Hi everyone,
Here is a Science Daily article about viruses and the latest understanding that they are living organisms that merit a place in the Tree of Life. They help spread diversity.
http://www.sciencedaily.com/releases/2015/09/150925142658.htm*************************************************************************
A new analysis supports the hypothesis that viruses are living entities that share a long evolutionary history with cells, researchers report. The study offers the first reliable method for tracing viral evolution back to a time when neither viruses nor cells existed in the forms recognized today, the researchers say.
Part of the confusion stems from the abundance and diversity of viruses. Less than 4,900 viruses have been identified and sequenced so far, even though scientists estimate there are more than a million viral species. Many viruses are tiny -- significantly smaller than bacteria or other microbes -- and contain only a handful of genes. Others, like the recently discovered mimiviruses, are huge, with genomes bigger than those of some bacteria.
Today, many viruses -- including those that cause disease -- take over the protein-building machinery of host cells to make copies of themselves that can then spread to other cells. Viruses often insert their own genetic material into the DNA of their hosts. In fact, the remnants of ancient viral infiltrations are now permanent features of the genomes of most cellular organisms, including humans. This knack for moving genetic material around may be evidence of viruses' primary role as "spreaders of diversity," Caetano-Anollés said.
"This tells you that you can build a tree of life, because you've found a multitude of features in viruses that have all the properties that cells have," Caetano-Anollés said. "Viruses also have unique components besides the components that are shared with cells."
In fact, the analysis revealed genetic sequences in viruses that are unlike anything seen in cells, Caetano-Anollés said. This contradicts one hypothesis that viruses captured all of their genetic material from cells. This and other findings also support the idea that viruses are "creators of novelty," he said.
The data suggest "that viruses originated from multiple ancient cells ... and co-existed with the ancestors of modern cells," the researchers wrote. These ancient cells likely contained segmented RNA genomes, Caetano-Anollés said.
Some scientists have argued that viruses are nonliving entities, bits of DNA and RNA shed by cellular life. They point to the fact that viruses are not able to replicate (reproduce) outside of host cells, and rely on cells' protein-building machinery to function. But much evidence supports the idea that viruses are not that different from other living entities, Caetano-Anollés said.
"Many organisms require other organisms to live, including bacteria that live inside cells, and fungi that engage in obligate parasitic relationships -- they rely on their hosts to complete their lifecycle," he said. "And this is what viruses do."
The lack of translational machinery in viruses was once cited as a justification for classifying them as nonliving, he said.
"This is no more," Caetano-Anollés said. "Viruses now merit a place in the tree of life. Obviously, there is much more to viruses than we once thought."
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For information.
Cheers.
Sriram
