A polydnavirus (PDV) or more recently, polydnaviriform
is a member of the family Polydnaviridae of insect viruses. There are two genera in the family: bracoform and Ichnoviriform. Polydnaviruses form a symbiotic relationship with parasitoid wasps. Ichnoviriforms (IV) occur in Ichneumonid wasps and Bracoviriforms (BV) in Braconid wasps. The larvae of wasps in both of those groups are themselves parasitic on Lepidoptera (moths and butterflies), and the polydnaviruses are important in circumventing the immune response of their parasitized hosts. Little or no sequence homology exists between BV and IV, suggesting that the two genera have been evolving independently for a long time.
Taxonomy
Bracoviriform
Bracoviriform altitudinis
Bracoviriform argentifrontis
Bracoviriform blackburni
Bracoviriform canadense
Bracoviriform congregatae
Bracoviriform crassicornis
Bracoviriform croceipedis
Bracoviriform curvimaculati
Bracoviriform demolitoris
Bracoviriform ectdytolophae
Bracoviriform facetosae
Bracoviriform flavicoxis
Bracoviriform flavipedis
Bracoviriform flavitestaceae
Bracoviriform fumiferanae
Bracoviriform glomeratae
Bracoviriform hyphantriae
Bracoviriform inaniti
Bracoviriform indiense
Bracoviriform insularis
Bracoviriform kariyai
Bracoviriform liparidis
Bracoviriform marginiventris
Bracoviriform melanoscelae
Bracoviriform nigricipitis
Bracoviriform ornigis
Bracoviriform paleacritae
Bracoviriform quadridentatae
Bracoviriform rubeculae
Bracoviriform schaeferi
Bracoviriform texani
Ichnoviriform
Ichnoviriform acronyctae
Ichnoviriform annulipedis
Ichnoviriform aprilis
Ichnoviriform arjunae
Ichnoviriform benefactoris
Ichnoviriform eribori
Ichnoviriform exiguae
Ichnoviriform flavicinctae
Ichnoviriform forcipatae
Ichnoviriform fugitivi
Ichnoviriform fumiferanae
Ichnoviriform geniculatae
Ichnoviriform infestae
Ichnoviriform interrupti
Ichnoviriform lymantriae
Ichnoviriform montani
Ichnoviriform pilosuli
Ichnoviriform rivalis
Ichnoviriform rostralis
Ichnoviriform sonorense
Ichnoviriform tenuifemoris
Ichnoviriform terebrantis
Structure
Viruses in Polydnaviridae are enveloped, with prolate ellipsoid and cylindrical geometries. Genomes are circular and segmented, composed of multiple segments of double-stranded, superhelical DNA packaged in capsid proteins. They are around 2.0–31kb in length.
Life cycle
Viral replication is nuclear. DNA-templated transcription is the method of transcription. The virus exits the host cell by nuclear pore export.
Parasitoid wasps serve as hosts for the virus, and Lepidoptera serve as hosts for these wasps. The female wasp injects one or more eggs into its host along with a quantity of virus. The virus and wasp are in a mutualistic symbiotic relationship: expression of viral genes prevents the wasp's host's immune system from killing the wasp's injected egg and causes other physiological alterations that ultimately cause the parasitized host to die. Transmission routes are parental.
Biology
These viruses are part of a unique biological system consisting of an endoparasitic wasp (parasitoid), a host (usually lepidopteran) larva, and the virus. The full genome of the virus is endogenous, dispersed among the genome of the wasp. The virus only replicates in a particular part of the ovary, called the calyx, of pupal and adult female wasps. 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, as the virion carries virulence genes instead of viral replication genes. It can be considered a type of viral vector.
Without the virus infection, phagocytic hemocytes (blood cells) will encapsulate and kill the wasp egg and larvae, but the immune suppression caused by the virus allows survival of the wasp egg and larvae, 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.