Giant viruses found in Austrian sewage fuel debate over potential fourth domain of life

[beam me up scottie]

Giant viruses found in Austrian sewage fuel debate over potential fourth domain of life
By Mitch Leslie
Apr. 6, 2017

Tourists visiting the town of Klosterneuburg in eastern Austria often head for the 12th century monastery or the nearby memorial to author Franz Kafka. Virologists and evolutionary biologists, however, may one day pay homage to the town's sewage treatment plant, which has yielded a genome that appears to be from the most cell-like viruses yet. These oddities challenge the controversial hypothesis that so-called giant viruses are descendants of a vanished group of cellular organisms—a fourth domain of life. Instead, the study argues, these outsized viruses have more pedestrian origins.

"I found (the work) very convincing," says environmental virologist Matthias Fischer of the Max Planck Institute for Medical Research in Heidelberg, Germany. "Based on the data available now, I would not put my money on the fourth domain hypothesis."

Most viruses are much smaller than cells and need few genes because they replicate by co-opting the machinery of their hosts. Certain bird and pig viruses, for example, get by with just two genes, compared with nearly 4400 genes in a common strain of the intestinal bacterium Escherichia coli. Because viruses cannot reproduce independently and lack other hallmarks of cellular organisms, biologists have typically blackballed them from the club of life.

The first report of giant viruses, in Science in 2003, jolted researchers. Not only are these viruses larger than many microorganisms, but they can carry more than 2500 genes, surpassing many bacteria. These behemoths required revisions to the evolutionary tree of life, some scientists contended. The standard tree has three main groups, or domains—bacteria, archaea, and eukaryotes. But several researchers proposed that giant viruses are leftovers of a fourth domain of life. In this view, their ancestors were now-extinct cells that over time ditched many genes and became parasites.

Other scientists, such as evolutionary biologist Eugene Koonin of the National Center for Biotechnology Information in Bethesda, Maryland, saw no need for a fourth domain. "It's crystal clear that these giant viruses belong to a group of viruses that includes much smaller ones," he says. He believes they evolved when some of these smaller viruses incorporated more and more DNA from hosts and became massive.

http://www.sciencemag.org/news/2017/04/giant-viruses-found-austrian-sewage-fuel-debate-over-potential-fourth-domain-life

[Historic NY]

1. Holy crap, where did they come from?

[beam me up scottie]

2. That's what scientists are debating.

In Giant Virus Genes, Hints About Their Mysterious Origin

As for the mysterious origins of giant viruses, they found some hints in the genes of the newly discovered Klosneuviruses. After sequencing their genes, Koonin and his colleagues found a surprisingly large number that are involved in making proteins, in addition to some for copying DNA.

Viruses aren't supposed to have those kinds of genes, especially not such a comprehensive set of them, because they aren't capable of doing those things themselves. It's one reason why they're usually so small, because instead of carrying around all the genetic machinery for replication, they just hijack the machinery of their host cells.

But if viruses can't reproduce on their own, then what's the point of carrying around a bunch of genetic baggage?

***

Based on that family tree, they conclude that giant viruses likely started out as much smaller viruses, snowballing into giant ones over many generations as they gathered bits and pieces of genetic material from various hosts they infected.

If that's true, it would mean they are like genetic hoarders, collecting lots of genes and rarely throwing any out, until they balloon into virus Godzillas.

***

According to another hypothesis, giant viruses didn't start small and get big. They started enormous, as actual cellular life forms, and shrank down over time. They might have even originated from a so-called "fourth domain" of life that no longer exists. (The current three domains are Eukarya, which includes us, Archaea and Bacteria).

"We all have questions about how they evolved, what is their origin and how they contributed to life and the evolution of life on Earth," says Abergel. "I don't think anybody has a real answer."

***

Rodrigo Araujo Rodrigues, a virologist and evolutionary biologist at the Federal University of Minas Gerais in Brazil, thinks there's compelling evidence for both hypotheses.

***

One thing that everyone in the field can agree on is how dramatically the discovery of giant viruses has turned scientific assumptions upside down.

"The discovery of giant viruses is clearly opening other ways of thinking. Everybody is understanding that we never understood anything about viruses in general," says Abergel. "This is one of the main accomplishments of the discovery of giant viruses, that everything has to be reconsidered."

http://www.npr.org/sections/health-shots/2017/04/06/522478901/in-giant-virus-genes-hints-about-their-mysterious-origin

Of course there's always a chance it was the aliens...

[Laffy Kat]

3. Quick! Someone write a grade B movie pitch. nt

[beam me up scottie]

4. I'm ready!