Plant Virus Biodiversity and Ecology

FAQ (without definitive answers)

• Does virus phylogeny map onto plant phylogeny? What role have viruses played in the phylogeny of major plant groups?
  • At the level of viral families and higher taxonomic levels, no mapping is obvious. Most viral families contain members that are found in plant species of a variety of families, Conversely, chances are good that members of a plant family are hosts for at least one virus of each viral family.
  • At a finer level of virus taxonomy, there are indications that some taxonomic divisions segregate viral family members according to host of isolation. For example, reasonable subdivision of the tobamovirus genus produces assemblages that contain viruses of exclusively cucurbit, exclusively legume, or exclusively solanaceous plants.
• Do virus communities map onto plant phylogeny?
• Within a plant species, do virus communities vary predictably according to environmental gradients? How do virus communities vary as a function of environmental gradients known to be important for vascular plants?
  • The presence or absence of a vectored virus in a particular plant species at any one location will depend on the presence of vectors for its transmission. Vectors respond to environmental and climatic factors.
  • Some viruses are known to have optimal temperatures for replication, such that at suboptimal temperatures there may be insufficient virus for efficient transmission.
• Does virus load vary as a function of stress?
  • Stress on a plant will induce a non-specific resistance response which will limit new infection.
• Do common plant species have more virus species than rare plant species?
  • This is difficult to determine from current data. The primary "common" plant species examined are crop species. Host range determinations rarely address the susceptibility of rare plant species.
• Is viral diversity correlated with geographic range?
• Does 'distance decay' exist in virus communities?
  • Epidemiological studies suggest that vector behavior is the major determinant of the pattern of distribution of viruses newly introduced into a field.
• Do virus abundance distributions conform to expectation from the Unified Neutral Theory?
• Are virus abundance distributions lognormal?
• Is tree age correlated with virus diversity?
  • Tree age is likely correlated with virus diversity. Old citrus trees contain more distinguishable strains of citrus tristeza virus than do young trees.
  • Whether there is a correlation with virus biodiversity also is not clear. Fruit trees in the northwest are known to bear several distinct viral species. So, the information for an answer may exist.
• Are there seasonal changes in virus communities?
  • Certainly where annual hosts are among the hosts. The young seedlings most likely do not have viruses. Viruses will be acquired from vectors. If the viruses are too pathogenic or when frost comes, the host will die--and the virus may then disappear from the community.
  • Seasonal changes in the biodiversity of viruses in perennials are not expected, though the levels of such viruses may oscillate with season. Also, additions of new viral species to the community would be expected to occur seasonally.
• Are annuals less likely to have high viral loads? Are some habitats more conducive to high viral loads than others?
  • Viral load, the amount of virus present per mass of tissue, depends on many factors. A major one is the virus itself, since some viral species accumulate to high levels and others do not.
  • Annuals would be expected to have a lower biodiversity of viruses per plant as their chances for becoming infected are less than for annuals.
• Is virus species biodiversity related to plant biodiversity?
  • Within a given geographic area, one would expect a positive correlation. However, it is unlikely that this expectation has been tested yet.
• Are native species likely to have higher viral loads than exotic species? Are successful invaders relatively virus-free? Are exotic plant species more or less prone to infection than native species?
  • A recent study suggests that "successful" exotic species are more pathogen resistant than their native relatives.
• Does a diverse plant community confer protection from viruses?
• Can viruses control plant fitness in nature? If so, do they do so in a density-dependent manner?
  • Some virus plant interactions result in a decreased plant fitness. However, it is not clear how important this is. It could be argued that a virus should increase the fitness of at least some species in order for the virus to be successful. One scenario to imagine how this might happen is that a virus may have negligible effect on the growth and health of a plant species when tested in monoculture. In the presence of other susceptible plant species, species that experience a decrease in fitness as a result of infection by this virus, the plant gains a competitive advantage by having a virus that can decrease fitness of competitors.
  • Plant density is important when transmission is needed.
• How are virus communities affected by fire and grazing?
  • Reestablishment of virus communities can come from
    • Seeds, for seed transmitted viruses
    • Roots and crowns that survive fire or grazing
    • Invasion from non-burned or non-grazed neighboring regions.
• To what degree does the 'extended phenotype' (sensu Dawkins) of a virus facilitate its dispersal?
  • There has been research on the hypothesis that the attractiveness of virus-induced pgiment patterns to insects is an important component of the dispersal of some viruses.
• What is the role of viruses in the maintenence of plant biodiversity?
  • Unknown?
• How many plant virus 'species' are there'?
  • Currently recognized by the ICTV, there are about 900. How many there actually are is unknown and highly dependent on how one defines a "species".
• Does clonality facilitate infection?

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Page posted May 8, 2006