TWiV 443: On a leaf, no one can hear you scream

The TWiVsters reveal the puppet master: an RNA virus injected with wasp eggs that paralyzes the ladybug so that she protects the cocoon until the adult emerges.

You can find TWiV #443 at microbe.tv/twiv, or listen below.

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Bodyguard manipulation by a virus

Coccinella septempunctata with Dinocampus coccinellae cocoonHost behavior alteration by viruses is known to assist the development of another organism. An example is a parasitoid wasp that injects viruses along with eggs into a caterpillar. The viral genomes encode proteins that subvert the caterpillar immune response, allowing the wasp larva to develop. A similar strategy may enable safe development of a wasp by a ladybeetle.

The parasitoid wasp D. coccinellae lays its eggs inside a ladybeetle. After 20 days of larval development, a prepupa emerges from the ladybeetle and fabricates a cocoon between the beetle’s legs. At the same time, the ladybeetle becomes paralyzed. It remains on top of the cocoon (pictured; image credit), protecting it until an adult wasp emerges. Remarkably, some ladybeetles then resume their normal lives!

Given what we know about how parasitoid viruses can alter the manipulation of their hosts, it was only logical to search for a virus that paralyzes the ladybeetle. Sequencing of RNA from the heads of parasitized ladybeetles revealed the presence of an RNA virus which the authors call D. coccinellae paralysis virus, DcPV. The virus is a new member of a Iflaviruses, a family of picornavirus-like, (+) strand RNA viruses that infect insects. DcPV was found in wasps in Poland, Japan, and The Netherlands, confirming its cosmopolitain nature.

Viral particles were observed in cells lining the wasp oviducts, but not in the lumen. Viral genomes were undetectable in wasp eggs, became more abundant during hatching, and ceased to replicate in adult wasps. The levels of virus in the ladybeetle abdomen and head increase with time to egress, suggesting that it was transmitted from the wasp larvae to the host. In ladybeetles where the wasp egg did not develop, viral replication does not occur.

DcPV appears to be neurotropic. Before larval egression, no changes were observed in the nervous system of the ladybeetle, but glial cells were full of virus particles. After egression, vaculoles developed in glial cells and neurons degenerated. This damage was less severe in beetles that survived and recovered from paralysis. An expansion of glial cells in these hosts might explain how normal brain functions were restored.

Insects respond to infection with an RNA-based antiviral response. Components of the RNA based immune system were down-regulated during larval development, possibly by viral proteins, allowing virus to invade the nervous system. Resumption of the antiviral reponse might enable recovery of the ladybeetle after emergence of the wasp.

It appears that DcPV is a wasp symbiont that manipulates the behavior of the ladybeetle host to ensure development of wasp offspring. This hypothesis can be tested by removing DcPV from infected wasps, or by adding DCpV to uninfected hosts, and determining the effect on larval development.

We now realize that animals are actually holobionts: an aggregate of eukaryotes, bacteria, and viruses. Therefore host-parasite interactions are really holobiont-holobiont interactions.

TWiV 434: Live long and pupate

The esteemed TWiVumvirate reveal the discovery of a new negative stranded RNA virus of wasps that regulates longevity and sex ratio of its parasitoid host.

You can find TWiV #434 at microbe.tv/twiv, or listen below.

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TWiV 372: Latent viral tendencies

TWiVOn episode #372 of the science show This Week in Virology, the TWiV-osphere introduces influenza D virus, virus-like particles encoded in the wasp genome which protect its eggs from caterpillar immunity, and a cytomegalovirus protein which counters a host restriction protein that prevents establishment of latency.

You can find TWiV #372 at microbe.tv/twiv

TWiV 213: Not bad for a hobby

On the final episode of the year of the science show This Week in Virology, the TWiV team reviews twelve cool virology stories from 2012.

You can find TWiV #213 at www.microbe.tv/twiv.

TWiV #21: Viruses of bacteria

twiv_aa_2001In episode #21 of This Week in Virology, Vincent, Dick, and Alan are joined by Max Gottesman, who has researched viruses of bacteria – bacteriophages – for many years. They discuss an unusual wasp-virus symbiosis, influenza transmission and absolute humidity, how mosquitoes survive Dengue virus infection, and bacteriophages.

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An unusual symbiosis between wasp and virus

2975832795_0d8af1c9d4_mEndoparasitic wasps inject their eggs into moth or butterfly larvae, which are cannibalized as the eggs mature and develop into adult wasps. The wasp larvae survive in the caterpillars because the eggs are injected together with virus particles called polydnaviruses. These viruses replicate in cells of the caterpillar, and their genomes express proteins that modify host defenses and physiology. Polydnaviruses are unusual because their genomes encode no structural proteins. A new study reveals that the wasp genome provides viral structural proteins that are probably used to package the polydnavirus genome.

The genome packaged within polydnavirus capsids consists of multiple circles of dsDNA (30 for the Cotesia congregata bracovirus for a total of 560 kb). The viral DNA is integrated into the wasp genome and is therefore transmitted vertically. However, virions are produced only in the wasp ovaries. When the polydnavirus is injected into the caterpillar, no genome replication occurs, although viral mRNAs and proteins are produced. These proteins are essential for successful maturation of the wasp in the larval host.

To identify the polydnavirus capsid proteins, the authors sought related genes expressed in wasp pupal ovaries at times of maximal viral particle production. By sequencing DNA copies of wasp mRNAs, they identified 22 genes related to those of nudiviruses, ancient viruses similar to baculoviruses. The genes encode subunits of DNA-dependent RNA polymerase and structural proteins. Curiously, no genes encoding DNA polymerase were identified, suggesting that polydnavirus DNA is copied by wasp enzymes. The implication is that the enzymes for polydnavirus mRNA and capsid synthesis are produced in the wasp ovary from nudivirus genes.

There are other examples of defective viruses that do not encode capsid proteins. The RNA genome of hepatitis delta virus is encapsidated within hepatitis B virus particles, which is provided in co-infected cells. Similar satellites of plants depend upon co-infection with a helper virus to provide capsid proteins. The wasp polydnaviruses are unique because the capsid proteins are not provided by another virus, but by the wasp host. In turn, the wasp needs the virus to survive – closing the circle on a highly unusual symbiotic relationship.

A. Bezier, M. Annaheim, J. Herbiniere, C. Wetterwald, G. Gyapay, S. Bernard-Samain, P. Wincker, I. Roditi, M. Heller, M. Belghazi, R. Pfister-Wilhem, G. Periquet, C. Dupuy, E. Huguet, A.-N. Volkoff, B. Lanzrein, J.-M. Drezen (2009). Polydnaviruses of Braconid Wasps Derive from an Ancestral Nudivirus Science, 323 (5916), 926-930 DOI: 10.1126/science.1166788