Thursday, April 13, 2006

Viruses - they're not just diseases anymore

There's been a rash of articles on viruses recently. Carl Zimmer posted on the genetic remnants of retroviruses that infected the genomes of organisms millennia ago; it turns out that they've left markers that allow us to track the evolutionary history of many lineages of animals. Even more recently Zimmer posted on a virus that may cause prostate cancer, as well as speculation that some primates may be resistant to HIV because they carry copies of the virus's genetic material in their own DNA.

And, in March, a PLOS Biology article (Wren et al. 2006) announced the start of a project to study the biodiversity and ecology of plant viruses.
The Plant Virus Biodiversity and Ecology (PVBE) project has been initiated to survey the biodiversity of viruses affecting vascular plants, including their endophytic fungi, in The Nature Conservancy's Tallgrass Prairie Preserve of Oklahoma, home to over 700 plant species.
The article reports that only about 2,000 species of viruses have been identified, yet cites a paper (Breitbart and Rohwer 2005) reporting that "there are an estimated 5,000 viral genotypes in 200 liters of seawater and possibly a million different viral genotypes in one kilogram of marine sediment." In short, there are a lot of viruses out there that haven't been identified yet, and it's likely that a lot of those viruses affect plants in the wild:
approximately 60% of plants surveyed in a Costa Rican region containing about 7,000 plant species total were positive for double-stranded RNA, a marker suggesting the presence of viruses.
However, the only type of plant viruses that have been studied in any depth to date are those that cause diseases in crops. Since the authors argue that it's likely the majority of viruses do not cause disease, but instead interact with their hosts in non-pathogenic relationships, by gaining an understanding of plant viruses they may "revolutionize" plant physiology and ecology:
Not only will these efforts help us understand virus ecology, but there is also great potential for revolutionizing plant ecology. The extended phenotype of a virus may affect a plant's local adaptation to its environment. Endophytes, mycorrhizae, or other symbionts could potentially mediate such interactions. Given that many plant viruses are generalists with respect to host species, it is theoretically possible that such effects may be ecosystem-wide. RNA silencing [6] has dramatic but unexplored ecological implications. As a hypothetical example, a bison-borne virus could silence genes for antigrazing defenses, thus facilitating its transmission.
This is a good example of how little we know about the world around us: there are probably thousands of viruses with myriad functions living in the plants we see on a daily basis, yet we have no idea that they're even there, much less any clue what they're doing. Hopefully in a few years we'll hear back from Wren et al. and learn more.

Breitbart M, Rohwer F (2005) Here a virus, there a virus, everywhere the same virus? Trends Microbiol 13: 278–284. (abstract)

Wren JD, Roossinck MJ, Nelson RS, Scheets K, Palmer MW, et al. (2006) Plant Virus Biodiversity and Ecology. PLoS Biol 4(3): e80 (full text)

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