麻豆淫院

How long have viruses been around? No one knows. Scientists at Portland State University have begun taking the first steps toward answering this question.

Viruses are a curious lot. The standard drawing of the tree of life, the one you find on the inside back cover of biology textbooks, is divided into three branches: Archaea, Bacteria and Eukarya. Viruses don鈥檛 make it onto the page.

That makes sense, some scientists argue, because they鈥檙e not alive. They can鈥檛 reproduce on their own; they require the cozy environment of living cells for their survival. Others disagree. Not only are viruses alive, they say, but genetic evidence indicates that they may have been the first forms of life on Earth, predating cellular life.

But really, says Ken Stedman, associate professor of biology at Portland State University, 鈥淣obody knows how old viruses are.鈥� Stedman and his colleagues are working to change that.

Unlike bacteria, for which there is an undisputed rock record going back as far as 3.5 billion years, there is no known fossil record for viruses. 鈥淎s far as we know,鈥� Stedman says, 鈥渘o-one has looked.鈥� It鈥檚 possible, he concedes, that there have been efforts, unsuccessful efforts that no one knows about, to uncover fossil viruses. But 鈥淚f you don鈥檛 find something, you鈥檙e not likely to report it.鈥�

In an attempt to learn how long viruses have been around, Stedman鈥檚 lab, in work led by his doctoral student, Jim Laidler, is trying to figure out what kind of biosignatures viruses might leave behind. This lab-first approach, Stedman said, is necessary because 鈥渨e don鈥檛 know what we鈥檙e looking for yet.鈥�

Much of the work of Stedman鈥檚 group is done in hot springs. Silica hot springs, in particular, are known to provide a good preservation environment for bacteria. Silica precipitates out of the hot water as the water cools, and bacteria that are present can become entombed in the silica-rich rock that forms. In the process, the shapes and the chemical signatures of these bacteria can be preserved.

Stedman鈥檚 group wanted to see if they could induce a similar process using viruses instead of bacteria. They first virus they chose was Bacteriophage T4, 鈥渟ort of the prototypical virus,鈥� which has a well-known shape. It 鈥渓ooks like the lunar lander,鈥� Stedman said.

Mixing the virus with silica-rich hot-spring water yielded a positive result, briefly. For a few days, under the microscope, virus shapes could be seen embedded within the silica. But 鈥渁fter about a week it starts to look like a blob,鈥� Stedman said. Not too hopeful a result, if the ultimate goal is to look for visual evidence of viruses in rocks billions of years old.

But Stedman and his colleagues are not deterred. They have a number of additional avenues of investigation planned. One is to try the experiment with viruses that come from hot springs, where Stedman does much of his research. 鈥淚t may be that these are much more resistant鈥� to disintegration, he said, that they 鈥渇orm a structure鈥� that is 鈥渂etter conserved鈥� over time.

Another approach researchers plan to pursue is looking for chemical signatures. Although the physical shape of the virus doesn鈥檛 stick around for long, it鈥檚 possible that a detectable chemical signal may remain for longer.

Alternatively, because some viruses have lipid coatings, similar to the lipids found in the walls of living cells, lipids may provide a detectable chemical signature. Virus lipids can differ chemically from the lipids found in their microbial hosts, and that distinction may prove useful in the chemical detection of fossil viruses. 鈥淲e don鈥檛 know if there is actually a different signature between the host and the virus,鈥� Stedman said.

So far, research has focused on the silicification of individual virus particles. Some viruses, however, particularly those that live inside algae, tend to cluster together in large numbers. They 鈥渁lmost form crystalline arrays,鈥� Stedman said. These larger masses may fossilize in ways that are easier to detect.

Laidler plans to explore all these various approaches in the lab, and also to study hot-spring sites in search of effects that can be clearly identified on a short time scale. If those efforts pan out and clear chemical biosignatures of virus signatures can be identified, Stedman says, 鈥渢hose then could become something that we can now start to look at back through the rock record.鈥�

Results of Laidler鈥檚 initial work on virus silicification was published in the July/August 2010 issue of the journal Astrobiology. He and Stedman hope that future research into fossil viruses will help to fill in gaps in biologists鈥� knowledge about the role viruses played in the development of life on Earth. It may even answer the question: which came first, the virus or the cell?