Research on viral origins suggests new definition of virus may be needed

Feb 12, 2009 by Diana Yates
A parasitic Cotesia wasp preparing to oviposi onto a Manduca caterpillar. Rights-protected photo courtesy Alex Wild (myrmecos.net)

(PhysOrg.com) -- The strange interaction of a parasitic wasp, the caterpillar in which it lays its eggs and a virus that helps it overcome the caterpillar’s immune defenses has some scientists rethinking the definition of a virus.

In an essay in the journal Science, Donald Stoltz, a professor of microbiology and immunology at Dalhousie University, in Halifax, Nova Scotia, and James Whitfield, a professor of entomology at the University of Illinois, report that a new study also appearing in Science shows how the diverse ways in which viruses operate within and among the organisms they encounter may not be fully appreciated. The study, from a team of researchers led by the Université François Rabelais, in Tours, France, found that the genes that encode a virus that helps wasps successfully parasitize caterpillars are actually integrated into the wasps’ own chromosomes. These genes, which they show to be related to those from another known group of viruses, are an indivisible part of the wasp’s genetic heritage; they are passed down from one generation to another of parasitoid wasps.

While it is not unusual for virus DNA to become embedded in the chromosomes of their hosts, in this case the wasp is not the only “host” of the virus. The viral genes do replicate (copy themselves) inside the wasp (the permanent host), but they actually target - and act upon - the immune system of the caterpillar (a more transient host).

“The unique thing about these viruses is that the organism into whose DNA their genes are embedded in is not the same one that their genes are actually targeted to operate on,” Whitfield said. “So it’s sort of like having two hosts, except that there’s not a complete life cycle in either host.”

The virus is beneficial to the wasp and depends on the wasp for its own survival, suggesting a kind of obligate mutualism that is not normally seen in viruses, Whitfield said.

Researchers have known for about 40 years that some species of parasitoid wasps inject these viruses, known as polydnaviruses, into the body cavities of caterpillars at the same time that they lay their eggs in the caterpillars. Because these “virus-like particles” have become an integral part of the wasp genome, some researchers have suggested they should no longer be considered viruses.

“It’s true that the wasp DNA and the viral DNA are now combined into the same genome, so maybe it’s not productive to think of them as separate entities,” Whitfield said. “But on the other hand, if you really want to understand them well, it does help to know where things come from.”

Whitfield and Stoltz have each spent decades studying the interplay of parasite and pathogens that makes up the life cycle of the parasitoid wasps. In their essay, they suggest that taxonomists of viruses take a new look at how viruses are defined.

“Many virology texts won’t even mention polydnaviruses,” Whitfield said. “The issue we bring up is: Do we want to call these viruses? And if not, why not? Because they certainly started out as viruses. And if so, then we have to change the definition of viruses to somehow specify what it is that a virus has to contain, and what it has to do, to be considered a virus.”

Provided by University of Illinois at Urbana-Champaign

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vivcollins
not rated yet Feb 12, 2009
I had a bad moment reading this when I thought about his conclusions, and then birds the common cold and us
bmcghie
not rated yet Feb 12, 2009
Don't worry, until the birds have integrated the virus and start spewing it in an active form as a biological weapon. Very cool stuff!
tkjtkj
not rated yet Feb 13, 2009
"The virus is beneficial to the wasp and depends on the wasp for its own survival..."

The above seems to perpetuate the myth that viruses are 'alive'.. they are NOT.. they are
collections of NON-living molecules .. 'Survival' is a term best used with respect to living organisms. Any biologist should appreciate that viruses are nothing but a kind of 'messenger' that exchanges info between and among living organisms. Bacteria 'ingest' them, expel them, and manufacture multiple copies of them .. but they do NOT give them 'life' , anymore than the IP packets here are alive on the net!
j. anderson, md
tkjtkj@gmail.com
moj85
3 / 5 (1) Feb 13, 2009
Uh huh. MD, eh? doubt that highly. Anyone with an 'md' would know there are different definitions to 'living'. Viruses can be classified as 'alive' if you define living as being able to pass on genetic information to future generations. In that way, viruses are alive. And way better at passing on that information than pretty much anything else (Think of how fast viruses can propagate). IP packets are a poor analogy because the IP packet has an endpoint and isn't propagated indefinitely.
Adriab
not rated yet Feb 13, 2009
I don't think this work would really challenge the definition of a virus. In this case a virus is acting symbiotically with the wasp. Pretty cool stuff though.
tkjtkj
not rated yet Feb 13, 2009
moj85 .. you think that calling me a liar helps you get your point across? do you know its illegal to represent oneself as having the m.d. degree? Its also illegal to defame a person without proof. Which proof would you like?
tkjtkj
not rated yet Feb 13, 2009
moj85
And you are entirely wrong .. 'being able to transmit genetic information' is NOT any independent criteria for 'life'.
There are such things as: independent reproduction, metabolism, respiration, ... all of which you seem to ignore.

moj85
not rated yet Feb 13, 2009
Sore subject for you, is it, tkjtkj? I was merely pointing out that your definition of life is not the only one. There are many different views. How can independent reproduction be classified? There are many symbiotic relationships that exist in 'living' things, such as parasitic wasps, that would not be able to reproduce without the caterpillars/insects they inject their eggs into. That is not independent reproduction.

My point being, there isn't one way to look at it. Without genetic information, none of your criteria for life would work. Respiration is around because DNA (or RNA) codes for it to exist.
superhuman
5 / 5 (1) Feb 14, 2009
The term virus can be understood to mean viral particle only and in this sense viruses are not alive as they cannot replicate. Although this is what is often meant by virus I think it we should stick to the term virion in this case.

On the other hand virus can be thought of as a life form, not a normal one but rather a complex, composite life form which is made of a core viral genome and a set of host cells capable of propagating it.

I think this definition better captures what viruses really are although some might find it weird that a given cell may serve as a basis for more then one life form. It's not so strange if you accept that genetic material is what differentiates various life forms, viruses change genetic material of host cells, either temporary or permanently, so in the process of infection the host cell is turned into a different life form - a virus.

A given cell can become more then one viral life at a time if it is infected by two or more viruses, also in the case of some viruses whose infection is not permanent the cell may revert to being the host after some time.

Using this definition the vasp and the virus are a single life form and those cells of caterpillars which are also able to propagate virus particles should also be considered part of this composite life form.

In the end the problem of definition is not really that important, all our classifications are just attempts to draw artificial lines where no such lines exist in reality. While it works well in many cases it's important to remember it's only an approximate tool which serves to organize knowledge, nothing more.
Sean_W
1 / 5 (2) Feb 14, 2009
"'Survival' is a term best used with respect to living organisms."

Not to be picky Dr. Anderson but why? I have heard both specialists and laypeople using the term to indicate resisting damage decay or destruction.

"Any biologist should appreciate that viruses are nothing but a kind of 'messenger' that exchanges info between and among living organisms."

And are dependent on a host for their continued perpetuation - there survival.

And expressing doubt about someone's claim to credentials is not defamation. Not even close. If it were, con men would be a lot safer since no one would risk calling their bluff by asking for proof of credentials.

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