The University of Arizona
Endoparasitoids and Polydnaviruses
This page relies heavily on two sources:
1. Max D. Summers and Sulayman D Dib-Hajj titled: Polydnavirus-facilitated
endoparasite protection against host immune defenses.
2. Schmidt, O., U. Theopold, M.R. Strand (2001). Innate immunity and its
evasion and suppression by hymenopteran endoparasitoids. BioEssays 23:
The reference list at the end contains several additional references to
(1) Cotesia congregata (a Braconid) and parasitism of Manduca
(2) Microplitis demolitor (a Braconid) and parasitism of
(3) Campoletis sonorensis (an Ichneumonid) and parasitism of several
(4) A few other endoparasitoid-host systems
Short Basic Introduction
These pages discuss examples of lepidoptera larvae (caterpillar)
parasitized by wasps (Hymenoptera) of the Ichneumonidae and Braconidae.
These wasps may (often? always?) contain a virus integrated into the
The virus helps the wasp by allowing the wasp egg avoid encapsulation (and
hence its death) by the lepidopteran larvae's immune cells.
"Permissive" hosts are those caterpillars in which the eggs escape
detection and the wasp flourishes, and the caterpillar nearly always
"Non-permissive" hosts either lack something the endoparasitoid needs to
develop or else the host recognizes the egg as foreign, hemocytes
encapsulate it, and thereby kill it.
There are about 225,000 species of parasitoid wasps (see Schmidt, 2001 for
further citation on that number), but each species is generally only
successful parasitizing one or a few host species.
The virus does not replicate in the lepidopteran larvae. The virions are
capable of infecting cells of the caterpillar, particularly relevant to the
killing is the ability to infect the hemocytes.
Virions are only formed in the wasp, notably in the calyx cells of the
female reproductive tract. They are released into the calyx fluid and get
injected into the caterpillar when the wasp lays her egg.
Introduction in More Depth
During DNA replication of the wasp (prior to either mitosis or meiosis;
both male and female wasps), the virus DNA is also replicated, just like
any other part of the wasp genome. To that extent it IS part of the wasp
genome. The virus is, therefore, vertically transmitted to all the wasp's
In the reproductive tract of the female, the virus is expressed and more
copies of its DNA are made and packaged with appropriate proteins and it
acquires a double coat: one membrane layer from the nucleus, another from
the cell membrane. The DNA packaged in these virions consists of multiple
chromosomes. Hence the name Polydnavirus. There are several copies of some
chromosomes in the final virion.
When the wasp injects her egg or eggs into the host caterpillar, the virus
is also injected along with the calyx fluid. The calyx fluid contains
several proteins (among other things less well characterized...).
In several systems (lepidopteran host, wasp, virus) studied, the virus
plays a key role in handicapping the lepidopteran larvae so it cannot mount
an effective cell-based immune response against the injected egg. This
particular page addresses that. In addition, there is evidence that the
production of antibacterial proteins by the fat body is also compromised
and a paragraph about that is included.
Future pages will address the roles of the calyx proteins and the role of
the developing wasp in changing the physiology of the lepidopteran host so
the developing wasp has a more suitable environment, to the detriment of
the lepidopteran host! The developing wasp needs several days to develop.
For example, C. sonorensis infecting H. virescens requires
9-12 days to develop. (Ref Summers and Dib-Hajj)
This section covers the structure of the virion, the genes encoded, and the
viral proteins expressed.
The viral genome
- double-stranded DNA genome
- integrated into wasp genome (Summers and Dib-Hajj say "non-randomly
- Virions are formed in the specialized calyx cells of the female wasp.
- Virus buds from cells and accumulates in oviduct with egg and other
- In at least the Ichneumonid Campoletis sonorensis, the viral DNA
is excised from the genome of the calyx cells of the female (Cui,
- Immunosuppression of lepidopteran host's immune response against the
- Developmental arrest of lepidopteran host.
Immunosuppression of lepidopteran host's immune response against the
- Endoparasite needs to evade host's immune response.
- Normal lepidopteran larvae response to large foreign material is to
encapsulate it. For review see, (Stoltz, 1993)
- Endoparasite needs means to evade encapsulation.
There are two key hemocytes involve in the cell-based immune response of
lepidoptera. They are granular cells, sometimes called granulocytes, and
- Purified polydnavirus caused suppression of Heliothis virescens'
ability to encapsulate Campoletis sonorensis eggs (Edson, 1981 and
perhaps Theilmann, 1986, both cited in Summer and Dib-Hajj)
- Inhibition of encapsulation correlates with changes in the number of
circulating hemocytes and behavior of plasmatocytes (Davies, 1987).
- Inhibition of encapsulation correlates with hemocyte cytoskeletal
rearrangements, loss of ability to spread on foreign surfaces, and
promotion of apoptosis (Schmidt, 2001 gives several citations for this).
The plasmatocytes are inhibited from spreading and the granular cells are
stimulated to undergo apoptosis when Microplitis demolitor (and its
polydnavirus) infects P. includens (Strand, 1995).
More specifically, these cells must be infected to get the inhibition of
spreading and promotion of apoptosis.
- Summers and Dib-Hajj reference: Edson 1981 and perhaps Theilmann, 1986,
but also note several papers by Beckage in the reference list.
- Successfully parasitized C. sonorensis gain less weight than
non-parasitized larvae and pupation is delayed, extending the larval stage
considerably. (see Beckage 1993a; Beckage 1993b, cited in Summers and
Dib-Hajj) Presumably this is to give the endoparasitoid a suitable,
stable, environment for its development.
- The importance of the virus is shown by the following observation:
Injecting washed C. sonorensis eggs (so no virions and no other
contents of the calyx fluid) into H. virescens results in the eggs
being encapsulated. However if eggs plus calyx fluid, or eggs plus gradient
purified virions, are injected, then encapsulation is inhibited,
developmental arrest of H. virescens occurs, and the C.
The polydnavirus genome
The nucleocapsid structure
- In the wasp, the DNA is integrated into the chromosome; it is ALSO
found as episomal DNA.
- In the virion, the virus genome consists of multiple, covalently
closed, circular, double-stranded DNAs (refs cited in Summers and Dib-Hajj
include Fleming, 1993; Krell, 1991; Francki, 1991).
- Different virus species contain different numbers but the number of DNA
pieces can be less than 10 to more than 25 circles. (I hesitate to say
chromosomes because I don't know if these have a normal origin of
replication and histones or whatever.)
- Genome size ranges from 75 kbp for some virus species to over 250 for
- For C. sonorensis, the genome consists of over 28 DNA circles.
- Many of the genes of the C. sonorensis polydnavirus are part of
larger gene families; i.e., there are several versions, varying only
moderately at best, in the genome.
- The genes include sequences encoding introns.
- The polydnavirus of Microplitis demolitor also has gene
families. The two gene families (that I know about) investigated so far are
discussed below under gene families. See Trudeau, 2000 for more
For C. sonorensis, the polydnavirus is ellipsoid with two envelopes:
one from the nuclear membrane, the other acquired as it buds through the
calyx cell membrane into the oviduct.
The virus in the lepidopteran host
- The virion is capable of infecting several different tissues,
including the hemocytes.
- The virus apparently does not replicate in the lepidopteran host,
although it persists for a considerably length of time in certain tissues.
(Theilmann, 1986, Strand, 1992 cited in Summers and Dib-Hajj)
Viral genome expression
- Viral genes are expressed in the lepidopteran host (ref 40) and in
some tissues of the wasp.
- In the lepidopteran host, at least 12 different viral mRNAs were
detected in northern analysis at suitable times (starting 2 hours) after
parasitization. (Theilmann, 1988, Blissard, 1986a; Blissard, 1986b,
Blissard, 1987, Blissard, 1989, Theilmann, 1987 cited in Summers and
Proteins encoded by the genome
Glycosylated protein genes
- For C. sonorensis polydnavirus, several protein-encoding genes
encode proteins that are rich in cysteine, hence these are called the
Cysteine-Rich Gene Family (Summers and Dib-Hajj).
The terminology used goes: WHv1.0, WHv1.6, and WHv1.1 and means:
- DNA segment W (segments are labeled A through W by increasing size);
- Hv stands for Heliothis virescens since the virus genome is
being expressed in that insect
- The number corresponds to the RNA size in kb.
- The cysteine-rich proteins contain one (WHv1.0, WHv1.6) or two (WHv1.1)
cysteine motifs C-C-CC-C-C (Blissard, 1987, Dib-Hajj, 1993).
- WHv1.0, WHv1.6 contain five more conserved regions which are about
68-80% identical at the nucleotide and amino acid levels. One region, in
intron 2, has 92% similarity which is higher than the similarities of the
- Microplitis demolitor genome includes several genes which have
coding for the cysteine motif. These genes are called EGF-like because they
resemble the epidermal growth factor at their N termini (Strand, 1997).
Strand et al have named the proteins from these genes Egf0.4, Egf1.0 or
Egf.1.5 to reflect the similarity to EGF (the number refers to the size of
the mRNA in kb) (Trudeau, 2000)
Microplitis demolitor genome include a gene that encodes a protein
with hydrophobic domains at its deduced N and C termini. It has six copies
of a heavily glycosylated repeat element in its central domain. (Trudeau,
2000) The transcript is 1.8 kb and the gene has been named Glc1.8. This
gene is also expressed in hemocytes infected by the M. demolitor
polydnavirus. The protein encoded by Glc1.8 is at the hemocyte cell
surface (by immunocytochemistry, Trudeau 2000).
A fifth transcript found in infected hemocytes is 3.1 kb (Trudeau, 2000).
Venom-related gene family
Polydnavirus envelope proteins have epitopes in common with proteins from
wasp venom glands. When virions were incubated with antibodies to these
epitopes, the antibodies neutralized the effectiveness of the virions in
inhibiting the H. virescens immune response. (Webb, 1990, cited in
Summers and Dib-Hajj)
Repeat Gene Family
These lack introns; share a 540 bp consensus sequence present within an
CrV1 of Cotesia rubecula. For more information, see (Asgari, 2002)
Cotesia rubecula polydnavirus has a gene whose product, CrV1,
inactivates hemocytes. It is one of only a few genes expressed by this
virus in the lepidopteran host Pieris rapae. (Also unusual about
this polydnavirus system is that expression of viral genes occurs primarily
between 4-12 h after parasitization, then stops).
The protein CrV1 is expressed and secreted from infected hemocytes.
The secreted protein is modified, then is subsequently taken back into
A specific coiled-coil region of the protein is required for it to bind to
lipophorin and be taken into the hemocytes.
Effect of Virus Infection on Translation in Fat Body
In addition to a cell-based response to threats, the larvae also express
antibacterial proteins. When H. virescens are parasitized by C.
sonorensis this process is compromised (Shelby, 1998).
- In a normal immune response, several antibacterial proteins and lectins
increase in concentration in hemolymph.
The increase is due to increased transcription induced via a NFkB/IkB-like
signal transduction system.
- When H. virescens are parasitized by C. sonorensis,
increased transcription still occurs, but the immune-relevant proteins are
not synthesized (other proteins are synthesized normally).
- This suggests a specific inhibition of translation occurs. (The
phenomenon of reduced protein levels had already been seen for certain
other fat body transcripts involved in maturation of the larvae:
translation of 3 storage proteins and juvenile hormone esterase. Citations
in Summers and Dib-Hajj are to Shelby 1994, 1997)
A partial list of references on polydnaviruses.
Beckage, N. (1994). Characterization and biological effects of Cotesia
congregata polydnavirus on host larvae of the tobacco hornworm,
Manduca sexta. Arch. Insect Biochemistry and Physiology 26(2-3):
Asgari, S., O. Schmidt (2002). A coiled-coil region of an insect immune
suppressor protein is involved in binding and uptake by hemocytes. Insect
Biochemistry and Molecular Biology 32: 497-504.
Asgari, S., M. Hellers, O. Schmidt (1996). Host haemocyte inactivation by
an insect parasitoid: transient expression of a polydnavirus gene. J.
General Virology 77(10): 2653-2662.
Asgari, S., O. Schmidt (1994). Passive protection of eggs from the
parasitoid, Cotesia rubecula, in the host, Pieris rapae.
Journal of Insect Physiology 40(9): 789-795.
Beckage, N.E. (1993a). Parasites and pathogens of insects Eds. N. E.
Beckage, S. M. Thompson and B. A. Federici. San Diego, Academic. 25-57.
Beckage, N.E. (1993b). Receptor 3: 233-245.
Beckage, N.E. (1998). Modulation of immune responses to parasitoids by
polydnaviruses. Parasitology 116: s57-s64.
Beckage, N.E., I.d. Buron (1993). Lack of prothoracic gland degeneration in
developmentally arrested host larvae of Manduca sexta parasitized by
the Braconid wasp Cotesia congregata. Journal of Invertebrate
Pathology 61: 103-106.
Blissard, G.W., J.G.W. Fleming, S.B. Vinson, M.D. Summers (1986a). Journal
of Insect Physiology 32: 351-359.
Blissard, G.W., O.P. Smith, M.D. Summers (1987). Virology 160: 120-134.
Blissard, G.W., D.A. Theilmann, M.D. Summers (1989). Virology 169: 78-89.
Blissard, G.W., S.B. Vinson, M.D. Summers (1986b). Journal of Virology 169:
Cui, L.W., B.A. Webb (1996). Isolation and characterization of a member of
the cysteine-rich gene family from Campoletis sonorensis
polydnavirus. Journal of General Virology 77(part 4): 797-809.
Cui, L., A.I. Soldevila, B.A. Webb (2000). Relationships between
polydnavirus gene expression and host range of the parasitoid wasp
Campoletis sonorensis. Journal of Insect Physiology 46:
Cusson, M., C. Beliveau, M. Laforge, G. Bellemare, D. Stoltz (2000).
Disruption of spruce budworm metamorphosis by the polydnavirus of
Tranosema rostrale: endocrine and molecular mechanisms. Comparative
Biochemistry and Physiology Part B 126: S28.
Davies, D.H., M.R. Strand, S.B. Vinson (1987). Journal of Insect Physiology
Dib-Hajj, S.D., B.A. Webb, M.D. Summers (1993). Proceedings of the National
Academy of Science USA 90: 3765-3769.
Edson, K.M., S.B. Vinson, D.B. Stoltz, M.D. Summers (1981). Science 211:
Fleming, J.G.W., P.J. Krell (1993). Parasites and pathogens of insects Eds.
N. E. Beckage, S. M. Thompson and B. A. Federici. San Diego, Academic.
Francki, R.I.B., C.M. Fauquet, D.L. Knudson, F. Brown, Eds. (1991).
Classification and nomenclature of viruses. New York, Springer.
Gruber, A., P. Stettler, P. Heineger, D. Schumperli, B. Lanzrein (1996).
Polydnavirus DNA of the braconid wasp Chelonus inanitus is
integrated in the wasp's genome and excised only in later pupal and adult
stages of the female. Journal of General Virology 77: 2873-2879.
Harwood (1994). Purification and characterization of an early-expressed
polydnavirus-induced protein from the hemolymph of Manduca sexta
larvae parasitized by Cotesia congregata. Insect Biochemistry and
Molecular Biology 24(7): 685-698.
Harwood, S.H., A.J. Grosovsky, E.A. Cowles, J.W. Davis, N.E. Beckage
(1994). An abundantly expressed hemolymph glycoprotein isolated from newly
parasitized Manduca sexta larvae is a polydnavirus gene product.
Virology 205(2): 381-392.
Hayakawa, Y., K. Yazaki (1997). Envelope protein of parasitic wasp symbiont
virus, polydnavirus, protects the wasp eggs from cellular immune reactions
by the host insect. European Journal of Biochemistry 246: 820-826.
Krell, P.J., M.D. Summers, S.B. Vinson (1982). Journal of Virology 43:
Krell, P.J. (1991). Viruses of Invertebrates Ed. E. Kurstak. New York,
Lavine, M.D., N.E. Beckage (1995). Polydnaviruses: potent mediators of host
insect immune dysfunction. Parasitology Today 11(10): 368-378.
Li, X.S., B.A. Webb (1994). Apparent functional role for a cysteine-rich
polydnavirus protein in suppression of the insect cellular immune response.
Journal of Virology 68(11): 7482-7489.
Luckhart, S., B.A. Webb (1996). Interaction of a wasp ovarian protein and
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Immunology 20(1): 1-21.
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Pennacchio, F., S.B. Vinson, C. Malva (2000). Regulation of host endocrine
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Schmidt, O., U. Theopold, M.R. Strand (2001). Innate immunity and its
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Shelby, K.S., B.A. Webb (1994). Polydnavirus infection inhibits synthesis
of an insect plasma protein, arylphorin. Journal of General Virology 75:
Shelby, K.S., B.A. Webb (1997). Polydnavirus iinfection inhibits
translation of specific growth-associated host proteins. Insect
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Shelby, K.S., L. Cui, B.A. Webb (1998). Polydnavirus-mediated inhibition of
lysozyme gene expression and the antibacterial response. Insect Molecular
Biology 7(3): 265-272.
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Stoltz, D.B. (1993). Parasites and pathogens of insects. Eds. N. E.
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Strand, M.R., D.I. McKenzie, V. Grassl, B.A. Dover, J.M. Aiken (1992).
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Strand, M.R. (1994). Microplitis demolitor polydnavirus infects and
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Strand, M.R., L.L. Pech (1995). Microplitis demolitor polydnavirus
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