Insect Physiology:
The University of Arizona
Elizabeth Willott

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: 344-351.

The reference list at the end contains several additional references to
(1) Cotesia congregata (a Braconid) and parasitism of Manduca sexta
(2) Microplitis demolitor (a Braconid) and parasitism of Pseudoplusia includens
(3) Campoletis sonorensis (an Ichneumonid) and parasitism of several Lepidopteran hosts
(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 genome.

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 dies.
"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 offspring.

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)

A Polydnavirus

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 integrated")
• Virions are formed in the specialized calyx cells of the female wasp.
• Virus buds from cells and accumulates in oviduct with egg and other oviduct secretions.
• In at least the Ichneumonid Campoletis sonorensis, the viral DNA is excised from the genome of the calyx cells of the female (Cui, 2000).

Consequences

1. Immunosuppression of lepidopteran host's immune response against the wasp egg.
2. Developmental arrest of lepidopteran host.

Immunosuppression of lepidopteran host's immune response against the wasp egg

• 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.

Some examples:

• 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).

There are two key hemocytes involve in the cell-based immune response of lepidoptera. They are granular cells, sometimes called granulocytes, and plasmatocytes.

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.

Developmental arrest

• 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. sonorensis develop.

The polydnavirus genome

• 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 others.
• 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 detail).

The nucleocapsid structure
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 Dib-Hajj)

Proteins encoded by the genome
Cysteine-rich 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 flanking exons.
  
• 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)

Glycosylated protein genes
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 open-reading frame.

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 hemocytes.

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 Cells
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): 165-195.

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: 78-89.

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: 1397-1407.
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 33: 143-153.

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: 582-583.

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. 189-225.

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: 859-870.

Krell, P.J. (1991). Viruses of Invertebrates Ed. E. Kurstak. New York, Dekker. 141-177.

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 polydnavirus in host immune suppression. Developmental and Comparative Immunology 20(1): 1-21.

Olivera, B.M., J. Riveri, C. Clark, C.A. Ramilo, G.P. Corpuz, F.C. Abogadie, E.E. Mena, S.R. Woodward, et al. (1990). Diversity of Conus neuropeptides. Science 249: 257-263.

Pennacchio, F., S.B. Vinson, C. Malva (2000). Regulation of host endocrine system by the endophagous braconid Cardiochiles nigriceps and its polydnavirus. Comparative Biochemistry and Physiology Part B 126: S76.

Schmidt, O., U. Theopold, M.R. Strand (2001). Innate immunity and its evasion and suppression by hymenopteran endoparasitoids. BioEssays 23: 344-351.

Shelby, K.S., B.A. Webb (1994). Polydnavirus infection inhibits synthesis of an insect plasma protein, arylphorin. Journal of General Virology 75: 2285-2292.

Shelby, K.S., B.A. Webb (1997). Polydnavirus iinfection inhibits translation of specific growth-associated host proteins. Insect Biochemistry and Molecular Biology 27: 263-270.

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.

Stoltz, D.B., S.B. Vinson (1979). Advances in Virus Research 24: 125-171.

Stoltz, D.B. (1993). Parasites and pathogens of insects. Eds. N. E. Beckage, S. M. Thompson and B. A. Federici. San Diego, Academic. 167-187.

Strand, M.R., D.I. McKenzie, V. Grassl, B.A. Dover, J.M. Aiken (1992). Journal of General Virology 73: 1627-1635.

Strand, M.R. (1994). Microplitis demolitor polydnavirus infects and expresses in specific morphotypes of Pseudoplusia includens haemocytes. Journal of General Virology 75(11): 3007-3020.

Strand, M.R., L.L. Pech (1995). Microplitis demolitor polydnavirus induces apoptosis of a specific haemocyte morphotype in Pseudoplusia includens. Journal of General Virology 76(part 2): 283-291.

Strand, M.R., R.A. Witherell, D. Trudeau (1997). Two Microplitis demolitor polydnavirus mRNAs expressed in hemocytes of Pseudoplusia includens contain a common cysteine-rich domain. Journal of Virology 71(3): 2146-2156.

Summers, M.D., S.D. Dib-Hajj (1996). Polydnavirus-facilitated endoparasite protection against host immune defenses. Chemical Ecology : The Chemistry of Biotic Interaction Eds. T. Eisner and J. Meinwald. Washington DC, National Academy Press. 67-85.

Theilmann, D.A., M.D. summers (1986). Journal of General Virology 67: 1961-1969.

Theilmann, D.A., M.D. Summers (1987). Journal of Virology 61: 2589-2598.

Theilmann, D.A., M.D. summers (1988). Virology 167: 329-341.

Trudeau, D., M.R. Strand (1998). A limited role in parassitism for Microplitis demolitor polydnavirus. Journal of Insect Physiology 44: 795-805.

Trudeau, D., A.R. Witherall, M.R. Strand (2000). Characterization of two novel Microplitis demolitor polydnavirus mRNAs expressed in Pseudoplusia includens haemocytes. Journal of General Virology 81: 3049-3058.

Washburn, J.O., E.J. Haas-Stapleton, F.F. Tan, N.E. Beckage, L.E. Volkman (2000). Co-infection of Manduca sexta larvae with polydnavirus from Cotesia congregata increases susceptibility to fatal infection by Autographa california M nucleopolyhedrovirus. Journal of Insect Physiology 46: 179-190.

Webb, B.A. (1998). Polydnavirus biology, genome structure, and evolution. The Insect Viruses Eds. L. K. Miller and A. L. Ball. New York, Plenum Press. 1st edition, ed. 105-139.

Webb, B.A., M.D. Summers (1990). Proceedings of the National Academy of Science USA 87: 4961-4965.