoculum dose of one pathogen variant, varying the dose of a second variant, and then quantifying the infection response for the first variant. We simulated infection data in which we introduced deviations from independent action, experimental variation, or both. Simulations showed that our new procedure has many advantages over the existing tests of independent action, especially if only systemic-infection data are available. We also performed experimental tests of our new procedure using two marked Tobacco etch virus (TEV) variants. We found minor deviations from the independent action model, which were not detected by previous tests using existing methods, exemplifying the utility of this approach. We discuss the implications for TEV infection kinetics and consider how to reconcile different dose-dependent effects.

Shrinkage of genome size in a plant RNA virus upon transfer of an essential viral gene into the host genome

Nonretroviral integrated RNA viruses (NIRVs) are genes of nonretroviral RNA viruses found in the genomes of many eukaryotic organisms. NIRVs are thought to sometimes confer virus resistance, meaning that they could impact spread of the virus in the host population. However, a NIRV that is expressed may also impact the evolution of virus populations within host organisms. Here, we experimentally addressed the evolution of a virus in a host expressing a NIRV using Tobacco etch virus (TEV), a plant RNA virus, and transgenic tobacco plants expressing its replicase, NIb. We found that a virus missing the NIb gene, TEV-ΔNIb, which is incapable of autonomous replication in wild-type plants, had a higher fitness than the full-length TEV in the transgenic plants. Moreover, when the full-length TEV was evolved by serial passages in transgenic plants, we observed genomic deletions within NIb--and in some cases the adjacent cistrons--starting from the first passage. When we passaged TEV and TEV-ΔNIb in transgenic plants, we found mutations in proteolytic sites, but these only occurred in TEV-ΔNIb lineages, suggesting the adaptation of polyprotein processing to altered NIb expression. These results raise the possibility that NIRV expression can indeed induce the deletion of the corresponding genes in the viral genome, resulting in the formation of viruses that are replication defective in hosts that do not express the same NIRV. Moreover, virus genome evolution was contingent upon the deletion of the viral replicase, suggesting NIRV expression could also alter patterns of virus evolution.

Within-host spatiotemporal dynamics of plant virus infection at the cellular level

A multicellular organism is not a monolayer of cells in a flask; it is a complex, spatially structured environment, offering both challenges and opportunities for viruses to thrive. Whereas virus infection dynamics at the host and within-cell levels have been documented, the intermediate between-cell level remains poorly understood. Here, we used flow cytometry to measure the infection status of thousands of individual cells in virus-infected plants. This approach allowed us to determine accurately the number of cells infected by two virus variants in the same host, over space and time as the virus colonizes the host. We found a low overall frequency of cellular infection (<0.3), and few cells were coinfected by both virus variants (<0.1). We then estimated the cellular contagion rate (R), the number of secondary infections per infected cell per day. R ranged from 2.43 to values not significantly different from zero, and generally decreased over time. Estimates of the cellular multiplicity of infection (MOI), the number of virions infecting a cell, were low (<1.5). Variance of virus-genotype frequencies increased strongly from leaf to cell levels, in agreement with a low MOI. Finally, there were leaf-dependent differences in the ease with which a leaf could be colonized, and the number of virions effectively colonizing a leaf. The modeling of infection patterns suggests that the aggregation of virus-infected cells plays a key role in limiting spread; matching the observation that cell-to-cell movement of plant viruses can result in patches of infection. Our results show that virus expansion at the between-cell level is restricted, probably due to the host environment and virus infection itself.

Stability and fitness impact of the visually discernible Rosea1 marker in the Tobacco etch virus genome

Antirrhinum majus Rosea1 (Ros1) is an MYB-related transcription factor that induces anthocyanin biosynthesis in plant tissues, and has been shown to be suitable for visual tracking of virus infection in plants. However, activation of anthocyanin biosynthesis has far reaching effects on plant physiology and could consequently have negative effects on viral replication. Therefore, viruses carrying the Ros1 marker might have a low fitness and consequently rapidly lose the marker. To compare the stability of the Ros1 marker, we generated Tobacco etch virus (TEV) based constructs containing either Ros1 or the enhanced green fluorescent protein (eGFP) between the NIb and CP cistrons (TEV-Ros1 and TEV-eGFP, respectively). We measured the within-host competitive fitness of both viruses by direct competitions with a common competitor during infection of Nicotiana tabacum. The fitness of TEV-Ros1 was significantly lower than that of TEV-eGFP, and both recombinant viruses had a significantly lower fitness than the wild-type virus. Nevertheless, after seven weeks of infection in N. tabacum, similar levels of marker gene instability where found for both viruses. Despite lower fitness of the marked virus, Ros1 is therefore a viable alternative marker for tracking viral infection in plants.