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

Tobacco Mild Green Mosaic Virus (TMGMV) Isolates from Different Plant Families Show No Evidence of Differential Adaptation to Their Host of Origin

The relevance of tobamoviruses to crop production is increasing due to new emergences, which cannot be understood without knowledge of the tobamovirus host range and host specificity. Recent analyses of tobamovirus occurrence in different plant communities have shown unsuspectedly large host ranges. This was the case of the tobacco mild green mosaic virus (TMGMV), which previously was most associated with solanaceous hosts. We addressed two hypotheses concerning TMGMV host range evolution: (i) ecological fitting, rather than genome evolution, determines TMGMV host range, and (ii) isolates are adapted to the host of origin. We obtained TMGMV isolates from non-solanaceous hosts and we tested the capacity of genetically closely related TMGMV isolates from three host families to infect and multiply in 10 hosts of six families. All isolates systemically infected all hosts, with clear disease symptoms apparent only in solanaceous hosts. TMGMV multiplication depended on the assayed host but not on the isolate's host of origin, with all isolates accumulating to the highest levels in Nicotiana tabacum. Thus, results support that TMGMV isolates are adapted to hosts in the genus Nicotiana, consistent with a well-known old virus-host association. In addition, phenotypic plasticity allows Nicotiana-adapted TMGMV genotypes to infect a large range of hosts, as encountered according to plant community composition and transmission dynamics.

Tobamoviruses Show Broad Host Ranges and Little Genetic Diversity Among Four Habitat Types of a Heterogeneous Ecosystem

Host ranges of plant viruses are poorly known, as studies have focused on pathogenic viruses in crops and adjacent wild plants. High-throughput sequencing (HTS) avoids the bias toward plant-virus interactions that result in disease. Here we study the host ranges of tobamoviruses, important pathogens of crops, using HTS analyses of an extensive sample of plant communities in four habitats of a heterogeneous ecosystem. Sequences of 17 virus operational taxonomic units (OTUs) matched references in the Tobamovirus genus, eight had narrow host ranges, and five had wide host ranges. Regardless of host range, the OTU hosts belonged to taxonomically distant families, suggesting no phylogenetic constraints in host use associated with virus adaptation, and that tobamoviruses may be host generalists. The OTUs identified as tobacco mild green mosaic virus (TMGMV), tobacco mosaic virus (TMV), pepper mild mottle virus, and Youcai mosaic virus had the largest realized host ranges that occurred across habitats and exhibited host use unrelated to the degree of human intervention. This result is at odds with assumptions that contact-transmitted viruses would be more abundant in crops than in wild plant communities and could be explained by effective seed-, contact-, or pollinator-mediated transmission or by survival in the soil. TMGMV and TMV had low genetic diversity that was not structured according to habitat or host plant taxonomy, which indicated that phenotypic plasticity allows virus genotypes to infect new hosts with no need for adaptive evolution. Our results underscore the relevance of ecological factors in host range evolution, in addition to the more often studied genetic factors. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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.

Phylogenetic and Phylodynamic Analyses of Soybean Mosaic Virus Using 305 Coat Protein Gene Sequences

Soybean mosaic virus (SMV) of the family Potyviridae is the most devastating virus that infects soybean plants. In this study, we obtained 83 SMV coat protein (CP) sequences from seven provinces in Korea using RT-PCR and Sanger sequencing. Phylogenetic and haplotype analyses revealed eight groups of 83 SMV isolates and a network of 50 SMV haplotypes in Korea. The phylogenetic tree using 305 SMV CP sequences available worldwide revealed 12 clades that were further divided into two groups according to the plant hosts. Recombination rarely occurred in the CP sequences, while negative selection was dominant in the SMV CP sequences. Genetic diversity analyses revealed that plant species had a greater impact on the genetic diversity of SMV CP sequences than geographical origin or location. SMV isolates identified from Pinellia species in China showed the highest genetic diversity. Phylodynamic analysis showed that the SMV isolates between the two Pinellia species diverged in the year 1248. Since the divergence of the first SMV isolate from Glycine max in 1486, major clades for SMV isolates infecting Glycine species seem to have diverged from 1791 to 1886. Taken together, we provide a comprehensive overview of the genetic diversity and divergence of SMV CP sequences.

Genetic Differentiation and Migration Fluxes of Viruses from Melon Crops and Crop Edge Weeds

Weeds surrounding crops may act as alternative hosts, playing important epidemiological roles as virus reservoirs and impacting virus evolution. We used high-throughput sequencing to identify viruses in Spanish melon crops and plants belonging to three pluriannual weed species, Ecballium elaterium, Malva sylvestris, and Solanum nigrum, sampled at the edges of the crops. Melon and E. elaterium, both belonging to the family Cucurbitaceae, shared three virus species, whereas there was no virus species overlap between melon and the other two weeds. The diversity of cucurbit aphid-borne yellows virus (CABYV) and tomato leaf curl New Delhi virus (ToLCNDV), both in melon and E. elaterium, was further studied by amplicon sequencing. Phylogenetic and population genetics analyses showed that the CABYV population was structured by the host, identifying three sites in the CABYV RNA-dependent RNA polymerase under positive selection, perhaps reflecting host adaptation. The ToLCNDV population was much less diverse than the CABYV one, likely as a consequence of the relatively recent introduction of ToLCNDV in Spain. In spite of its low diversity, we identified geographical but no host differentiation for ToLCNDV. Potential virus migration fluxes between E. elaterium and melon plants were also analyzed. For CABYV, no evidence of migration between the populations of the two hosts was found, whereas important fluxes were identified between geographically distant subpopulations for each host. For ToLCNDV, in contrast, evidence of migration from melon to E. elaterium was found, but not the other way around. IMPORTANCE It has been reported that about half of the emerging diseases affecting plants are caused by viruses. Alternative hosts often play critical roles in virus emergence as virus reservoirs, bridging host species that are otherwise unconnected and/or favoring virus diversification. In spite of this, the viromes of potential alternative hosts remain largely unexplored. In the case of crops, pluriannual weeds at the crop edges may play these roles. Here, we took advantage of the power of high-throughput sequencing to characterize the viromes of three weed species frequently found at the edges of melon crops. We identified three viruses shared by melon and the cucurbit weed, with two of them being epidemiologically relevant for melon crops. Further genetic analyses showed that these two viruses had contrasting patterns of diversification and migration, providing an interesting example on the role that weeds may play in the ecology and evolution of viruses affecting crops.

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.

Population Genomics of Plant Viruses: The Ecology and Evolution of Virus Emergence

The genomics era has revolutionized studies of adaptive evolution by monitoring large numbers of loci throughout the genomes of many individuals. Ideally, the investigation of emergence in plant viruses requires examining the population dynamics of both virus and host, their interactions with each other, with other organisms and the abiotic environment. Genetic mechanisms that affect demographic processes are now being studied with high-throughput technologies, traditional genetics methods, and new computational tools for big-data. In this review, we discuss the utility of these approaches to monitor and detect changes in virus populations within cells and individuals, and over wider areas across species and communities of ecosystems. The advent of genomics in virology has fostered a multidisciplinary approach to tackling disease risk. The ability to make sense of the information now generated in this integrated setting is by far the most substantial obstacle to the ultimate goal of plant virology to minimize the threats to food security posed by disease. To achieve this goal, it is imperative to understand and forecast how populations respond to future changes in complex natural systems.