Polydnavirus

Ichnovirus Bracovirus

The Polydnaviruses (PDV) are a family of insect viruses that contain two genera: Ichnoviruses (IV) and Bracoviruses (BV). The genome of the virus is composed of multiple segments of double-stranded, superhelical DNA packaged in capsid proteins and a double layer (IV) or single layer (BV) envelope.

Biology
These viruses are part of a unique biological system consisting of an endoparasitic wasp (parasitoid), an insect (usually lepidopteran) larva, and the virus. The full genome of the virus is integrated into the genome of the wasps and the virus only replicates in specific cells in the female wasp's reproductive system. The virus is injected along with the wasp egg into the body cavity of a lepidopteran host caterpillar and infects cells of the caterpillar. The infection does not lead to replication of new viruses, rather it affects the caterpillar's immune system. Without the virus infection, phagocytic hemocytes (blood cells) will encapsulate and kill the wasp egg but the immune suppression caused by the virus allows for survival of the wasp egg, leading to hatching and complete development of the immature wasp in the caterpillar. Additionally, genes expressed from the polydnavirus in the parasitised host alter host development and metabolism to be beneficial for the growth and survival of the parasitoid larva. Thus, the virus and wasp have a symbiotic (mutualistic) relationship.

Characteristics
Both genera of PDV share certain characteristics:


 * the virus particles of each contain multiple segments of dsDNA with each segment containing only part of the full genome (much like chromosomes in eucaryotic organisms);


 * the genome of each is integrated into the host wasp genome;


 * the virus particles are only replicated (produced) in specific cell types in the female wasp reproductive organs.

However, the morphology of the two genera are different when observed by electron microscopy. Ichnoviruses tend to be ovoid (egg-shaped) while bracoviruses are short rods.

Also, as their names suggest, the ichnoviruses occur in ichneumonid wasps species and bracoviruses in braconid wasps.

Little or no sequence homology exists between BV and IV, suggesting that the two genera evolved independently.

Evolution
Nucleic acid analysis suggests a very long association of the viruses with the wasps (greater than 70 million years).

Two proposals have been advanced for how the wasp/virus association developed. The first suggests that the virus is derived from wasp genes. Many parasitoids that do not use PDVs inject proteins that provide many of the same functions, that is, a suppression of the immune response to the parasite egg. In this model, the braconid and ichneumonid wasps packaged genes for these functions into the viruses – essentially creating a gene-transfer system that results in the caterpillar producing the immune-suppressing factors. In this scenario, the PDV structural proteins (capsids) were probably “borrowed” from existing viruses.

The alternative proposal suggests that ancestral wasps developed a beneficial association with an existing virus that eventually led to the integration of the virus into the wasp’s genome. Following integration, the genes responsible for virus replication and the capsids were (eventually) no longer included in the PDV genome. This hypothesis is supported by the distinct morphology differences between IV and BV, suggesting different ancestral viruses for the two families. The IV have remarkable similarities to ascoviruses while BV may be related to baculoviruses