When is a virus not a virus?

A fascinating new post from Science Daily – for which, thanks Vaibhav Bhardwaj – raises again the problem of just exactly what is (and what is not) a virus?

The article in question describes a fascinating three-way interaction between a virus, a parasitic wasp, and a caterpillar – with a virus which happens to be part of the germline of the wasp, gets injected into the caterpillar as a free genome, and modifies its immune system so as to tolerate the wasp’s eggs.

From the article:

“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.”

Funny thing: that hasn’t stopped the International Committee on Taxonomy of Viruses (ICTV) from recognising as viruses agents such as Petunia vein clearing virus, a pararetrovirus (=DNA retrovirus) with an activatable integrated form, Banana streak virus, which is present in and activatable from the genomes of most Musa species, or even two whole  families of retrovirus-like retrotransposable elements in the Pseudoviridae and the Metaviridae.

So there is ample precedent for things that integrate into host genomes, that are also viruses – including, among vertebrates, where activatable endogenous retroviruses are the subject of much study – including a finding that prions may activate endogenous retroviruses in the human brain.

And what of polydnaviruses (family Polydnaviridae)?  Well, fascinating and unique beasts, these: the two genera so far described – Bracovirus and Ichnovirus – contain viruses which have a variable number of circular double-stranded DNA components, with components ranging in size from 2 to >31 kbp, for a total genome size between 150 – 250 kbp.  Both sets of viruses occur as integrated proviruses in the genomes of endoparasitic hymenopteran wasps, replicate by amplification of the host DNA, followed by excision of episomal genomes by site-specific recombination, and only produce particles by budding from (ichnoviruses) or lysis of (bracoviruses) calyx cells in the oviducts of female wasps during pupal-adult transition.  Moreover, the viruses in the two groups may well not be evolutionarily linked to one another, given that there is no antigenic or genome similarity, and the particles formed by the two groups are very different: ichnoviruses make ellipsoidal particles with double membranes containing one nucleocapsid; bracoviruses make single-enveloped particles containing one or more cylindrical nucleocapsids.  The latter may derive from nudiviruses, which appear to have contributed very substantially to wasp survival.

From another Science Daily article:

“What this means… is that nudiviruses infected wasps a few million years ago and that, over time, the viral DNA fully integrated into the wasp genome. As it currently stands, the wasps need the virus to survive, because the virus helps the insects lay eggs in caterpillars. The virus also needs the wasp to survive, because the virus can only replicate in the wasp’s ovaries. The virus cannot replicate inside the caterpillar, because all of its replication machinery is inside the wasp.”

 Particles are injected along with eggs into larvae of lepidopteran hosts; the DNA gets into secondary host cells and is expressed, but does not replicate -and this expression leads to some quite profound phsiological changes, many of which are responsible for successful parasitism.  The association between wasp and virus has been termed an “obligate mutualistic symbiosis”, and appears to have evolved over more than 70 million years.  It does nothing for the lepidopterans, however….

But nothing in this association would lead me to doubt their nature as viruses.  Viruses now dependent on a particular host, maybe, but there now appears to be a continuum in the virus life experience, from massive bigger-than-cell-genome mimiviruses, through parasitic viruses to tiny circoviruses – and including agents that have become irretrievably integrated into the genomes of other hosts, yet still have a partially independent existence, like pseudoviruses – and polydnaviruses.

Viruses are wonderful…B-)

One Response to “When is a virus not a virus?”

  1. Dorian Says:

    There’s also a kind of bacterial equivalent to nudiviruses – Wolbachia. These things are obligate intracellular parasites that are vertically transmitted in the oocyte cytoplasm. The relationship with their insect hosts seems to have moved more towards mutualism, and there are even some insects that become infertile if their Wolbachia are removed by antibiotic treatment. They have become dependent on their parasite/symbiont.

    Also, it appears that Wolbachia confers resistance to RNA virus infection on the host (http://www.ncbi.nlm.nih.gov/pubmed/19222304), so carrying them may have a significant survival advantage.

    Just thought you’d want to know.


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