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into host lepidopteran larvae. The viruses seem to be vertically transmitted and
integrated into the chromosome of the wasp ( Fleming and Summers 1991 ). Each
wasp species seems to carry a polydnavirus characteristic of that species. If one
species within a genus carries a polydnavirus, all are likely to do so ( Stoltz and
Whitfield 1992 ).
Insects possess immune mechanisms that protect them from microorganisms,
other invertebrates, and abiotic material ( Hultmark 1993, Gillespie et  al. 1997 ).
Protection occurs through constitutive factors or by inducible humoral and cel-
lular responses. Many behavioral, morphological, nutritional, and endocrine
factors determine whether the interactions between a host and a parasitoid
will lead to development of the parasitoid or to its destruction ( Fleming 1992 ).
Polydnaviruses alter the host insect's neuroendocrine and immune responses,
preventing encapsulation of parasitoid eggs and larvae by host hemocytes, and
influence development of the host to benefit the parasitoid ( Webb and Cui
1998, Shelby and Webb 1999 ). The virus replicates asymptomatically in the para-
sitoid, but it causes a pathogenic infection in the lepidopteran host ( Webb and
Cui 1998 ). The virus alone can induce altered immune responses in some hosts,
but in other hosts the venom injected by the wasp also must be present for the
full effect of the virus to occur. Parasitoid wasps seem to benefit significantly
from the polydnaviruses. The virus also clearly benefits if the parasitoid is able
to reproduce, because polydnaviruses are known to replicate only within their
host parasitoids.
Sequencing of genomes of Braco- and Ichnoviruses indicate that the rela-
tionship between viruses and host parasitoids has existed for at least 70 million
years ( Dupuy et  al. 2006 ). One hypothesis is that an ancestral Ichneumonoidea
genome contained genes that were transmitted to ancestral Braconidae and
Ichneumonidae. Dupuy et al. (2006) suggested the two wasp families each cap-
tured mobile elements or viruses whose replication and encapsidation machin-
ery were used to form the basis of the ancestral provirus. The integrated
viruses then captured wasp genes that were beneficial for a parasitic life cycle.
Convergent evolution led to the capture of related genes and the loss of viral
structural genes, with gene flow taking place between the wasp genome and
the virus leading to evolved proviruses that eventually developed in the braco-
nid and ichneumonid wasp lineages that contain the polydnaviruses.
The polydnavirus-parasitoid-lepidopteran system provides an unusual exam-
ple of an obligate mutualistic association between a virus and a parasitoid
that functions to the detriment of the lepidopteran. The origin of polydnavi-
ruses is unknown, as is how they became established in the parasitoid genome.
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