Emergence of new strains of Watermelon mosaic virus in South-eastern France: evidence for limited spread but rapid local population shift

Insights into the early transcriptomic response against watermelon mosaic virus in melon

BACKGROUND

Watermelon mosaic virus (WMV) is one of the most prevalent viruses affecting melon worldwide. Recessive resistance to WMV in melon has previously been reported in the African accession TGR-1551. Moreover, the genomic regions associated to the resistance have also been described. Nevertheless, the transcriptomic response that might infer the resistance to this potyvirus has not been explored.

RESULTS

We have performed a comparative transcriptomic analysis using mock and WMV-inoculated plants of the susceptible cultivar "Bola de oro" (BO) and a resistant RIL (Recombinant inbred line) derived from the initial cross between "TGR-1551" and BO. In total, 616 genes were identified as differentially expressed and the weighted gene co-expression network analysis (WGCNA) detected 19 gene clusters (GCs), of which 7 were differentially expressed for the genotype x treatment interaction term. SNPs with a predicted high impact on the protein function were detected within the coding regions of most of the detected DEGs. Moreover, 3 and 16 DEGs were detected within the QTL regions previously described in chromosomes 11 and 5, respectively. In addition to these two specific genomic regions, we also observde large transcriptomic changes from genes spread across the genome in the resistant plants in response to the virus infection. This early response against WMV implied genes involved in plant-pathogen interaction, plant hormone signal transduction, the MAPK signaling pathway or ubiquitin mediated proteolysis, in detriment to the photosynthetic and basal metabolites pathways. Moreover, the gene MELO3C021395, which coded a mediator of RNA polymerase II transcription subunit 33A (MED33A), has been proposed as the candidate gene located on chromosome 11 conferring resistance to WMV.

CONCLUSIONS

The comparative transcriptomic analysis presented here showed that, even though the resistance to WMV in TGR-1551 has a recessive nature, it triggers an active defense response at a transcriptomic level, which involves broad-spectrum resistance mechanisms. Thus, this study represents a step forward on our understanding of the mechanisms underlaying WMV resistance in melon. In addition, it sheds light into a broader topic on the mechanisms of recessive resistances.

Long-Term Monitoring of Aphid-Transmitted Viruses in Melon and Zucchini Crops: Genetic Diversity and Population Structure of Cucurbit Aphid-Borne Yellows Virus and Watermelon Mosaic Virus

Understanding the emergence and prevalence of viral diseases in crops requires the systematic epidemiological monitoring of viruses, as well as the analysis of how ecological and evolutionary processes combine to shape viral population dynamics. Here, we extensively monitored the occurrence of six aphid-transmitted viruses in melon and zucchini crops in Spain for 10 consecutive cropping seasons between 2011 and 2020. The most prevalent viruses were cucurbit aphid-borne yellows virus (CABYV) and watermelon mosaic virus (WMV), found in 31 and 26% of samples with yellowing and mosaic symptoms. Other viruses, such as zucchini yellow mosaic virus, cucumber mosaic virus, Moroccan watermelon mosaic virus, and papaya ring spot virus, were detected less frequently (<3%) and mostly in mixed infections. Notably, our statistical analysis showed a significant association between CABYV and WMV in melon and zucchini hosts, suggesting that mixed infections might be influencing the evolutionary epidemiology of these viral diseases. We then carried out a comprehensive genetic characterization of the full-length genome sequences from CABYV and WMV isolates by using the Pacific Biosciences single-molecule real-time (PacBio) high-throughput technology to assess the genetic variation and structure of their populations. Our results showed that the CABYV population displayed seven codons under positive selection, and although most isolates clustered in the Mediterranean clade, a subsequent analysis of molecular variance revealed a significant, fine-scale temporal structure, which was in part explained by the level of the variance between isolates from single and mixed infections. In contrast, the WMV population genetic analysis showed that most of the isolates grouped into the Emergent clade, with no genetic differentiation and under purifying selection. These results underlie the epidemiological relevance of mixed infections for CABYV and provide a link between genetic diversity and CABYV dynamics at the whole-genome level.

Dynamics of the rice yellow mottle disease in western Burkina Faso: Epidemic monitoring, spatio-temporal variation of viral diversity, and pathogenicity in a disease hotspot

The rice yellow mottle virus (RYMV) is a model in plant virus molecular epidemiology, with the reconstruction of historical introduction routes at the scale of the African continent. However, information on patterns of viral prevalence and viral diversity over multiple years at a local scale remains scarce, in spite of potential implications for crop protection. Here, we describe a 5-year (2015-9) monitoring of RYMV prevalence in six sites from western Burkina Faso (geographic areas of Bama, Banzon, and Karfiguela). It confirmed one irrigated site as a disease hotspot and also found one rainfed lowland (RL) site with occasional high prevalence levels. Within the studied fields, a pattern of disease aggregation was evidenced at a 5-m distance, as expected for a mechanically transmitted virus. Next, we monitored RYMV genetic diversity in the irrigated disease hotspot site, revealing a high viral diversity, with the current coexistence of various distinct genetic groups at the site scale (ca. 520 ha) and also within various specific fields (25 m side). One genetic lineage, named S1bzn, is the most recently emerged group and increased in frequency over the studied period (from 20 per cent or less in 2015-6 to more than 65 per cent in 2019). Its genome results from a recombination between two other lineages (S1wa and S1ca). Finally, experimental work revealed that three rice varieties commonly cultivated in Burkina Faso were not different in terms of resistance level, and we also found no significant effect of RYMV genetic groups on symptom expression and viral load. We found, however, that infection outcome depended on the specific RYMV isolate, with two isolates from the lineage S1bzn accumulating at the highest level at early infections. Overall, this study documents a case of high viral prevalence, high viral diversity, and co-occurrence of divergent genetic lineages at a small geographic scale. A recently emerged lineage, which comprises viral isolates inducing severe symptoms and high accumulation under controlled conditions, could be recently rising through natural selection. Following up the monitoring of RYMV diversity is required to confirm this trend and further understand the factors driving the local maintenance of viral diversity.

Genetic Variability and Evidence of a New Subgroup in Watermelon Mosaic Virus Isolates

Watermelon mosaic virus (WMV) is one of the important Potyviruses that infect cucurbits worldwide. To better understand the population structure of WMV in the United States (U.S.), 57 isolates were collected from cucurbit fields located in nine southern states. The complete coat protein gene of all WMV isolates was cloned, sequenced and compared with 89 reported WMV isolates. The nucleotide and amino acid sequence identities among the U.S. WMV isolates ranged from 88.9 to 99.7% and from 91.5 to 100%, respectively. Phylogenetic analysis revealed that all the U.S. WMV isolates irrespective of their geographic origin or hosts belonged to Group 3. However, the fifty-seven isolates made three clusters in G3, where two clusters were similar to previously reported subgroups EM1 and EM2, and the third cluster, containing nine WMV isolates, formed a distinct subgroup named EM5 in this study. The ratio of non-synonymous to synonymous nucleotide substitution was low indicating the occurrence of negative purifying selection in the CP gene of WMV. Phylogenetic analysis of selected 37 complete genome sequences of WMV isolates also supported the above major grouping. Recombination analysis in the CP genes confirmed various recombinant events, indicating that purifying selection and recombination are the two dominant forces for the evolution of WMV isolates in the U.S.

Emerging strains of watermelon mosaic virus in Southeastern France: model-based estimation of the dates and places of introduction

Where and when alien organisms are successfully introduced are central questions to elucidate biotic and abiotic conditions favorable to the introduction, establishment and spread of invasive species. We propose a modelling framework to analyze multiple introductions by several invasive genotypes or genetic variants, in competition with a resident population, when observations provide knowledge on the relative proportions of each variant at some dates and places. This framework is based on a mechanistic-statistical model coupling a reaction–diffusion model with a probabilistic observation model. We apply it to a spatio-temporal dataset reporting the relative proportions of five genetic variants of watermelon mosaic virus (WMV, genus Potyvirus, family Potyviridae) in infections of commercial cucurbit fields. Despite the parsimonious nature of the model, it succeeds in fitting the data well and provides an estimation of the dates and places of successful introduction of each emerging variant as well as a reconstruction of the dynamics of each variant since its introduction.

Assessment of the Current Status of Potyviruses in Watermelon and Pumpkin Crops in Spain: Epidemiological Impact of Cultivated Plants and Mixed Infections

Viral infections on cucurbit plants cause substantial quality and yield losses on their crops. The diseased plants can often be infected by multiple viruses, and their epidemiology may depend, in addition to the agro-ecological management practices, on the combination of these viral infections. Watermelon mosaic virus (WMV) is one of the most prevalent viruses in cucurbit crops, and Moroccan watermelon mosaic virus (MWMV) emerged as a related species that threatens these crops. The occurrence of WMV and MWMV was monitored in a total of 196 apical-leaf samples of watermelon and pumpkin plants that displayed mosaic symptoms. The samples were collected from 49 fields in three major cucurbit-producing areas in Spain (Castilla La-Mancha, Alicante, and Murcia) for three consecutive (2018-2020) seasons. A molecular hybridization dot-blot method revealed that WMV was mainly (53%) found in both cultivated plants, with an unadvertised occurrence of MWMV. To determine the extent of cultivated plant species and mixed infections on viral dynamics, two infectious cDNA clones were constructed from a WMV isolate (MeWM7), and an MWMV isolate (ZuM10). Based on the full-length genomes, both isolates were grouped phylogenetically with the Emergent and European clades, respectively. Five-cucurbit plant species were infected steadily with either WMV or MWMV cDNA clones, showing variations on symptom expressions. Furthermore, the viral load varied depending on the plant species and infection type. In single infections, the WMV isolate showed a higher viral load than the MWMV isolate in melon and pumpkin, and MWMV only showed higher viral load than the WMV isolate in zucchini plants. However, in mixed infections, the viral load of the WMV isolate was greater than MWMV isolate in melon, watermelon and zucchini, whereas MWMV isolate was markedly reduced in zucchini. These results suggest that the impaired distribution of MWMV in cucurbit crops may be due to the cultivated plant species, in addition to the high prevalence of WMV.

Ecological fitting is the forerunner to diversification in a plant virus with broad host range

The evolution and diversification of ssRNA plant viruses are often examined under reductionist conditions that ignore potentially much wider biotic interactions. The host range of a plant virus is central to interactions at higher levels that are organized by both fitness and ecological criteria. Here we employ a strategy to minimize sampling biases across distinct plant communities and combine it with a high-throughput sequencing approach to examine the influence of four habitats on the evolution of Watermelon mosaic virus (WMV). Local, regional and global levels of genetic diversity that correspond to spatial and temporal extents are used to infer haplotype relationships using network and phylogenetic approaches. We find that the incidence and genetic diversity of WMV were structured significantly by host species and habitat type. A single haplotype that infected 11 host species of a total of 24 showed that few constraints on host species use exist in the crop communities. When the evolution of WMV was examined at broader levels of organization, we found variation in genetic diversity and contrasting host use footprints that broadly corresponded to habitat effects. The findings demonstrated that nondeterministic ecological factors structured the genetic diversity of WMV. Habitat-driven constraints underlie host use preferences.

Distribution and evolution of the major viruses infecting cucurbitaceous and solanaceous crops in the French Mediterranean area

Plant viral diseases represent a significant burden to plant health, and their highest impact in Mediterranean agriculture is on vegetables grown under intensive horticultural practices. In order to understand better virus evolution and emergence, the most prevalent viruses were mapped in the main cucurbitaceous (melon, squashes) and solanaceous (tomato, pepper) crops and in some wild hosts in the French Mediterranean area, and virus diversity, evolution and population structure were studied through molecular epidemiology approaches. Surveys were performed in summer 2016 and 2017, representing a total of 1530 crop samples and 280 weed samples. The plant samples were analysed using serological and molecular approaches, including high-throughput sequencing (HTS). The viral species and their frequency in crops were quite similar to those of surveys conducted ten years before in the same areas. Contrary to other Mediterranean countries, aphid-transmitted viruses remain the most prevalent in France whereas whitefly-transmitted ones have not yet emerged. However, HTS analysis of viral evolution revealed the appearance of undescribed viral variants, especially for watermelon mosaic virus (WMV) in cucurbits, or variants not present in France before, as for cucumber mosaic virus (CMV) in solanaceous crops. Deep sequencing also revealed complex virus populations within individual plants with frequent recombination or reassortment. The spatial genetic structure of cucurbit aphid-borne yellows virus (CABYV) was related to the landscape structure, whereas in the case of WMV, the recurrence of introduction events and probable human exchanges of plant material resulted in complex spatial pattern of genetic variation.

Identification, genetic diversity and recombination analysis of Watermelon Mosaic Virus isolates

Watermelon mosaic virus (WMV) is an important virus causing adverse effects on cucurbits throughout the world. In this study, we recorded WMV infection in the watermelon (Citrullus lanatus)-growing area of Alwar and Sikar in districts of Rajasthan, India. The RT-PCR-based detection was performed to confirm the presence of WMV, by using potyvirus-degenerated coat protein primers. Further, the complete genome sequences of two WMV isolates were compared with previously reported genome sequences. The complete genome of each isolate was 10,030 nt long, excluding the poly-A tails. Phylogeny relationships of the WMV isolates in the present study revealed the presence of uneven evolutionary pressure among the different WMV viral genomic segments. The analysis revealed that all the WMV isolates were divided into three clusters and the Indian WMV isolates cluster together with the French isolate. Recombination analysis of WMV exhibited significant recombination hotspots in the P1, NIa-Pro and Nib-CP regions. Our finding highlights the importance of genetic variability and recombination analysis to provide a better understanding of WMV molecular diversity.

Genetic variability of watermelon mosaic virus isolates infecting cucurbit crops in Italy

Watermelon mosaic virus (WMV; genus Potyvirus, family Potyviridae) is responsible for serious cucurbit yield losses worldwide. Different WMV genetic groups have been characterized so far. Among these, the "classical" (CL) group has been present in the Mediterranean basin for 40 years, whereas the "emergent" (EM) group includes isolates that are associated with more-severe symptoms observed since 2000. Information on the spatial and temporal evolution of WMV isolates in Italy is currently sparse. In this study, 39 WMV isolates samples collected in different regions over the last two decades were analysed at two different genomic regions that are known to be highly variable and contain recombination breakpoints. Most of the isolates collected between 2002 and 2009 were found to belong to the CL group, whereas the isolates from 2012 onwards were classified as EM, indicating that EM isolates have progressively displaced the CL population in Italy. Although genetic variability was observed within both CL and EM groups and recombinant isolates were detected, no positive selection or haplotype geographic structure were inferred. This suggest that the shift from CL to EM populations was likely due to multiple introductions of EM isolates in different regions of Italy rather than from genetic differentiation of local populations. The progressive increase in prevalence of the highly virulent EM populations is a serious concern because of their symptom severity, and the presence of multiple EM variants that include recombinants necessitates new efforts to develop durable control strategies.

Resistance to the Emerging Moroccan Watermelon Mosaic Virus in Squash

Moroccan watermelon mosaic virus (MWMV) represents an emerging threat to cucurbit production in the Mediterranean Basin. We sequenced the near complete genome of MWMV-SQ10_1.1, a cloned Spanish isolate. MWMV-SQ10_1.1 has the typical potyvirus genomic structure, and phylogenetic analysis showed that it shared a common ancestor with other Mediterranean MWMV isolates. We used MWMV SQ10_1.1 to inoculate plants in a collection of commercial squash cultivars, including some described as potyvirus resistant. All inoculated plants from all cultivars showed severe infection symptoms. Twenty-four Cucurbita spp. accessions were then tested for their susceptibility to MWMV-SQ10_1.1. Plants of the C. ecuadorensis PI 432441 accession showed no symptoms and their enzyme-linked immunosorbent assay readings were similar to uninfected controls. Progeny analysis of F1 and F2 populations suggested that two recessive genes control PI 432441 resistance to MWMV. We hypothesized that this resistance could be associated with alleles of genes encoding the eukaryotic translation initiation factor 4E (eIF4E), particularly after determination of its recessive nature. A multiple sequence alignment including the two eIF4E ortholog sequences from PI 432441 (CeeIF4E1 and CeeIF4E2) identified three amino acid substitutions in CeeIF4E1 and two amino acid substitutions in CeeIF4E2 potentially involved in potyvirus resistance. Polymerase chain reaction markers for CeeIF4E1 and CeeIF4E2 were developed and used to genotype 156 F2 individuals already phenotyped; this analysis did not support an association of either CeeIF4E2 or CeeIF4E1 with MWMV resistance.

Capsicum annum, a new host of watermelon mosaic virus

The occurrence of Watermelon mosaic virus (WMV) in sweet pepper (Capsicum annuum L.) in Kurdistan province, Iran was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR) and partial characterization of coat protein. To the best of our knowledge, this is the first report of WMV infecting C. annuum, adding a new host to list of more than 170 species infected by this virus.

Emergence of rice yellow mottle virus in eastern Uganda: Recent and singular interplay between strains in East Africa and in Madagascar

Epidemics of rice yellow mottle virus (RYMV) have developed recently in eastern Uganda, close to Lake Victoria in East Africa. Unexpectedly, all isolates from the affected area belonged to a single strain (named S4ug), a strain that is different from the S4lv strain that has been prevalent in the Lake Victoria basin for the past five decades. Interestingly, the S4ug strain is most closely related at the genomic level (except ORF1) to the strain present in Madagascar (S4mg), 2000km away. The minor parent of the S4mg recombinant strain could not be detected. Molecular clock dating analysis indicated that the singular sequence of events - that associated the emergence of a new strain (S4ug), a modular recombination between closely related strains (S4mg and S4ug) and a long distance transmission (S4mg) - occurred recently, within the past few decades. This finding is at variance with the process of gradual strain dispersal and diversification over two centuries throughout Africa that was previously established.

A short motif in the N-terminal part of the coat protein is a host-specific determinant of systemic infectivity for two potyviruses

Although the biological variability of Watermelon mosaic virus is limited, isolates from the three main molecular groups differ in their ability to infect systemically Chenopodium quinoa. Mutations were introduced in a motif of three or five amino acids located in the N-terminal part of the coat protein, and differing in isolates from group 1 (motif: lysine-glutamic acid-alanine (Lys-Glu-Ala) or KEA, systemic on C. quinoa), group 2 (Lys-Glu-Thr or KET, not systemic on C. quinoa) and group 3 (KEKET, not systemic on C. quinoa). Mutagenesis of KEKET in an isolate from group 3 to KEA or KEKEA was sufficient to make the virus systemic on C. quinoa, whereas mutagenesis to KET had no effect. Introduction of a KEA motif in Zucchini yellow mosaic virus coat protein also resulted in systemic infection on C. quinoa. These mutations had no obvious effect on the disorder profile or potential post-translational modifications of the coat protein as determined in silico.

Comparative molecular epidemiology provides new insights into Zucchini yellow mosaic virus occurrence in France

Zucchini yellow mosaic virus (ZYMV, genus Potyvirus) causes important crop losses in cucurbits worldwide. In France, ZYMV epidemics are sporadic but occasionally very severe. This contrasts with Watermelon mosaic virus (WMV, genus Potyvirus) which causes regular and early epidemics. Factors influencing ZYMV epidemiology are still poorly understood. In order to gain new insights on the ecology and epidemiology of this virus, a 5-year multilocation trial was conducted in which ZYMV spread and populations were studied in each of the 20 plot/year combinations and compared with WMV. Search for ZYMV alternative hosts was conducted by testing weeds growing naturally around one plot and also by checking ZYMV natural infections in selected ornamental species. Although similar ZYMV populations were observed occasionally in the same plot in two successive years suggesting the occurrence of overwintering hosts nearby, only two Lamium amplexicaule plants were found to be infected by ZYMV of 3459 weed samples that were tested. The scarcity of ZYMV reservoirs contrasts with the frequent detection of WMV in the same samples. Since ZYMV and WMV have many aphid vectors in common and are transmitted with similar efficiencies, the differences observed in ZYMV and WMV reservoir abundances could be a major explanatory factor for the differences observed in the typology of ZYMV and WMV epidemics in France. Other potential ZYMV alternative hosts have been identified in ornamental species including begonia. Although possible in a few cases, exchanges of populations between different plots located from 500 m to 4 km apart seem uncommon. Therefore, the potential dissemination range of ZYMV by its aphid vectors seems to be rather limited in a fragmented landscape.

Melon RNA interference (RNAi) lines silenced for Cm-eIF4E show broad virus resistance

Efficient and sustainable control of plant viruses may be achieved using genetically resistant crop varieties, although resistance genes are not always available for each pathogen; in this regard, the identification of new genes that are able to confer broad-spectrum and durable resistance is highly desirable. Recently, the cloning and characterization of recessive resistance genes from different plant species has pointed towards eukaryotic translation initiation factors (eIF) of the 4E family as factors required for the multiplication of many different viruses. Thus, we hypothesized that eIF4E may control the susceptibility of melon (Cucumis melo L.) to a broad range of viruses. To test this hypothesis, Cm-eIF4E knockdown melon plants were generated by the transformation of explants with a construct that was designed to induce the silencing of this gene, and the plants from T2 generations were genetically and phenotypically characterized. In transformed plants, Cm-eIF4E was specifically silenced, as identified by the decreased accumulation of Cm-eIF4E mRNA and the appearance of small interfering RNAs derived from the transgene, whereas the Cm-eIF(iso)4E mRNA levels remained unaffected. We challenged these transgenic melon plants with eight agronomically important melon-infecting viruses, and identified that they were resistant to Cucumber vein yellowing virus (CVYV), Melon necrotic spot virus (MNSV), Moroccan watermelon mosaic virus (MWMV) and Zucchini yellow mosaic virus (ZYMV), indicating that Cm-eIF4E controls melon susceptibility to these four viruses. Therefore, Cm-eIF4E is an efficient target for the identification of new resistance alleles able to confer broad-spectrum virus resistance in melon.

Viruses of cucurbit crops in the Mediterranean region: an ever-changing picture

Cucurbit crops may be affected by at least 28 different viruses in the Mediterranean basin. Some of these viruses are widely distributed and cause severe yield losses while others are restricted to limited areas or specific crops, and have only a negligible economic impact. A striking feature of cucurbit viruses in the Mediterranean basin is their always increasing diversity. Indeed, new viruses are regularly isolated and over the past 35 years one "new" cucurbit virus has been reported on average every 2 years. Among these "new" viruses some were already reported in other parts of the world, but others such as Zucchini yellow mosaic virus (ZYMV), one of the most severe cucurbit viruses and Cucurbit aphid-borne yellows virus (CABYV), one of the most prevalent cucurbit viruses, were first described in the Mediterranean area. Why this region may be a potential "hot-spot" for cucurbit virus diversity is not fully known. This could be related to the diversity of cropping practices, of cultivar types but also to the important commercial exchanges that always prevailed in this part of the world. This chapter describes the major cucurbit viruses occurring in the Mediterranean basin, discusses factors involved in their emergence and presents options for developing sustainable control strategies.

Simple sequence repeat markers linked to QTL for resistance to Watermelon mosaic virus in melon

A population of recombinant inbred lines (RIL) derived from a cross between the Watermelon mosaic virus (WMV) resistant genotype TGR-1551 and the susceptible Spanish cultivar 'Bola de Oro' has been evaluated for WMV resistance in spring, fall and growth chamber conditions. The quantitative trait loci (QTL) analyses detected one major QTL (wmv) on linkage group (LG) XI close to the microsatellite marker CMN04_35. This QTL controls the resistance to WMV in the three environmental conditions evaluated. Other minor QTLs affecting the severity of viral symptoms were identified, but they were not detected in all the assayed environments. The screening of the marker CMN04_35 in an F(2) progeny, derived from the same cross, confirmed the effect of this QTL on the expression of WMV resistance also in early generations, which evidences the usefulness of this marker for a marker assisted selection program.

Search for factors involved in the rapid shift in Watermelon mosaic virus (WMV) populations in South-eastern France

Watermelon mosaic virus (WMV, genus Potyvirus, family Potyviridae) was reported for the first time in France in 1974, and it is now the most prevalent virus in cucurbit crops. In 2000, new strains referred as 'emerging' (EM) strains were detected in South-eastern France. EM strains are generally more severe and phylogenetically distinct from those previously reported in this country and referred as 'classic' (CL) strains. Since 2000, EM strains have been progressively replacing CL strains in several areas where they co-exist. In order to explain this rapid shift in virus populations, the biological properties of a set of 17 CL and EM WMV isolates were compared. No major differences were observed when comparing a limited host range including 48 different plant species or cultivars. Only two species were differential; Chenopodium quinoa was systemically infected by CL and not by EM isolates whereas Ranunculus sardous was systemically infected by EM and not by CL isolates. A considerable variability was observed in aphid transmission efficiencies but this could not be correlated to the CL or EM types. Two subsets of five isolates of each group were used to compare aphid transmission efficiencies from single and double (CL-EM) infections using six different cucurbit and non-cucurbit hosts. EM isolates were generally better transmitted from mixed CL-EM infections than CL isolates and CL transmission rates were significantly lower from double than from single infections. Cross-protection was only partial between CL and EM strains leading to frequent double infections, and only a slight asymmetry was observed in cross-protection efficiencies. Since double infections occur very commonly in fields, the preferential transmission of EM from mixed CL-EM infections could be one of the factors leading to the displacement of CL isolates by EM isolates.

Recombination in natural populations of watermelon mosaic virus: new agronomic threat or damp squib?

Since their introduction in south-eastern France around 1999, new, 'emerging' (EM) strains of watermelon mosaic virus (WMV) coexist with the 'classic' (CL) strains present for more than 40 years. This situation constitutes a unique opportunity to estimate the frequency of recombinants appearing in the few years following introduction of new strains of a plant RNA virus. Molecular analyses performed on more than 1000 isolates from epidemiological surveys (2004-2008) and from experimental plots (2009-2010), and targeting only recombinants that became predominant in at least one plant, revealed at least seven independent CL/EM or EM/EM recombination events. The frequency of recombinants involving at least one EM parent in the natural populations tested was on the order of 1 %. No new recombinant was detected for more than 1 year, and none but one in more than one location. In tests comparing host range and aphid transmissibility, the new recombinants did not display a better fitness than their 'parental' isolates. No recombinant was detected from artificial mixed infections of CL and EM isolates of various hosts after testing more than 1500 subcultures obtained after single-aphid transmission. These results constitute one of the first estimations of the frequency of recombinants in natural conditions for a plant RNA virus. This suggests that although viable recombinants of WMV are not rare, and although recombination may potentially lead to new highly damaging strains, the new recombinants observed so far had a lower fitness than the parental strains and did not emerge durably in the populations.

Genetic Diversity of Watermelon mosaic virus in Slovakia and Iran Shows Distinct Pattern

Although Watermelon mosaic virus (WMV) is one of the main cucurbit pathogens and has a worldwide distribution, reliable data on its molecular variability is still limited to some geographical regions. The genetic diversity of 36 WMV isolates from Slovakia and Iran were studied by sequence analysis targeting two opposite genomic regions (P1 and NIb-CP). Phylogenetic analysis using partial sequences of the P1 gene showed that Slovak WMV isolates had greater diversity, representing two groups (group 1 and group 2), whereas all Iranian isolates belonged to a single group (group 2), with relatively low divergeance. Interestingly, in the NIb-CP region, all analyzed Slovak and Iranian isolates clustered within the group 1, thereby illustrating the phylogenetic discrepancies between the two analyzed genomic regions. Based on these data, one-half of analyzed Slovak isolates and all Iranian WMV isolates showed a switch in affiliation based on considered genomic region, clearly indicating their recombinant nature. This work provides further evidence of the significant contribution of recombination to the evolutionary history of WMV and outlines the necessity to target more than a single genome fragment for accurate typing of WMV isolates.

A single amino acid change in HC-Pro of soybean mosaic virus alters symptom expression in a soybean cultivar carrying Rsv1 and Rsv3

It is generally believed that infidelity of RNA virus replication combined with R-gene-driven selection is one of the major evolutionary forces in overcoming host resistance. In this study, we utilized an avirulent soybean mosaic virus (SMV) mutant to examine the possibility of emergence of mutant viruses capable of overcoming R-gene-mediated resistance during serial passages. Interestingly, we found that the emerged progeny virus induced severe rugosity and local necrotic lesions in Jinpumkong-2 (Rsv1 + Rsv3) plants, while SMV-G7H provoked a lethal systemic hypersensitive response. Genome sequence analysis of the emerged progeny virus revealed that the mutation in CI that had caused SMV-G7H to lose its virulence was restored to the original sequence, and a single amino acid was newly introduced into HC-Pro, which means that the symptom alteration was due to this single amino acid mutation in HC-Pro. Our results suggest that SMV HC-Pro functions as a symptom determinant in the SMV-soybean pathosystem.

Barriers to gene flow between emerging populations of Watermelon mosaic virus in Southeastern France

Since 1999, "emerging" (EM) strains of Watermelon mosaic virus (WMV) have been detected in cucurbit crops of southeastern France, probably as a result of recent introductions. Population genetic approaches were used to study the structure of EM isolates in southeastern France and to identify factors involved in their spatial distribution. A population clustering method (SAMOVA) and a maximum-difference algorithm (Monmonier's algorithm) were combined to visualize and quantify barriers to gene flow between populations. Both methods yielded similar results and two main barriers were identified. A North/South oriented barrier may be related to physical obstacles to gene flow (Rhône River, presence of an area with few cucurbit crops). Although the barrier was very strong, some "crossing" events were detected. A second barrier, oriented Northwest to Southeast, was not correlated with obvious geographical features. The two methods used here are complementary and confirm the limited spread of WMV-EM isolates. This approach can be useful in epidemiology studies to characterize the structure of viral populations and identify barriers to gene flow.

Mixed infections of Pepino mosaic virus strains modulate the evolutionary dynamics of this emergent virus

Pepino mosaic virus (PepMV) is an emerging pathogen that causes severe economic losses in tomato crops (Solanum lycopersicum L.) in the Northern hemisphere, despite persistent attempts of control. In fact, it is considered one of the most significant viral diseases for tomato production worldwide, and it may constitute a good model for the analysis of virus emergence in crops. We have combined a population genetics approach with an analysis of in planta properties of virus strains to explain an observed epidemiological pattern. Hybridization analysis showed that PepMV populations are composed of isolates of two types (PepMV-CH2 and PepMV-EU) that cocirculate. The CH2 type isolates are predominant; however, EU isolates have not been displaced but persist mainly in mixed infections. Two molecularly cloned isolates belonging to each type have been used to examine the dynamics of in planta single infections and coinfection, revealing that the CH2 type has a higher fitness than the EU type. Coinfections expand the range of susceptible hosts, and coinfected plants remain symptomless several weeks after infection, so a potentially important problem for disease prevention and management. These results provide an explanation of the observed epidemiological pattern in terms of genetic and ecological interactions among the different viral strains. Thus, mixed infections appear to be contributing to shaping the genetic structure and dynamics of PepMV populations.

An Amino Acid Deletion in Wheat streak mosaic virus Capsid Protein Distinguishes a Homogeneous Group of European Isolates and Facilitates Their Specific Detection

The tritimovirus Wheat streak mosaic virus (WSMV) is widespread throughout the world and represents a severe threat to cereal crop production. To increase knowledge of genetic diversity of WSMV in Europe, until now scarce, capsid protein (CP) sequences of several Czech, French, Italian, Slovak, and Turkish isolates have been determined. A multiple alignment of CP nucleotide sequences using available WSMV sequences revealed only limited sequence variation among 3 previously sequenced European isolates and the 14 European isolates sequenced in this study. Moreover, these isolates were characterized by an identical 3-nucleotide deletion, resulting in the lack of the Gly₂₇₆₁ codon within the CP region of the polyprotein. The results indicate that this monophyletic group of isolates (designated as WSMV-ΔE) is common and widely dispersed throughout the European continent. The close relationship of WSMV-ΔE isolates implies a single common ancestor and, presumably, subsequent dispersal throughout Europe from a single focus. We developed two simple assays for specific and accurate detection of WSMV-ΔE isolates. First, a conserved ClaI restriction site in the core CP gene sequence unique to WSMV-ΔE isolates was used for restriction fragment length polymorphism analysis of amplified polymerase chain reaction (PCR) products. Second, the conserved and specific codon gap in WSMV-ΔE sequences was used as a target to design specific primers functional in one-step reverse-transcription PCR detection of WSMV-ΔE isolates.